Neurobiology ofAging, Vol. 14, pp. 635-636, 1993 Printed in the U.S.A. All fightsreserved. 0197-4580/93 $6.00 + .00 Copyright© 1993PergamonPress Ltd. The Longitudinal Assessment of Recognition Memory in Aged Rhesus Monkeys MARK B. MOSS Department of Anatomy and Neurobiology, Boston University School of Medicine, Boston, MA 02118-2394 THESE behavioral data come from a multidisciplinary study on the neurobiology of aging in rhesus monkeys as part of a program project funded by the National Institute on Aging to Boston Uni- versity in collaboration with the Yerkes Regional Primate Re- search Center in Atlanta. Morphological data from some of these behaviorally characterized monkeys will be presented by Dr. Jo- hannes Tigges, Dr. Alan Peters, Dr. Thomas Kemper, and Dr. Douglas Rosene. Our behavioral data complement those presented by Drs. Rapp and Bachevalier on age-related memory loss in the monkey and reinforce the observation that performance in aged monkeys is characterized by marked individual differences. Be- fore presenting our behavioral findings, I would like to briefly address the issue of what constitutes an old monkey. Published life span data from three cohorts of rhesus monkeys at the Yerkes Primate Center (a total of 763 animals) indicate that 50% survive to the age of 16 years, 25% survive to the age of 25 years, and less than 10% survive beyond the age of 30 years (4). Review of life span curves for humans shows that half of the population is still alive by the age of 80, whereas for monkeys, this point on the survival curve corresponds to the age of 16. Accordingly, mon- keys 25-29 years of age, the range of the monkeys that were used in these studies, likely represent the oldest of the old. One of the objectives of our program has been to maintain, to the extent possible, parallel studies of nonhuman primates and humans. As an example of this approach, one the behavioral tasks we have developed to assess visual recognition memory in the monkey, and one I will discuss today, has been used extensively to assess memory function in human clinical populations. The delayed recognition span task (DRST) is a visual recognition memory task that can be administered using several different stim- ulus classes. For the spatial condition of the DRST in monkeys, the animal is initially presented with an 18-well test board with one well baited and covered by a disk. The animal is simply required to displace the disk to obtain the reward. The screen is lowered, and during the 10-s inter-trial interval, a new identical disk is placed on the board over another baited well; the previous disk is left in place and is now unbaited. The animal must displace the new disk to obtain the reward and for the first trial is performing in effect, a nonmatching to position task. If the animal succeeds in displacing the new disk the next trial, another disk is placed on the testing board and the animal is required to displace this additional disk. This sequence continues until the animal commits its first error, i.e., displacing a previously rewarded disk. The number of correct responses before the first error is committed constitutes the "recognition span." The color condition of the DRST is administered in a similar fashion to that for the spatial condition. This time the monkey is required to displace a disk of a particular color; on the subsequent trial, a new color is added to the board, but this time the previously presented stimulus is moved to a different location on the board to preclude the possibility of using spatial cues. Thus, this form of the DRST serves as a nonspatial visual recognition memory task. As in the spatial condition, the sequence continues until the animal commits the first error and the number of correct responses until the first error constitutes the recognition span. Ten such sequences are given in a daily session, typically for 5 to 10 sessions. For administration of this task to humans, the stimulus matrix is expanded and verbal reinforcement is used instead of food re- ward, but the test is otherwise carded out in a similar fashion to that with monkeys. We have administered the DRST to normal, young, adult, and aged individuals, as well as to a variety of patients with age-related cognitive disorders such as Alzheimer's disease and Parkinson's disease (1,3). We administered the spa- tial and color DRST to a group of young (mean age 18.5 years) normal adults and compared their performance to that of aged, normal adults (mean age of 67.5 years). For the young adults, the spatial span approached the ceiling of 16 positions, whereas for the aged group, the mean spatial recognition span was only 11.5. A similar pattern emerged for the color condition. Hence, the DRST and the recognition span it yields appears to be a useful measure in detecting age-related changes in memory function in humans. In our earlier study of aged monkeys, we compared the per- formance of four young adults (5-7 years) to normal aged (25-27 years) monkeys on both the spatial and color conditions of the DRST. We found that aged monkeys were impaired relative to young adults on both conditions of the task, a finding consistent with the data from normal aged humans mentioned earlier. We have since had the opportunity to assess the spatial and color recognition spans in this same group of animals one year following their initial assessment. Performance in the young adult group was quite consistent. On the spatial span, three of the four animals showed no change in performance and the fourth evidenced a slight decline of less than 10%. For the color span, again, three animals in the groups showed no change in performance over the 1-year interval. The fourth animal evidenced slightly improved performance. In the group of five aged monkeys, a very different pattern of performance than that seen in the young adults was observed for the two tasks. On the spatial condition, two animals showed no change in performance, one showed a slight improvement, one showed a mild loss and the last evidenced a marked loss. In con- trast to their performance on the spatial condition, none of the animals showed an appreciable loss in recognition span on the color condition. In fact, two animals showed a slight improve- ment. The apparent greater sensitivity to performance of the spa- tial span as compared to color span by aged monkeys is similar to that seen following ablations of the hippocampus (2). Whether or 635