Visual filling-in

How is the blind spot filled in? The natural blind spot is a retinal region devoid of photoreceptors, where the head of the optic nerve joins the retina (Figure 1A). Close your left eye and extend your right arm straight in front of you. Spread your fingers wide, palm down, and gaze at your thumbnail. Now wiggle your little finger. It will fall on your blind spot and you will be unable to see it! The blind spot is quite large, about 6° in diameter — large enough to hold a dozen moons side by side. So it makes a big hole in the visual field — yet we do not see a hole. It is true that the retina of the other eye covers this area, but even with the other eye closed the area of the blind spot looks ‘filledin’ with the color or texture of the surround. If the surround is red or black or striped, then the region of the blind spot appears to be filled in with red or black or with stripes. Also, traumatic damage to the retina, or the slow degeneration caused by glaucoma, can result in a blind spot or scotoma: a scotoma is also perceptually filled-in, so effectively that the patient is often not aware of it, which can lead to undesirable delay in seeking medical treatment.