Microcircuitry of M and L cone midget ganglion cell pathways in the primate fovea

Midget ganglion cells constitute 80% of all ganglion cells and are believed to be the retinal substrates of both high spatial acuity and color vision, two capacities that separate the visual system of the Old World monkey from that of the rest of the mammalian world. Yet, very little is known about the synaptic architecture of the midget ganglion cell, whether it involves additional elements, or whether the midget circuits for S, M and L cones in the Old World retina are identical. In this thesis, nearly 200 midget ganglion cells and their connections to midget bipolar and amacrine cells were studied in electron micrographs of 319 ultrathin serial sections through the central retina (or fovea) of the Old World monkey, macaca fascicularis. Each of 104 cones contacted a single OFF and a single ON midget pathway consisting of a midget bipolar and a midget ganglion cell. The OFF and ON pathways connected to the same cone were nearly identical in size, number of bipolar $\rightarrow$ ganglion cell synapses, and number of amacrine cell $\rightarrow$ bipolar cell and amacrine to $\rightarrow$ ganglion cell synapses. For both the OFF and ON pathways, most of the amacrine synapses to the midget cells were from amacrine processes postsynaptic to the midget bipolar in the same pathway. These amacrine cells collect from every bipolar terminal within their reach. The midget circuits from adjacent cones differed remarkably. Some cones had midget circuits with about 30 bipolar $\rightarrow$ ganglion cell synapses, while other cones had midget circuits that were larger with about 50 bipolar synapses. These two cone types in the mosaic of 104 cones were about equal in number and were arranged in small, randomly formed clusters of like cones. This result is similar to a recent microspectrophotometric assay of the primate cone mosaic in which M and L cones were found in about equal numbers arranged in small clusters of like type. Thus, the two types of midget circuit identified here in all likelihood correspond to M and L cone circuits