The development of stereoacuity in infant rhesus monkeys.

The emergence of stereopsis at 3-4 months postnatal in human infants is striking and has led to speculation that its rapid onset and subsequent development must be due to a dramatic reorganization of the brain. Stereopsis has never been measured in infant monkeys, but previous studies have demonstrated that many other visual functions develop four times faster in infant monkeys than in humans. We made longitudinal assessments of stereoacuity in 11 infant rhesus monkeys. A forced-choice preferential-looking technique was used to present random-dot stereograms during testing. By 8 weeks after birth, all of the monkeys were responding to at least coarse levels of disparity (1760" [seconds]), and by 13 weeks of age, all were responding to the relatively fine level of 88" disparity. Age of onset for stereopsis in monkeys was at about one-quarter the age when it occurs in humans, as expected. However, subsequent development proceeded at a similar absolute rate in monkeys and humans. The findings are discussed relative to the neural mechanisms which might be responsible for the differing rates of development.