Agnosia

Agnosia is a neurological disorder characterized by an inability to process sensory information. Often there is a loss of ability to recognize objects, persons, sounds, shapes, or smells while the specific sense is neither defective nor is there any significant memory loss. It is usually associated with brain injury or neurological illness, particularly after damage to the occipitotemporal border, which is part of the ventral stream. Agnosia affects only a single modality, such as vision or hearing. More recently, a top-down interruption is considered to cause the disturbance of handling perceptual information.

Visual agnosia is a broad category that refers to a deficiency in the ability to recognize visual objects. Visual agnosia can be further subdivided into two different subtypes: apperceptive visual agnosia and associative visual agnosia.

Individuals with apperceptive visual agnosia display the ability to see contours and outlines when shown an object, but they experience difficulty if asked to categorize objects. Apperceptive visual agnosia is associated with damage to one hemisphere, specifically damage to the posterior sections of the right hemisphere.

In contrast, individuals with associative visual agnosia experience difficulty when asked to name objects. Associative agnosia is associated with damage to both the right and left hemispheres at the occipitotemporal border. A specific form of associative visual agnosia is known as prosopagnosia. Prosopagnosia is the inability to recognize faces. For example, these individuals have difficulty recognizing friends, family and coworkers. However, individuals with prosopagnosia can recognize all other types of visual stimuli.

Speech agnosia, or auditory verbal agnosia, refers to "an inability to comprehend spoken words despite intact hearing, speech production and reading ability". Patients report that they hear sounds being produced, but that the sounds are fundamentally unrecognizable or untranslatable.

Despite an inability to process what the speaker is saying, some patients have been reported to recognize certain characteristic information about the speaker's voice (such as being a man or woman).

Agnosia can result from strokes, dementia, or other neurological disorders. It may also be trauma-induced by a head injury, brain infection, or hereditary. Additionally, some forms of agnosia may be the result of developmental disorders. Damage causing agnosia usually occurs in either the occipital or parietal lobes of the brain. Although one modality may be affected, cognitive abilities in other areas are preserved.

Patients who experience dramatic recovery from blindness experience significant to total agnosia.