Infants' vowel discrimination in modulated noise

Abstract

Adult listeners detect and discriminate target sounds better in amplitude modulated noise than in unmodulated noise. This study examined infants' ability to take advantage of masker modulation to improve sensitivity to a target. Listeners were 7–9-month-old infants and 18–30-year-old adults. Vowel discrimination in noise was tested. Listeners learned to respond when a repeated vowel changed from /a/ to /i/ or from /i/ to /a/. An observer-based method was used to assess sensitivity to the vowel change. The maskers were speech-spectrum noise either unmodulated, amplitude modulated with the envelope of single-talker speech or sinusoidally amplitude modulated at 8 Hz with a 75% modulation depth. The overall level of the maskers was 60 dB SPL. The level of the vowels, chosen to yield an average d' of 1 in the unmodulated masker, was 46 dB SPL for adults and 58 dB SPL for infants. Adults' d' was substantially and significantly higher in both modulated maskers than in the unmodulated masker. Infants' d' was also significantly higher in the two modulated maskers than in the unmodulated masker, but the improvement due to modulation was significantly smaller for infants than for adults. [Work supported by NIDCD R01DC00396 and P30DC04661.]