Learning of a hiding task and a delayed response task in infant rhesus monkeys.

The ability of the prefrontal cortex to learn was studied in 2-4-month-old male rhesus monkeys. Either the classical Delayed Response task with an opaque screen during the delay period or a Delayed Response task without a screen (the 'Hiding task') were performed every day beginning when the monkeys were 2 months old. The criterion for having learned the task was a correct performance rate of > 80% for two consecutive days. The Hiding task with a 0-s delay, training of which began at 50 days, was learned at 67 days. When the monkeys had reached 4 months of age, they were able to perform the Delayed Response task with a 3-s delay. During learning when the performance rate was low, monkeys tended to make perseverative errors, and tended to select the target position erroneously that was ipsilateral to the hand used. The rate of perseveration errors decreased linearly as the performance rate increased. Thus, 2-4-months-old monkeys in which synapses are produced in great abundance in the cortex, were capable of learning a Delayed Response task with a 3-s delay.