SPATIAL RECEPTIVE-FIELD STRUCTURE OF CAT RETINAL W-CELLS

We have used frequency-domain methods to characterize the spatial rece ptive-field structure of cat retinal W cells. For most ON- and OFF-cen ter tonic and phasic W cells, measurements of responsivity to drifting gratings at various spatial frequencies could be adequately described by a difference-of-Gaussians (DOG) function, consistent with the pres ence of center and surround mechanisms that are approximately Gaussian in shape and whose signals are combined additively. Estimates of the responsivity of the center mechanisms of tonic and phasic W cells were similar, but both were significantly lower than the corresponding val ues for X or Y cells. The width of the center mechanisms of tonic W ce lls, phasic W cells, and Y cells did not differ significantly from eac h other, but all were significantly larger than the width of X-cell ce nters. Surround parameters did not vary significantly among the four g roups of ganglion cells. Measurements of contrast gain in both tonic a nd phasic W cells gave values that were significantly lower than in X or Y cells. Virtually all of the phasic W cells in our sample displaye d evidence of spatial non-linearities in their receptive fields, in th e form of either d.c. responses to drifting sine-wave gratings or seco nd harmonic responses to counterphased gratings. The spatial resolutio n of the mechanism underlying these nonlinearities was typically highe r than that of the center mechanism of these cells. Most tonic W cells exhibited linear spatial summation, although a subset gave strong sec ond harmonic responses to counterphased gratings. Spatial-responsivity measurements for most ON-OFF and directionally selective W cells were not adequately described by DOG functions. These cells did, however, show evidence of spatial nonlinearities similar to those seen in phasi c W cells. Suppressed-by-contrast cells gave both modulated and unmodu lated responses to drifting gratings which both appeared to involved r ectification, but which differed from each other in both spatial resol ution and contrast gain. These data confirm earlier reports that the r eceptive fields of tonic and most ON- or OFF-center phasic W cells app ear to include classical center and surround mechanisms. However, the receptive fields of some phasic cells, as well as ON-OFF and direction ally selective W cells may have quite different structures. Our result s also suggest that phasic, ON-OFF, directionally selective, suppresse d-by-contrast, and a subset of tonic W cells may all receive nonlinear inputs with characteristics similar to those described in the recepti ve fields of retinal Y cells. If so, this has important implications f or identifying and understanding the presynaptic circuitry of W cells, as well as the nature of their output to both telencephalic and midbr ain visual targets.