Developmental changes in sound localization precision under conditions of the precedence effect

The ability to give perceptual priority to an original sound source and ignore later-arriving echoes of that sound is termed the law of the first wave-front , or the precedence effect (PE). Little attention has been paid to the influence that echoes exert on localization accuracy for the leading sound. The present study investigated localization precision of children and adults in the presence of a simulated echo, using the minimal audible angle (MAA) task, which indicates the smallest change in the location of a sound that can be reliably discriminated. Three age groups were tested: 18-months, 5-years, and adults. Each age group was tested with one single-source (SS) stimulus, and two precedence effect (PE) stimuli: LEAD, in which the original sound shifted from midline and the echo remained at midline, and LAG, where the reverse occurred. Subjects were tested using an adaptive, 2-down/1-up, psychophysical algorithm. For all age groups, MAA thresholds were smallest for SS, larger for LEAD and largest for LAG. For all three stimulus conditions, the 18-month-olds\u27 thresholds were significantly larger than those of either 5-year-olds or adults. Five-year-olds\u27 MAA thresholds for SS sounds were very near to those of adults. However, their thresholds for the PE stimuli were significantly higher than those of adults\u27, and closer to those of 18-month-olds. When a lagging sound is inaudible as a separate auditory event, the auditory system presumably treats the leading and lagging sound as components of the same auditory percept, and uses both signals to compute the position of the sound source. The leading sound, which signals the onset of an auditory event, is assigned perceptual dominance thereby diminishing the nervous system\u27s interaural sensitivity for the later-arriving echo. This and related work has raised important questions concerning the neural mechanisms involved in spatial hearing in adults and children, especially those aspects which involve an active suppression of superfluous signals