The stellate varicose amacrine cell is positioned to provide a second layer of inhibition specific to the primate midget system

Abstract

Purpose : Human vision served by the midget system evolved to deduce the reflectance properties of objects. To do this, the visual system separates information about reflectance from information about the properties of the illuminant. Some of this information is available at the edges of objects because illumination changes occur gradually across a scene while reflectance changes rapidly at the edges. Images processed through the primate midget system extract this reflectance information by being filtered through stages such that the color appearance of objects is determined solely by the contrast at their edges (Fig 1A). A first stage of this filtering process takes place in the outer retina by lateral inhibition from horizontal cells. We sought to characterize the circuitry of the inner retina specific to the midget system that might provide an additional stage of filtering.

Methods : We used serial block-face scanning electron microscopy in the macaque central retina (Patterson et al., 2019, Sci Rep) to reconstruct neurons in the inner retina that receive synaptic input from midget ON bipolar cells.

Results : We reconstructed a type of wide-field amacrine cell (n=2 for synaptic analysis, n=3 for morphology) that receives 97% of its input from and provides 98% of its output as reciprocal inhibitory feedback onto midget ON bipolar cells. Based on their stratification (78% IPL depth), the characteristic morphology of their dendritic trees and their dendritic varicosities, these were identified as ON stellate varicose cells. The dendritic trees of reconstructed cells overlapped extensively, forming dense coverage of the midget ON bipolar cell terminals (Fig 1B). We reconstructed 99 presynaptic midget ON bipolar cells and found that each made on average 2.3 ribbons synapses onto and received 1.5 reciprocal feedback synapses from an ON stellate varicose cell.

Conclusions : A dense network of ON stellate varicose amacrine cells specific to midget bipolar cells could provide a second layer of inhibition in the inner retina. By virtue of the delay of the inhibition from an additional synapse, the interaction between bipolar cell terminals and amacrine cells could act as a temporal filter contributing to the removal of signals other than those produced by contrast edges in a scene.

This is a 2021 ARVO Annual Meeting abstract.

 

A. Color appearance due to contrast at edges. B. Reconstructed stellate varicose amacrine cells.