Trends Neurosci May 1986;9:186-192

Microcircuitry and functional architecture of the cat retina.

Sterling P, Freed M, Smith RG

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104, USA.

Neurons in cat retina belong to specific types. Each type is characterized by a specific correspondence between morphology and physiology and forms a regular array that connects lawfully to the arrays of certain other types. Two circuits have been traced quantitatively through these arrays from photoreceptors to alpha- and beta- ganglion cells. The 'cone-bipolar circuit' appears to convey the center-surround receptive field to ganglion cells, using cones in daylight and rods (via gap junctions to cones) in twilight. A 'rod-bipolar circuit' appears to convey the quantal signal and the pure center receptive field to the ganglion cells in starlight.

Eur J Neurosci Feb 1992;4:506-520

Parallel Circuits from Cones to the On-Beta Ganglion Cell.

Cohen E, Sterling P

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104, USA.

Neural integration depends critically upon circuit architecture; yet the architecture has never been established quantitatively (numbers of cells and synapses) for any vertibrate local circuit. Here we describe circuits in the cat retina that connect cones to the on-beta ganglion cell. This cell type is important because on- and off-beta cells contribute about 50% of the optic nerve fibers and the major retinal input to the striate cortex. Three adjacent on-beta cells in the area centralis and their bipolar connections to cones were reconstructed from electron micrographs of 279 serial section. The beta dendritic field is 34±2 micrometers in diameter and encompasses 35 cones. All of these cones connect to the beta cell via 14-17 bipolar cells. These bipolar cells were shown previously by cluster analysis to be of four types (b1-b4); three of these types (b1, b2 and b3) provided 97% of the bipolar contacts to the beta cell, in the ratio 4:2:1. On average, bipolar cells nearest the centre of the beta dendritic field contribute more synapses than those towards the edge, but the peaked distribution of bipolar synapses across the dendritic field is only slightly broader than the optical pointspread function of the cat's eye, and is narrower by half than the centre of the ganglion cell receptive field. This implies that the distribution of bipolar synapses across the beta cell dendritic field contributes little to the extent or shape of the receptive field. Since all three bipolar circuits connect to the same set of cones, they must carry the same spatial and chromatic information; they might convey different temporal frequencies. The numbers of bipolar synapses (mean±SD = 154±8) and amacrine synapses (59±5) converging on three adjacent beta cells are remarkably constant (SD approx. equals 5% of the mean). Thus, as the circuits repeat locally, the fundamental design is accurately reproduced.

Vis Neurosci 1994 Mar;11(2):261-269

Conductances evoked by light in the ON-beta ganglion cell of cat retina.

Freed MA, Nelson R

Laboratory of Neurophysiology, National Institute of Neurological Disorders and Stroke, Bethesda, MD 20892.

When a bar of light (215 x 5000 microns) illuminates the receptive field of an ON-beta ganglion cell of cat retina, the cell depolarizes. Intracellular recording from the cat eyecup preparation shows that this depolarization is due to an increase in conductance (2.4 +/- 0.6 nS). Different phases of this depolarization have different reversal potentials, but all of these reversal potentials are more positive than the cell's resting potential in the dark. When the light is turned on, there is an initial transient depolarization; the reversal potential measured for this transient is positive (23 +/- 11 mV). As the light is left on, the cell partially repolarizes to a sustained depolarization; the reversal potential measured for this sustained depolarization is close to zero (-1 +/- 5 mV). When the light is turned off, the cell repolarizes further; the reversal potential measured for this repolarization is negative (-18 +/- 7 mV), but still above the resting potential in the dark (-50 mV). To explain this variety of reversal potentials, at least two different synaptic conductances are required: one to ions which have a positive reversal potential and another to ions which have a negative reversal potential. Comparing the responses to broad and narrow bars suggests that these two conductances are associated with the center and surround, respectively. Finally, since an ON-beta cell in the area centralis receives about 200 synapses, these results indicate that a single synapse produces an average conductance increase of about 15 pS during a near-maximal depolarization.

J Comp Neurol 1993 Mar 1;329(1):68-84

OFF-alpha and OFF-beta ganglion cells in cat retina. I: Intracellular electrophysiology and HRP stains.

Nelson R, Kolb H, Freed MA

Laboratory of Neurophysiology, National Institute of Neurological Diseases and Stroke, National Institutes of Health, Bethesda, Maryland 20892.

Nature 1996 Jun 13;381(6583):613-615

Absence of spectrally specific lateral inputs to midget ganglion cells in primate retina.

Calkins DJ, Sterling P

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104, USA.

Visual information is conveyed to the brain by the retinal ganglion cells. Midget ganglion cells serve fine spatial vision by summing excitation from a receptive field 'centre', receiving input from a single cone in the central retina, with lateral inhibition from a receptive field 'surround', receiving input from many surrounding cones. Midget ganglion cells are also thought to serve colour opponent vision because the centre excitation is from a cone of one spectral type, while the surround inhibition is from cones of the other type. The two major cone types, middle(M)- and long-(L)wavelength sensitive, are equally numerous and randomly distributed in the primate central retina, so a spectrally homogeneous surround requires that the cells mediating la teral interactions (horizontal or amacrine cells) receive selective input from only one cone type. Horizontal cells cannot do this because they receive input indiscriminately from M and L cones. Here we report that the amacrine cells connected to midget g anglion cells are similarly indiscriminate. The absence of spectral specificity in the inhibitory wiring raises doubt about the involvement of midget ganglion cells in colour vision and suggest that colour opponency may instead be conveyed by a different type of ganglion cell.

Vision Res 1996 Nov;36(21):3373-3381

Foveal cones form basal as well as invaginating junctions with diffuse ON bipolar cells.

Calkins DJ, Tsukamoto Y, Sterling P

Mahoney Institute for Neurological Sciences, University of Pennsylvania, Philadelphia 19104-6058, USA. calkins@mpih-frank-furt.mpg.d400.de

The response of a mammalian bipolar cell is generally thought to be determined by the location and morphology of synapses from the cone terminal: ON bipolar cells are believed to be depolarized strictly at invaginating contacts and OFF bipolar cells hy perpolarized at basal contacts. This hypothesis was re-investigated in the macaque fovea (1 deg nasal) using electron micrographs of serial sections. We determined the number of invaginating sites available and then identified the contacts to bipolar cell s with axons in the ON level of the inner plexiform layer. A cone terminal forms about 20 active zones marked by ribbons. A few active zones house two invaginating dendrites, so there are 22 invaginating sites per cone. A midget ON bipolar cell collects 1 8 invaginating contacts from one cone, thus only about four invaginating sites remain for diffuse ON bipolar cells. Two diffuse ON cells were reconstructed; each collects about 25 contacts from an estimated 10 cones. Only three or four of these contacts a re invaginating; the rest are basal, adjacent to the triad. This suggests that basal contacts can be depolarizing. The distance from the vesicle release site at active zones to an invaginating contact is 140 +/- 40 nm; to a basal contact adjacent to the t riad is 500 +/- 160 nm, and to the next nearest basal contact is 950 +/- 370 nm.

Nature 1994 Sep 1;371(6492):70-72

M and L cones in macaque fovea connect to midget ganglion cells by different numbers of excitatory synapses.

Calkins DJ, Schein SJ, Tsukamoto Y, Sterling P

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104.

J Comp Neurol 1986 Aug 1;250(1):1-7

Accumulation of (3H)glycine by cone bipolar neurons in the cat retina.

Cohen E, Sterling P

Cone bipolar neurons in the cat retina were studied in serial sections prepared as electron microscope autoradiograms following intravitreal injection of (3H)glycine. The goal was to learn whether the cone bipolar types that accumulate glycine correspo nd to the types thought on other grounds to be inhibitory. About half of the cone bipolars in a given patch of retina showed specific accumulation of silver grains. The specificity of accumulation was similar to that shown by glycine-accumulating amacrine s. All of the cone bipolars arborizing in sublamina b accumulated glycine but none of the cone bipolars arborizing in sublamina a did so. The types of cone bipolars accumulating glycine did not match the types thought to be inhibitory. Cone bipolar types CBb1 and CBb2 both form gap junctions with the glycine-accumulating AII amacrine, thus raising the possibility that glycine might accumulate in these cone bipolars by diffusion from the AII cell or vice versa. Thus it is logically impossible to tell which of these three cells contains a high-affinity uptake mechanism for glycine and consequently which of the three might actually use glycine as a neurotransmitter.

Philos Trans R Soc Lond B Biol Sci 1990 Dec 29;330(1258):305-321

Demonstration of cell types among cone bipolar neurons of cat retina.

Cohen E, Sterling P

Department of Anatomy, University of Pennsylvania, Philadelphia 19104.

We identified all the cone bipolar cells (80) in a small patch of one retina and then studied in detail the complete subset (42) that sends axons to sublamina b of the inner plexiform layer. The point was to learn whether the 'types' suggested previous ly, based on a few examples from a large population, could be substantiated or whether there would be intermediate forms. Tissue from the area centralis (1 degree eccentricity), was prepared as a series of 279 ultrathin sections and photographed in the el ectron microscope. Thirteen cells were reconstructed completely and parcelled into five categories (b1-b5) based on external morphology. For nine of these cells (two from categories b1-b4 and one from b5) most of the synaptic inputs and outputs were ident ified. When these nine cells were parcelled according to their synaptic patterns, they sorted into the same five categories. The remaining 29 cells in the population, though not reconstructed, were studied in detail by tracing their processes through the series. Ten of these cells, those near the margin of the series, were incomplete. The other 19 cells had essentially the same distribution of morphologies and synaptic patterns as the subset studied by total reconstruction: when plotted in multiparametric space, they formed distinct clusters corresponding to the five morphological categories. There was no hint of intermediate forms. That all the neurons in the population sort into some cluster (no intermediate forms), and that each neuron sorts into the s ame cluster by different criteria, argues that the clusters represent natural types. Each type forms a regular array in the region studied with an axonal 'coverage factor' that is close to one.

