Infants' detection of speech in noise.

Localization responses to a speech phrase masked by white noise were obtained from infants 6, 12, 18, and 24 months of age and from adults. The masking noise was presented continuously from two loudspeakers located 45 degrees to each side of the infant. During a trial the speech phrase was presented through one of the loudspeakers. A head turn to the signal (correct response) was rewarded by activating an animated toy on top of the speaker. The intensity of the signal was varied over trials (method of constant stimuli) to determine thresholds (defined as the intensity corresponding to 65% correct head turns) at each of two levels of masking noise, 42 and 60 dBC. Thresholds for the speech signal were comparable across all infant groups for both levels of masking noise. Increasing the masking noise from 42 to 60 dBC resulted in a threshold shift of comparable magnitude for infants and adults. However, adult thresholds were approximately 10-12 dB lower than those of infants at both masking levels.