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2. Seeing: Hacks 13–33 > 16. Map Your Blind Spot

Map Your Blind Spot

Find out how big your visual blind spot is and how your brain fills the hole so you don’t notice it.

Coating the back of each eye are photoreceptors that catch light and convert it to nerve impulses to send to the brain. This surface, the retina, isn’t evenly spread with receptors—they’re densest at the center and sparse in peripheral vision [See the Limits of Your Vision]. There’s also a patch that is completely devoid of receptors; light that falls here isn’t converted into nerve signals at all, leaving a blind spot in your field of view—or actually two blind spots, one for each eye.

In Action

First, here’s how to notice your blind spot (later we’ll draw a map to see how big it is). Close your left eye and look straight at the cross in Figure 2-6. Now hold the book flat about 10 inches from your face and slowly move it towards you. At about 6 inches, the black circle on the right of the cross will disappear, and where it was will just appear grey, the same color as the page around it.


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