The Diverse Role of ipRGCs in Visual Perception Beyond Non-Image-Forming Functions

The intrinsically photosensitive retinal ganglion cells (ipRGCs) are the third class of photoreceptors apart from rods and cones, containing the photopigment melanopsin and are characterized into different subtypes (M1-M6) that support conscious visual perception. They transmit the light information to the brain through complex circuits serving both non-image-forming and image-forming functions. Of the total population, there are four main ipRGCs, such as M2, M4, M5 and M6 that project substantially to the image-forming visual pathway and target the dorsal lateral geniculate nucleus (dLGN). Recent research has shown that they can modulate the excitatory and inhibitory balance in the primary visual cortex (V1) and thereby boost cortical computations for orientation discrimination. However, the projections of these ipRGCs to the V1 and how they integrate melanopsin signals with the conventional retinal ganglion cells is not yet clear. In this review, we summarize the morphological, physiological and behavioural characteristics of the ipRGC population that project to dLGN and contribute to image processing. Further research to understand how they can potentially enhance sensory representations in the visual cortex would offer therapeutic advantage in several eye diseases causing blindness.