Infant speech-sound discrimination in noise

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Abstract
The effects of noise on 7‐ to 11‐month‐old infants’ speech‐sound discrimination (/ba/vs/ga/) were determined using a conditioned head‐turn procedure. Variation in performance as a function of signal‐to‐noise ratio (S/N) was estimated by testing each infant at four S/N’s (−8, 0, 8, and 16 dB). Adults were tested for comparison at four S/N’s (−12, −8, −4, and 0 dB). The S/N’s were chosen based on pilot data. Performance varied monotonically with S/N for both age groups, but infants required greater S/N than adults to achieve comparable levels of performance. Both groups were also tested using an adaptive (1‐up, 1‐down) threshold procedure with a 3‐dB step size. There was a group mean difference in threshold of 5.8‐dB S/N favoring the adults. Weighted group psychometric functions, derived from the responses obtained in the adaptive runs, showed good correspondence with the data points at the four S/N’s. The slopes of these functions were the same (7.5%/dB) for infants and adults. The results suggest that infants are at a greater disadvantage than adults when processing speech in noise and that concern over the effects of a noisy environment on the acquisition of language is justified. In addition, the adaptive threshold procedure can be used as an efficient way to estimate the limits of discrimination ability as a function of S/N or intensity, both for individual subjects and for groups of subjects, in developmental research.

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