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O. Packer, J. Verweij, J. L. Schnapf, D. M. Dacey; Primate S Cones Have Blue-Yellow Opponent Receptive Fields. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2849.
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In primate retina, the 'blue-yellow', small bistratified ganglion cell type receives ON signals from short (S) wavelength sensitive cones and OFF signals from long (L) and middle (M) wavelength sensitive cones. Electron microscopic reconstruction suggests that spectral antagonism arises from parallel S-ON and L+M-OFF cone bipolar cell inputs to the small bistratified dendritic tree (Calkins et al., J. Neurosci.18:3373, 1998). Here we show however that blue-yellow opponency is already fully present in the receptive field of the S cone itself.
Whole-cell recordings were made from S cones in isolated, dark-adapted peripheral retina of macaque monkeys. S cones were identified from their responses to silent substitution stimuli that selectively modulated light absorption in L, M, or S cone photopigment. The spatial and chromatic properties of S cone receptive fields were characterized from spots, annuli, and drifting sinusoidal gratings that selectively modulated either S or L+M cone responses.
S cones could be reliably distinguished from L and M cones by consistently smaller diameter inner segment profiles throughout their length as well as a tendency to disrupt the packing geometry of the cone mosaic. Thus far over 80 identified S cones have been recorded. In addition to being hyperpolarized by S cone stimulation, all were depolarized by selective L+M cone stimulation. By contrast L and M cones lacked any response to S cone stimulation. The spatial receptive fields measured in 21 S cones were well fit by a Gaussian function, e-(x/r)2, where x is distance and radius r = 13 ± 6 µm (mean ± SD) for the S cone center and 21 ± 12 µm for the L+M cone surround. No evidence was found for an S cone contribution to the surround.
Macaque S cones can be anatomically identified and selectively targeted for electrical recording. Blue-yellow spectral opponency originates in the S cones. Considering the optical blur of the stimulus and the cone-to-cone spacing, the S cone receptive field roughly comprises a center of one S cone, and an antagonistic surround of the adjacent L and M cones. The receptive field surround is consistent with an inhibitory feedback signal via the selective S cone contacts of H2 horizontal cells.
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