Center and surround mechanisms of opponent-color X and Y ganglion cells of retina of macaques.

1. Opponent-color ganglion cells of macaques can be classed as X or Y. Cells with a cone-specific receptive-field organization (center and surround receiving input from spectrally different cone types) have a linear summation, whereas cells with a cone-mixed organization (center and surround partly mediated by input from the same cone type) have a nonlinear summation. 2. Pure center and pure surround responses of Y-cells have a fast decay and show conspicuous transients at stimulus offset and onset; pure responses of X-cells have a slow decay and show fewer transients, especially at stimulus offset. 3. Sensitivity profiles based on pure responses elicited in conditions of chromatic adaptation of the opponent responses show that Y-cells have unimodal center and unimodal surround profiles, whereas X-cells have unimodal center and bimodal surround profiles. 4. Responses receiving contribution from both opponent mechanisms (mixed) have different time course and pattern in X- and Y-cells. Mixed response of Y-cells show a discontinuity in cell firing during the transient (on) component of cell activity, which has a higher sensitivity than other waveform changes produced by concurrent stimulation of the opponent mechanism. This discontinuity occurs with stimulus conditions that also elicit proximal negative responses in the local electroretinogram and appears to be due to a centrally located process having some degree of rectification.     

Spatiospectral properties of goldfish retinal ganglion cells

1. Responses of single ganglion cells from isolated goldfish retinas were recorded during presentation of various spatial and spectral stimuli. Each cell was classified along several spatial [spatial summation class, spatial contrast sensitivity function (CSF), and response to contrast] and spectral (Red-ON, Red-OFF or Red-ON/OFF, and spectral opponency/nonopponency) dimensions. 2. Linearity of spatial summation was determined from responses to contrast-reversal sinusoidal gratings positioned at various locations across the receptive field of the cell. CSFs were derived from responses to sinusoidal gratings of various spatial frequencies and contrasts, drifting across the cell's receptive field at a rate of 4 Hz. Response to contrast was determined from responses to variations in contrast of a sinusoidal grating of optimal spatial frequency. Spectral classifications were based on responses to monochromatic stimuli presented separately to the center and surround portions of the receptive field. 3. Linearity of spatial summation (X-, Y-, and W-like) was independent of the cell's spectral properties; for example, an X-like cell could be classified as either a Red-ON, Red-OFF, or Red-ON/OFF center cell and as spectrally opponent or nonopponent. 4. There were differences in response to contrast across spectral categories. Red-OFF center cells were very sensitive to contrast compared with Red-ON center cells. Spectrally nonopponent cells were more responsive to contrast than spectrally opponent cells. 5. There were dramatic differences across the spectral categories in relative sensitivity to low spatial frequency stimuli; however, the spatial resolution (i.e., sensitivity to high spatial frequencies) of each spectral classification appeared to be similar.(ABSTRACT TRUNCATED AT 250 WORDS)     

The contrast sensitivity of retinal ganglion cells of the cat

1. Spatial summation within cat retinal receptive fields was studied by recording from optic-tract fibres the responses of ganglion cells to grating patterns whose luminance perpendicular to the bars varied sinusoidally about the mean level. 2. Summation over the receptive fields of some cells (X-cells) was found to be approximately linear, while for other cells (Y-cells) summation was very non-linear. 3. The mean discharge frequency of Y-cells (unlike that of X-cells) was greatly increased when grating patterns drifted across their receptive fields. 4. In twenty-one X-cells the relation between the contrast and spatial frequency of drifting sinusoidal gratings which evoked the same small response was measured. In every case it was found that the reciprocal of this relation, the contrast sensitivity function, could be satisfactorily described by the difference of two Gaussian functions. 5. This finding supports the hypothesis that the sensitivities of the antagonistic centre and surround summating regions of ganglion cell receptive fields fall off as Gaussian functions of the distance from the field centre. 6. The way in which the sensitivity of an X-cell for a contrast-edge pattern varied with the distance of the edge from the receptive field centre was determined and found to be consistent with the cell's measured contrast sensitivity function. 7. Reducing the retinal illumination produced changes in the contrast sensitivity function of an X-cell which suggested that the diameters of the summating regions of the receptive field increased while the surround region became relatively ineffective.     

