Seeing without rods or cones: contributions of intrinsically-photosensitive retinal ganglion cells to the image-forming visual system

Abstract

It was long thought that rods and cones were the only components of the mammalian retina capable of conveying light information to the brain. Recently, a novel class of transduction-capable retinal ganglion cells containing the photopigment melanopsin were discovered in the mammalian retina identified as an “intrinsically photosensitive retinal ganglion cell” or ipRGC. Most of the functionality associated with ipRGCs has been linked to nonperceptual, non-cortical visual operations such as circadian (day-night) phase modulation and pupillary constriction. More recently, however, two subpopulations of ipRGCs have been identified called M1 and M2 cells, with the latter showing “blue-yellow” chromatic opponency that possibly links to brightness or colour pattern vision – properties associated with the retinogeniculostriate, or image-forming visual system. The present study expands on the current understanding of these putative image-forming non-traditional photoreceptor systems. To this end, I developed two stimulus paradigms that target short-wavelength-sensitive cones (S-cones) to tease out the unique contributions of ipRGCs that have neural associations with S-cone visual functioning. In the first paradigm, I measured detection thresholds using short-wavelength selective stimuli that are temporally presented with either an onset or offset “sawtooth” profile to ascertain ipRGC input to the S-OFF, “brightness” pathways. The results revealed differences in the asymmetry between S-ON and S-OFF pathways dependent upon adapting field conditions that were expected to influence ipRGCs over other photoreceptors. In the second experiment, I used a modification of an S-cone contrast sensitivity task employing homochromatic “blue” sine-grating gabors of varying spatial frequencies to directly test ipRGC involvement in spatial pattern vision. The results from the second experiment showed a slight advantage to the perception of low spatial frequency gabors superimposed on chromatic adapting fields that were expected to influence ipRGCs more than the others. Preliminary evidence supporting a spatial tuning property of ipRGCs was also found. Overall, these findings suggest that ipRGCs have measurable influences on conscious, image-forming perceptions, and shed further light on the microcircuitry of the retinogeniculate pathway.