ASA 125th Meeting Ottawa 1993 May

4aBV4. Human infants' sensitivity to interaural level differences.

Daniel H. Ashmead

D. Wesley Grantham

Walter Murphy

Anne Marie Tharpe

Div. of Hear. and Speech Sci., Vanderbilt Univ., and Bill Wilkerson Ctr., 114 19th Ave. S., Nashville, TN 37212

Infants' sensitivity to interaural differences was assessed using a lateralization paradigm. Infants aged 5 to 6 months heard speech sounds with interaural level or time differences (ILD, ITD), while an observer made forced choice judgments about the side of the louder (or leading) sound. In experiment 1 (N=10), the stimuli were 10 dB ILD, 100 dB ILD (monaural), and 400 (mu)s ITD. Performance was well above chance (70%, 74%, and 78% correct), indicating appropriate responses to ``easy'' interaural differences. In experiment 2 (N=16), a group threshold for ILD sensitivity was determined, using ILDs of 2, 5, 8, and 100 dB (as well as the 10 and 100 dB ILD trials from experiment 1). The threshold estimate was 6.6 dB, far higher than adult thresholds of 0.5 to 1 dB. These findings, together with previous work on infants' free field sound localization, suggest that ILDs are not a strong basis for sound localization in this age range. [Work supported by NIH.]