Philos Trans R Soc Lond B Biol Sci 1990 Dec 29;330(1258):323-328

Convergence and divergence of cones onto bipolar cells in the central area of cat retina.

Cohen E, Sterling P

Department of Anatomy, University of Pennsylvania, Philadelphia 19104.

In the central area of cat retina the cone bipolar cells that innervate sublamina b of the inner plexiform layer comprise five types, four with narrow dendritic fields and one with a wide dendritic field. This was shown in the preceding paper (Cohen &a mp; Sterling 1990 a) by reconstruction from electron micrographs of serial sections. Here we show by further analysis of the same material that the coverage factor (dendritic spread x cell density) is about one for each of the narrow-field types (b1, b2, and b4). The same is probably true for the other narrow-field type (b3), but this could not be proved because its dendrites were too fine to trace. The dendrites of types b1, b2, and b4 collect from all the cone pedicles within their reach and do not bypa ss local pedicles in favour of more distant ones. The dendrites of type b5, the wide-field cell, bypass many pedicles. On average 5.1 +/- 1.0 pedicles coverage on a b1 bipolar cell; 6.0 +/- 1.2 converge on a b2 cell and 5.7 +/- 1.5 converge on a b4 cell. Divergence within a type is minimal: one pedicle contacts only 1.2 b1 cells, 1.0 b2 cells, and 1.0 b4 cells. Divergence across types is broad: each pedicle apparently contacts all four types of the narrow-field bipolar cells that innervate sublamina b. Ea ch pedicle probably also contacts an additional 4-5 types of narrow-field bipolar cell that innervate sublamina a. There are several possible advantages to encoding the cone signal into multiple, parallel, narrow-field pathways.

J Neurophysiol 1991 Feb;65(2):352-359

Microcircuitry related to the receptive field center of the on-beta ganglion cell.

Cohen E, Sterling P

Department of Anatomy, School of Medicine, University of Pennsylvania, Philadelphia 19104.

1. We have investigated the anatomic basis for the Gaussian-like receptive field center of the on-beta ("X") ganglion cell in the area centralis of cat retina. Three adjacent on-beta cells were reconstructed from electron micrographs of 279 s erial sections cut vertically through a patch of retina at approximately 1 degree eccentricity. 2. All the bipolar synapses on these cells were identified, and about one-half of these were traced to type b1 bipolar cells, which formed a regular array in t he plane of the retina. 3. On average, seven b1 cells contributed to a beta cell: bipolar axons near the middle of the beta dendritic field tended to give many contacts (12-33 contacts); axons near the edge of the field tended to give few contacts (3-4 co ntacts). 4. Each b1 cell collected from four to seven cones, and the mean number of cones converging through the b1 array to a beta cell was 30. 5. Assuming equal effectiveness for all b1->beta cell synapses, a spatial weighting function was derived fro m these results. The mean radius of this function at 1/e amplitude for three beta cells was 18.0 +/- 1.1 (SD) microns. This is considerably narrower than the corresponding measurements of the beta cell receptive field center (28 +/- 3 microns) at this ecc entricity. 6. It is concluded, in agreement with previous work, that all cones encompassed by the beta cell's dendritic field and those slightly beyond it connect directly to the beta cell via the b1 bipolar cell array. However, the center of the beta cel l receptive field is still broader by approximately 60%. This suggests that pooling of cone signals may occur at the level of the outer plexiform layer.

J Comp Neurol 1979 Dec 15;188(4):599-627

Microcircuitry of cat visual cortex: classification of neurons in layer IV of area 17, and identification of the patterns of lateral geniculate input.

Davis TL, Sterling P

Neurons in the cerebral cortex have been classified primarily by their differences in axonal and dendritic branching patterns observed in material impregnated by the Golgi method. Although these morphological differences are widely believed to reflect differences in connectivity, very little is actually known about the patterns of synaptic input to different cell types. We have obtained such information for 32 adjacent neurons in layer IVab of cat cortical area 17 by reconstructing them from electron m icrographs of 150 serial sections. Synaptic terminals from the lateral geniculate nucleus were labeled in this material by anterograde degeneration and their distribution, as well as that of normal terminals containing flat or round vesicles, was recorded . The neurons were divided into seven classes based on differences in size, shape, dendritic branching pattern and synaptic input. Class I cells were pyramidal with apical and basilar dendrites, dendritic spines, exclusively flat-vesicle terminals on the somas (11/100 micron2), and geniculate terminals on the basilar dendrites. Class II cells were large stellates (20 micron diameter) with dark cytoplasm and numerous flat-vesicle and round-vesicle terminals on the somas (48/100 micron2). Geniculate termin als contacted the cell bodies and primary, secondary, and tertiary dendrites. The Class III cell was stellate with varicose dendrites, a sparse distribution of flat-vesicle terminals (8/100 micron2) on the soma, and both geniculate and round-vesicle termi nals on the dendrites. Class IV cells had radially elongated somas with sharply tapered apical and basilar dendrites bearing spines. There was a medium distribution of flat-vesicles terminals (17/100 mu2), to the somas while geniculate terminals were rest ricted to the secondary dendrites. Class V cells were multipolar with flat-vesicle terminals on the somas (11/100 micron2) and a few geniculate terminals on the dendrites. Class VI cells were mostly small (as small as 7 micron diameter), with a sparse dis tribution on the somas of both flat-vesicle terminals (7/100 micron2). Two cells had geniculate terminals on their somas. Class VII cells had sharply tapered apical and basilar dendrites, both flat-vesicle and round-vesicle terminals on the somas (14/100 micron2), and no geniculate input. The results make clear that the neurons in layer IVab are quite heterogeneous, not merely in their intrinsic morphology, but also in their patterns of connectivity. The geniculate input is not funneled to a single type o f neuron but diverges widely, contacting at least six different cell types, and may form on each a pattern that is characteristic for the type. The reconstruction approach, in providing a detailed identification of the synaptic patterns on substantial num bers of adjacent cells, should make it possible to address directly certain unanswered questions about cortical circuitry...

J Comp Neurol 1987 Jun 1;260(1):76-86

Pattern of lateral geniculate synapses on neuron somata in layer IV of the cat striate cortex.

Einstein G, Davis TL, Sterling P

The distribution of geniculate synapses on neuron cell bodies in layers IVab and IVc of cat area 17 was studied. Electron microscope autoradiography was used to identify geniculate terminals that were labeled by anterograde transport of radioactivity i njected into the A-laminae of the lateral geniculate nucleus. Thirty-eight cell bodies (19 in layer IVab and 19 in layer IVc) were examined in a series of 138 consecutive sections. Two pyramidal somas were studied and had no geniculate contacts. All of th e other somas studied were nonpyramidal, and of these, 85% received geniculate contacts. The proportion of somas receiving somatic geniculate input differed in layers IVab and IVc. In layer IVab, 70% of the nonpyramidal somas received geniculate contacts; in IVc, 100%. Such high percentages indicate that geniculate afferents synapse with more types of layer IV neuron than the aspinous neurons that synthesize gamma-aminobutyric acid (GABA) (Freund et al., '85b). The pattern of input to somas was so diverse that it was impossible to form groups of neurons based on only this criterion. We wondered if it would be possible to form groups of neurons based on a range of characteristics among which would be pattern of synaptic input. To this end, pyramidal neuron s and neurons that contained a cytoplasmic laminated body (CLB) (Winfield, '79; Einstein et al., '84) were treated as two separate classes. We found fair agreement among the features of these neurons within their own classes, with the CLB-cells in layer I Vab and IVc forming separate groups. Among the remaining neurons there was too little agreement within the range of features to enable us to treat them in this manner. Geniculate somatic contacts in both sublayers were of 2 forms, those with round vesicle s and asymmetric thickenings (RA) and those with pleomorphic vesicles and symmetric thickenings (PS) (Einstein et al., '87). The distribution of these forms varied: some cells received contacts exclusively from one form or the other; other cells received contacts from both. On one cell that bore 33 somatic geniculate terminals, 61% were RA and 39% were PS. Such substantial numbers of geniculate contacts located near the site of impulse initiation are likely to contribute significantly to the receptive fie ld properties of this neuron, and the possible effects are discussed.

J Comp Neurol 1987 Jun 1;260(1):76-86

Pattern of lateral geniculate synapses on neuron somata in layer IV of the cat striate cortex.

Einstein G, Davis TL, Sterling P

The distribution of geniculate synapses on neuron cell bodies in layers IVab and IVc of cat area 17 was studied. Electron microscope autoradiography was used to identify geniculate terminals that were labeled by anterograde transport of radioactivity i njected into the A-laminae of the lateral geniculate nucleus. Thirty-eight cell bodies (19 in layer IVab and 19 in layer IVc) were examined in a series of 138 consecutive sections. Two pyramidal somas were studied and had no geniculate contacts. All of th e other somas studied were nonpyramidal, and of these, 85% received geniculate contacts. The proportion of somas receiving somatic geniculate input differed in layers IVab and IVc. In layer IVab, 70% of the nonpyramidal somas received geniculate contacts; in IVc, 100%. Such high percentages indicate that geniculate afferents synapse with more types of layer IV neuron than the aspinous neurons that synthesize gamma-aminobutyric acid (GABA) (Freund et al., '85b). The pattern of input to somas was so diverse that it was impossible to form groups of neurons based on only this criterion. We wondered if it would be possible to form groups of neurons based on a range of characteristics among which would be pattern of synaptic input. To this end, pyramidal neuron s and neurons that contained a cytoplasmic laminated body (CLB) (Winfield, '79; Einstein et al., '84) were treated as two separate classes. We found fair agreement among the features of these neurons within their own classes, with the CLB-cells in layer I Vab and IVc forming separate groups. Among the remaining neurons there was too little agreement within the range of features to enable us to treat them in this manner. Geniculate somatic contacts in both sublayers were of 2 forms, those with round vesicle s and asymmetric thickenings (RA) and those with pleomorphic vesicles and symmetric thickenings (PS) (Einstein et al., '87). The distribution of these forms varied: some cells received contacts exclusively from one form or the other; other cells received contacts from both. On one cell that bore 33 somatic geniculate terminals, 61% were RA and 39% were PS. Such substantial numbers of geniculate contacts located near the site of impulse initiation are likely to contribute significantly to the receptive fie ld properties of this neuron, and the possible effects are discussed.

