The goal of this research project is to determine at what age and with what underlying mechanism young infants perceive the visual world as stable despite movements of the eyes, head, and body. Adults perceive the visual world as stable despite self-produced movements primarily by means of reflexive eye movements triggered by visual and vestibular stimulation. These eye movements serve to reduce or eliminate those movements of the retinal image that are created by self-produced movements. The vestibulo-ocular reflex (VOR), triggered by rotations or translations of the head, counter-rolls the eyes in the direction opposite to head movement. The optokinetic reflex (OKR) moves the eyes in the direction of retinal image movement. These reflexes are supplemented by a fine-grained analysis of retinal image velocities located near the current direction of gaze and converted into eye movement signals for the smooth pursuit system. In a typical visual environment with objects at many different distances, the observer must select a depth-plane for fixation and suppress those retinal image motions created by objects at nearer and farther distances. When the visual scene moves, either because of object movement or movement of the observer, the three oculomotor systems -- VOR, OKR, and smooth pursuit -- are activated in concert to stabilize the attended object despite the movements of other retinal images. These experiments will present 2- to 6-month-old human infants either with moving visual displays while they are held in a stable head/body position or with stationary visual displays while their head/body is moved laterally. The former condition stimulates the OKR and smooth pursuit systems, while the latter condition stimulates all three systems to reveal the role of the VOR. At issue is how effectively infants are able to keep the object of attention stabilized on the retina despite movements of the visual surround and/or movements of the head/body. Based on earlier work showing that smooth pursuit does not emerge until 2 months of age and depth perception does not emerge until 4 months of age, Aslin predicts that if the smooth pursuit system is the critical factor, then effective stabilization should emerge at 2 months of age. However, if segregation of objects in depth is the critical factor, then effective stabilization should not emerge until 4 months of age. These results from normal infants will provide a baseline for understanding the development of three oculomotor systems that are known to be essential for adults' perception of a stable visual world.
