Modified Spectral Tilt Affects Older, but Not Younger, Infants' Native-Language Fricative Discrimination

PURPOSE: It is important to ensure that hearing aid fitting strategies for infants take into account the infant's developing speech perception system. As a way of exploring this issue, this study examined how 6- and 9-month-olds with normal hearing perceive native-language speech in which the natural spectral shape was altered to emphasize either high-frequency (positive spectral tilt) or low-frequency (negative spectral tilt) information. METHOD: Discrimination was tested using a visual habituation procedure. Forty-eight 6-month-olds and forty-eight 9-month-olds were presented with a fricative contrast, /f/-/s/, in 1 of 3 conditions: (a) as unmodified speech; (b) with a -6 dB/octave tilt; or (c) with a +6 dB/octave tilt. RESULTS: Six-month-olds showed evidence of discriminating /f/-/s/ in all 3 conditions, but 9-month-olds showed such evidence only in the unmodified condition. CONCLUSIONS: The findings suggest that the perceptual reorganization that emerges for consonants at the end of the first year affects 9-month-olds' discrimination of native speech sounds. Perceptual reorganization is usually indexed by a decline in the ability to discriminate nonnative speech sounds. In this study, 6-month-olds demonstrated an acoustic-based sensitivity to both modified and unmodified native speech sounds, but 9-month-olds were most sensitive to the unmodified speech sounds that adhered to the native spectral profile.