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Abstract:
Photopic vision arises from three classes of cones (L, M and S) as well as from the recently discovered intrinsically photosensitive retinal ganglion cells (ipRGCs), which contain melanopsin. Much is known about the brain targets of signals originating from cones. Considerably less is known about the projections of the ipRGCs, particularly in the human brain. Here we use BOLD fMRI to measure neural responses in humans to spectral modulations that selectively target melanopsin while minimizing the responses of the cones, and compare these to responses generated using cone-targeted spectral modulations. Four observers viewed large-field (27.5째 diameter, central 5째 obscured) sinusoidal flicker (1-16 Hz, log spaced) at 470 cd/m2 during BOLD fMRI. Using the method of silent substitution and a digital light synthesizer, spectral modulations were directed at melanopsin, L+M, L-M, or S cones; an isochromatic modulation was also studied. Estimates of photopigment spectral sensitivities used to produce modulations accounted for observer age and stimulus size. Anatomical regions of interest were defined for the lateral geniculate nucleus (LGN; using volumetric templates), for primary visual cortex (V1) and for extrastriate areas (V2, V3 and hV4) (Benson VSS2012). Cortical voxels were restricted to >5째 eccentricity as an extra precaution against contamination from changes in spectral sensitivity at the fovea. There was a robust melanopsin-driven response in both LGN and V1-hV4. In LGN, the melanopsin response was bandpass, maximal at 8 Hz with no measurable response below 2 Hz. The melanopsin response in V1 and V2 through hV4 was also bandpass. Across areas the responses to cone-directed stimuli differed with the direction of the spectral modulation, and differed as well from the melanopsin-driven responses. Importantly, S-cone responses were distinct from melanopsin responses in both LGN and cortex.
