Some recent findings on the development of human binocularity: a review.

Evidence on the development of binocular function in infancy is reviewed. (1) Visual evoked potentials (VEP) may be recorded from infants in response to dynamic random dot stimuli which alternate between positive and negative binocular correlation. Such responses can only arise in neurones receiving binocular input. (2) Infants' looking behaviour may be shown to depend on the presence of binocular disparity in the stimulus (either random-dot or line stereograms). Results of these techniques agree that binocular function normally develops initially between 2 and 4 months of age. Our own data using VEP show a median age of first binocular response of 13 weeks but with marked individual variations. Binocular development involves the interplay of sensory interaction and oculomotor coordination, but it is unlikely that alignment of the two eyes is the dominant constraint determining the onset of binocular vision. It is possible, but not yet established, that the detection of binocular correlation may precede the ability to discriminate stereoscopic disparities. Infants in the first 3 months of life show an asymmetry of monocular optokinetic nystagmus (MOKN). The response to temporalwards field motion which they lack is driven in cat by a pathway via binocular cortex: thus the development of this response in human infants might depend on development of binocularity. However, the correlation across individual infants between the age of onset of binocularity and the age at which symmetrical MOKN is attained is relatively weak. It is possible that the neuroanatomical basis of MOKN control differs between cat and human.