Agnosia

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Agnosia

An impairment in the recognition of stimuli in a particular sensory modality. True agnosias are associative defects, where the perceived stimulus fails to arouse a meaningful state. An unequivocal diagnosis of agnosia requires that the recognition failure not be due to sensory-perceptual deficits, to generalized intellectual impairment, or to impaired naming (as in aphasia). Because one or more of these conditions frequently occur with agnosia, some clinical scientists have questioned whether pure recognition disturbances genuinely exist; but careful investigation of appropriate cases has affirmed agnosia as an independent entity which may occur in the visual, auditory, or somesthetic modalities. See Aphasia

The patient with visual object agnosia, though quite able to identify objects presented auditorily or tactually, cannot name or give other evidence of recognizing visually presented objects. Because visual object agnosia is a rather rare disorder, knowledge of its underlying neuropathology is incomplete. Most reported cases have shown bilateral occipital lobe lesions, with the lesion extending deep into the white matter and often involving the corpus callosum. Prosopagnosia is the inability to recognize familiar faces. Persons well known to the individual before onset of the condition, including members of the immediate family, are not recognized. In many instances, individuals fail to recognize picture or mirror images of themselves. Isolated impairment of reading is frequently considered to be an exotic form of aphasia. Logically, however, it may be considered as a visual-verbal agnosia (also referred to as pure word blindness or alexia without agraphia). Individuals with this disorder show a marked reduction in their ability to read the printed word, though their writing and other language modalities remain essentially intact.

The term auditory agnosia is most often used to indicate failure to recognize nonverbal acoustic stimuli despite adequate hearing sensitivity and discrimination. In most well-documented cases of agnosia for sounds, the subjects have had bilateral temporal lobe lesions. Auditory-verbal agnosia (or pure word deafness) is a disturbance in comprehension of spoken language, in the presence of otherwise intact auditory functioning and essentially normal performance in other language modalities. The person's speech expression is remarkably intact in comparison with the gross impairment in understanding speech. Like its visual analog, visual-verbal agnosia, this is a disconnection syndrome. It is produced by damage to the left primary auditory cortex (or the tracts leading to it) coupled with a lesion to the corpus callosum. Phonagnosia is a disturbance in the recognition of familiar voices. The person has good comprehension of what is spoken, but the speaker cannot be identified. See Brain, Hearing (human), Hemispheric laterality, Psychoacoustics, Vision

Agnosia

 

a disorder of the recognition processes with little or no loss of basic sensory abilities and with no loss of consciousness. It occurs after the higher parts of the brain have been affected, primarily the sincipital and parietooccipitalic areas of the left (in right-handed persons) cerebral hemisphere.

Visual agnosia, or “mind blindness,” is characterized by the sufferer’s inability to recognize an object even though he has perfect vision and can name the color and shape of the object. Noting small and immaterial details of the object, the sufferer is unable to isolate its essential characteristics, to determine its significance, and to name it, despite full powers of speech. Sometimes, he can recognize separate objects but cannot grasp the total picture and connect its separate elements. In some cases, the patient loses his ability to connect a chain of events shown to him on a series of cards. When the occipital or lower sincipital area is afflicted, spatial orientation is disrupted. The patient cannot distinguish right from left, loses his sense of direction, becomes disoriented in familiar situations, and so on. This is known as apractognosia. Sometimes agnosia applies only to the ability to recognize letters, numbers, and musical notes. Auditory agnosia, or “mind deafness,” is characterized by the inability to recognize objects by their sounds, although the hearing apparatus is unimpaired. The sufferer does not recognize a clock by its ticking, water by its bubbling and dripping, and so on. In this case, known as sensory aphasia, understanding of speech is also disrupted. Tactile agnosia—the inability to recognize objects by touch—is also known as astereognosis. With the eyes closed, the patient cannot recognize by touch objects familiar to him, although with his eyes open he recognizes them instantly.

A special form of agnosia is the inability to recognize one’s own body, which occurs when the major part of the right cerebral hemisphere is afflicted. The patient does not recognize his own arms, legs, and so forth; he feels many arms and legs on his own body (polymelia) while understanding the absurdity of this feeling. Phantom pains and feeling in amputated limbs belong to this form of agnosia, as well as the patient’s failure to recognize his defective vision, hearing, or paralysis.

Z. IA. RUDENKO

agnosia

[ag′nozh·ē·ə]
(medicine)
Loss of the ability to recognize persons or objects and their meaning.