Abstract
The natural blind spot in the visual field has been known as a large oval region that cannot receive any optical input because it corresponds to the retinal optic disk containing no rod/cone-photoreceptors. Recently, stimulation inside the blind spot was found to enhance, but not trigger, the pupillary light reflex. However, it is unknown whether blind-spot stimulation also affects visual perception. We addressed this question using psychophysical brightness-matching experiments. We found that a test stimulus outside the blind spot was judged as darker when it was accompanied by a consciously unexperienced blue oval inside the blind spot; moreover, the pupillary light reflex was enhanced. These findings suggested that a photo-sensitive mechanism inside the optic disk, presumably involving the photopigment melanopsin, contributes to our image-forming vision and provides a ‘reference’ for calibrating the perceived brightness of visual objects.
Highlights
The blind spot (BS)[1,2,3,4] corresponds to the optic disk on the retina, where blood vessels and ganglion-cell axons converge to form the optic nerve, which leads away from the eyeball to the brain
We examined whether BS illumination affected the point of subjective equality (PSE) in brightness, i.e., the luminance of the reference arc that appeared just as bright as the test arc (Fig. 1b)
We found that blue light illumination inside the BS is invisible, but reduces the brightness of a white light outside the BS (Figs 2 and 4)
Summary
The blind spot (BS)[1,2,3,4] corresponds to the optic disk on the retina, where blood vessels and ganglion-cell axons converge to form the optic nerve, which leads away from the eyeball to the brain For this reason, the optic disk contains no photoreceptors (rods or cones), and no visual events can be received within the blind spot. We found that, even though the BS has no rods/cones, the involuntary pupil reflex— known as the short-latency pupillary light reflex (PLR)7,8—in response to light increments outside the BS was enhanced by concurrent light exposure within the BS8 This enhancement was not related to perceptual filling-in; instead, direct light stimulation inside the BS was the key factor. It is unknown whether and how incident light within the optic disk influences our perceptual judgement
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