The retina is the first stage of visual processing. It encodes elemental features of visual scenes. Distinct cone bipolar cells provide the substrate for this to occur. They encode visual information, such as color and luminance, a principle known as parallel processing. Few studies have directly examined whether different forms of spatial information are processed in parallel among cone bipolar cells. To address this issue, we examined the spatial information encoded by mouse ON cone bipolar cells, the subpopulation excited by increments in illumination. Two types of spatial processing were identified. We found that ON cone bipolar cells with axons ramifying in the central inner plexiform layer were tuned to preferentially encode small stimuli. By contrast, ON cone bipolar cells with axons ramifying in the proximal inner plexiform layer, nearest the ganglion cell layer, were tuned to encode both small and large stimuli. This dichotomy in spatial tuning is attributable to amacrine cells providing stronger inhibition to central ON cone bipolar cells compared with proximal ON cone bipolar cells. Furthermore, background illumination altered this difference in spatial tuning. It became less pronounced in bright light, as amacrine cell-driven inhibition became pervasive among all ON cone bipolar cells. These results suggest that differential amacrine cell input determined the distinct spatial encoding properties among ON cone bipolar cells. These findings enhance the known parallel processing capacity of the retina.
Dendritic and parallel processing of visual threats in the retina control defensive responses
