The influence of intrinsically-photosensitive retinal ganglion cells on the spectral sensitivity and response dynamics of the human pupillary light reflex
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Abstract
Historically, it was assumed that the light-evoked neural signals driving the human
pupillary light reflex (PLR) originated exclusively from rod and cone photoreceptors.
However, a novel melanopsin-containing photoreceptive cell class has recently been
discovered in the mammalian retina. These intrinsically-photosensitive retinal ganglion
cells (ipRGCs) project to the pretectum, the retinorecipient area of the brain responsible
for the PLR. This study was therefore designed to examine the relative contribution
of rod, cone and the melanopsin photoresponses of ipRGCs to the human PLR. We establish
that the melanopsin photoresponse of ipRGCs contributes significantly to the maintenance
of half maximal pupilloconstriction in response to light stimuli of 30s or longer,
even at low photopic irradiances. Furthermore, we show that the melanopsin photoresponse
contributes significantly to three-quarter maximal pupilloconstriction in response
to light stimuli as short as 2s. We also demonstrate that cone photoresponses driving
pupilloconstriction adapt considerably and contribute little after 30s, but rod photoresponses
adapt less and contribute significantly to the maintenance of pupilloconstriction
in response to steady-state light stimuli at irradiance levels which are below the
threshold of the melanopsin photoresponse.