Cover your left eye. Find a spot directly in front of you (a thumbtack on a wall works well). Extend your right arm straight in front of you, so your thumb covers the spot on the wall. Slowly move your arm outward (away from your nose, to the right), but continue looking forward at your previously marked spot. Pay attention to your moving thumb. Notice that when your thumb reaches a certain point, it disappears! Try it using your left eye and left thumb.
This blind spot exists because your optic nerve (which transmits visual information from the eye to the brain), has no light detecting cells like the rest of the retina does. You don't typically notice your blind spot because the visual fields of each eye overlaps and your brain is able to "fill in" the missing visual information. When we test peripheral vision at TotalVision, we use this blind spot, called the "physiological blind spot," as one means of assessing the accuracy and reliability of our peripheral vision testing.
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