Temporal characteristics of melanopsin inputs to the human pupil light reflex

Abstract

Rods, cones and melanopsin containing intrinsically photosensitive retinal ganglion cells (ipRGCs) operate in concert to regulate pupil diameter. The temporal properties of intrinsic ipRGC signalling are distinct to those of rods and cones, including longer latencies and sustained signalling after light offset. We examined whether the melanopsin mediated post-illumination pupil response (PIPR) and pupil constriction were dependent upon the inter-stimulus interval (ISI) between successive light pulses and the temporal frequency of sinusoidal light stimuli. Melanopsin excitation was altered by variation of stimulus wavelength (464 nm and 638 nm lights) and irradiance (11.4 and 15.2 log photons cm(-2) s(-1)). We found that 6s PIPR amplitude was independent of ISI and temporal frequency for all melanopsin excitation levels, indicating complete summation. In contrast to the PIPR, the maximum pupil constriction increased with increasing ISI with high and low melanopsin excitation, but time to minimum diameter was slower with high melanopsin excitation only. This melanopsin response to briefly presented pulses (16 and 100 ms) slows the temporal response of the maximum pupil constriction. We also demonstrate that high melanopsin excitation attenuates the phasic peak-trough pupil amplitude compared to conditions with low melanopsin excitation, indicating an interaction between inner and outer retinal inputs to the pupil light reflex. We infer that outer retina summation is important for rapidly controlling pupil diameter in response to short timescale fluctuations in illumination and may occur at two potential sites, one that is presynaptic to extrinsic photoreceptor input to ipRGCs, or another within the pupil control pathway if ipRGCs have differential temporal tuning to extrinsic and intrinsic signalling.

Keywords: Intrinsically photosensitive retinal ganglion cells (ipRGCs); Melanopsin; Post-illumination pupil response (PIPR); Pupil light reflex; Summation.

Figure 1

Representative 100ms 2-pulse pupil responses for conditions M⁺_BH, M⁻_RH, M⁻_RL, and M⁻_BL (clockwise from top left). In each panel the data are vertically offset by 20% as a function of the 2-pulse inter-stimulus interval. Pupil diameter is expressed as percentage baseline on the left axis, and millimetres for the 1024ms condition only on the right axis. The control condition shows the pupillary response to the single 100 ms pulse. The insets show the first 15 seconds of the pupillary trace after stimulus onset. The vertical line indicates the timing of the second pulse of the longest ISI condition (1024 ms). The dashed vertical line denotes the timing of the 6 s PIPR. The grey lines are the best fitting exponential functions (Equation 1) used to derive the 6 s PIPR. The data in each panel are for a single observer.

Figure 2

Average 100ms (panels A,C,E) and 16ms (panels B,D,F) 2-pulse normalised minimum pupil timing and diameter, and 6s PIPR as a function of ISI. Panels 2A,B show the time at which maximum constriction is reached and Panels 2C,D show the diameter of this constriction. Panels 2E,F show the 6 s PIPR. Asterisks in panel 2D denote a significant difference from 512 ms data point (Bonferroni adjusted pairwise comparisons, familywise p = .05). Conditions are coded: M⁺_BH, open blue squares; M⁻_RH, open red circles; M⁻_RL, filled red circles and M⁻_BL, filled blue circles.

Figure 3

Representative sinusoidal pupil responses for conditions M⁺_BH, M⁻_RH, M⁻_RL, and M⁻_BL (clockwise from top left). In each panel the data are vertically offset by 20% as a function of stimulus frequency. Pupil diameter is expressed as percentage baseline on the left axis, and millimetres for the 0.24 Hz condition only on the right axis. The insets show the first 15 seconds of the pupillary trace after stimulus onset. The grey lines are the best fitting exponential functions (Equation 1) used to derive the 6 s PIPR. The data in each panel are for a single observer.

Figure 4

Average sinusoidal data depicting 6s PIPR, peak-trough amplitude, phase and Impulse Response Function. Panel 4A shows the 6 s PIPR as a function of stimulus frequency. Panel 4B shows the peak-trough amplitude and Panel 4C the phase lag in degrees between the input stimulus and the pupil’s measured response. Panel 4D shows the derived Impulse Response Function for frequencies 0.24 to 1.98 Hz. Conditions are coded: M⁺_BH, dashed blue line; M⁻_RH, dashed red line; M⁻_RL, solid red line and M⁻_BL, solid blue line.