Neurological Substrate Of The Disorder

The damage responsible for these disorders is usually caused by cerebrovascular disease, usually embolic stroke, that involves the midportion of the first temporal gyrus bilaterally (at least one case has been reportedly caused by a hemorrhage in an auditory structure in the brain stem, the inferior colliculi). If caused by unilateral damage, the lesion is usually deep in the posterior temporal lobe of the hemisphere dominant for speech (usually the left hemisphere). In such cases, there may be signs that any residual abilities that are present depend on right hemispheric linguistic abilities. For example, the person may report hearing a word that is semantically related to the actual word spoken ("horse" for "pony"). Some authors have suggested that damage to the primary auditory cortex produces pure word deafness (agnosia for spoken language only), whereas damage to the auditory association cortex produces difficulties with nonspeech sounds. Damage to both regions would produce global auditory agnosia. The pattern of disabilities in auditory agnosia differs from that of the more common Wernicke aphasia in that the receptive language disturbance is limited to the auditory domain while the patient's spoken language is relatively good. In early accounts of the syndrome, a normal audiogram led to the assumption that basic auditory analyses (apperception) were normal and that the disorder arose because of an inability to link the sound to its meaning (association). This interpretation can still be found in current texts and the syndrome is frequently attributed to a disconnection of auditory cortex from Wernicke's area, with the possible implication that both are capable of normal functioning.

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