The influence of pitch contour on vowel discrimination in infants

Because the harmonics are farther apart in high- than in low-pitched vowels, it was predicted that it should be difficult to extract the frequencies of the formant resonances, and therefore, to discriminate high-pitched vowels. Infant-directed speech is very high in pitch, yet studies showing that young infants can discriminate vowels have used low-pitched male voices. The present study showed that 6-month olds were able to discriminate /i/ and /I/ when the pitch was 240 Hz, but were unable to do so when the pitch was 340 Hz, demonstrating that the high pitch of infant-directed speech indeed impedes vowel discrimination. How, then, do infants learn vowel discrimination? The answer may lie with the large pitch contours that are also characteristic of infant-directed speech. Because the harmonic structure follows the frequency of the fundamental, but the formant structure remains relatively constant across frequency shifts, a second prediction was made: discrimination should be better for vowels containing pitch contours than for those with steady pitch. This is exactly what was found. The addition of a downward pitch contour at both high (440–340 Hz) and low (240–140 Hz) pitches significantly improved infants discrimination of /i/ and /I/. [Research supported by NSERC.]