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Marina Saito, Kentaro Miyamoto, Ikuya Murakami; Spot illumination within the blind spot affects the absolute threshold for light in a normal region of the visual field.. Journal of Vision 2020;20(11):943. https://doi.org/10.1167/jov.20.11.943.
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Spot illumination directly entering the natural blind spot is invisible to us because of the lack of rod/cone photoreceptors there. However, our previous research (Saito et al., 2018) demonstrated that spot illumination projected to the blind spot affects our brightness perception in a normal region of the visual field, suggesting that there exists a mechanism that makes it possible to receive spot illumination within the blind spot. Nevertheless, we still do not understand the relationship between the blind-spot illumination and other performances such as light detection threshold and contrast sensitivity. The present study aimed to test whether the light detection threshold in a normal region of the visual field changes when the blind spot is simultaneously illuminated. Observers viewed stimuli with only the right eye open. We compared the absolute threshold for a light stimulus flashed in the left visual hemifield with and without simultaneous illumination within the blind spot of the right eye. We found that the light detection threshold increased when the blind spot was illuminated by low-luminance blue light (t(17) = -2.99, p < .05). Next, we determined contrast threshold for a light stimulus within a gray background in the left hemifield with and without simultaneous illumination within the blind spot of the right eye. The contrast threshold did not change when we simultaneously illuminated the blind spot (t(17) = -1.19, p > .05). Therefore, we suggest that there exists a mechanism receiving illumination within the blind spot and that illumination projected to the blind spot is invisible to us but affects our absolute threshold for light elsewhere. A possible account involves melanopsin photopigment that is reportedly expressed along axons passing through the optic disk.
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