Abstract
Humans and nonhuman primates experience a protracted period of cognitive development, and the accrual of cognitive capacities parallels the maturation of the prefrontal cortex (PFC). To characterize developmental improvements in visuospatial working memory and examine the underlying maturation of the PFC that enables it, we trained 8 adolescent macaques (2F, 6M) to perform variations of the Oculomotor Delayed Response (ODR) task, and recorded neural activity in the PFC at two time points, at the ages of 3.4±0.2 years and 4.0±0.2 years. Eight animals performed the ODR task with a 1.5 s delay, four animals performed a version of the task with a longer, 3 s delay period, and four animals also performed the ODR + distractor task, in which a distractor appears during the 3s delay. Overall performance in the ODR task (percentage of correct trials among trials completed) for the 8 monkeys was 78% in 301 sessions. Monkeys with lower average performance exhibited higher variability across sessions. Between the two time points, modest improvements in performance in the ODR task and ODR + distractor task was observed, with the greatest improvement for the most difficult tasks and conditions. Development between the two time points was characterized by an increase in the percentage of neurons that responded to the task (98/401= 24% and 84/261=32% respectively, chi-square = 4.76, p = 0.03). However, the firing rates of the neurons that did respond to the task were similar at the two time points (6.98 vs. 6.43 spikes/s for the delay period, 1-way ANOVA, p = 0.62). Our results provide insights on prefrontal neuronal activity during working memory at the earliest time points achieved and indicate progressively greater engagement of the prefrontal cortex as development advances.