Abstract
Purpose :
Parallel visual pathways originate in diverse retinal ganglion cell (GC) populations. In the mouse retina nearly 40 GC types have been recognized and a substantial fraction of these show direction selectivity (DS) (Sabbah et al., Nature 546:492, 2017). By contrast in the primate ~17 GC types have been identified and DS has not been reported (Crook et al., The New Vis. Neurosci. 2014; Masri, et al., J. Comp. Neurol. 2017). However, a single GC type, the recursive bistratified cell, shares morphological features with ON-OFF DS cells of non-primate mammals and has been proposed as a likely DSGC (Dacey, The Cog. Neurosci. 2004; Moritoh, et al., Plos One 8: e54667, 2013).
Methods :
Rhodamine dextran was injected at physiologically identified locations in superior colliculus of macaque monkeys (Crook et al., J Neurosci 28:12654, 2008). After retrograde transport of dye photodynamic staining in the vitro retina (Dacey, Neuron 37:15, 2003) was used to visualize GCs and selectively target the recursive bistratified cell for extracellularly recording. Bars that varied in speed and size were moved across the receptive field in directions spanning 360 deg at 30 deg intervals. Directional tuning was characterized by the directions of motion that evoked the greatest (preferred) and smallest (null) spike discharge and a DS index given by the preferred and null response difference relative to the preferred response.
Results :
Unlike DSGCs in non-primate mammals where multiple populations show extensive dendritic overlap the recursive bistratified cells formed a single mosaic that tiled with little dendritic overlap comprising ~1.5% of the total GC population in the retinal periphery (Fig. top). Recursive cells show ON-OFF light responses and are strongly DS (Fig. bottom) with indexes ranging from 1.0 - 0.11 (mean +/- std = 0.46 +/- 0.18; n=23) and preferred directions that varied across cells.
Conclusions :
In macaque monkey retina a single GC type, the recursive bistratified cell shows ON-OFF direction selectivity and is present as a single population that tiles the retina with little dendritic overlap. We hypothesize that the low density of DSGCs in primate may explain a lower total number of GC types relative to other mammals and that the variable direction preferences found thus far may be related to retinal position relative to the foveal specialization unique to primate.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.