J Comp Neurol 1996 Jan 15;364(3):556-566

ON-OFF amacrine cells in cat retina.

Freed MA, Pflug R, Kolb H, Nelson R

National Institute of Neurological Disorders and Stroke, Maryland 20892, USA.

We studied the morphology, photic responses, and synaptic connections of ON-OFF amacrine cells in the cat retina by penetrating them with intracellular electrodes, staining them with horseradish peroxidase, and examining them with the electron microsco pe. In a sample of seven cells, we found two different morphological types: the A19, which ramifies narrowly in stratum 2 (sublamina a) of the inner plexiform layer, and the A22, which ramifies mostly in stratum 4 (sublamina b) but extends some dendrites to sublamina a. Both of these cell types have axon-like processes that extend > 800 microns from the conventional dendritic arbor. ON-OFF amacrine cells in our sample had receptive fields (1.7 +/- 0.3 mm diameter) that were broader than their dendritic arbors (425 +/- 35 microns diameter) and that extended over the region of axon-like processes. In addition, we found many features in common with ON-OFF amacrine cells in poikilotherm vertebrates: a broad receptive field without surround antagonism, two sizes of spike-like events, narrow dynamic range (1 log unit intensity), and excitatory postsynaptic potentials at light on and light off. Two A19 amacrine cells were examined in the electron microscope: most synaptic inputs (93 and 76%, respectively) to either cell were from amacrine cells, with minor inputs from cone bipolar cells. Synaptic outputs were to bipolar, amacrine, and ganglion cells, including the OFF-alpha cell.

J Comp Neurol 1983 Sep 20;219(3):295-304

Four types of amacrine in the cat retina that accumulate GABA.

Freed MA, Nakamura Y, Sterling P

Roughly one-quarter of neurons in the amacrine cell layer accumulate exogenous gamma-aminobutyric acid (GABA). Some of these (8%) are interplexiform cells; the remainder are true amacrine cells. We partially reconstructed, from serial electron microsco py autoradiograms, 25 GABA-accumulating amacrines and distinguished four types based on cytoplasmic appearance, soma size and shape, and the form of primary and secondary processes. Type 1 had a large (609 +/- 60 microns3), dark soma, and multiple, medium -diameter (0.6 microns) processes splayed from the soma margins like the appendages from a crab. Type 2 had a medium (360 +/- 40 microns3), helmet-shaped, pale soma, and medium-diameter (0.8 microns) processes that branched in sublamina alpha. Type 3 had a small (267 +/- 44 microns3), dark, pyriform soma. The latter formed a single stout (3.0 microns) process that bifurcated in the middle of sublamina alpha. Type 4 had a very large, pale soma (860 microns3). This was pyriform, tapering into a stout (2.0 m icrons) process that descended into the middle of sublamina alpha where it emitted smaller tangential processes. It is to be expected that each of these amacrine cell types will have distinct functions in neurotransmitter retinal circuitry.

J Comp Neurol 1987 Dec 15;266(3):445-455

Rod bipolar array in the cat retina: pattern of input from rods and GABA-accumulating amacrine cells.

Freed MA, Smith RG, Sterling P

Department of Anatomy, University of Pennsylvania, Philadelphia 19104.

The potential and actual connections between rod and rod bipolar arrays in the area centralis of the cat retina were studied by electron microscopy of serial ultrathin sections. In the region studied there were about 378,000 rods/mm2 and 36,000-47,000 rod bipolars/mm2. The tangential spread of rod bipolar dendrites was 11.2 microns in diameter, and the "coverage factor" for the rod bipolar cell was 3.5-4.6. We estimate that about 37 rods potentially converge on a rod bipolar cell and that one rod potentially diverges to about four rod bipolar cells. The actual connections, however, are less than this by about half: 16-20 rods actually converge on a bipolar cell and one rod actually diverges to slightly less than two rod bipolar cells. The deg ree of convergence appears to reflect a compromise between the need to signal graded stimulus intensities (requiring wide convergence) and the need to maintain a good signal/noise ratio (requiring narrow convergence). Amacrine varicosities that provide re ciprocal contact at the rod bipolar dyad were studied in serial electron microscopic autoradiograms following intraocular administration of 3H-GABA or 3H-glycine. More that 90% of the reciprocal amacrine processes accumulated GABA in a specific fashion. T his information, in conjunction with Nelson's recordings from the rod bipolar and amacrine cells postsynaptic at the dyad (Nelson et al: Invest. Ophthalmol. 15:946-953, '76; Kolb and Nelson: Vision Res. 23:301-312, '83), suggests that feedback at the rod bipolar output might be positive.

Proc Natl Acad Sci U S A 1992 Jan 1;89(1):236-240

Computational model of the on-alpha ganglion cell receptive field based on bipolar cell circuitry.

Freed MA, Smith RG, Sterling P

Laboratory of Neurophysiology, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD 20892.

The on-alpha ganglion cell in the area centralis of the cat retina receives approximately 450 synapses from type b1 cone bipolar cells. This bipolar type forms a closely spaced array (9 microns), which contributes from 1 to 7 synapses per b1 cell throu ghout the on-alpha dendritic field. Here we use a compartmental model of an on-alpha cell, based on a reconstruction from electron micrographs of serial sections, to compute the contribution of the b1 array to the on-alpha receptive field. The computation shows that, for a physiologic range of specific membrane resistance (9500-68,000 omega.cm2) and a linear synapse, inputs are equally effective at all points on the on-alpha dendritic tree. This implies that the electrotonic properties of the dendritic tree contribute very little to the domed shapes of the receptive field center and surround. Rather, these shapes arise from the domed distribution of synapses across the on-alpha dendritic field. Various sources of "jitter" in the anatomical circu it, such as variation in bipolar cell spacing and fluctuations in the number of synapses per bipolar cell, are smoothed by the overall circuit design. However, the computed center retains some minor asymmetries and lumps, due to anatomical jitter, as found in actual alpha-cell receptive fields.

J Neurosci 1988 Jul;8(7):2303-2320

The ON-alpha ganglion cell of the cat retina and its presynaptic cell types.

Freed MA, Sterling P

University of Pennsylvania Medical School, Department of Anatomy, Philadelphia 19104.

Anatomical circuits converging onto the ON-alpha (Y) ganglion cell were studied by computer-assisted reconstruction of substantial portions of 2 alpha cells from electron micrographs of serial sections. The alpha cells in the area centralis were labele d by a Golgi-like retrograde filling with horseradish peroxidase, and certain presynaptic amacrine processes were labeled by uptake of 3H-glycine. About 4400 synapses contacted the alpha cell. Eighty-six percent were from amacrine cells; the rest were fro m bipolar cells. About one-quarter of the amacrine synapses were specifically labeled by 3H-glycine and probably belong to the A4 amacrine. The bipolar inputs were provided by several types: cone bipolar CBb1 (85%), cone bipolar CBb5 (2%), the rod bipolar (5%), and some unidentified cone bipolars (11%). Contacts from each type occurred in specific strata, with the consequence that they tended to form spots or annulli over the alpha dendritic field. The CBb1 bipolars formed a moderately dense array (8000/m m2), with a nearest-neighbor distance of 8.6 +/- 1.3 microns. Most members of the array (84%) contacted the alpha cell, providing 1-7 synapses (average, 2.7 +/- 1.6). The placement of contacts from an individual CBb1 followed certain rules: they were rest ricted to a parent branch of the alpha arbor or to 2 daughter branches, but almost never crossed a branch of the alpha arbor. The synaptic territory of an individual CBb1 was not shared with other b1s (or cone bipolars of any sort), although it was shared with amacrine contacts. Rod bipolar cells also formed a very dense array (54,500/mm2) in the alpha dendritic field, but only a few of these (3%) contacted the alpha cell. The concentric receptive field of the CBb1, combined with the spatial organization of its array, is used to predict the CBb1 contribution to the alpha cell receptive field; this contribution resembles the spatial and temporal organization of the alpha receptive field itself.

Proc Natl Acad Sci U S A 1996 Dec 10;93(25):14598-14601

Phospholipase C beta 4 is involved in modulating the visual response in mice.

Jiang H, Lyubarsky A, Dodd R, Vardi N, Pugh E, Baylor D, Simon MI, Wu D

Department of Pharmacology and Physiology, University of Rochester, NY 14642, USA.

Expression of G protein-regulated phospholipase C (PLC) beta 4 in the retina, lateral geniculate nucleus, and superior colliculus implies that PLC beta 4 may play a role in the mammalian visual process. A mouse line that lacks PLC beta 4 was generated and the physiological significance of PLC beta 4 in murine visual function was investigated. Behavioral tests using a shuttle box demonstrated that the mice lacking PLC beta 4 were impaired in their visual processing abilities, whereas they showed no defi cit in their auditory abilities. In addition, the PLC beta 4-null mice showed 4-fold reduction in the maximal amplitude of the rod a- and b-wave components of their electroretinograms relative to their littermate controls. However, recording from single r od photoreceptors did not reveal any significant differences between the PLC beta 4-null and wild-type littermates, nor were there any apparent differences in retinas examined with light microscopy. While the behavioral and electroretinographic results in dicate that PLC beta 4 plays a significant role in mammalian visual signal processing, isolated rod recording shows little or no apparent deficit, suggesting that the effect of PLC beta 4 deficiency on the rod signaling pathway occurs at some stage after the initial phototransduction cascade and may require cell-cell interactions between rods and other retinal cells.

