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Abstract:
The relative amplitude of pupil response to melanopsin stimulation is a stable individual difference.
Melanopsin-containing retinal ganglion cells contribute to several visual functions, including the light-evoked pupil response. As variations in melanopsin function may contribute to clinical disturbances (such as light sensitivity), we examined if individual variation to melanopsin stimulation can be seen in the human pupil response, and if such differences are stable.
Following a pre-registered protocol (https://osf.io/76u9x/), we measured in each of 25 people during two separate sessions the pupil response to 3-second unipolar spectral pulses. These were designed to selectively stimulate either melanopsin (Mel) or the L, M, and S cones (LMS). The pulses produced 400% nominal contrast upon the targeted retinal mechanism and were presented against a rod-saturating background (~40 cd/m2 for LMS, ~100 cd/m2 for Mel).
The amplitude of pupil constriction to Mel and LMS pulses was correlated across subjects, consistent with variation in non-specific pupil responsiveness. We therefore obtained the ratio of Mel and LMS response in each subject. This relative Mel response was found to vary substantially between subjects (0.0× – 1.6×), and to a degree greater than was easily attributed to within-subject measurement error. This individual difference was consistent across measurement sessions (Spearman's rho = 0.60). Simulations indicate that within-session measurement error accounts for much of the non-reproduced between-session variation.
We find substantial, stable, individual variation in the relative pupil response to melanopsin stimulation. In future studies, we wish to determine if this variation in pupil response is related to other putative melanopsin-driven functions, such as discomfort from light.
