Visual agnosia

Visual agnosia is the inability of the brain to make sense of or make use of some part of otherwise normal visual stimulus, and is typified by the inability to recognize familiar objects or faces. This is distinct from blindness, which is a lack of sensory input to the brain due to damage to the eye or optic nerve. Visual agnosia is often due to damage, such as stroke, in posterior parietal lobe in the right hemisphere of the brain. The specific symptoms can vary depending on the cause of the agnosia. Some sufferers are unable to copy drawings, but are able to manipulate objects with good dexterity. Commonly patients can describe objects in their visual field in great detail, including such aspects as color, texture, and shape, but are unable to recognize them. Similarly, patients can often describe familiar objects from memory despite their visual problems.

The careful analysis of the nature of visual agnosia has led to improved understanding of how normal vision works.