Agnosia: Auditory

Causes

Auditory agnosia is most commonly caused by strokes on both sides affecting the upper temporal lobe. It can also occur after temporal lobe damage resulting from herpes simplex encephalitis. Many cases of auditory agnosia reported after cortical lesions do not actually meet a strict definition of agnosia based on the presence of normal hearing. This may reflect the evolution from cortical deafness to auditory agnosia that is observed in a number of cases. In addition, many patients with auditory agnosia caused by stroke are from an elderly population in which deafness resulting from abnormal cochlea function is common.

Lifelong (congenital) forms of agnosia have also been described. In particular, the disorder commonly known as tone deafness or tune deafness can be characterized as a congenital agnosia called congenital amusia.

Overlap between Apperceptive Agnosias

In apperceptive forms of agnosia, there is often an overlap between the domains of perception that are affected. Deficits in the recognition of single words (word deafness), environmental sounds (environmental-sound agnosia), and music (amusia) commonly coexist, especially in patients with agnosia resulting from damage to the upper temporal lobe on both sides. This reflects overlap in the aspects of auditory perception required for the analysis of these different sounds, where the profile of specific deficits depends on the particular aspect of auditory analysis that is affected by the cortical lesions. The degree of overlap between domains affected by agnosia is likely to be underestimated in view of the practical difficulty of carrying out a comprehensive and rigorous assessment of all of these, and that it is unusual for patients to undergo a systematic assessment of complex sound analysis.

Some studies of patients with apperceptive forms of agnosia have examined temporal aspects of complex sound perception, especially in patients with word deafness, using approaches that include click counting and discrimination of tone sequences or rhythms. Many of the reported patients with word deafness in which such temporal deficits have been described also suffered deficits in other domains, as would be predicted on the basis of an apperceptive agnosia caused by a deficit in temporal analysis. Deficits in examining the frequency structure (spectrum) of sound have not been systematically explored in agnosia, but might be relevant to deficits in musical perception, in particular, which is less robust than speech is to degraded spectral structure.

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