Non-linear spatial summation in cat retinal ganglion cells at different background levels

Summary Cat retinal ganglion cells were identified as X-cells (linear) or Y-cells (non-linear) on the basis of the spatial summation properties of their receptive fields. For each cell, the degree of non-linearity in spatial summation was assessed at a number of different mean luminance levels in order to determine how spatial linearity depended on mean luminance. The stimuli were counterphase sinusoidal gratings whose contrast was sinusoidally modulated in time. A grating with one bar centered on the receptive field was used to measure the contrast sensitivity of the mechanisms which produced responses at the stimulus frequency. A grating with a zero crossing centered on the receptive field was used to measure the contrast sensitivity of mechanisms responsible for the non-linear frequency doubled responses of Y-cells. As the mean luminance was reduced from low photopic to scotopic, the contrast sensitivity decreased for both the linear and non-linear responses. The ratio of non-linear to linear sensitivity in Y-cells changed less with background than did either contrast sensitivity. In some Y-cells this ratio decreased slightly at low luminance levels, but in others it did not. X-cells appeared to sum signals linearly at all levels of illumination. X-cells and Y-cells could still be distinguished on the basis of their spatial summation properties in the scotopic range.     

Velocity tuning of cells in dorsal lateral geniculate nucleus and retina of the cat

Velocity tuning curves were measured for on-center cells in the dorsal lateral geniculate nucleus of the cat using a stimulus approximately the height and one-fourth the width of the hand-plotted receptive-field center. The standard stimulus strength was 1 log unit above the mesopic background luminance. Lateral geniculate Y-cells had significantly higher preferred velocities than geniculate X-cells when cells with receptive fields having the same range of retinal eccentricities were compared. Preferred velocity increased for both classes of cells as a function of retinal eccentricity. For all geniculate cells, preferred velocity increased with stimulus strength, showing an approximately threefold increase in preferred velocity for each log unit of stimulus strength. Preferred velocity was measured for on-center retinal ganglion cells with receptive fields at the same range of retinal eccentricities as the geniculate sample and under the same stimulus conditions. Preferred velocities of retinal ganglion Y-cells were significantly higher than those of ganglion X-cells, and as for geniculate cells, preferred velocities increased with increasing stimulus strength. However, the classes were better separated in the geniculate than in the retina; with geniculate X-cells having lower preferred velocities than retinal X-cells, and the geniculate Y-cells having higher preferred velocities than retinal Y-cells. For retinal ganglion cells, smaller receptive-field center sizes of the X-cells than the Y-cells could account in large part for the lower preferred velocities of the X-cells. However, for geniculate cells, differences in receptive-field center size could not account as well for the differences in preferred velocity between X- and Y-cells. Furthermore, field size differences could not account for the differences in preferred velocity between ganglion and geniculate cells of the same functional class. Experiments comparing responses to moving stimuli and flashed stationary stimuli show that stimuli moving at high velocities are in effect equivalent to brief-duration flashes, and responses are governed by the same laws of temporal summation in both cases. When velocity tuning curves were measured with long bars that enhanced peripheral inhibition, geniculate X- and Y-cells were better separated than ganglion X- and Y-cells, not only with respect to preferred velocity but also, with respect to velocity selectivity (width of the velocity tuning curve) and differential velocity sensitivity (slope of the leg of the velocity tuning curves ascending from low velocities to the peak).(ABSTRACT TRUNCATED AT 400 WORDS)     

Velocity selectivity in the cat visual system. I. Responses of LGN cells to moving bar stimuli: a comparison with cortical areas 17 and 18

