Recognition memory function in early senescent rhesus monkeys

Abstract

Assessment of recognition memory was performed in a group of 12 rhesus monkeys, 19–24 years old (an age estimated to be comparable with early senescence in humans—i.e., 60s to early 70s). Their performance was compared with that of 14 young adult animals (5–14 years old, comparable to humans from the late teens to early 40s) on two tasks of recognition memory: trial-unique delayed nonmatching to sample (DNMS) and the delayed recognition span task (DRST). The DNMS is a benchmark visual recognition memory task that requires the identification of a novel object when paired with a familiar object that was presented 10 sec earlier. After reaching learning criterion, the memory demands of the task are increased by lengthening the interval between stimulus presentation and recognition up to 600 sec and lengthening the list length of samples presented before recognition up to 10 objects. The DRST is a task used to assess memory capacity by determining memory span for number of stimuli in different stimulus conditions. In the present study, memory span was determined for spatial, color, and object stimuli using both a unique series of stimuli and an embedded repeated series. On DNMS, only 4 of 12 early senescent monkeys were impaired, compared with the range of young adult monkeys in the acquisition of the task, but as a group, they were significantly impaired on the delay and list performance measures. On the DRST, early senescent monkeys, as a group, were impaired on both the spatial condition and the object condition of the task, but not on the color condition. The results reveal a cognitive profile of early senescent monkeys that, unlike monkeys of more advanced age, is characterized by a relatively moderate impairment in recognition memory and memory span.