Delayed response of human melanopsin retinal ganglion cells on the pupillary light reflex

A recent study has shown that retinal ganglion cells containing the photopigment melanopsin, which are intrinsically photosensitive in primates, project to the pupillary control centre in the pretectum. The aim of this study was to investigate how melanopsin retinal ganglion cells (mRGCs) contribute to the pupillary pathway.

We designed and built a novel multi-primary stimulation system to control stimulation of the three cone types and mRGCs independently in the human eye. We measured the latency and amplitude of transient pupillary responses to three types of test stimuli modulating excitations of mRGCs and cones (mRGC, luminance and the light flux stimuli).

It was found that the transient pupillary response to mRGC stimuli has a longer latency than that to luminance and the light flux stimuli when an onset of sinusoidal stimulus was used.

The results indicate that we successfully demonstrated the pupillary response to mRGCs under conditions where mRGCs are isolated in humans. Furthermore, the data confirm that the delayed response disappeared when the stimulus is presented as a square-wave pulse and not weighted by a sinusoid. The similarity of time courses for the earlier phase of pupillary responses to all stimuli suggested that these transient pupillary responses were driven by a single mechanism, which is perhaps associated with cone-mediated signals.