How monkeys acquire a new way of seeing

In an experiment on perceptual learning, monkeys were given the opportunity to watch on television the "private behaviour" of another monkey (which did not know it was being watched.) The subjects were shown monkey X for twenty sessions in a row, followed by monkey Y for twenty sessions, followed by monkey X again fro twenty sessions. The subjects' "interest" in the stimulus monkey remained roughly level within each block of twenty sessions, but increased in a step-like way at the changeover from X to Y, and again from Y to X. These results are interpreted as evidence that the subjects gained little or no extra insight into the nature of private behaviour through watching the same monkey in successive sessions; the critical factor in their perceptual education was the comparison between one monkey's behaviour and another's.