Melanopsin and Cone Photoreceptor Inputs to the Afferent Pupil Light Response

Abstract

Background: Retinal photoreceptors provide the main stage in the mammalian eye for regulating the retinal illumination through changes in pupil diameter, with a small population of melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) forming the primary afferent pathway for this response. The purpose of this study is to determine how melanopsin interacts with the three cone photoreceptor classes in the human eye to modulate the light-adapted pupil response. Methods: We investigated the independent and combined contributions of the inner and outer retinal photoreceptor inputs to the afferent pupil pathway in participants with trichromatic color vision using a method to independently control the excitations of ipRGCs, cones and rods in the retina. Results: We show that melanopsin-directed stimuli cause a transient pupil constriction generated by cones in the shadow of retinal blood vessels; desensitizing these penumbral cone signals uncovers a signature melanopsin pupil response that includes a longer latency (292 ms) and slower time (4.1x) and velocity (7.7x) to constriction than for cone-directed stimuli, and which remains sustained post-stimulus offset. Compared to melanopsin-mediated pupil responses, the cone photoreceptor-initiated pupil responses are more transient with faster constriction latencies, higher velocities and a secondary constriction at light offset. The combined pupil responses reveal that melanopsin signals are additive with the cone signals. Conclusions: The visual system uses the L-, M-, and S-cone photoreceptor inputs to the afferent pupil pathway to accomplish the tonic modulations of pupil size to changes in image contrast. The inner retinal melanopsin-expressing ipRGCs mediate the longer-term, sustained pupil constriction to set the light-adapted pupil diameter during extended light exposures.

Keywords: cone; intrinsically photosensitive retinal ganglion cells; melanopsin; photoreceptor; pupil light reflex.

Figure 1

Pupil responses to temporal white noise. (A) Temporal white noise is presented for 5,000 ms then repeated; pupil diameter is steady during continuous presentation of the temporal white noise. (B) A 1,000 ms blank equal to the time average chromaticity and retinal illuminance of the orange field (no pulse, only field) is inserted within the temporal white noise; this blank field does not cause a pupil constriction. Panels show the average ±95% confidence limits for each of three observers (traces vertically offset; ~100 trials per observer). Pupil responses are normalized to the diameter at 2 s (vertical line) during each 5,000 ms repeat.

Figure 2

Light-adapted pupil responses measured under photoreceptor isolating conditions and with combined cone- and melanopsin-directed stimuli. (A) Melanopsin-directed pupil responses (17% Weber contrast in all measurements) measured with temporal white noise (without penumbral cones; green lines) and without temporal white noise (with penumbral cones; cyan lines). (B) +L+M cone luminance directed pupil responses (10% Weber contrast; grey lines) and the combined+ L+M cones and melanopsin responses (orange lines). (C) S-cone directed pupil responses (10% Weber contrast; blue lines) and the combined S-cone and melanopsin responses (orange lines). (D) +L–M directed pupil responses (6% Weber contrast; red lines) and the combined +L–M and melanopsin responses (orange lines). In all panels the data show the average ±95% confidence limits of 4 observers (~100 trials per observer). Dotted vertical lines indicate the onset and offset of the incremental pulses. Left column shows the PLR with 5,000 ms incremental pulses; right column shows the PLR with 1,000 ms incremental pulses. The average light-adapted baseline pupil diameter for all observers across all conditions was 4.43 mm ± 0.21 (mean ± SEM).

Figure 3

Photoreceptor-directed and combined pupil light responses (PLR). (A) Photoreceptor-directed PLR. (B) Combined melanopsin- and cone-directed PLR. (C) Melanopsin contributions to the combined PLR [difference between the data in (A,B)]. The PLR traces are an overlay of the average pupil responses from Figure 2 (n = 4 observers) on the same timescale for the 5,000 ms stimulus pulse (left panels) and 1,000 ms stimulus pulse (right panels). Stimulus contrasts are specified within the panels.