Primate retina: cell types, circuits and color opponency.

The link between morphology and physiology for some of the cell types of the macaque monkey retina is reviewed with emphasis on understanding the neural mechanism for spectral opponency in the light response of ganglion cells. An in vitro preparation of the retina is used in which morphologically identified cell types are selectively targeted for intracellular recording and staining under microscopic control. The goal is to trace the physiological signals from the long (L), middle (M) and short-wavelength sensitive (S) cones to identified cell types that participate in opponent and non-opponent signal pathways. Heterochromatic modulation photometry and silent substitution are used to characterize L-, M- or S-cone inputs to the receptive fields of distinct horizontal cell, bipolar cell, ganglion cell and amacrine cell types. The majority of the retinal cell types await detailed analysis, and knowledge of the mechanisms of opponency remains incomplete. However results thus far have established: (1) Horizontal cell interneurons make preferential connections with the three cone types, but cannot provide a basis for spectral opponency in the circuitry of the outer retina. (2) A morphologically distinctive bistratified ganglion cell type transmits a blue-ON yellow-OFF spectral opponent signal to the parvocellular division of lateral geniculate nucleus. The morphology of this ganglion cell type suggests a simple synaptic mechanism for blue yellow opponency via converging input from an S-cone connecting ON-bipolar cell and an L - M cone connecting OFF-bipolar cell. (3) Midget ganglion cells, whose axons project to the parvocellular layers of the lateral geniculate nucleus and are assumed to be the origin of red/green opponent signals, show a non-opponent, achromatic physiology when recorded in the retinal periphery the underlying circuitry for red green opponency thus remains controversial, and (4) recent recordings from identified bipolar and amacrine cells in macaque suggest that a more complete accounting of opponent circuitry is a realistic goal.