J Neurosci 1995 Nov;15(11):7673-7683

How retinal microcircuits scale for ganglion cells of different size.

Kier CK, Buchsbaum G, Sterling P

Department of Bioengineering, University of Pennsylvania, School of Medicine, Philadelphia 19104, USA.

Ganglion cell receptive field centers are small in central retina and larger toward periphery. Accompanying this expansion, the distribution of sensitivity across the centers remain Gaussian, but peak sensitivities decline. To identify circuitry that might explain this physiology, we measured the density of bipolar cell synapses on the dendritic membrane of beta (X) and alpha (Y) ganglion cells and the distribution of dendritic membrane across their dendritic fields. Both central and peripheral beta cells receive bipolar cell synapses at a density of approximately 28/100 microns2 of dendritic membrane; central and peripheral alpha cells receive approximately 13/100 microns2. The distribution of dendritic membrane across the dendritic field is dome-like; therefore, the distribution of bipolar cell synapses is also dome-like. As the dendritic field enlarges, total postsynaptic membrane increases with field radius, but only linearly. Consequently, density of postsynaptic membrane in the dendritic field declines, and so does density of synapses within the field. The results suggest a simple model in which the receptive field center's Gaussian profile and peak sensitivity are both set by the density of bipolar cell synapses across the dendritic field.

J Comp Neurol 1983 Apr 20;215(4):465-471

Granule cells in the rat olfactory tubercle accumulate 3H-gamma-aminobutyric acid.

Krieger NR, Megill JR, Sterling P

The rat olfactory tubercle contains high concentrations of gamma-aminobutyric acid (GABA) and its synthetic enzyme, glutamic acid decarboxylase (GAD). We previously demonstrated that GABA and GAD are most concentrated in the polymorphic layer of the tubercle and relatively absent from the plexiform and pyramidal layers. Here we report that the granule cells (the islands of Calleja) in the polymorphic layer accumulate 3H-GABA. 3H-GABA (34.5 Ci/mmole; 1.5 microliter) was injected into the tubercle and an hour later the rat was perfused with a mixture of paraformaldehyde and glutaraldehyde. The tissue was osmicated, dehydrated, and embedded in epon. Silver grains were sparse over the pyramidal and polymorphic cell bodies but numerous over the granule cell bodies in the islands of Calleja and dendrites in the surrounding neuropil. Grain densities for the granule cells were 41/100 micrometer3 compared to 4.2 for the pyramidal and polymorphic cells. Within the island, all the granule cells appeared to be labeled. These results, combined with previous demonstrations of the presence in this region of endogenous GABA and GAD, suggest that the granule neurons of the rat olfactory tubercle are GABA-ergic. These neurons also appear to receive dopamine input and therefore form part of a circuit that includes targets for both major and minor tranquilizers.

J Neurosci 1984 Dec;4(12):2920-2938

Microcircuitry of bipolar cells in cat retina.

McGuire BA, Stevens JK, Sterling P

We have studied 15 bipolar neurons from a small patch (14 X 120 micron) of adult cat retina located within the area centralis. From electron micrographs of 189 serial ultrathin sections, the axon of each bipolar cell was substantially reconstructed with its synaptic inputs and outputs by means of a computer-controlled reconstruction system. Based on differences in stratification, cytology, and synaptic connections, we identified eight different cell types among the group of 15 neurons: one type of rod bipolar and seven types of cone bipolar neurons. These types correspond to those identified by the Golgi method and by intracellular recording. Those bipolar cell types for which we reconstructed three or four examples were extremely regular in form, size, and cytology, and also in the quantitative details of their synaptic connections. They appeared quite as specific in these respects as invertebrate "identified" neurons. The synaptic patterns observed for each type of bipolar neuron were complex but may be summarized as follows: the rod bipolar axon ended in sublamina b of the inner plexiform layer and provided major input to the AII amacrine cell. The axons of three types of cone bipolar cells also terminated in sublamina b and provided contacts to dendrites of on-beta and other ganglion cells. All three types, but especially the Cb1, received gap junction contacts from the AII amacrine cell. Axons of four types of cone bipolar cells terminated in sublamina a of the inner plexiform layer and contacted dendrites of off-beta and other ganglion cells. One of these cone bipolar cell types, CBa1, made reciprocal chemical contacts with the lobular appendage of the AII amacrine cell. These results show that the pattern of cone bipolar cell input to beta (X) and probably alpha (Y) ganglion cells is substantially more complex than had been suspected. At least two types of cone bipolar contribute to each type of ganglion cell where only a single type had been anticipated. In addition, many of the cone bipolar cell pathways in the inner plexiform layer are available to the rod system, since at least four types of cone bipolar receive electrical or chemical inputs from the AII amacrine cell. This may help to explain why, in a retina where rods far outnumber the cones, there should be so many types of cone bipolar cells.

J Neurosci 1986 Apr;6(4):907-918

Microcircuitry of beta ganglion cells in cat retina.

McGuire BA, Stevens JK, Sterling P

We reconstructed from electron micrographs of 189 serial ultrathin sections a major portion of the dendritic tree of an on-beta ganglion cell through its sixth order of branching. One hundred three contacts from three cone bipolar cells were identified. Forty-seven contacts were from a single CBb1 cone bipolar. These were distributed widely over the dendritic tree but were frequently found on the slender "basal tuft" dendrites. Twenty-two additional contacts from a second CBb1 cell were found but not studied in detail. Thirty-four contacts were from a single CBb2 cone bipolar; these also were distributed widely but were primarily on the branches of the main dendritic arborization. A major portion of the dendritic tree of an off-beta cell was also reconstructed through its seventh order of branching. Thirty-five contacts from two cone bipolar cells were identified. Twenty-three contacts were from a single CBa1 cone bipolar and 12 widely distributed over the off-beta cell dendritic tree. We propose that the photopic receptive field center of a beta cell corresponds to the envelope of the receptive fields of the bipolar cells that connect it to the cones. The center response of a beta cell may be generated by a "push-pull" mechanism. For the on-beta cell there would be excitation at light on from CBb1 and disinhibition from CBb2 and the reverse at light off. For the off-beta cell there would be inhibition at light on from CBa2 and withdrawal of excitation from CBa1. Should the bipolars have antagonistic surrounds (so far reported only for CBb1), the beta cell surrounds as well as their centers might be generated by this push-pull mechanism.

J Comp Neurol 1981 Nov 1;202(3):385-396

Neurons and glia in cat superior colliculus accumulate [3H]gamma-aminobutyric acid (GABA).

Mize RR, Spencer RF, Sterling P

We have examined by autoradiography the labeling pattern in the cat superior colliculus following injection of tritiated gamma-aminobutyric acid (GABA). Silver grains were heavily distributed within the zonal layer and the upper 200 micrometer of the superficial gray. Fewer grains were observed deeper within the superficial gray, and still fewer were found within the optic and intermediate gray layers. The accumulation of label was restricted to certain classes of neuron and glia. Densely labeled neurons were small (8-12 micrometer in diameter) and located primarily within the upper 200 micrometer. Dark oligodendrocytes and astrocytes showed a moderate accumulation of label while pale oligodendrocytes and microglia were unlabeled. Label was also selectively accumulated over several other types of profile within the neuropil, including presynaptic dendrites, axons, and axon terminals.

J Comp Neurol 1982 Apr 1;206(2):180-192

Two types of GABA-accumulating neurons in the superficial gray layer of the cat superior colliculus.

Mize RR, Spencer RF, Sterling P

Two types of neuron in the upper superficial gray layer of the cat superior colliculus accumulated exogenous 3H-gamma-aminobutyric acid intensely. The first type was a horizontal cell with a fusiform cell body, horizontal dendrites, a low synaptic density, but a high percentage of cortical synaptic contacts. This cell had presynaptic dendrites. The second type was a granule cell (type A) with a small round cell body, thin and obliquely oriented dendrites, a moderate synaptic density, and few cortical synaptic contacts. These two types differed in size, shape, dendritic morphology, and patterns of synaptic input. They likely participate in different inhibitory mechanisms. Four types of unlabeled neurons were also identified. Type B granule cells were found only within the upper subdivision of the superficial gray layer. They had moderate-sized cell bodies, a high synaptic density, and numerous somatic spines. A third type of granule cell (type C) was found only in the deep subdivision of the superficial gray. This type had a low synaptic density and spines that contained synaptic vesicles. Vertical fusiform and stellate forms were also found. We conclude that at least six types of neurons populate the upper superficial gray layer of the cat superior colliculus.

Proc Natl Acad Sci U S A 1980 Jan;77(1):658-661

Interplexiform cell in cat retina: identification by uptake of gamma-[3H]aminobutyric acid and serial reconstruction.

Nakamura Y, McGuire BA, Sterling P

After intravitreal injection of gamma-[3H] aminobutyric acid (GAB), 2% of the neurons at the outer margin of the inner plexiform layer were intensely labeled. Reconstructions of these neurons from serial electron microscope autoradiograms showed that they are interplexiform cells, which synapse on bipolar processes in the outer plexiform layer and on amacrine and bipolar processes in the inner plexiform layer.

J Comp Neurol 1995 Oct 23;361(3):479-490

Retinal neurons and vessels are not fractal but space-filling.

Panico J, Sterling P

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104-6058, USA.

Many branched patterns in nature are hypothesized to be fractal, i.e., statistically self-similar across a range of scales. We tested this hypothesis on the two-dimensional arbors of retinal neurons and blood vessels. First, we measured fractalness on synthetic fractal and nonfractal patterns. The synthetic fractal patterns exhibited self-similarity over a decade of scale, but the nonfractal "controls" showed hardly any self-similarity. Neuronal and vascular patterns showed no greater self-similarity than the controls. Second, we manipulated a synthetic fractal pattern to remove its self-similarity and found this to be reflected in a loss of measured fractalness. The same manipulation of the nonfractal control and also of the neural and vascular patterns did not alter their measured fractalness. Third, we "grew" patterns of branched line segments according to a variety of nonfractal algorithms. These patterns were, if anything slightly more fractal than the neural and vascular patterns. We conclude that the biological patterns studied here are not fractal. Finally, we measured extended versions of these patterns: a contiguous array of homotypic neuron arbors and a vascular pattern with a high degree of total detail. These patterns showed a "fractal dimension" of 2, which implies that down to some cut-off scale they fill space completely. Thus, neural and vascular patterns might best be described as quasi-regular lattices.

