What are Visual Agnosias?

The History of Agnosias

Although relatively rare, agnosias have been recognized at least since the time of classical Greek civilization. Thucydides suggested that agnosias develop because of the plague as early as 430 BC. Hippocrates also mentioned symptoms of agnosias in his writings "On Sacred Disease." The term agnosia is derived from the Greek "a" meaning not, and "gnosis" meaning to know. Broadly, the term refers to the failure to know or recognize an object or scene despite good basic vision.

Systematic experimental research on visual agnosias began with Monk's 1877 observation of the effects of certain brain lesion on dogs. Although able to walk without bumping into objects, the dogs behaved abnormally when presented with food, or a whip. This suggested that the dogs were able to see but not recognize objects, an effect that Monk termed "seelenblindsheit".

Freud coined the term "visual agnosia" in 1891, using it to distinguish between perception and recognition. Freud's term is still used today to refer to a neurologically based inability to recognize or identify familiar objects in the absence of a primary visual problem (i.e., acuity, brightness discrimination and visual fields are all intact), a psychiatric disorder, or other serious cognitive or intellectual loss (e.g., aphasia, alexia). Typically, agnosias are acquired disorders due to brain lesions (e.g., trauma, stroke, tumor, or carbon monoxide poisoning) that impair functioning of one or more higher order visual centers .