Velocity selectivity of 92 LGN cells was measured quantitatively using long, narrow light or dark bars of high contrast in N2O-anesthetized and paralyzed cats. The optimal velocities of the main responses to a moving light bar, representing center responses (i.e., due to entering the ON center or leaving the OFF center) were significantly lower for X-cells than for Y-cells. The velocity upper cutoffs were significantly higher for Y-cells than for X-cells, whereas responses to slow movement were significantly stronger in X-cells than in Y-cells. The velocity range over which secondary responses were found was significantly lower for X-cells than for Y-cells. The velocity characteristics of LGN cells were compared with those measured under precisely the same experimental conditions in areas 17 and 18. Overall, the LGN cells were sensitive to much faster velocities than cortical cells. The differences between these cortical areas were found to be much larger than the differences in velocity selectivity observed in the LGN between X- and Y-cells or within the X and Y classes. In particular, the ubiquitous presence of cells responding only to very low velocities (less than 10 degrees/s) in area 17 subserving central vision cannot directly reflect LGN velocity selectivity, since such extreme preference for low velocities was not found in the LGN sample. Changes in eccentricity had much less effect on the velocity characteristics in the LGN than in the cortex. The latency of responses to a moving light bar as estimated using a spatial lag-velocity method was on average 46 and 37 ms for X-ON and Y-ON cells as opposed to 75 and 68 ms for X-OFF and Y-OFF cells, respectively. These latencies were slightly shorter than the ON and OFF latencies (time to peak) measured with stationary presentations of the same light bar (averages 61 and 53 ms for X-ON and Y-ON, 113 and 93 ms for X-OFF and Y-OFF). For a moving dark bar the average latency was 35 and 29 ms for X-OFF and Y-OFF cells, respectively, whereas it was 47 and 54 ms for X-ON and Y-ON cells. There were no significant differences in response strength between ON and OFF cells nor between X- and Y-cells. Many Y-OFF cells had nonlinear spatial lag-velocity relationships. This indicates a shift in response origin from distal to more proximal parts of the receptive field when going from low to high velocities.(ABSTRACT TRUNCATED AT 400 WORDS)     

Nonlinear measurement and classification of receptive fields in cat retinal ganglion cells

Originally, modeling of ganglion-cell responses in cat was based mainly on linear analysis. This is satisfactory for those cells in which spatial summation of excitation is approximately linear (X-cells) but it fails for Y-cells, where summation has strong nonlinear components. Others have shown the utility of using sinusoidal analysis to study harmonic and intermodulation nonlinearities in the temporal frequency domain. We have used Wiener-kernel analysis to obtain directly both temporal and spatial impulse responses and their nonlinear interactions. From these, we were able to predict accurately the responses that a counterphase modulated grating elicited in both X-cells and Y-cells. In addition, we show that the first-order responses can measure the two-dimensional spatial features of the receptive field with high resolution. Thus, nonlinear analysis of responses to white-noise stimuli may be sufficient to both classify and measure the receptive fields of many different types of ganglion cells.     

Adaptation and dynamics in X-cells and Y-cells of the cat retina

Summary On the basis of the spatial summation properties of their receptive fields, cat retinal ganglion cells were classified as either X-cells (linear) or Y-cells (non-linear). Responses were then obtained to a small, centered spot, square-wave modulated in time and superimposed on various levels of diffuse, steady background illumination. When fully dark-adapted, both X-cells and Y-cells produced responses that were entirely sustained. When well lightadapted but still in the scotopic range, both cell types produced largely transient responses with only a very small sustained component. The sustained or transient nature of responses is, therefore, not an invariant characteristic of X-cells and Y-cells in the scotopic range. We also conclude that the mechanism which controls the center's sensitivity in the scotopic range is similar though not identical in the two types of cells.     

Receptive-field properties of adult cat’s retinal ganglion cells with regenerated axons

Receptive-field properties of retinal ganglion cells (RGCs) that had regenerated their axons were studied by recording single-unit activity from strands teased from peripheral nerve (PN) grafts apposed to the cut optic nerve in adult cats. Of the 286 visually responsive units recorded from PN grafts in 20 cats, 49.7% were classified, according to their receptive-field properties, as Y-cells, 39.5% as X-cells, 6.6% as W-cells, and 4.2% were unclassified. The predominant representation of Y-cells is consistent with a corresponding morphological study (Watanabe et al. 1993a), which identified α-cells as the RGC type with the largest proportion of regenerating axons. Among the X-cells, we only found ON-center types, whereas both ON-center and OFF-center Y-cells were found. As in intact retinas, the receptive-field center sizes of Y-cells and W-cells were larger than those of X-cells at corresponding displacements from the area centralis. Within the 10° surrounding the area centralis, the receptive fields of X-cells with regenerated axons were larger than those in intact retinas, suggesting that some rearrangement of retinal circuitry occurred as a consequence of degeneration and regeneration. Receptive-field center responses of Y-, X-, and W-type units with regenerated axons were similar to those found in intact retinas, but the level of spontaneous activity of Y- and X-type units was, in general, less than that of intact RGCs. Receptive-field surrounds were weak or not detected in more than half of the visually responsive RGCs with regenerated axons. Received: 17 November 1997 / Accepted: 19 June 1998     