Biophys J 1994 Jul;67(1):57-63

Rate of quantal transmitter release at the mammalian rod synapse.

Rao R, Buchsbaum G, Sterling P

Department of Bioengineering, University of Pennsylvania, Philadelphia 19104.

Under scotopic conditions, the mammalian rod encodes either one photon or none within its integration time. Consequently the signal presented to its synaptic terminal is binary. The synapse has a single active zone that releases neurotransmitter quanta tonically in darkness and pauses briefly in response to a rhodopsin isomerization by a photon. We asked: what minimum tonic rate would allow the postsynaptic bipolar cell to distinguish this pause from an extra-long interval between quanta due to the stochastic timing of release? The answer required a model of the circuit that included the rod convergence onto the bipolar cell and the bipolar cell's signal-to-noise ratio. Calculations from the model suggest that tonic release must be at least 40 quanta/s. This tonic rate is much higher than at conventional synapses where reliability is achieved by employing multiple active zones. The rod's synaptic mechanism makes efficient use of space, which in the retina is at a premium.

Neuron 1995 Mar;14(3):561-569

Mammalian rod terminal: architecture of a binary synapse.

Rao-Mirotznik R, Harkins AB, Buchsbaum G, Sterling P

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104.

The mammalian rod synapse transmits a binary signal (one photon or none) using tonic, rapid exocytosis. We constructed a quantitative, physical model of the synapse. Presynaptically, a single, linear active zone provides docking sites for approximately 130 vesicles, and a "ribbon" anchored to the active zone provides a depot for approximately 640 vesicles. Postsynaptically, 4 processes invaginate the terminal: 2 (known to have low affinity glutamate receptors) lie near the active zone (16 nm), and 2 (known to have high affinity glutamate receptors) lie at a distance (130-640 nm). The presynaptic structure seems designed to minimize fluctuations in tonic rate owing to empty docking sites, whereas the postsynaptic geometry may permit 1 vesicle to evoke an all-or-none response at all 4 postsynaptic processes.

In: Neurobiology and clinical aspects of the outer retina (Archer S, Djamgoz MBA, Vallerga S eds), pp 325-348. London UK: Chapman & Hall, Ltd.

Functional architecture of mammalian outer retina and bipolar cells.

Sterling P, Smith RG, Rao R, Vardi N (1995)

Department of Neuroscience, School of Medicine, University of Pennsylvania, Philadelphia 19104-6058.

Function in the outer retina has mainly gbeen studied by recording in situ from single neurons. In lower vertebrates this approach to bipolar cells has been extremely fruitful (e.g. Chapter 12), but in mammals bipolar cell recordings can be counted on the fingers of (at most) two hands (Nelson and Kolb, 1983; Dacheux and Raviola, 1986). And, considering that the recordings include both rod bipolar and multiple types of cone bipolar cell (Chapter 11), the electrophysiological data regarding mammalian bipolar neurons are thinly spread. On the other hand, in lower vertebrates information essential to understanding the contribution of the outer retina to image processing (such as optics, sampling frequencies, and synaptic circuitry) hardly exists. So, in lower vertebrates how single neuron responses in the outer contribute to vision remains unclear.

Yet, single cell recording is not the only possible approach to understanding retinal function. An alternative strategy is to determine complete circuit structure ('wiring diagram') plus the chemical architecture and to incorporate this informatlion, together with the optics and ganglion cell electrophysiology, into various computational models. Then, one might calculate backwards to the properties of the bipolar cells and photoreceptors. Such an effort leads to specific predictions regarding the photoreceptor and bipolar cell function, and with this approach a little electrophysiology goes a surprisingly long way. At least, that is our argument in this review. We emphasize cat retina, which is known in most detail, but also note recent data from reabbit and primate that indicate conservation of certain basic circuits and functions.

J Neurosci Methods 1987 Sep;21(1):55-69

Montage: a system for three-dimensional reconstruction by personal computer.

Smith RG

Department of Anatomy, School of Medicine, University of Pennsylvania, Philadelphia 19104-6058.

This paper describes a simplified system for serial section three-dimensional (3-D) reconstruction. A set of 9 software programs runs on a standard personal computer and produces camera-ready illustrations suitable for publication. The user enters trace points on a digitizing tablet from sections that have been already aligned. A 3-D view of the reconstructed object is generated which can be displayed with hidden lines removed. Analysis of volume, surface area and autoradiographic grain density are performed automatically. A relational database query language allows display and analysis of a selected subset of the data. The system runs under the UNIX operating system which allows the programs to be easily transported to new hardware or modified for other purposes.

J Neurosci Methods 1992 Jul;43(2-3):83-108

NeuronC: a computational language for investigating functional architecture of neural circuits.

Smith RG

Department of Anatomy, University of Pennsylvania, Philadelphia 19104-6058.

A computational language was developed to simulate neural circuits. A model of a neural circuit with up to 50,000 compartments is constructed from predefined parts of neurons, called "neural elements". A 2-dimensional (2-D) light stimulus and a photoreceptor model allow simulating a visual physiology experiment. Circuit function is computed by integrating difference equations according to standard methods. Large-scale structure in the neural circuit, such as whole neurons, their synaptic connections, and arrays of neurons, are constructed with procedural rules. The language was evaluated with a simulation of the receptive field of a single cone in cat retina, which required a model of cone-horizontal cell network on the order of 1000 neurons. The model was calibrated by adjusting biophysical parameters to match known physiological data. Eliminating specific synaptic connections from the circuit suggested the influence of individual neuron types on the receptive field of a single cone. An advantage of using neural elements in such a model is to simplify the description of a neuron's structure. An advantage of using procedural rules to define connections between neurons is to simplify the network definition.

Vis Neurosci 1995 May;12(3):545-561

Simulation of an anatomically defined local circuit: the cone-horizontal cell network in cat retina.

Smith RG

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104-6058, USA.

The outer plexiform layer of the retina contains a neural circuit in which cone synaptic terminals are electrically coupled and release glutamate onto wide-field and narrow-field horizontal cells. These are also electrically coupled and feed back through a GABAergic synapse to cones. In cat this circuit's structure is known in some detail, and much of the chemical architecture and neural responses are also known, yet there has been no attempt to synthesize this knowledge. We constructed a large-scale compartmental model (up to 50,000 compartments) to incorporate the known anatomical and biophysical facts. The goal was to discover how the various circuit components interact to form the cone receptive field, and thereby what possible function is implied. The simulation reproduced many features known from intracellular recordings: (1) linear response of cone and horizontal cell to intensity, (2) some aspects of temporal responses of cone and horizontal cell, (3) broad receptive field of the wide-field horizontal cell, and (4) center-surround cone receptive field (derived from a "deconvolution model"). With the network calibrated in this manner, we determined which of its features are necessary to give the cone receptive field a Gaussian center-surround shape. A Gaussian-like center that matches the center derived from the ganglion cell requires both optical blur and cone coupling: blur alone is too narrow, coupling alone gives an exponential shape without a central dome-shaped peak. A Gaussian-like surround requires both types of horizontal cell: the narrow-field type for the deep, proximal region and the wide-field type for the shallow, distal region. These results suggest that the function of the cone-horizontal cell circuit is to reduce the influence of noise by spatio-temporally filtering the cone signal before it passes through the first chemical synapse on the pathway to the brain.

Vis Neurosci 1990 Nov;5(5):453-461

Cone receptive field in cat retina computed from microcircuitry.

Smith RG, Sterling P

The receptive-field profile of the cone in cat-retina was computed. The computation was based on (1) the known anatomical circuit connecting cones via narrow-field bipolar cells to the on-beta ganglion cell; (2) the known physiological receptive-field profile of the on-beta (X) cell at the corresponding eccentricity; and (3) a model in which the beta receptive field arises by linear superposition of cone receptive fields. The computed cone receptive field has a center/surround organization with a center almost as broad as that of the beta cell center. The cone surround is comparably broad to that of the beta cell but somewhat lower in peak amplitude. The problems to which the center/surround receptive field are the solution, namely, signal compression and noise reduction, apparently must be solved before the first synapse of the visual pathway.

Proc Natl Acad Sci U S A 1984 Jun;81(12):3898-3900

Numbers of specific types of neuron in layer IVab of cat striate cortex.

Solnick B, Davis TL, Sterling P

Layer IVab of the visual cortex (area 17) of the cat contains about 51,400 neurons per mm3, including about 400-1200 per mm3 of each of three categories of neuron believed from previous work to represent discrete types. Each type forms about 0.5-1.5% of all the IVab neurons, which suggests that the total number of types in this layer might be much greater than previously supposed, perhaps as many as 50 or more. From their densities and estimates of their dendritic fields, we calculate that each type completely "covers" layer IVab in the tangential plane but only by a small factor (1.3-4.2).

J Comp Neurol 1980 Aug 15;192(4):737-749

Neurons in cat lateral geniculate nucleus that concentrate exogenous [3H]-gamma-aminobutyric acid (GABA).

Sterling P, Davis TL

About one-quarter of the neurons in the A-laminae of the cat lateral geniculate selectively accumulate exogenous [3H]-gamma-aminobutyric acid (GABA), its analog, [3H]-2,4-diaminobutyric acid (DABA), and the GABA agonist, [3H] muscimol. These neurons are small (12-18 micrometers diameter) and lack a laminar body, which suggests that they correspond to the class III cell identified in Golgi material. GABA and DABA are also accumulated by F-terminals which are post-synaptic to retinal terminals and presynaptic to relay cell dendrites. It is suggested that GABA may be the transmitter for these small neurons which appear to mediate by means of local circuits a feed-forward inhibition onto the relay cells.

