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Sara Patterson; Differences between the S-OFF and L/M-OFF contacts inform the role of OFF midget bipolar cells in the perception of yellow. Journal of Vision 2017;17(15):15. doi: 10.1167/17.15.15.
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© ARVO (1962-2015); The Authors (2016-present)
Theoretically, separate channels are responsible for sensations of red, green, blue and yellow. The physiology of the S-OFF (yellow-ON) pathway responsible for yellow sensations is most controversial. Conclusions conflict about S-OFF midget bipolar cells as a substrate, however, most evidence is negative. OFF-bipolar-to-S-cone connections are not observed in rodents and, except for occasional contacts, they are not observed in marmosets, making it unlikely that an entire S-OFF midget pathway evolved in de novo in Old World primates. Moreover, serial electron microscopy (EM) of macaque peripheral retina did not reveal extensive OFF-bipolar contacts to S-cones. While there are weak inputs to some peripheral OFF-midget ganglion cells, (L+M)-S midget ganglion cells haven't been reported from electrophysiological recordings of the central retina. The only exceptions to these negative findings are reports of S-OFF midget bipolars from EM reconstructions near the fovea of macaques. Using serial EM of parafoveal retina, we confirmed S-OFF midget bipolars and reconstructed neighboring pairs of S-OFF and L/M-OFF midget bipolars. All S- and L/M-cones contacted an OFF-midget bipolar, which provided the sole input to an OFF-midget ganglion cell. However, these contacts differed between L/M- and S-OFF bipolar cells, and when combined with the distinctive morphologies of reconstructed H1 and H2 horizontal cells, generate distinctly different center-surround receptive fields. Our findings suggest that OFF-midget bipolars generally avoid S-cones in primates but they are more promiscuous in macaques near the fovea. While the role of S-OFF-midgets in vision remains unclear, it seems unlikely they are the sole substrate for yellow sensations.
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