The effect of controlled photopigment excitations on pupil aperture

Abstract

In addition to rods and cones, the human retina contains melanopsin which has been identified recently in the body and dendrites of a few ganglion cells. The intrinsically photosensitive retinal ganglion cells (ipRGCs) are good candidates for controlling the tonic pupil aperture but their spectral sensitivity is close to those of rods and S-cones which are other candidates. Our study aims at identifying the stimulus for the pupil response when the luminance is constant and the spectrum of the light changes. A light booth was equipped with five types of coloured light emitting diodes (LEDs): Blue, Cyan, Green, Orange and Red. The intensity of each LED type could be adjusted to control the light spectrum. Illumination pairs were prepared ensuring the exclusive variation of excitation of one receptor type and silent substitution for others. Because the range of the possible controlled changes of excitation was narrow, we also prepared illumination pairs ensuring silent substitution for luminance rather than for L-cones and M-cones independently. Photographs of the observer's eyes were taken following one minute of adaptation to each illumination and the ratio of pupil to iris diameter was measured. No differential pupillary response was observed with a variation of rod, melanopsin or S-cone excitation alone. A differential pupillary response could only be obtained with a variation of the melanopsin stimulus of sufficient high contrast with or without a concurrent variation of rods.

Melanopsin Contributions to the Representation of Images in the Early Visual System

“…A definitive assessment of melanopsin’s contribution to form vision must therefore come from studies of the intact visual system. Recently, we and others have adopted the technique of receptor silent substitution [21, 22] to study melanopsin, using carefully calibrated changes in spectral composition to generate stimuli differentially visible to melanopsin versus rods and cones [13, 20, 23, 24, 25, 26]. This approach achieves the objective of studying melanopsin in animals with intact vision but so far has been restricted to application of spatially diffuse stimuli.…”

Domain of metamers exciting intrinsically photosensitive retinal ganglion cells (ipRGCs) and rods

“…In a previous experiment [24], we had not succeeded in isolating the ipRGC response using silent substitution. No differential pupillary response could be observed with a variation of rod, ipRGC, or S-cone excitation alone.…”

“…In such cases, the ipRGC contrast was sufficiently high. [24]. As it is possible to achieve with seven LEDs an ipRGC contrast higher than with five LEDs, it would be valuable to attempt exclusive ipRGC isolation at our moderate photopic luminances.…”

“…The difference of spectral content may be captured by any other photopigment. A few experiments have already been conducted to correlate the pupil aperture with the spectral content of illumination in primates and humans [3,10,11,13,17,23,24].…”

“…We have hypothesized that the spectral template of the melanopsin photopigment is the same as the template of rod-and-cone rhodopsin. Thus, starting with the low density absorbance spectra of the visual pigments proposed by Stockman and Sharpe [27], we applied a shift of the template along the frequency axis, so as to position the peak at 482 nm [3,11,24]. Then, we reconstructed the action spectrum of the ipRGCs at the corneal plane.…”