Aged monkeys exhibit behavioral deficits indicative of widespread cerebral dysfunction.

To determine whether the decline of behavioral abilities with aging in monkeys is selective or widespread, we examined 18 monkeys ranging from 3 to 34 years of age on a wide variety of tests with the ultimate goal of correlating behavioral deficits with age-related changes in the brain. In our initial study we found impaired visual recognition ability in the aged monkeys (43). In the present study, we assessed the same animals on tests of spatial memory, visual habit formation, visuospatial orientation, visually guided reaching, motor skill learning, and reaction time, these categories having been chosen to test the integrity of different cerebral systems. There were three major findings. First, age-related impairments were observed in nearly all test categories, though often not on easy versions of the tests, suggesting that the deficits observed were in the specific abilities measured and not an artifact of lowered motivation or other general disability. Second, the behavioral decline began in the late teens for certain spatial abilities but did not affect other abilities until the late 20's, suggesting that although the cerebral dysfunction eventually becomes widespread, the cerebral systems underlying spatial abilities are compromised by aging earlier than others. Finally, the finding of correlations between scores of aged animals primarily within test categories as opposed to across categories suggests that different animals have different patterns of cerebral involvement.