Agnosia is a neuropsychological disorder characterized by the inability to recognize common objects, persons, or sounds, in the absence of perceptual disability. There are three major types of agnosia: visual agnosia, auditory agnosia, and tactile agnosia. Agnosia is caused by lesions to the parietal and temporal lobes of the brain, regions involved in storing memories and associations of objects. The condition may arise following head trauma or stroke, or following carbon monoxide poisoning or anoxia.
Agnosia, from the Greek "not knowing," describes a collection of disorders where the ability to recognize objects or sounds or retrieve information about them is impaired, in the absence of other perceptual difficulties, including memory, intellectual capabilities, and the capacity for communication. The disorder can affect visual, auditory or tactile object recognition, but visual agnosia is the most common form of the condition, and most often expressed as an inability to recognize people.
In addition to being the most common form of agnosia, visual agnosias are also the best understood. Lissauer was the first scientist to provide a detailed account of agnosia (1888). He hypothesized that disorders in visual object recognition could be classified as either apperceptive agnosia or associative agnosia. This classification continues to be used today although there is some debate as to whether the deficits occur as a dichotomy or as a spectrum.
Apperceptive agnosics can see, but they lack higher-level visual perception, which interferes with object information gathering. Apperceptive agnosics fail shape-recognition and shape-copying tests. In an attempt to copy a drawing of a circle, a patient with apperceptive agnosia my draw a series of concentric scribbles. Conversely, associative agnosics have normal perception, but fail to draw on stored memories or knowledge associated with the object, such as its name, or the way it feels when picked up.
APPERCEPTIVE VISUAL AGNOSIA Carbon monoxide poisoning is a frequent cause of apperceptive visual agnosia. The ensuing brain damage is frequently profuse and located in the posterior region of the brain. Simultanagnosia, a syndrome related to apperceptive visual agnosia, describes a condition where scenes containing multiple objects cannot be interpreted as a whole. Instead patients with simultanagnosia, recognize only portions of the scene at one time, and fail to describe the overall nature of the scene and comprehend its meaning.
Individuals capable of seeing only one object at a time are said to have dorsal simultanagnosia. The condition is associated with lesions in the posterior parietal cortex, which are frequently bilateral. Patients with ventral simultanagnosia retain the ability to recognize whole objects, but the rate of recognition is impaired. The left inferior temporo-occipital cortex is generally implicated in the deficit.
ASSOCIATIVE VISUAL AGNOSIA Even when perception remains intact, some people have difficulty recognizing objects. For these people, who lack language or communication disorders or intellectual impairment, and who are able to create good copies of objects, the deficit lies in retrieving stored information about the object that would permit identification. However, many people can provide semantic information about the object without being able to provide the name. For example, the word "kangaroo" may remain elusive, but descriptors, such as "found in Australia" and "has a pouch" may be offered in its place. Many associative visual agnosics have difficulty recognizing faces (prosopagnosia) or words (pure alexia), others specific types of objects, such as tools, or animals.
Prosopagnosia was first described by Quaglino and Borelli in 1867. Although deficits in face recognition occur in a variety of neurological diseases, including Alzheimer's and Huntington's diseases, Asperger's syndrome and autism, the term is best reserved for situations where impaired face recognition appears in absence of other neurological symptoms. Patients are often uncomfortable in social situations, although many learn to recognize people using other visual cues, such as hairstyles, glasses, or scars.
Prosopagnosia can be diagnosed using the Warrington Memory Test for faces, or the Benton Face Recognition test. Although the latter will not indicate prosopagnosia, failing the test does help quantify the degree of impairment. Neuroimaging of the adult with prosopagnosia often reveals lesions in the lingual and fusiform gyri of the medial occipitotemporal cortex, which are frequently bilateral. Children who have acquired the condition in utero or genetically, however, may not show these cortical lesions.
Auditory agnosics fail to ascribe values to verbal or non-verbal sounds. Individuals with pure word deafness have intact hearing, but are unable to understand the spoken word, typically the result of bilateral trauma to the temporal cortico-subcortical regions of the brain. Nonverbal auditory agnosics fail to associate sounds with specific objects or events, such as a dog's bark or the slamming of a door. In these patients, the lesions tend to locate to the right hemisphere.
Tactile agnosia, also called astereognosis, is often difficult to recognize as we rarely identify objects solely by feel. Information about the object, including its weight, size, and texture are not given any value. Lesions in the somatosensory cortex are thought to be responsible for the condition.
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National Institute on Deafness and Other Communication Disorders (NIDCD), National Institutes of Health. Bldg. 31, Rm. 3C35, Bethesda, MD 20892-2320, (301) 496-7243. firstname.lastname@example.org. <http://www.nidcd.nih.gov>.
National Organization for Rare Disorders (NORD). P.O. Box 1968 (55 Kenosia Avenue), Danbury, CT 06813-1968. (203) 744-0100 or (800) 999-NORD (6673); Fax: (203) 798-2291. email@example.com. <http://www.rarediseases.org>.
Hannah M. Hoag, MSc