Midlatency auditory evoked responses: differential abnormality of P1 in Alzheimer's disease.

The human 'P1' middle latency evoked potential is postulated to be generated in the thalamus by a cholinergic component of the ascending reticular activating system. To test the hypothesis that P1 and its generator substrate are abnormal in Alzheimer's disease (AD), a disorder of marked cholinergic deficiency, recordings of middle latency responses to click stimuli were carried out. Comparisons between the AD and age-matched control groups indicated normal auditory brain-stem and Pa responses but a significant decrease in P1 amplitude. This P1 abnormality suggests that the midbrain cholinergic cells in AD may be dysfunctional.