POTASSIUM-EVOKED DIRECTIONALLY SELECTIVE RESPONSES FROM RABBIT RETINAL GANGLION-CELLS

Previous studies have shown that directionally selective (DS) retinal ganglion cells cannot only discriminate the direction of a moving obje ct but they can also discriminate the sequence of two flashes of light at neighboring locations in the visual field: that is, the cells elic it a DS response to both real and apparent motion. This study examines whether a DS response can be elicited in DS ganglion cells by simply stimulating two neighboring areas of the retina with high external K+. Extracellular recordings were made from ON-OFF DS ganglion cells in s uperfused rabbit retinas, and the responses of these cells to focal ap plications of 100 mM KCl to the vitreal surface of the retina were mea sured. All cells produced a burst of spikes (typically lasting 50-200 ms) when a short pulse (10-50 ms duration) of KCl was ejected from the tip of a micropipette that was placed within the cell's receptive fie ld. When KCl was ejected successively from the tips of two micropipett es that were aligned along the preferred-null axis of a cell, sequence -dependent responses were observed. The response to the second micropi pette was suppressed when mimicking motion in the cell's null directio n, whereas an enhancement during apparent motion in the opposite direc tion frequently occurred. Sequence discrimination in these cells was e liminated by the GABA antagonist picrotoxin and by the Ca2+-channel bl ocker omega-conotoxin MVIIC, two drugs that are known to abolish direc tional selectivity in these ganglion cells. The spatiotemporal propert ies of the K+-evoked sequence-dependent responses are described and co mpared with previous findings on apparent motion responses of ON-OFF D S ganglion cells.