Infants’ localization of sounds in the median sagittal plane: Effects of signal frequency

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Abstract
The purpose of this research was to determine if infants, like adults, show differential localization performance in the median sagittal plane (MSP) as a function of the spectrum of the signal. Infants 6-18 months of age were seated in a dark room facing an array of nine loudspeakers, with one loudspeaker positioned at ear level, 0.degree., and four each positioned above and below ear level at 4.degree., 8.degree., 12.degree. and 16.degree.. A two-alternative, forced-choice procedure was used in which a sequence of noise bursts was presented at 0.degree. and then shifted vertically, above or below 0.degree., and continued to be presented until the infant made a directional head and/or eye movement; correct responses were visually reinforced. For each of three bandpass noice conditions (< 4 kHz, 4-8 kHz, 8-12 kHz), minimum audible angle (MAA) for each listener, i.e., the smallest of the four angular shifts in vertical sound location that the listener could reliably detect, was estimated. Results indicated that MAA systematically decreased with increasing age, revealing an increasingly finer partitioning of auditory space. Moreover, performance of each age revealed the importance of high frequencies for localization in the MSP. Infants did not reliably localize the low-pass signal (< 4 kHz) and showed the best performance to the signal comprising the highest frequencies (8-12 kHz). These findings reveal systematic age-related improvements in sound localization abilities during infancy, and suggest that spectral cues similar to those for adults operate for infants in vertical localization.