Three sets of experiments are proposed on different aspects of auditory localization in human infants: (1) newborns' response to acoustic information; (2) localization of sound in 3-dimensional space; and (3) acuity of localization in the horizontal plane. Our previous work has estimated that the newborn can directionally discriminate sounds located on the right vs the left, indicated by head turns toward the sound source. However, this response is fragile and occurs only under certain stimulus conditions. The proposed studies will specify some critical aspects of the stimulus, including duration of sound, spectral properties, and rate of presentation. This work should lead to a better understanding of acoustic information processing in the auditory system at birth. Two new procedures will be used to explore the older infant's ability to localize sound. One method consists of recording infants' reaching in total darkness to sounding objects placed within reach in a variety of positions. We will test infants' sensitivity to acoustic information that specifies a sound's location in the azimuth and the distance away from the infant's body. Do intensity and spectrum changes furnish information the infant can use to localize sound? Does the infant have a boundary between objects within and beyond reach? While similar questions have been asked in relation to infants' response to visual depth cues, they have not been asked in relation to auditory space. The second method is the application of psychophysical techniques to measurement of infants' acuity of sound localization. The infant detects a shift in sound from a center loudspeaker to an off- centered loudspeaker, yielding an estimate of minimal audible angle similar to that found for adults. The newborn work concentrates on delineating critical aspects of a sound's eliciting properties while the older infant work seeks to refine and exploit the new methodologies to achieve a more complete picture of auditory development in the first year of life.
