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Abstract

Purpose : Light of different wavelengths has been shown to modulate pupil response differently when the whole retina was illuminated, but its effect has not been fully investigated when applied locally at the corresponding area of blind-spot (BS) on the retina, called optic disc (OD). Although there are no image-forming photoreceptors on the OD, a pupillary light response (PLR) is expected when a blue stimulus is presented on the BS due to expression of melanopsin on the axons of intrinsically photosensitive retinal ganglion cells (ipRGCs) on the OD. Melanopsin activation has also been suggested to increase retinal dopamine levels (DA), which enhances contrast sensitivity (CS) for stimuli with high spatial frequencies (SF).
We tested the hypothesis that stimulating BS with blue light (1) creates comparable PLR to fovea or periphery stimulation, and (2) increases the CS, suggesting a melanopsin-triggered increase of retinal DA.

Methods : First, participants could adjust a circular stimulus to the size of the BS, while looking at a fixation and changing size and position until the stimulus was invisible.
Experiment 1: Six blue stimuli appeared monocularly for 80ms followed by 10s inter-trial interval on the three locations respectively. A custom-built pupilometer was developed to record the PLR in 6 participants (age 30.5 ± 6.9). Blinks were removed, and PLR was corrected to the 500ms pre-stimulus baseline.
Experiment 2: CS for 1.3, 3, 6, 12 and 18 cycle per degree (cpd) was measured with Freiburg Visual Acuity Test (FrACT) before and 20min after stimulating BS with pulses of blue light flickering at 15 Hz for 1min binocularly in 5 participants (age 29.0± 5.6 years).

Results : Exp. 1: A repeated ANOVA of PLR revealed no difference among BS, fovea and periphery (p = 0.56). A paired T-test comparison in the BS condition and the fovea showed no significant difference (p = 0.75).
Exp. 2: A paired T-test in higher (6, 12, 18 cpd) SF revealed a significant increase of CS (p < 0.02) after BS stimulation, but not in lower (1.3 and 3 cpd) SF (p = 0.35).

Conclusions : Pupil constricts when OD is exclusively illuminated with blue light in absence of classical photoreceptors' stimulation, which suggests a PLR via activation of melanopsin on the axons of ipRGCs.
Stimulating OD with blue light leads to an increase in CS at higher SF which suggests a melanopsin-triggered modulation of retinal DA.