the disorders













visual form agnosia


"a neurological virus"

"a unifying vision"

"an active process"

We think of the world as objectively containing objects, whether or not we see them or otherwise perceive them. But the computational and physiological study of visual perception shows conclusively that the objects we see are in fact the end product of a sophisticated process of perceptual construction. We see objects because we create objects. One simple bit of evidence: If you close your eyes, all objects disappear. You have ceased to create them, so you cannot see them. If you then open your eyes, they reappear. You have reengaged your visual processes that construct objects, so you once again see the objects that you create. If you close your eyes and touch an object, you cease to create the visual object and instead create a tactile object. Remove your hand, and the tactile object also disappears. You construct all the objects you experience by sight, touch, and any other modality of perception. Another bit of evidence for this comes from visual form agnosia: As a result of brain injury due to, say, carbon monoxide poisoning, a person can end up unable to see objects, although their acuity is fine and they can see colors and motions. The normal ability to assemble edges, colors, and motions into visual objects is gone.

It is hard to imagine the visual world of such a person. Does their visual world look garbled, granular, nondescript, blurred, blended, or scrambled? It has certainly lost an important component of its meaning, viz., its objects. Note, however, that a person suffering from visual form agnosia has not lost the concept of object, nor the ability to perceive objects from other modalities. They can, for instance, recognize an object by touch, or a person by the sound of their voice. It is the specifically visual construction of objects that has been impaired or destroyed. In a complementary neuropsychological condition called Charles Bonnet syndrome the patient has the opposite problem: They cannot stop creating objects that are fictitious. They hallucinate people, dogs, even entire city streets with traffic and buses. Sometime the hallucinated objects are entirely visual. But occasionally they have associated sounds and even tactile feels. In Charles Bonnet syndrome the normal constraints on object construction are removed, and we create objects that are not useful. In visual form agnosia we fail to be able to create objects that are useful. Both syndromes point to the same conclusion: We create all the objects that we see.