Melanopsin driven enhancement of cone-mediated visual processing.

The precise control of visual sensitivity to variations in external lighting is critical for optimising human visual function in a changing environment. In photopic illumination, the cone photoreceptors and their post-receptoral pathways have a primary role in regulating these adjustments; it is not fully understood how a small population of melanopsin-expressing, intrinsically photosensitive retinal ganglion cells (ipRGCs) can interact with the three cone photoreceptor classes to modulate human visual function. Here we investigated interactions between these inner and outer retinal photoreceptor classes in participants with trichromatic colour vision under conditions that independently controlled the excitations of ipRGCs, cones and rods in the retina. In the peripheral retina, we show that interaction between melanopsin- and cone-directed signals affect conscious, image-forming vision. The interaction patterns are inconsistent with any potential effects arising from artefacts due to open-field and penumbral-cone contrasts, or rod intrusion. For cone signals mediated via each of the three primary visual pathways, melanopsin activation enhances contrast sensitivity. The contrast response functions indicate this enhancement is a more generalised facilitation effect that onsets at ∼9% melanopsin contrast. The implication for human vision is that the contrast sensitivity of cone-mediated visual processes can be modulated by the level of melanopsin excitation in the stimulus light.