Yet this is not always true, as you will see from (d). Notice the vertical illusory strip running through the parallel horizontal lines. Aim your left eye's blind spot on the blue disk to make it vanish. Now the question is, Do you fill in the missing segments of horizontal lines running through the blind spot? Or do you fill in the vertical illusory strip? The answer depends on the spacing of the lines.

Why does filling in occur? It is unlikely that the visual system evolved this ability for the sole purpose of dealing with the blind spot (after all, the other eye usually compensates). Filling in is probably a manifestation of what we call surface interpolation, an ability that has evolved to compute representations of continuous surfaces and contours that occur in the natural world--even ones that are sometimes partly occluded (for example, a cat seen behind a picket fence looks like one whole cat, not like a cat sliced up). Physiologists (especially Leslie G. Ungerleider of the National Institute of Mental Health, Ricardo Gattass of the Federal University of Rio de Janeiro and Charles D. Gilbert of the Rockefeller University) have now begun to explore the neural mechanism of this process by monitoring the manner in which single neurons in the visual centers respond to objects partially covered by the blind spot or by opaque occluders.

If you get bored playing with your natural blind spot, try this. Toward the right side of your TV screen tape a tiny (half a centimeter in diameter) bit of white cardboard with a black spot in its center. Next, turn the TV to a channel that isn't broadcasting so that you see just twinkling "snow." Affix a two-centimeter-square patch of thick gray cardboard (about the same color as the TV snow) 12 centimeters or so away from the white cardboard. Stand a meter away from the TV set. If you open both eyes and stare at the small black dot steadily for 15 seconds, the large gray square will vanish completely, and the region "vacated" by it becomes filled in with the snow--you hallucinate the snow where none exists! Remarkably, if you now look away at a gray wall, you will see a square patch of dots twinkling in the region where the filling in occurred. Even a solitary red blob seen against a background of green blobs will disappear in the same manner--the green blobs fill in. The brain, it would seem, abhors a vacuum.

These experiments show how little information the brain actually takes in while you inspect the world and how much is supplied by your brain. The richness of our individual experience is largely illusory; we actually "see" very little and rely on educated guesswork to do the rest.

This article was originally published with the title Mind the Gap.

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