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Infants 8 to 28 weeks of age were tested to determine the smallest sound shift off midline and along the horizontal axis that could be reliably discriminated, that is, minimum audible angle. Both soundshift and no-shift trials were presented. For each trial, an observer decided, based solely on the infant's behavior, whether the sound shifted location (i.e., an observer-based yes-no task). To isolate sensory from nonsensory factors, analyses were performed on d' scores. The results indicated increasing localization acuity with age. The video records revealed numerous auditory orienting behaviors (e.g., alerting, quieting, and lateral head and eye movements) and highlighted the value of not requiring that specific behavioral responses be performed on sound-shift trials. Possible factors contributing to these developmental trends are discussed.

Newborns' localization of sounds was examined in two experiments that utilized different psychophysical procedures and imposed different task demands. The results of both experiments were consistent in indicating that neonates not only differentiate the hemifield of a sound source but have some capacity to localize a sound within the hemifields. Adjustment of their initial head turn angle following a within-hemifield shift in location of an ongoing sound indicated that head orientation in neonates is elicited not only by sound onset but also by changes in location of an ongoing sound. Thus, multiple stimulus parameters impact on this neonatal response. Results are related to research on sound localization in older infants, and discussed in light of early development of the central auditory system.

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