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Joris Vincent, Geoffrey K Aguirre, David H Brainard; Adaptation to melanopic stimulation does not affect cone-mediated flicker sensitivity. Journal of Vision 2019;19(10):72c. doi: https://doi.org/10.1167/19.10.72c.
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© ARVO (1962-2015); The Authors (2016-present)
Melanopsin-containing retinal ganglion cells (ipRGCs) are known to contribute to reflexive visual functions (e.g., pupil constriction). Histological and ERG studies in rodents indicate that the ipRGCs modulate cone signals via recurrent axon collaterals, a mechanism by which melanopsin stimulation could alter visual perception. Here we test if changes in the melanopsin stimulation level of a steady background affect thresholds for detecting flicker seen only by the cones. Threshold sensitivity for 5 Hz LMS cone-directed flicker was measured on 2 pairs of adapting fields that differed only in their melanopsin stimulation or only in their LMS cone stimulation; each pair differed by a 350% isochromatic contrast step. Stimuli were generated using a digital light synthesis engine with 56 effective primaries and presented as a 27.5° diameter field, of which the central 5° was occluded. The three authors served as participants and viewed the stimuli monocularly with a pharmacologically dilated pupil. During each of four separate sessions the participant adapted to each background for 5 minutes prior to collection of 120 trials of a two-interval forced-choice paradigm. Flicker threshold expressed as differences in LMS cone excitation did not differ between the high- and low-melanopic backgrounds for any of the three participants (thresholds for the three participants on the high-melanopic background were 0.89×, 1.02×, 1.03× the thresholds on the low-melanopic background). Thresholds on the high-LMS background were higher (by 3.81×, 4.14×, 4.50×) than on the low-LMS background. The elevations in response to LMS background change serve as a positive control and are consistent with the 4.5× increase expected from Weber’s law. A substantial increase in the melanopic content of a steady adaptation field does not affect sensitivity to LMS cone-directed flicker. This finding speaks against the possibility that cone sensitivity is regulated by signals arising from tonic melanopsin stimulation.
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