Figure 11.

Summary of the OFF midget circuit illustrating a synaptic basis for S-OFF opponent pathways in foveal and near-peripheral retinae. A, An L+M ON surround in the S cone is formed by H2 horizontal cell negative feedback to S cones (Packer et al., 2010). This receptive field structure is transmitted via an S-OFF midget bipolar cell and, subsequently, an OFF midget ganglion cell, creating an S-OFF private line circuit (curved arrows). These OFF midget ganglion cells would thus show center-surround organization and S OFF, L+M ON opponency. Inset at lower left shows a schematic foveal cone mosaic of S (blue), L (red), and M (green) cones in relation to an S-OFF midget circuit reconstruction taken from Figure 3, B and C. B, In the retinal periphery, L+M ON surrounds are formed in each cone via negative feedback from H1 horizontal cells (to L and M cones) and H2 horizontal cells (to S cones), which are inherited by individual midget bipolar cells and transmitted to midget ganglion cells (curved arrows), which collect input from three to five midget bipolar cells in the near retinal periphery (∼4 mm from the fovea; Wool et al., 2018). Converging S, L, and M cone inputs create a receptive field in the midget ganglion cell with complex chromatic properties—(S+L) OFF,M ON; (S+M) OFF,L ON; or (S+L+M) OFF—depending on the relative strength of individual cone inputs (Wool et al., 2018). Inset at lower right shows the peripheral cone mosaic, as in A, in spatial relation to a near peripheral midget ganglion-cell dendritic tree at ∼4 mm from the foveal center.