Auditory function in Alzheimer's disease.

The pattern of cerebral degeneration in Alzheimer's disease (AD) patients suggests that basic auditory capacities should be normal in AD, whereas progressively higher levels of auditory function should be increasingly impaired. To test this hypothesis, we administered tests of auditory capacities associated with primary auditory cortex (sound localization and perception of complex tones) and auditory association cortex (phoneme discrimination, timbre discrimination, and tonal memory) to 19 mildly to moderately demented AD patients, 21 elderly control subjects (ECS), and 14 young control subjects (YCS). The results showed significant differences between YCS and ECS on phoneme discrimination with synthetic speech and on tonal memory. The AD group differed from the ECS group on sound localization, one measure of synthetic speech discrimination, and timbre discrimination. Performance did not correlate with age, dementia severity, or duration of illness on any test condition. These findings indicate that although AD is accompanied by specific auditory deficits, the increase in neuropathologic change between primary auditory and auditory association cortices is not reflected in an increased impairment of functions that are mediated by these areas. Degraded aural language comprehension, which is characteristic of AD, likely reflects disruption of language processes, rather than dysfunction specific to auditory circuits.