The man who saw faces in everything

Some people with impaired vision see patterned carpets, miniature figures, or faces that are not there—and know perfectly well they are not there. Oliver Sacks describes Charles Bonnet syndrome as a failure of input rather than reason: when the visual system receives less information, image-making circuits may supply activity of their own.

The experiences vary, and hallucinations can also have neurological, psychiatric, drug-related, or other causes requiring assessment. The syndrome’s useful correction is narrow: seeing an unreal image does not by itself show that a person has lost judgement. Perception is not a camera feed delivered to a passive witness. The brain is always helping to construct the visible world; reduced input can make that contribution startlingly apparent.