You Don’t See the World Exactly as It Is

An exit creates a blind spot

The optic nerve leaves the eye through an area of the retina without the light-sensitive cells that detect the image elsewhere. This creates the normal physiological blind spot. With both eyes open, their fields help cover one another; even with one eye, the gap usually does not announce itself. ^[1]

You do not generally experience this as a visible patch of darkness. What you perceive depends on the surrounding scene as well as on the missing retinal input. Researchers studying perceptual filling-in investigate how the edges around the blind spot affect the appearance of what lies within it. ^[2]

Try a small disappearing act

Use the cross and dot below. Close your left eye and keep your right eye looking at the cross. Slowly move closer to or farther from the screen without shifting your gaze. At a suitable distance, the dot to the right may disappear. The geometry depends on screen size and viewing distance. ^[1]

The dot has not left the page. Its image has moved onto the blind spot of your right eye. This exercise is a demonstration of normal vision, not a test for eye disease. If the effect does not appear, try the source's printable version.

A useful picture, rather than a pixel-perfect copy

The surprising lesson is that a complete-looking experience does not require complete input at every point. Your visual system produces a coherent view despite a normal gap in the signal.

That does not mean the world is imaginary or that any interpretation is equally good. It means perception has mechanisms worth understanding. A simple disappearing dot lets you encounter one of them directly.

Next time an image seems obvious, you have a useful extra question: which part comes from the available signal, and which part depends on the context in which I am seeing it?