Abstract
The photopigment melanopsin is expressed in a subset of
retinal ganglion cells, rendering them intrinsically photo-
sensitive in the absence of cone and rod input. Because
cones, rods and melanopsin have broad and overlapping
spectral sensitivities, no monochromatic or narrowband
light allows for the selective study of melanopsin-mediated
responses, and psychophysical and physiological output will
be nonspecific. The method of silent substitution has
emerged as a powerful research tool to study the role
of melanopsin in visual functions, including the pupillary
light reflex, psychophysical detection and discrimination
of brightness, and neuroendocrine outputs. Here, I will
review (a) silent substitution and its physical realisation
using spectrally tuneable or LED-based multi-primary
light sources, (b) challenges in isolating melanopsin func-
tion (individual differences, retinal inhomogeneities, cones
in the shadow of the retinal blood vessels) and (c) emerg-
ing areas of investigation.
