Figure 1.

Spectral distributions of experimental light stimuli, their predicted effects on the color-opponent inputs to ipRGCs, and a theoretical phase response curve. (a) Spectrum of the experimental white light with chromaticity coordinates 0.333, 0.333. (b) Spectrum of the LED-derived experimental “blue” light with a spectral peak at 476 nm. (c) (Left) Illustration of the color vision circuitry for S-ON and S-OFF types of primate ipRGCs. (Right) Illustration of the spectrally opponent response of an S-ON ipRGC with S - (L + M) cone inputs. (d) The product of wavelength-by-wavelength multiplication of the spectral distribution of the white light (a) times the spectrally opponent response of an ipRGC (c). Integration of the curve in (d) across wavelength yields the predicted very small relative response (0.0015) of the ipRGC to the white light. (e) The product of multiplication of the spectral distribution of the blue light (b) times the spectrally opponent response of an ipRGC (c). Integration across wavelengths yields the predicted large relative response of the ipRGC to the blue light, which we have normalized to 1. The blue light is expected to be about 667 times more effective at activating the S-cone color vision circuit than the white light. (f) Phase response curve based on Khalsa et al. (2003) that is aligned with earth time so that the beginning of the internal biological night occurs at sunset and the end of the internal biological night occurs before wake time just after sunrise as indicated below the x-axis of the curve. (g) Image of sunset in Seattle, Washington, illustrating how contrasting short and long wavelength light near the horizon produces a stimulus capable of driving spectrally opponent inputs to ipRGCs, making them act as sunrise/sunset detectors.