Abstract
Although there are no rod/cone photoreceptors in the optic disc corresponding to the blind spot in the visual field, the performance of light detection decreases when another light stimulation is directly applied to the blind spot (Saito et al., VSS2019). Both scattered light outside the blind spot and melanopsin expressed inside the blind spot can be possibly involved in the reception of the light stimulus inside the blind spot. Melanopsin is a kind of photopigment that exists not only in the cell body of intrinsically photosensitive retinal ganglion cells but also along their axons passing through the blind spot, whereas its effect on light detectability remains unknown. To examine which mechanism is more likely to explain the above phenomenon, we conducted an experiment that took advantage of the difference in the temporal characteristics between the two, the melanopsin cells sluggishly dwindling their responses after the light stimulus is turned off, and the scattered light disappearing instantly. We compared the detection performance for a test stimulus in three conditions. In the NBS condition, no blind spot stimulation was applied; in the BS condition, the blind spot stimulus was presented before and during the test stimulus; in the aBS condition, the blind spot stimulus was turned off before the test stimulus was turned on. The results showed that the detection performances in the BS and aBS conditions were worse than in the NBS condition, and there was no significant difference in the detectability between the BS condition and the aBS condition. In other words, even if the blind spot stimulus and the test stimulus did not temporally overlap, the effect of the blind spot stimulation remained. This indicates that melanopsin is more likely to explain the photoreception in the blind spot.