Motor equivalence in the production of /∫/

Abstract

To explore the idea that speech motor goals are acoustic targets, upper lip protrusion and tongue blade fronting were examined in the sibilant /∫/ for evidence of motor equivalence in eight speakers of American English. Positive correlations across multiple repetitions of /∫/ (motor equivalence) would occur if the upper lip compensated with more protrusion when the tongue blade was further forward and vice versa. This motor equivalence would serve to maintain an adequate front cavity size for good acoustic separation from /s/ and enhance the acoustic stability of /∫/. It was hypothesized that motor equivalence would be found among less prototypical tokens, as elicited in a ‘‘clear+fast’’ speaking condition. Acoustic spectral analyses indicated excellent acoustic separation between the two sibilants, even in the ‘‘clear+fast’’ condition. There were significant positive correlations of the tongue-blade and upper lip movements for 28% of all possible cases, with considerable individual variation. When motor equivalence could be compared across the speaking condition, the tokens produced with the motor equivalent pattern were acoustically less prototypical. No occurrences of negative movement correlations were found. One possible source of individual variation may be the use of saturation (quantal) effects, which could also enhance acoustic stability. [Work supported by NIH.]