Effects of Stimulus Size on Ganglion Cell Responses in the Light Adapted Murine Retina

Abstract

Abstract: : Purpose: To study the effects of increasing stimulus size on the response properties of retinal ganglion cells in the mouse. Methods: Single-unit responses were recorded from optic nerve fibers of anesthetized mice in vivo. For each cell the receptive field center was mapped on a tangent screen, using small spots generated by a handheld ophthalmoscope. Area response functions were then obtained by presenting computer-generated flashing spots whose diameter varied from 1 to 50 deg., presented on a uniform background of 25 cd/m². Full-field illumination was 50x80 deg. The spot was turned on (50 cd/m²) and off (0 cd/m²) at 0.1 Hz. Each spot diameter was presented eight times. Average PSTHs were accumulated over all trials at each stimulus size. The center sign of the response (ON, OFF, ON/OFF), the response amplitude and its duration were measured from the average PSTH and a transient/sustained index was computed. Results: Using our hand-held ophthalmoscope and small diameter computer-generated spots, we have encountered approximately equal proportions of ON- and OFF- center ganglion cells. Using the same stimulation conditions, no ON/OFF-center cells have been encountered. The majority of both ON- and OFF-center cells responded optimally to a stimulus whose diameter was between 4-21 deg, and their responses were attenuated when both smaller and larger diameter stimuli were centered on the receptive field. Increasing to full-field illumination produced a significant decrease in the peak firing rate of most cells by an average factor of 0.55±0.07 (±SEM, p less than 0.001), indicating the presence of an inhibitory surround. In contrast, increasing the stimulus size had little or no effect on the duration of the response or on its transient/sustained index. In ~10% of the OFF-center cells, full-field illumination elicited an ON/OFF response, probably due to the presence of strong center–surround interactions. In another 10% of the cells the center response was completely suppressed by full-field stimulation. Conclusions: These data suggest that in the mouse, as in other mammalian retinas, center-surround receptive field interactions are important in shaping of RGC response. Thus, full-field stimulation cannot be used to accurately predict either the center sign or the peak response for many cells.

Keywords: ganglion cells • receptive fields • electrophysiology: non-clinical