Infant-adult differences in unmasked thresholds for the discrimination of consonant-vowel syllable pairs

Speech sound discrimination thresholds were obtained for two speech sound contrasts (/ba/ vs. /da/ and /ba/ vs. /ga/) for infant and adult subjects. The stimuli were computer-generated synthetic tokens. An adaptive (one-up, one-down) threshold procedure was used with the visual reinforcement infant speech discrimination procedure for the infant subjects. Adults were tested using the same apparatus and threshold-tracking protocol as the infants. There was a 28-dB difference in threshold for discrimination of /ba/ versus /da/ and a 25-dB difference in threshold for discrimination of /ba/versus/ga/ between the infants and the adults. The differences reveal that to reach a criterion level of performance on a simple speech perception task, infants require much greater stimulus intensity than do adults. This has implications for our understanding of normal auditory development, for our notions of hearing impairment in infants and for the role of intensity in research studies of infant speech perception.