appear to be associated with equally distinct microcircuits. Blue-ON/yellow-OFF cells have spatially coextensive ON and OFF fields that are derived directly from ON- and OFF-center bipolar cell inputs to a bistratified dendritic tree. The blue-ON response derives from a direct excitatory input from the blue-cone bipolar cell and in this sense defines a cone-typespecific labeled line from the S-cone to the Blue-ON ganglion cell.

The midget cell system of the primate central retina has long been linked to red–green opponency, but the underlying circuitry remains a puzzle. On the one hand, physiological mapping of L- or M-cone inputs supports the labeled line model of cone type-specific connections to both the center and the surround of the midget cell receptive field. The “private line” from a single cone to a single midget ganglion cell can account for a pure cone center response. On the other hand, there is as yet no identified anatomical basis for a cone-typespecific center in the peripheral retina, where the larger receptive fields of midget ganglion cells receive convergent, additive input from both L- and M-cones. Neither is there a known anatomical basis for a cone-type-specific receptive-field surround. Horizontal cell types, believed important for surround formation, receive additive input from L- and M-cones. Thus, a simple model of opponency in which bipolar cells mediate a cone-selective center and horizontal cells mediate a cone-selective inhibitory surround must be discarded. What are the alternatives? Can the circuits of the inner retina—the connections among bipolar, amacrine, and ganglion cell types—generate selective L- and M-cone inhibitory pathways? Or is there truly a nonselective mixed surround? Finally, do other red–green (and blue-yellow) cell types and circuitries exist that remain to be discovered?

Continued electron microscopic study of the circuits of the primate fovea and more detailed physiological analysis of identified interneurons are needed to answer these questions. The retinal mechanisms for spectral opponency are likely to reside in the morphology and response properties of a few distinct bipolar and amacrine cell mosaics and their associated microcircuits. Because the first intracellular recordings from identified bipolar and amacrine cell types have been achieved (55), working out this circuitry is now a realistic and exciting goal.

I thank Barry B. Lee, Donna Stafford, Steve Buck, Joel Pokorny, and Vivianne C. Smith, who collaborated on various aspects of the in vitro studies; Pat and Keith Boro for technical help; and Christine Curcio, Kate Mulligan, Beth Peterson, Helen Sherk, and Lubert Stryer who offered helpful comments on the manuscript. This work was supported by Public Health Service Grants EY 06678 (to D.M.D.), EYO1730 (to the Vision Research Core), and RR00166 (to the Regional Primate Center at the University of Washington).

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