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Abstract:
Introduction: In the human visual field, there is a region known to be blind for conscious vision, called the blind-spot. The blind-spot corresponds to part of retina where retinal ganglion cells bundle through the optic disc to form the optic nerve. Since there are no cones and rods in the optic disc, no image is formed at that region and it is effectively a blind zone to the observer. Nevertheless, intrinsically photosensitive retinal ganglion cells (ipRGCs) can express melanopsin also at their axons up to the head of the optic nerve (Hattar, Liao, Takao, Berson, & Yau, 2002). It has been shown that pupil response could be modulated by duration and luminance when the retina was stimulated full-field (Park, Moura, Raza, Rhee, & Kardon, 2011). Here, we investigated whether duration and luminance of a blue light stimulus could also modulate pupil response, when applied locally on the blind-spot.
Methods: Pupil response was recorded in five adult participants. The right eye was covered with an eye patch and the left eye was recorded with an eye tracker (EyeLink 1000 Eye Tracking system, SR Research Ltd., Ottawa, Ontario, Canada), after the inbuilt calibration. The maximal pupil constriction, which followed the stimulus presentation, was expressed as pupillary change by dividing the amplitude by the pre-stimulus baseline, according to the standards in pupillography (Kelbsch et al., 2019). The stimulus consists of blue circular discs presented on the optic disc with varying stimulus Luminance (10, 5, and 1 cd/m²) and Duration (3000, 300, and 30 ms).
To calibrate the stimulus on the blind-spot, the participant looked at a fixation target and adjusted a disc on the screen to the position and radius of the blind-spot. The stimulus is on the blind-spot when the participant reports it as invisible.
The experiment was started with a 20 secs fixation phase. The stimulus presentation consists of 6 blue stimulus flashes, separated by 6 secs blank intertrial intervals.
Results: A repeated measure ANOVA revealed a significant main effect for Luminance (F(6,2) = 32.0, p < 0.001) and Duration (F(6,2) = 10.8, p < 0.01), but not for their interaction (F(12,4) = 2.1, p = 0.617) for pupillary change. A Tukey corrected post-hoc test showed that pupillary change is higher in 10 and 5 cd/m² compared to 1 cd/m² (t(6) = -8.0, p < 0.001; t(6) = -4.1, p = 0.020) as well as in 10 compared to 5 cd/m² (t(6) = -3.92, p = 0.023). Regarding Duration, a Tukey corrected post-hoc test showed that pupillary change is higher in 3000 ms compared to 30 ms (t(6) = -4.6, p = 0.011). Furthermore, strong correlations between pupillary responses and both duration and luminance were observed (r(2) > 0.83, p < 0.001).
Conclusion: Additional to full-field (Park et al., 2011), we showed that pupillary response to blue light stimulation of the blind-spot depends on luminance and duration as well. These results suggest that excitability of melanopsin on the head of optic nerve is related to both duration and luminance.
