The dynamics of the cat retinal Y cell subunit.

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Abstract
1. The dynamics of the subunit mechanism of individual cat Y retinal ganglion cells are investigated. In order to isolate the response of the non-linear subunit mechanism, the visual stimuli were sine gratings of a spatial frequency sufficiently high so that contrast reversal of the grating elicited no fundamental response in any spatial phase. For study of the non-linear subunit mechanism, the contrast of the spatial sine grating was varied in time by a temporal modulation signal, consisting of either a square wave or a sum of sinusoids. 2. The responses of twenty-three Y ganglion cells (sixteen on-centre, seven off-centre) to these two stimulus types were measured at a range of contrasts. Responses to the sum-of-sinusoids signal were characterized by the second-order frequency kernel. The overall size of the second-order frequency kernel was approximately proportional to contrast. The deviation from proportionality suggested a power-law scaling, with a power in the range 0.8-0.9. 3. Square-wave responses, as characterized by the post-stimulus histogram, demonstrated identical responses at both reversals of the grating. A similar contrast dependence was observed in the overall size of the square-wave responses. 4. In order to attempt to predict the square-wave responses from the sum-of-sinusoids responses, the second-order frequency kernel measured at each contrast level was fitted with a lumped linear-static non-linear-linear model. In eighteen of twenty-three cells (eleven on-centre, seven off-centre), this model provided an adequate description of the response to the sum-of-sinusoids stimulus. In these cells, the linear-static non-linear-linear model accurately reproduced the square-wave response. 5. In the remaining five ganglion cells (all on-centre), the second-order frequency kernel could not be fitted by a linear-static non-linear model. This diversity of dynamical properties among Y cells was not apparent from the respones of these Y cells to the square-wave temporal stimulus. 6. In the eighteen Y ganglion cells that were fitted well with the linear-static non-linear-linear model, substantial variation of the dynamical parameters was found. However, there were systematic differences between the dynamics of the typical on-centre and off-centre ganglion cells. These differences relate to both linear stages of the model, and are not merely consequences of the lower firing rate of the off-centre cells. 7. In these ganglion cells, the dynamics of the first linear filter were similar to the linear dynamics of the X cell centre. There was a tendency for greater transience in the first linear filter of the Y cell compared with that of the X cell. However, most of the greater transience of the Y cell response originates in the second linear filter. The non-linearity per se does not contribute to the transience of the Y cell.