J Neurosci 1988 Feb;8(2):623-642

Functional architecture of rod and cone circuits to the on-beta ganglion cell.

Sterling P, Freed MA, Smith RG

Department of Anatomy, University of Pennsylvania, Philadelphia 19104.

Photoreceptors connect to the on-beta ganglion cell through parallel circuits involving rod bipolar (RB) and cone bipolar (CB) neurons. We estimated for a small patch in the area centralis of one retina the 3-dimensional architecture of both circuits. This was accomplished by reconstructing neurons and synapses from electron micrographs of 189 serial sections. There were (per mm2) 27,000 cones, 450,000 rods, 6500 CBb1, 30,300 RB, 4100 All amacrines, and 2000 on-beta ganglion cells. The tangential spread of processes was determined for each cell type, and, with the densities, this allowed us to calculate the potential convergence and divergence of each array upon the next. The actual numbers of cells converging and diverging were estimated from serial sections, as were the approximate numbers of chemical synapses involved. The cone bipolar circuit showed narrow convergence and divergence: 16 cones->4 CBb1->1 on-beta 1 cone->1 CBb1->1.2 on-beta This circuit is thought to contribute significantly to the on-beta cell's photopic receptive field because the CBb1 has a center-surround receptive field whose center diameter is greater than the spacing between adjacent CBb1s. Consequently, the receptive fields of the CBb1s converging on a beta cell are probably largely concentric and thus mutually reinforcing in their contributions to the on-beta. The rod bipolar circuit showed a wider convergence and divergence: 1500 rods->100 RB->5 AII->4 CBb1->1 on-beta 1 rod->2 RB->5 AII->8 CBb1->2 on-beta The 1500 rods converging via this circuit account for the spatial extent of the beta cell's dark-adapted receptive field. This convergence also accounts for the ganglion cell's maintained discharge, which is thought to arise from about 6 quantal "dark events" per second. This many dark events would appear in the ganglion cell if each rod in the circuit contributed 0.004 dark events per second, and this is close to what has been measured in monkey rods (Baylor et al., 1984). Divergence in this circuit serves to expand the number of copies of the quantal signal (1 rod->8 CBb1) and so to engage large numbers of chemical synapses that provide amplification. Reconvergence at the last stage (8 CBb1->2 on-beta) may reduce (by signal averaging) the synaptic noise that would otherwise accumulate along the pathway.

J Neurosci 1986 May;6(5):1314-1324

Molecular specificity of defined types of amacrine synapse in cat retina.

Sterling P, Lampson LA

The inner plexiform layer of cat retina contains synaptic structures belonging to 50 or more types of "identified" neurons. To learn whether there are antigens confined to subsets of these synaptic structures, we raised monoclonal antibodies to homogenates of neural retina. Binding patterns of these antibodies were visualized by the peroxidase-antiperoxidase method and studied in serial, ultrathin sections by electron microscopy. Four antibodies stained the synaptic varicosities of certain amacrine cells. Many of the stained varicosities formed reciprocal synapses with a rod bipolar axon terminal, but only about half of the reciprocal synapses associated with a rod bipolar were stained. Other stained varicosities formed synapses with cone bipolar axons, ganglion cell dendrites, and unstained amacrine processes. The patterns were essentially the same for each antibody and were not altered by staining with the antibodies two at a time; therefore, it is likely that all four antibodies stain the same subset of synaptic structures. These patterns would be accounted for if there were staining of all the synaptic varicosities of three of the four types of identified amacrine reciprocally connected to the rod bipolar (A6, A8, A13). This localization suggests that the antigen responsible for the binding pattern is not associated with synaptic transmission. Staining is present in the inner plexiform layer during the period of synaptogenesis and consequently the antibodies are serving as markers for following the development of identified synapses in an identified neural circuit.

Brain Res 1980 Dec;2(3):265-293

A systematic approach to reconstructing microcircuitry by electron microscopy of serial sections.

Stevens JK, Davis TL, Friedman N, Sterling P

To observe certain quantitative features of neuronal geometry and microcircuitry, it is necessary to reconstruct neurons from electron micrographs of serial, ultra-thin sections. We describe here an approach to preparing, photographing, and analyzing moderately long series (100-500 sections). A series is prepared using an assembly line approach: one operator cuts while a second mounts ribbons of sections using various mechanical aids. Photographs are taken in the electron microscope at low magnification and high accelerating voltage. Sequential negatives are aligned using an image combiner and copied, using quasi-coherent illumination, onto 35 mm film. The resulting "movie' is mounted on a precision film transport mounted on an X-Y stage controlled by stepping motors. The movie is viewed through a high resolution video system while a video storage device and switching system permit rapid alternation between frames for comparisons. The profiles of a process in successive frames are "microaligned' by small adjustments of the transport's X-Y position. The absolute X-Y biological coordinates for each frame and the correction necessary to bring it into alignment are stored in a Z80 microprocessor as a process vector. When the movie is re-examined with the stepping motors under control of the computer, the microaligned process shows almost no frame-to-frame jitter. The process vector may be used to generate a "branch schematic' of the neuron. The microaligned profiles can also be digitized and displayed as a reconstruction using a PDP 11/34 computer. Uses of the approach are presented with examples from the cat retina and visual cortex.

Science 1980 Jan 18;207(4428):317-319

Toward a functional architecture of the retina: serial reconstruction of adjacent ganglion cells.

Stevens JK, McGuire BA, Sterling P

Twenty adjacent ganglion cells in cat retina were partially reconstructed from electron micrographs of serial thin sections. Cells were classified by size and by dendritic branching patterns as alpha, beta, or gamma cells. The alpha and beta cells were further subdivided by differences in the laminar distribution of their dendrites in the inner plexiform layer. The distribution of synaptic contacts on the cells was distinctive for each of the five major classes. Contacts on the alpha and beta cells were mainly on the dendrites in the sublamina in which a cell's major dendritic arborization was contained.

Vision Res 1992 Oct;32(10):1809-1815

Gap junctions between the pedicles of macaque foveal cones.

Tsukamoto Y, Masarachia P, Schein SJ, Sterling P

Department of Anatomy, Hyogo College of Medicine, Japan.

Cone terminals ("pedicles") in the fovea of macaque retina were studied in electron micrographs of serial sections. Pedicles were sheathed in glia except for small (0.2 microns 2) fenestrations, 4.8 +/- 1.7 per pedicle. At each fenestration the membranes of adjacent pedicles were contiguous and marked by an adherent junction, which in turn was invariably associated with gap junctions. There were 3.2 +/- 1.4 gap junctions per adherent junction and thus, about fifteen gap junctions per pedicle. The gap junctions were small, 1.6 x 10(-3) +/- 1.8 x 10(-3) microns 2 (mean +/- SD) and were formed indiscriminately with all neighboring pedicles. An upper bound was estimated of 170 connexons per pedicle and thus a coupling conductance of 1.7 x 10(4) pS. Available psychophysical data suggest that the junctions are uncoupled at high luminance. They may couple at lower luminance where spatial averaging would improve contrast sensitivity without cost to spatial acuity.

Proc Natl Acad Sci U S A 1990 Mar;87(5):1860-1864

"Collective coding" of correlated cone signals in the retinal ganglion cell.

Tsukamoto Y, Smith RG, Sterling P

Department of Anatomy, School of Medicine, University of Pennsylvania, Philadelphia 19104.

The signals in neighboring cones are partially correlated due to local correlations of luminance in the visual scene. By summing these partially correlated signals, the retinal ganglion cell improves its signal/noise ratio (compared to the signal/noise ratio in a cone) and expands the variance of its response to fill its dynamic range. Our computations prove that the optimal weighting function for this summation is dome-shaped. The computations also show that (assuming a particular space constant for the correlation function) ganglion cell collecting area and cone density are matched at all eccentricities such that the signal/noise ratio improves by a constant factor. The signal/noise improvement factor for beta ganglion cells in cat retina is about 4.

J Comp Neurol 1995 Jan 16;351(3):374-384

Specific cell types in cat retina express different forms of glutamic acid decarboxylase.

Vardi N, Auerbach P

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104.

We studied the expression of glutamate decarboxylase (GAD), GAD65 and GAD67, in cat retina by immunocytochemistry. About 10% of GABAergic amacrine cells express only GAD65 and 30% express only GAD67. Roughly 60% contain both forms of the enzyme, but GAD67 is present only at low levels in the majority of these double-labeled amacrine cells. The staining pattern in the inner plexiform layer (IPL) for the two GAD forms was also different. GAD65 was restricted to strata 1-4, and GAD67 was apparent throughout the IPL but was strongest in strata 1 and 5. This indicates that somas, as well as their processes, are differentially stained for the two forms of GAD. Cell types expressing only GAD65 include interplexiform cells, one type of cone bipolar cell, and at least one type of serotonin-accumulating amacrine cell. Cell types expressing only GAD67 include amacrine cells synthesizing dopamine, amacrine cells synthesizing nitric oxide (NO), and amacrine cells accumulating serotonin. Cholinergic amacrine cells express a low level of both GAD forms. Our findings in the retina are consistent with previous observations in the brain that GAD65 expression is greater in terminals than in somas. In addition, in retina most neurons expressing GAD67 also contain a second neurotransmitter as well as GABA, and they tend to be larger than neurons expressing GAD65. We propose that large cells have a greater demand for GABA than small cells, and thus require the constant, relatively unmodulated level of GABA that is provided by GAD67.

Vis Neurosci 1994 Jan;11(1):135-142

Horizontal cells in cat and monkey retina express different isoforms of glutamic acid decarboxylase.

Vardi N, Kaufman DL, Sterling P

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104.

