Melanopsin expressing intrinsically photosensitive Retinal Ganglion Cells (ipRGCs) entirely control the post-illumination pupil response (PIPR) from 6 s post-stimulus to the plateau dur-ing redilation after light offset. However, the photoreceptor contributions to the early redila-tion phase of the PIPR (< 6 s post-stimulus) have not been reported. Here, we evaluated the photoreceptor contributions to the early phase PIPR (0.6 s to 5.0 s) by measuring the spec-tral sensitivity of the criterion PIPR amplitude in response to 1 s light pulses at five narrow-band stimulus wavelengths (409, 464, 508, 531 and 592 nm). The retinal irradiance producing a criterion PIPR was normalised to the peak and fitted by either a single photopig-ment nomogram or the combined melanopsin and rhodopsin spectral nomograms with the +L+M cone photopic luminous efficiency (Vλ) function. We show that the PIPR spectral sen-sitivity at times 1.7 s after light offset is best described by the melanopsin nomogram. At times < 1.7 s, the peak PIPR sensitivity shifts to longer wavelengths (range: 482 to 498 nm) and is best described by the combined photoreceptor nomogram, with major contributions from melanopsin and rhodopsin. This first report of melanopsin and rhodopsin contributions to the early phase PIPR is in line with the electrophysiological findings of ipRGC and rod sig-nalling after the cessation of light stimuli and provides a cut-off time for isolating photorecep-tor specific function in healthy and diseased eyes.
Adhikari, P., Feigl, B., & Zele, A. J. (2016). Rhodopsin and melanopsin contributions to the early redilation phase of the post-illumination pupil response (PIPR). PLoS ONE, 11(8). https://doi.org/10.1371/journal.pone.0161175