Classification of cat retinal ganglion cells into X- and Y-cells with a contrast reversal stimulus

Summary A contrast reversal (alternating phase) stimulus was used to study the responses of 150 retinal ganglion cells from 15 adult cats. Because the majority of the cells did not show perfect linear spatial summation, a ratio of the firing rates at two time periods was used to express the degree of nonlinearity. Y-cells showed a high degree of nonlinearity, and their mean null ratio was significantly lower than that of X-cells. With the stimulus at the null position, X-cells had an unmodulated discharge rate which was significantly higher than maintained activity, while the firing rate of Y-cells was lower than maintained activity. With the stimulus placed at an eccentric position in the receptive field, X-cells responded in a sustained manner, while Y-cells respond transiently. Because of these observations, we conclude that X-cells correspond to the sustained cells, while Y-cells correspond to the transient cells.     

Precision, reliability, and information-theoretic analysis of visual thalamocortical neurons

High-order statistics of neural responses allow one to gain insight into neural function that may not be evident from firing rate alone. In this study, we compared the precision, reliability, and information content of spike trains from X- and Y-cells in the lateral geniculate nucleus (LGN) and layer IV simple cells of area 17 in the cat. To a stochastic, contrast-modulated Gabor patch, layer IV simple cells responded as precisely as their primary inputs, LGN X-cells, but less reliably. LGN Y-cells were more precise and reliable than LGN X-cells. Also, within each LGN cell type, 1) responses to the same stimulus were nearly identical if they shared the same center sign and 2) responses of neurons with the same center sign were nearly identical to the responses of neurons of opposite center sign if the stimulus' contrasts were inverted. These results suggest simple cells receive highly precise and synchronous LGN input, resulting in precise responses. Nonetheless, the response precision of simple cells was greater than expected. Finally, information-theoretic calculations of our cell responses revealed that 1) LGN X-cells encoded information at half the rate of LGN Y-cells but 2.5 times the rate of layer IV simple cells; 2) LGN cells encoded information in their responses using temporal patterns, whereas simple cells did not; and 3) simple cells used more of their information capacity than LGN X-cells. We propose mechanisms that simple cells might use to ensure high precision.     

Properties of X- and Y-cells in the rabbit retina

Bevelled glass microelectrodes were used to record spike potentials extracellularly from the ganglion cells of the rabbit retina. Good responses were obtained from the isolated retina or eye-cup preparation for at least 12 hr. Using a contrast reversal stimulus, 63.8% (67/105) of the units showed linear spatial summation (X-cells), and 21.0% (22/105) showed nonlinear spatial summation (Y-cells). The X- and Y-cells in the rabbit retina had physiological properties which were similar to those in cat retina. Directionally selective cells (15.2%) were found to respond poorly, if at all, to the contrast reversal stimulus.     

Development of neuronal response properties in the cat dorsal lateral geniculate nucleus during monocular deprivation

We measured response properties of X- and Y-cells from laminae A and A1 of the dorsal lateral geniculate nucleus of monocularly lid-sutured cats at 8, 12, 16, 24, and 52-60 wk of age. Visual stimuli consisted of small spots of light and vertically oriented sine-wave gratings counterphased at a rate of 2 cycles/s. In cats as young as 8 wk of age, nondeprived and deprived neurons could be clearly identified as X-cells or Y-cells with criteria previously established for adult animals. Nonlinear responses of Y-cells from 8- and 12-wk-old cats were often temporally labile; that is, the amplitude of the nonlinear response of nondeprived and deprived cells increased or decreased suddenly. A similar lability was not noted for the linear response component. This phenomenon rarely occurred in older cats. At 8 wk of age, Y-cell proportions (number of Y-cells/total number of cells) in nondeprived and deprived A-laminae were approximately equal. By 12 wk of age and thereafter, the proportion of Y-cells in deprived laminae was significantly lower than that in nondeprived laminae. At no age was there a systematic difference in response properties (spatial resolution, latency to optic chiasm stimulation, etc.) for Y-cells between deprived and nondeprived laminae. Spatial resolution, defined as the highest spatial frequency to which a cell would respond at a contrast of 0.6, was similar for nondeprived and deprived X-cells until 24 wk of age. In these and older cats, the mean spatial resolution of deprived X-cells was lower than that of nondeprived X-cells. This difference was noted first for lamina A1 at 24 wk of age and later for lamina A at 52-60 wk of age. The average latency of X-cells to optic chiasm stimulation was slightly greater in deprived laminae than in nondeprived laminae. No such difference was seen for Y-cells. Cells with poor and inconsistent responses were encountered infrequently but were observed far more often in deprived laminae than in nondeprived laminae. Lid suture appears to affect the development of geniculate X- and Y-cells in very different ways. Not only is the final pattern of abnormalities quite different between these cell groups, but the developmental dynamics of these abnormalities also differ.     

