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H Lu, HM Petry; Long-Term Changes in the Temporal Modulation Sensitivity of Retinal Ganglion Cells in Tree Shrews Reared in Red Light . Invest. Ophthalmol. Vis. Sci. 2002;43(13):4781.
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Purpose: The tree shrew possesses dichromatic color vision based on SWS and LWS cones. Red-light-rearing (RLR) selectively deprives the SWS cones and produces a differential stimulation of color and luminance channels. Behavioral studies have shown that RLR shrews have poorer color discrimination (Petry & Kelly 1991), but enhanced motion sensitivity at high temporal frequencies (Callahan & Petry 1995). This electrophysiological study explored underlying cellular mechanisms. Methods: Extracellular recordings were made from single optic tract fibers in 13 deeply anesthetized adult tree shrews (8 normals and 5 red-light-reared from birth for 12-25 weeks in Kodak 1A filtered tungsten light). Using tungsten-in-glass electrodes and standard electrophysiological methods, receptive fields (RFs) were plotted; the sustained or transient nature of the response characterized; and chromatic responsiveness tested. Of 93 cells recorded, temporal-modulation-sensitivity functions (tMSFs) were obtained from 48 cells (26 normal and 22 RLR). RF center responses to temporally-modulated stimuli were determined at 7 temporal frequencies (2-60Hz) by sinusoidally modulating stimulus luminance above and below screen luminance (34 cd/m2). Modulation depth (contrast) was varied systematically by method of constant stimuli. Results: The tMSFs of RLR cells peaked at higher temporal frequencies (mean=23Hz) compared to normal tMSFs (mean=12Hz; p<0.01). This finding held regardless of whether the cells' responses were sustained or transient in nature. All sustained cells from normal shrews peaked <20Hz (mean 7.1 Hz), whereas 42% of sustained cells in RLR shrews peaked ≷20 Hz (mean 18.1Hz). Transient cells of RLR shrews also peaked at higher frequencies (27.5Hz (RLR) ≷ 18.9 Hz (normals), p<0.05). Conclusion: Our finding that retinal ganglion cells in RLR shrews are tuned to higher temporal frequencies is consistent with our previous behavioral result that RLR shrews are better at detecting temporal modulations at high temporal frequencies. This finding of an enhancement of temporal vision (paired with our previous finding of a deficit in color vision) implies competitive interactions during post-natal development between cells contributing to different visual functions. That this result was observed in retinal ganglion cells argues for an early locus of this competition.
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