On the relevance of locus equations for production and perception of stop consonants

We examined the possible relevance of locus equations to human production and perception of stop consonants. The orderly output constraint (OOC) of Sussman, Fruchter, and Cable (1995) claims that humans have evolved to produce speech such that F2 at consonant release and F2 at vowel midpoint are linearly related for consonants so that developing perceptual systems can form representations in an F2ons-by-F2vowel space. The theory claims that this relationship described by locus equations can distinguish consonants, and that the linearity of locus equations is captured in neural representations and is thus perceptually relevant. We investigated these claims by testing how closely locus equations reflect the production and perception of stop consonants. In Experiment 1, we induced speakers to change their locus equation slope and intercept parameters systematically, but found that consonants remained distinctive in slope-by-intercept space. In Experiment 2, we presented stop-consonant syllables with their bursts removed to listeners, and compared their classification error matrices with the predictions of a model using locus equation prototypes and with those of an exemplar-based model that uses F2ons and F2vowel, but not locus equations. Both models failed to account for a large proportion of the variance in listeners' responses; the locus equation model was no better in its predictions than the exemplar model. These findings are discussed in the context of the OOC.