The neurotransmitter used by horizontal cells in mammals has not been identified. GABA has been the leading candidate, but doubt has remained because of failure to clearly demonstrate the GABA synthetic enzyme, glutamic acid decarboxylase (GAD) in these cells. Because GAD was recently shown to exist as two isoforms, 65 kDa and 67 kDa, we considered whether there might be a mismatch between the forms of GAD expressed in horizontal cells and the probes used to detect it. Accordingly, we stained sections of mammalian retina with antibodies specific for each isoform. Cat horizontal cells of both types (A and B) were immunoreactive for GAD67 but negative for GAD65; monkey horizontal cells of both types (H(I) and HII) were positive for GAD65 and negative for GAD67. The findings reconcile previous, apparently conflicting, observations and strengthen considerably the hypothesis that mammalian horizontal cells are GABAergic.

J Comp Neurol 1989 Oct 22;288(4):601-611

Structure of the starburst amacrine network in the cat retina and its association with alpha ganglion cells.

Vardi N, Masarachia PJ, Sterling P

Department of Anatomy, School of Medicine, University of Pennsylvania, Philadelphia 19104-6058.

To investigate indirect pathways to ganglion cells we studied the starburst amacrine cell network and its relationship to the alpha ganglion cell. Starburst cells were identified by an antiserum to choline acetyltransferase and alpha cells by injection of Lucifer yellow. The density of on and off starburst cells peaks at the area centralis and decreases with eccentricity by a factor of seven. The fine amacrine processes, interrupted by distinct varicosities, arborize in a planar fashion in the inner plexiform layer. The on network, at the junction of strata 3 and 4, and the off network, in stratum 2, have a similar appearance. Neighboring starburst processes run in intimate association to form a network of bundles. As bundles cross each other, loops of irregular size and shape are formed. The loops are smallest in the area centralis and increase by a factor of three towards the periphery; correspondingly, bundle length per unit area decreases with eccentricity. However, the number of varicosities/bundle length stays constant with eccentricity as does the number of processes per bundle. Segments of the starburst network associate over fairly long distances with dendrites of alpha ganglion cells. About 26% of the alpha ganglion dendritic tree shows such association, and this is significantly greater than would be expected if the alpha and starburst processes were independent. We conclude that the functional unit of the starburst cell is a linear bundle of processes and that the starburst network may connect synaptically to the alpha cell.

J Comp Neurol 1992 Jun 15;320(3):394-397

Immunoreactivity to GABAA receptor in the outer plexiform layer of the cat retina.

Vardi N, Masarachia P, Sterling P

Department of Anatomy, University of Pennsylvania, Philadelphia 19104-6058.

The distribution of GABAA receptor in the outer plexiform layer of cat retina was studied by immunocytochemistry with monoclonal antibodies. Staining was observed at the base of the cone pedicle, extracellularly, in association with the "triad" synaptic complex. Some bipolar dendrites and the basal processes that interconnect the cone pedicles were also stained. Rod spherules and horizontal cells were negative. The findings support the idea that the cone horizontal cells are GABAergic.

Vis Neurosci 1993 May;10(3):473-478

Identification of a G-protein in depolarizing rod bipolar cells.

Vardi N, Matesic DF, Manning DR, Liebman PA, Sterling P

Department of Anatomy, University of Pennsylvania, Philadelphia 19104-6058.

Synaptic transmission from photoreceptors to depolarizing bipolar cells is mediated by the APB glutamate receptor. This receptor apparently is coupled to a G-protein which activates cGMP-phosphodiesterase to modulate cGMP levels and thus a cGMP-gated cation channel. We attempted to localize this system immunocytochemically using antibodies to various components of the rod phototransduction cascade, including Gt (transducin), phosphodiesterase, the cGMP-gated channel, and arrestin. All of these antibodies reacted strongly with rods, but none reacted with bipolar cells. Antibodies to a different G-protein, G(o), reacted strongly with rod bipolar cells of three mammalian species (which are depolarizing and APB-sensitive). Also stained were subpopulations of cone bipolar cells but not the major depolarizing type in cat (b1). G(o) antibody also stained certain salamander bipolar cells. Thus, across a wide range of species, G(o) is present in retinal bipolar cells, and at least some of these are depolarizing and APB-sensitive.

Vision Res 1994 May;34(10):1235-1246

Subcellular localization of GABAA receptor on bipolar cells in macaque and human retina.

Vardi N, Sterling P

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104-6058.

The subcellular distribution of GABAA receptor in the macaque and human retina was studied by immunocytochemistry with monoclonal antibodies for the alpha and beta subunits with a particular focus on bipolar cells. Immunoreactivity to GABAA receptor was present on dendritic tips of all bipolar cells. The stain was strongest on bipolar membranes in apposition to horizontal cell processes. Stain was concentrated on the tips of flat and invaginating cone bipolar cells at the base of the cone pedicle and on the invaginating tips of rod bipolar cells. Stain on the cone pedicle membrane was restricted to sites of apposition to stained bipolar dendrites; pedicle membrane in apposition to horizontal cell processes was unstained. Stain was also present on bipolar axon terminals in both on and off strata of the inner plexiform layer. All bipolar cell somas stained faintly; horizontal and Muller cell somas were unstained. The alpha and beta subunits distributed similarly in monkey and human retina. Presence of GABAA receptor on the bipolar dendritic tips suggests that horizontal cells directly affect bipolar cells. Thus, GABAA receptor might mediate the receptive field surround of both off and on bipolar cells. Presence of GABAA receptor on bipolar axon terminals suggests that much of the inhibition feeding back from GABAergic amacrine to bipolar cells is GABAA-mediated.

Neuron 1996 Jun;16(6):1221-1227

Evidence that vesicles on the synaptic ribbon of retinal bipolar neurons can be rapidly released.

von Gersdorff H, Vardi E, Matthews G, Sterling P

Department of Neurobiology and Behavior, State University of New York, Stony Brook, 11794-5230, USA.

We relate the ultrastructure of the giant bipolar synapse in goldfish retina to the jump in capacitance that accompanies depolarization-evoked exocytosis. Mean vesicle diameter is 29 +/- 4 nm, giving 26.4 aF/vesicle, so the maximum evoked capacitance (150 fF within 200 ms) represents fusion of about 5700 vesicles. Two terminals contained, respectively, 45 and 65 ribbon-type synaptic outputs, and a fully loaded ribbon tethers about 110 vesicles. Thus, the tethered pool, about 6000 vesicles, corresponds to the rapidly released pool. Further, the difference between small and large terminals in number of tethered vesicles matches their difference in capacitance jump. This suggests, within a "fire and reload" model of exocytosis, that the ribbon translocates synaptic vesicles very rapidly to membrane docking sites, supporting a maximum release rate of 500 vesicles/active zone/s, until the population of tethered vesicles is exhausted.

Vardi N. Morigiwa K.

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104, USA.

ON cone bipolar cells in rat express the metabotropic receptor mGluR6.

Retina

Peter Sterling

The retina is a thin sheet of neural tissue lining the posterior hemisphere of the eyeball. It is actually part of the brain itself (~0.5%), evaginating from the lateral wall of the neural tube during embryonic development. The optic stalk grows out from the brain toward the ectoderm, inducing it to form an optical system (cornea, pupil, lens), which projects a physical image of the world onto the retina. The retina's task is to convert this optical image into a "neural image" for transmission down the optic nerve to a multitude of centers for further analysis. The task is complex - which is reflected in the synaptic organization.

A closer look at this apparently simple design (three interconnected layers and five broad classes of neuron) reveals additional complexity (Figs. 6.2, 6.3). Each neuron class is represented by several or many specific types. Each cell type is distinguished from others in its class by its characteristic morphology, connections, neurochemistry, and function (Rodieck and Brening, 1983; Sterling, 1983). This diversity, amounting to some 80 cellular types (Kolb et al., 1981; Sterling, 1983; Vaney, 1990), was puzzling at first, but a broad explanation has gradually emerged: it is impossible to encode all the information in an optical image using a single neural image. Therefore, the retina uses different cell types to create parallel circuits for different light levels - daylight, twilight, and starlight - but these share certain circuit compaonents and use the same final pathways to the brain (Smith et al., 1986).

This chapter describes key cell types and their interconnection in parallel circuits. It also discusses how the functional architecture of a circuit depends on the functional architecture of its synapses. Finally, it suggests how the flow of visual information shifts between circuits that are specialized for different light levels and how the circuits are switched. The chapter focuses on mammalian retina because that is where the combined knowledge of circuitry and cell physiology is best known. Early efforts centered on cat, so specific measurements, counts, etc., cited here refer to cat central retina. But recent efforts have broadened to include rabbit, rat, monkey, and human. These demonstrate strongly conserved patterns in the circuitry, as well as special adaptations, and some of both will be mentioned.

Alpha subunit of Go localizes in the dendritic tips of ON bipolar cells.

Vardi N.

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104, USA.

The metabotropic glutamate receptor (mGluR6), expressed by rod bipolar cells and ON cone bipolar cells, activates a trimeric guanine nucleotide-binding protein (G-protein) that ultimately closes a cation channel. The G-protein remains unidentified, but the alpha subunit of Go (Go(alpha)) has been suggested as a candidate because it is present in rod bipolar cells. However, the precise subcellular distribution of Go within the rod bipolar cell, and its distribution among cone bipolar cells was not determined. This information is important in assessing the hypothesis that Go couple mGluR6 to its effector. Here I report the distribution of Go (alpha subunit) by immunostaining in several mammalian retinas. The overall distribution is conserved across mammalian species: strongest in the dendrites of ON bipolar cells, moderate in their somas, weak in their axons, and absent from their terminals. Go(alpha) is also present in some amacrine somas and processes. In monkey fovea, where rods and rod bipolar cells are absent, Go(alpha) is present in about half of the bipolar somas which occupy the upper tiers of the bipolar layer, and are therefore identified as ON cone bipolar cells. Ultrastructurally, in monkey and cat, Go(alpha) is present in the dendritic tips of rod bipolar cells and ON cone bipolar cells, which are identified by their invaginating contacts. It is absent from OFF cone bipolar dendrites, which are identified by their flat contacts. It is also absent from axons entering the inner plexiform layer, and their terminals. In the primary dendrites, stain for Go(alpha) mainly associates with the plasma membrane, but in the dendritic tips it is also present in the cytosol. Apparently, Go(alpha) is expressed by the same bipolar cells that also express mGluR6, and is concentrated at the same subcellular location. Thus, Go(alpha) could serve to couple mGluR6 to later stages of its signaling cascade.

