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In the human, cone photoreceptors (L, M, and S) and the melanopsin-containing, intrinsically photosensitive retinal ganglion cells (ipRGCs) are active at daytime light intensities. Signals from cones are combined both additively and in opposition to create the perception of overall light and color. Similar mechanisms seem to be at work in the control of the pupil's response to light. Uncharacterized however, is the relative contribution of melanopsin and S cones, with their overlapping, short-wavelength spectral sensitivities. We measured the response of the human pupil to the separate stimulation of the cones and melanopsin at a range of temporal frequencies under photopic conditions. The S-cone and melanopsin photoreceptor channels were found to be low-pass, in contrast to a band-pass response of the pupil to L- and M-cone signals. An examination of the phase relationships of the evoked responses revealed that melanopsin signals add with signals from L and M cones but are opposed by signals from S cones in control of the pupil. The opposition of the S cones is revealed in a seemingly paradoxical dilation of the pupil to greater S-cone photon capture. This surprising result is explained by the neurophysiological properties of ipRGCs found in animal studies.

Original publication

DOI

10.1073/pnas.1400942111

Type

Journal article

Journal

Proc Natl Acad Sci U S A

Publication Date

28/10/2014

Volume

111

Pages

15568 - 15572

Keywords

ipRGCs, melanopsin, opponency, pupillary light response, Humans, Light Signal Transduction, Models, Biological, Photic Stimulation, Pupil, Retinal Cone Photoreceptor Cells, Rod Opsins, Time Factors