The normal visual brain consists of two components; the dorsal and the ventral stream. Each stream has been associated with a particular aspect of vision. The ventral stream deals with the ‘what is it’ question, whereas the dorsal stream focuses on the ‘where is it’ question.

Ventral Stream

The ventral stream consists of the inferotemporal cortex located within the brain. The ventral stream processes information to answer the ‘what’ question of visual stimuli. Studies have been conducted on animals where the area of the brain involved with the ventral stream has been damaged or removed. In 1969, Schneider conducted an experiment with golden hamsters. He found that hamsters with a damaged ventral stream were unable to learn patterns (Allard, 2004). Ungerleider and Mishkin (1982) found similar results in monkeys with damage to this part of the brain. Essentially, subjects are able to perceive where the object is but cannot identify what it is.

Visual agnosia is a disorder of this stream. The literal definition ‘seeing without knowledge’, visual agnosioa impairs a person’s capability to perceive what an object is (Allard, 2004). Visual recognition of everyday objects is impaired; however, identification can be made if patients are presented with other stimuli, such as tactile sensations, of the object to be identified. Many patients with visual agnosia are also unable to recognize faces of family members and friends (Allard, 2004). Onset of this disorder usually occurs later in life where damage to the brain is the result of trauma or disease. However, special cases have been found where the damage to the ventral stream occurs early in life so that the visual brain develops without a ventral stream (see The Incredible Case of S.B.)

Dorsal Stream

The dorsal stream consists of the posterior parietal cortex. It is responsible for the gathering of visual information to make decisions about where an object is located. This information is then transferred to the somatosensory cortex where decisions regarding proper motor actions are made. Schneider (1969) continued his experiments on golden hamsters that had the parts of the brain associated with the dorsal stream removed. These hamsters could learn pattern recognition, but were unable to learn spatial discrimination. Once again similar results were found by Ungerleider and Mishkin (1982) with monkeys. For both experiments, damage to the posterior parietal cortex produced difficulty in “landmarking” tasks (Allard, 2004).

Optic ataxia is a disorder of the dorsal stream where people experience difficulty in reaching for objects in a goal oriented task. It is a visuo-motor deficit where the perception of object location in space is intact, but the patient is unable to perform the required movements to interact with the object. For example, individuals with optical ataxia would be able to verbally describe and located an object, ex a basketball; however, when asked to interact with that object, the individuals would be unable to either move in the right direction or place their hand in appropriate locations to pick the ball up.

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