Regional differences in GABA and GAD immunoreactivity in rabbit horizontal cells.

Johnson MA. Vardi N.

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104-6058, USA.

Microcircuitry and mosaic of a blue-yellow ganglion cell in the primate retina.

Calkins DJ. Tsukamoto Y. Sterling P.

Department of Ophthalmology, University of Rochester Medical Center, New York

Neurochemistry of the mammalian cone 'synaptic complex'.

Vardi N. Morigiwa K. Wang TL. Shi YJ. Sterling P.

Department of Neuroscience, University of Pennsylvania School of Medicine, Philadelphia, 19104-6058, USA.

"Knocking out" a neural circuit.

Sterling P.

Department of Neuroscience, University of Pennsylvania, Philadelphia 19104, USA.

Transmitter concentration at a three-dimensional synapse.

Rao-Mirotznik R. Buchsbaum G. Sterling P.

Department of Bioengineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.

Differential expression of ionotropic glutamate receptor subunits in the outer retina.

Morigiwa K. Vardi N.

Department of Physiology, Osaka University Medical School, Japan.

Localization of type I inositol 1,4,5-triphosphate receptor in the outer segments of mammalian cones.

Wang TL. Sterling P. Vardi N.

Department of Neuroscience, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.

Calcium enters the outer segment of a vertebrate photoreceptor through a cGMP-gated channel and is extruded via a Na/Ca, K exchanger. We have identified another element in mammalian cones that might help to control cytoplasmic calcium. Reverse transcription-PCR performed on isolated photoreceptors identified mRNA for the SII- splice variant of the type I receptor for inositol 1,4,5-triphosphate (IP3), and Western blots showed that the protein also is expressed in outer segments. Immunocytochemistry showed type I IP3 receptor to be abundant in red-sensitive and green-sensitive cones of the trichromatic monkey retina, but it was negative or weakly expressed in blue-sensitive cones and rods. Similarly, the green-sensitive cones expressed the receptor in dichromatic retina (cat, rabbit, and rat), but the blue-sensitive cones did not. Immunostain was localized to disk and plasma membranes on the cytoplasmic face. To restore sensitivity after a light flash, cytoplasmic cGMP must rise to its basal level, and this requires cytoplasmic calcium to fall. Cessation of calcium release via the IP3 receptor might accelerate this fall and thus explain why the cone recovers much faster than the rod. Furthermore, because its own activity of the IP3 receptor depends partly on cytoplasmic calcium, the receptor might control the set point of cytoplasmic calcium and thus affect cone sensitivity.

Deciphering the retina's wiring diagram.

Sterling P.

Department of Neuroscience, University of Pennsylvania, 123 Anatomy/Chemistry Bldg., Philadelphia, Pennsylvania 19104-6058, USA.

By simultaneously recording from retinal ganglion cells while stimulating a single cone, Chichilnisky and Baylor demonstrate that the strength of physiological connections within a retinal microcircuit is linearly proportional to the number of anatomically defined synapses.

Over the last few decades, considerable effort has been devoted to constructing a detailed 'wiring diagram' for the retina, that is, a quantitative map of all its excitatory and inhibitory synaptic connections1, 2. Propelling this effort has been a faith that the diagram would ultimately make functional sense—that we would be able to 'read' the diagram of a neural circuit just as we do for an electronic circuit. Were this faith to prove unjustified—if, for example, the physiological strength of an input proved to be unrelated to the number of anatomically defined synapses—then the retina's synaptic diagram would be far less informative than we might otherwise hope. It is certainly reasonable to question the usefulness of a purely structural diagram, given that neurons contain nonlinear elements such as voltage-gated channels and second-messenger cascades. However, a report by Chichilnisky and Baylor in this issue of Nature Neuroscience3 provides reassurance. By analyzing how individual cones contribute to the receptive field of a retinal gangion cell, the authors provide support for a key hypothesis derived from the wiring diagram, that the strength of an excitatory input is proportional to the number of chemical synapses that underlie it.

Retinal ganglion cells are the output neurons of the retina. They are separated by two synapses from the photoreceptors, and the intervening wiring suggests that ganglion cells are capable of considerable integration. Cones are connected laterally to each other through electrical synapses, and they project forward via glutamatergic synapses onto bipolar cells. Similarly, the bipolar cells are connected laterally via electrical synapses and also make forward connections via glutamatergic synapses onto the wide-spreading dendrites of ganglion cells. Thus, a ganglion cell receives connections, via bipolar cells, from many cones (convergence), whereas a given cone is connected to several ganglion cells (divergence). Quantitative anatomical studies have revealed that a ganglion cell receives many synapses from a bipolar cell that is aligned with the center of its dendritic tree, but few synapses from bipolar cells at the edge of the dendritic tree. Most of the synapses formed by these bipolar cells are onto neighboring ganglion cells with which they are more closely aligned.

This wiring causes a 'dome-like' distribution of excitatory synapses across a ganglion cell's dendritic tree. The distribution of light sensitivity across the central region of a ganglion cell's receptive field is also dome-shaped, and so it was natural to hypothesize that this might reflect the distribution of synaptic inputs4, 5. Furthermore, the fact that a single bipolar cell can form synapses with more than one ganglion cell suggests an explanation for the overlap between the receptive field centers of neighboring ganglion cells6, 7. These hypotheses could be tested by recording simultaneously from several adjacent ganglion cells while stimulating the overlying cones one at a time. A method for multiple recordings has been available for several years8, but cones are so closely packed in the intact retina that it is difficult to stimulate them one at a time. That is what Chichilnisky and Baylor have now achieved in a technical and analytical tour de force that confirms both hypotheses.

The authors placed a small piece of peripheral retina from a macaque monkey in a perfusion chamber, with the ganglion cell layer contacting a multi-electrode array. This allowed simultaneous recording of the spike trains from many ganglion cells. To stimulate individual cones, they projected the image of a color monitor onto the photoreceptors through reducing optics, so that a single pixel would span about 25 m, comparable to the distance between adjacent cones. Each pixel could be driven by the monitor's red, green and blue guns, and by adjusting their relative intensities, a given pixel could be made to emit light that was optimal for one of the three cone types, which are sensitive to short (S), medium (M) or long (L) wavelengths. By using a variety of flickering patterns, it was possible to calculate the average stimulus displayed in the period just preceding a spike and hence to identify the combination of gun intensities that was most likely to trigger a spike in a given ganglion cell.

This approach allowed the authors to cleanly identify the well known 'blue/yellow' (BY) ganglion cells9. These cells fire selectively in response to an increase in the intensity of blue light and a decrease in the intensity of yellow light (that is, a mixture of red and green light; Fig. 1a). The cell responds oppositely to excitation of S and M+L cones, despite the fact that all bipolar synapses excite the BY cell. The reason is that bipolar types selective for S or M+L cones respond oppositely to the glutamate released from cones. S bipolar dendrites express a metabotropic receptor (mGluR6) that closes a cation channel, whereas M+L bipolar dendrites express an ionotropic receptor that opens a cation channel. This molecular difference between the two classes of bipolar cells is largely responsible for the spectral opponency of the BY ganglion cells, and thus represents a key mechanism for blue/yellow perception10.

The BY ganglion cell was ideal for the authors' purpose. S cones comprise only ten percent of all cones, and in the coarse mosaic of peripheral retina (Fig. 1b), they are spaced widely enough to be stimulated individually by the flash of a single pixel. By stimulating individual S cones and recording from BY ganglion cells, the authors could probe for divergence and convergence of the inputs to the ganglion cells, and they could directly test for linearity of S-cone summation.

A single S cone signal did indeed diverge to provide input to adjacent BY ganglion cells, contributing strongly to one BY cell and weakly to another. Because the two BY cells were recorded simultaneously, their different responses to stimulation of the same cone could not be attributed to a change in cone photosensitivity; rather, it must have been caused by a difference in the cone's synaptic connections. Furthermore, two S cones did indeed converge onto a given BY cell, with one cone always dominating and the other contributing only weakly. Finally, a plot of the BY cell's average spike rate versus stimulus intensity for illumination of both S cones neatly superimposed onto plots of rate versus stimulus intensity for illumination of either S cone separately. The finding that all three curves superimpose proves that the signals from both S cones combine linearly. Thus, despite the existence of nonlinear mechanisms, such as sodium action potentials in ganglion cell dendrites11, the overall circuit provides a linear summation of S cone signals.

These functional results clearly match the structural microcircuit (Figs. 1c and 2). An S cone provides most of its synapses to one S bipolar cell and a few synapses to the neighboring bipolar cells10. In turn, an S bipolar cell provides most of its synapses to one BY ganglion cell and a few synapses to the neighbors10. Thus, the synaptic connections from one S cone diverge to several BY cells, and connections from several S cones converge upon one BY cell, with one cone predominating. The structural circuit also shows the BY cell receiving twofold more synapses from S bipolar cells than from M+L bipolar cells10. This might explain why a BY cell responds faster to S cones than to M and L cones3 ( Fig. 1a), because speed increases with amplification.

The BY cell's microcircuit, now based on functional as well as structural evidence, suggests an explanation for a striking result from human psychophysics13. When M and L cones are desensitized by exposure to yellow light, it is possible to perceive a tiny blue flash that is designed to excite a single S cone. Sensitivity to this flash shows a punctate distribution, corresponding to the distribution of S cones14. This punctate perceptual sensitivity might now be explained b