Aging in the rhesus monkey: debilitating effects on short-term memory.

The performance of aged rhesus monkeys (18 years and older) was compared to that of young control monkeys (three to five years old) in three experiments designed to define and evaluate the presumed short-term memory impairment associated with aging. An automated, indirect delayed-response procedure was used with special emphasis directed toward controlling or eliminating potentially confounding variables such as attention, motivation, learning disabilities, etc. It was shown that the aged monkeys do suffer from a profound a specific impairment in short-term memory (STM), performing normally on the shortest dealy interval and showing progressively greater impairment as the retention interval was increased. A subsequent study varied deprivation level and demonstrated that it is unlikely that differences in motivation could account for the age-related STM deficits observed on the delayed-response task. Further studies indicated that alterations in stimulus availability did not differentially affect the performance of the two age groups to any measurable extent, suggesting that differences in stimulus processing abilities are neigher necessary nor sufficient conditions for the deficit found in the first experiment. These results suggest that the delayed-response deficity in old monkeys is directly related to age-associated changes in those areas of the nervous system which are important for the expression of short-term memory.