The response to prism deviations in human infants

Previous research has suggested that infants are unable to make a correctiv e eye movement in response to a small base-out prism placed in front of one eye before 14-16 weeks [1]. Three hypotheses have been proposed to explain this early inability, and each of these makes different predictions for th e time of onset of a response to a larger prism. The first proposes that in fants have a 'degraded sensory capacity' and so require a larger retinal di sparity (difference in the position of the image on the retina of each eye) to stimulate disparity detectors [2]. This predicts that infants might res pond at an earlier age than previously reported [1] when tested using a lar ger prism. The second hypothesis proposes that infants learn to respond to larger retinal disparities through practice with small disparities [3]. Acc ording to this theory, using a larger prism will not result in developmenta lly earlier responses, and may even delay the response, The third hypothesi s proposes that the ability to respond to prismatic deviation depends on ma turational factors indicated by the onset of stereopsis (the ability to det ect depth in an image on the basis of retinal disparity cues only) [4,5], p redicting that the size of the prism is irrelevant. To differentiate betwee n these hypotheses, we tested 192 infants ranging from 2 to 52 weeks of age using a larger prism. Results showed that 63% of infants of 5-8 weeks of a ge produced a corrective eye movement in response to placement of a prism i n front of the eye when in the dark. Both the percentage of infants who pro duced a response, and the speed of the response, increased with age. These results suggest that infants can make corrective eye movements in response to large prismatic deviations before 14-16 weeks of age. This, in combinati on with other recent results [6], discounts previous hypotheses.