Receptive field analysis: responses to moving visual contours by single lateral geniculate neurones in the cat

1. Responses of single geniculate cells to moving light and dark bars and light/dark edges were studied in cats anaesthetized with nitrous oxide/oxygen (70%/30%).2. Over 95% (230 out of 241) of geniculate cells had antagonistic centre-surround receptive fields. Their responses could be characterized as centre-activated or centre-suppressed depending on the receptive field type (ON- or OFF-centre) and the contrast between stimulus and the background (brighter or darker than the background). Moving light and dark edges evoked responses which were very similar to the responses evoked by these stimuli in simple cells of striate cortex.3. A number of cells (45) with antagonistic centre-surround receptive fields were classified according to their X/Y (sustained/transient) properties. Units with sustained properties (X-cells) did not increase their firing rate with an increase of stimulus velocity and some of them showed a clear-cut preference for slow movement (around 1-2 degrees /sec). On the other hand, units with transient properties (Y-cells) showed a clear-cut preference for fast-moving stimuli (50-100 degrees /sec.)4. Elongation of the stimulus beyond the antagonistic surround in both X- and Y-cells produced a clear-cut reduction of amplitude of both centre and surround components of the response. Thus the existence of a suppressive field component beyond the antagonistic surround is confirmed.5. About 5% of cells had receptive fields which did not have an antagonistic centre-surround organization but gave a mixed ON-OFF discharge from the central region of the field. Around the central region there was a silent suppressive zone. These units were not directionally selective, responded preferentially to fast-moving stimuli (25-100 degrees /sec) and had a substantial (spontaneous) maintained activity.     

Selectivity and spatial distribution of signals from the receptive field surround in macaque V1 neurons

The responsiveness of neurons in V1 is modulated by stimuli placed outside their classical receptive fields. This nonclassical surround provides input from a larger portion of the visual scene than originally thought, permitting integration of information at early levels in the visual processing stream. Signals from the surround have been reported variously to be suppressive and facilitatory, selective and unselective. We tested the specificity of influences from the surround by studying the interactions between drifting sinusoidal gratings carefully confined to conservatively defined center and surround regions. We found that the surround influence was always suppressive when the surround grating was at the neuron's preferred orientation. Suppression tended to be stronger when the surround grating also moved in the neuron's preferred direction, rather than its opposite. When the orientation in the surround was 90 degrees from the preferred orientation (orthogonal), suppression was weaker, and facilitation was sometimes evident. The tuning of surround signals therefore tended to match the tuning of the center, though the tuning of the surround was somewhat broader. The tuning of suppression also depended on the contrast of the center grating-when the center grating was reduced in contrast, orthogonal surround stimuli became relatively more suppressive. We also found evidence for the tuning of the surround being dependent to some degree on the stimulus used in the center-suppression was often stronger for a given center stimulus when the parameters of the surround grating matched the parameters of the center grating even when the center grating was not itself of the optimal direction or orientation. We also explored the spatial distribution of surround influence and found an orderly relationship between the orientation of grating patches presented to regions of the surround and the position of greatest suppression. When surround gratings were oriented parallel to the preferred orientation of the receptive field, suppression was strongest at the receptive field ends. When surround gratings were orthogonal, suppression was strongest on the flanks. We conclude that the surround has complex effects on responses from the classical receptive field. We suggest that the underlying mechanism of this complexity may involve interactions between relatively simple center and surround mechanisms.     

