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Andrew Salzwedel, Matthew Mauck, Jay Neitz, Edgar DeYoe; S-Cone Induced Cortical activity Despite an ON-Pathway Defect; a BOLD fMRI-Based Case Study. Journal of Vision 2011;11(15):16. doi: 10.1167/11.15.16.
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Rods and S-cones synapse with ON-bipolar cells that use the same metabotropic glutamate receptor, mGluR6. Strikingly, in individuals where mGluR6 signaling is interrupted by GRM6 mutations, rod vision is lost, but normal color vision is retained despite the absence of S-ON bipolar input. Here we report the first neuroimaging study of a GRM6-/- patient. BOLD fMRI was used to measure cortical activity in visual areas V1–V3, V4, VO, V3a, and MT in response to diffuse stimuli presented monocularly, that were chromatically modulated to stimulate specifically either S or M+L cones at frequencies of 1–20 Hz. Robust cortical activity was elicited in both conditions and the overall pattern was similar to control subjects. This suggests that the H2 horizontal cell to OFF-bipolar pathway—not S-cone to ON- bipolar and small-bistratified cells—are responsible for normal blue- yellow hue perception. Moreover, we propose that inner retina crossover inhibition between ON- and OFF-pathways explains the remarkably normal photopic vision in GRM6 patients. Some quantitative differences were evident, most notable was attenuation of both S and M+L signals at high temporal frequencies (>7 Hz) suggesting that the higher level, ON-OFF pathway interactions shape the temporal properties of both S and M/L pathways in normal observers.
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