Aging, memory, and the cholinergic system: a study of dichotic listening.

Impairment of cholinergic neural function has been proposed as a cause of memory and cognitive (M/C) disorders of the aged since with cholinergic blockade young subjects show the M/C pattern of the elderly. To validate this hypothesis, we compared the performance of young normal subjects, young scopolamine-treated subjects, and aged subjects, using a different type of behavioral task, a dichotic listening test, which primarily measures neural channel capacity. With simultaneous dichotic presentation, undrugged young subjects reported both messages correctly in 35% of trials. Aged subjects scored double-correct responses in only 19% of trials, a highly significant difference (p less than 0.001). Following treatment with 1.0 mg scopolamine, young subjects also scored double correct responses in only 19% of trials, a highly significant reduction from their undrugged performance (p less than 0.01), but virtually the same as that of normal aged subjects. Similar findings were obtained with a staggered dichotic presentation. These results extend the observation that cholinergic blockade in young subjects reproduces the pattern of M/C decline of the aged; thus providing further for the hypothesis that this decline may result from impaired function in cholinergic neurons. The neurobiologic implications of cholinergic impairment are discussed.