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Jonathan Winawer, Hiroshi Horiguchi; Human trichromacy revisited. Journal of Vision 2013;13(15):T7. doi: 10.1167/13.15.7.
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For more than a century, transduction of light energy into neural signals in the primate nervous system was thought to take place only in the photoreceptor layer of the retina. The presence of the photopigment melanopsin within certain retinal ganglion cells (mRGCs / ipRGC) was a surprising and significant discovery. Since its discovery about a decade ago, melanopsin has often been described as a “non-visual pigment” due to its role in functions like pupil dilation and circadian rhythms. More recently, however, investigations have begun to test whether melanopsin absorptions might in fact be visible to healthy human subjects. In this talk I will present results from a recent publication addressing this question (Horiguchi et al, 2013). We asked whether light absorbed by melanopsin can be detected by healthy human subjects. To answer this requires delivering intense (above rod saturation), well-controlled lights using four independent primaries. We collected detection thresholds to many four-primary stimuli using a novel apparatus we built for this experiment. Threshold measurements in the fovea are explained by Trichromatic theory, with no need to invoke a fourth photopigment. In the periphery, where melanopsin is present, threshold measurements deviate from Trichromatic theory; at high photopic levels, sensitivity is explained by absorptions in four, not three, photopigment classes. We consider a series of hypotheses to explain the tetrasensitivity at high photopic levels in the periphery humans. The most likely hypothesis is that in healthy human subjects melanopsin absorptions influence visibility.
Horiguchi H, Winawer J, Dougherty RF, Wandell BA. Human Trichromacy Revisited. ProcNatlAcadSciU S A. 2013;110(3):E260–9.
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