Summary: The Visual System

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The eyeball is circular because this is the best shape for gathering light into a focused image. The first few structures of the eye (the cornea, the lens, and the fluid filled chambers) all refract light. This helps bring light to a focus on the receptor surface–the retina–at the back of the eye.

Nearsightedness (myopia) occurs when the focal plane is in front of the retina; farsightedness (hyperopia) occurs when the focal plane is in back of the retina. Corrective lenses align the focal plane with the retina.

Hallucinations and illusions show the difference between sensation and perception. The gestalt psychologists were fascinated with illusions such as ambiguous figures, because they showed perception involved central (brain) influences and were not just simple responses to incoming stimuli.

Depth perception provides many examples of unconscious inference in the brain. Cues from the sensory information are used to draw conclusions about the distance of objects and surfaces.

The problem of extracting depth information from visual information was recognized early in the study of perception. Helmholtz used the phrase unconscious inference to describe the brain's ability to calculate depth using various depth cues.

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