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. 2006 Oct 18;26(42):10826–10846. doi: 10.1523/JNEUROSCI.2091-06.2006

Figure 18.

Figure 18.

Proposed scheme for processing of red–green cone signals by the early visual system. This scheme reconciles the existence of both double-opponent cells and complex-equiluminant cells within primary visual cortex. Complex-equiluminant cells lack the first-order receptive-field structure of double-opponent cells, yet like double-opponent cells, show responses to equiluminant color boundaries. Type I cells in the LGN form the building blocks for both populations of cells. Single-opponent cells and simple cells may be intermediate stages. A given ON-center luminance simple cell probably gets input from both L-ON and M-ON type I cells given that there is not a separate retinal mosaic for red-ON and green-ON cells (Lee, 1996). Type I cells respond poorly to spatial color contrast, but the color contrast that nulls the response “varie(s) widely from cell to cell” (Hubel and Livingstone, 1990b) and is not restricted to psychophysical equiluminance (Logothetis et al., 1990). This produces neurons with orientation tuning and no equiluminance null, “complex-equiluminance” cells, because the various inputs, each with their own null points, compensate for each other. This compensatory effect would be enhanced with the addition of magnocellular input. Double-opponent cells also lack an equiluminance null yet preserve the opponency of the type I inputs within each receptive-field subregion. The push–pull structure of double-opponent cells (Fig. 3) suggests that they receive both excitatory and inhibitory input. The separate excitatory and inhibitory inputs are shown for a single hypothetical red-ON double-opponent cell (open arrow, inhibitory input; filled arrows, excitatory input). Given four kinds of red–green type I cells and two kinds of synapses (excitatory and inhibitory), several other wiring diagrams can be invented for a red-ON-center double-opponent cell. The double-opponent neuron is depicted with a crescent-shaped surround having two hot spots, reflecting the receptive fields of actual double-opponent cells (Fig. 6).