Retesting reverse-reward performance in capuchin monkeys (Sapajus apella) after 16 years: evidence of aging-related decline.

In reverse-reward contingency tasks, the subject chooses between two potential rewards differing in value. To receive the higher-value reward, the subject must reach toward the lower-value reward; any reach toward the higher-value reward results in receiving the lower-value reward. In studies on animals, the rewards are usually food items differing in quantity or quality. Sixteen years after participating in reverse-reward tests, captive adult capuchin monkeys (Sapajus apella) were retested under the same conditions. We asked whether there would be evidence of memory of previous learning, or aging-related effects on performance. As previously, monkeys first experienced a "quantity" condition and then switched to a "quality" condition, or vice-versa. The two best-performing monkeys 16 years earlier showed significantly poorer performance in the present study, indicating possible age-related decline in reverse-reward competence. By contrast, a monkey not yet fully adult 16 years earlier but now 23 years old learned the R-R contingency in the quality condition, which suggests that inhibitory control ability in this species can be expressed at least into their 20s. Other individuals' performances were similar to those of 16 years ago, some associated with side preferences. Assessments of age-related changes in inhibitory control need to consider task characteristics and behavioral biases. Combining longitudinal and cross-sectional approaches may be optimal for clarifying the development and later decline of cognitive abilities across the lifespan.