Spatial distribution of suppressive signals outside the classical receptive field in lateral geniculate nucleus

A suppressive surround modulates the responsiveness of cells in the lateral geniculate nucleus (LGN), but we know nothing of its spatial structure or the way in which it combines signals arising from different locations. It is generally assumed that suppressive signals are either uniformly distributed or balanced in opposing regions outside the receptive field. Here, we examine the spatial distribution and summation of suppressive signals outside the receptive field in extracellular recordings from 46 LGN cells in anesthetized marmosets. The receptive field of each cell was stimulated with a drifting sinusoidal grating of the preferred size and spatial and temporal frequency; we probed different positions in the suppressive surround with either a large half-annular grating or a small circular grating patch of the preferred spatial and temporal frequency. In many of the cells with a strong suppressive surround (29/46), the spatial distribution of suppression showed clear deviation from circular symmetry. In the majority of these of cells, suppressive signals were spatially asymmetrical or balanced in opposing areas outside the receptive field. A suppressive area was larger than the classical receptive field itself and spatial summation within and between these areas was nonlinear. There was no bias for suppression to arise from foveal or nasal retina where cone density is higher and no other sign of a systematic spatial organization to the suppressive surround. We conclude that nonclassical suppressive signals in LGN deviate from circular symmetry and are nonlinearly combined.     

Influence of contrast on the responses of marmoset lateral geniculate cells to drifting gratings

The responses of lateral geniculate nucleus (LGN) cells in the common marmoset (Callithrix jacchus) to drifting luminance or cone isolating gratings of different spatial frequencies and contrasts were measured. The response noise, defined as the variability of the responses to single sweeps in the complex plane, was independent of stimulus contrast and spatial frequency but increased with increasing overall responsiveness of the cell. The signal-to-noise ratio of parvocellular (PC) cells was smaller than of magnocellular (MC) cells. At each contrast, the response amplitude as a function of spatial frequency could be described with a difference of Gaussians model. With this model, the sizes and the peak sensitivities of the receptive field centers and surrounds were estimated. It was found that receptive field center and surround sizes of LGN cells decrease slightly with increasing contrast. Further, the peak sensitivity decreases with increasing contrast. The two factors are involved in a decrease in responsivity (the response per unit contrast) with increasing contrast which is compatible to response saturation for low spatial frequency stimuli. PC cells did not saturate as much to luminance stimuli although some saturation was found with cone isolating gratings. We found that the response phase lag of both PC and MC cells decreased with increasing contrast, which cannot be explained on the basis of linear response behavior. Apparently the phase of LGN cell responses to drifting gratings is altered in comparison with the retinal inputs by additional nonlinearities.     

Spatial properties of neurones in the lateral geniculate nucleus of the pigmented ferret

Summary We used quantitative electrophysiological techniques to study the spatial properties of single units recorded extracellularly in the lateral geniculate and perigeniculate nuclei of the adult pigmented ferret. All neurones examined had approximately circular receptive fields, whose central regions gave responses antagonistic to those elicited from the surrounds. We presented sinusoidally modulated grating patterns, either drifting or counterphased, to obtain spatial frequency tuning curves, contrast sensitivity functions and to assess linearity or non-linearity of each neurone's response. In the ferret, as in other species, two types of lateral geniculate neurone could be distinguished, and we have termed these X-cells and Y-cells; both groups responded briskly to visual stimulation but X-cells gave sustained and linear responses whereas Y-cells responded transiently and non-linearly. Perigeniculate cells gave linear responses. For neurones in the lateral geniculate and perigeniculate nuclei, both the limit of spatial resolution (acuity) and optimum spatial frequency were inversely related to receptive field eccentricity and the diameter of the receptive field centre. We recorded geniculate neurones in the ferret with acuities up to 3 cycles deg^-1 and contrast sensitivities up to 114, values that are lower than those found previously for many geniculate cells in the cat.     

The periphery effect in cat retinal ganglion cells: variation with functional class and eccentricity

Summary We have studied the responses of ganglion cells of the cat retina to visual stimulation remote from the center of their receptive field. Following previous work, this response is termed the periphery effect (PE). Cells were identified as Y-, X- or W-class from the latency of their response to optic chiasm stimulation and from their receptive field properties. The strength of the PE elicited by a rotating windmill or counterphased grating stimulus was measured for ganglion cells of all major classes. The PE was consistently stronger in Y- than in X-cells, and the strength of the effect in both X- and Y-cells increased significantly with retinal eccentricity. A PE was elicited from about 47% of W-cells studied. In some (36%) the effect was excitatory, as for X- and Y- cells; in others (11%) it was inhibitory. Despite this heterogeneity, the PE in W-cells increased significantly with eccentricity. These variations of the PE with eccentricity and cell class have implications for the circuitry of the inner plexiform layer.