Covert contrasts and categorical perception: a comparison of two measurement tools.

This study considers adults' perception of subtle differences in the error productions of /r/ and /l/ in young children. Given that these two phonemes share a common substitution [w], the potential covert contrast between these three sounds was investigated. The utility of two perceptual rating scales was compared: a linear visual analogue scale (VAS) and a triangular scale that considered the possibility of covert contrast among /r,l,w/. Adult listeners rated speech productions from children with cochlear implants (CIs) and speech age-matched children with normal hearing (NH) as they produced VCV or CV productions with /r,l,w/ in syllable-initial position. Mahalanobis distances of the listener ratings for each stimulus on each rating scale were calculated for analysis. Ultimately, there was no difference between the ratings of the two speaker populations. However, there was a significant difference in the amount of variability captured with each scale. The triangular scale reliably captured more variability in the productions than the linear VAS. The significant effect of scale type on the spread of listeners' responses provides support for the claim that the type of rating system used can capture different aspects of a listener's perceptions, thereby suggesting that the type of rating system used in a study may significantly influence that study's findings.