How does prosody influence speech categorization?

A recent study (Kim & Cho, 2013, The Journal of the Acoustical Society of America) reported that the perception of a prosodic boundary leads to a shift in a stop-identification function in English, so that stops with a relatively long VOT are accepted as voiced if occurring after a major prosodic boundary. Even Korean learners of English showed such a shift. This shift would seem to result from compensation for post-boundary lengthening effects (or domain-initial strengthening) and thereby help to overcome the invariance problem in speech perception. In two experiments, we ask how this effect comes about. The first experiment tested whether a simple adjustment to a change in overall speaking rate would be sufficient to account for the shift. Results showed that while the global speaking-rate change modulates phonetic categorization in a similar way as a change in the prosodic boundary strength, the speaking-rate effect is not sufficient to explain the boundary effect. That is, there was a more robust shift in a stop identification function with localized slowing down of the final syllable due to an intonational phrase (IP) boundary than with global slowing down of speaking rate. The second experiment therefore investigated the contribution of an F0 cue to the observed perceptual shift and found that the presence or absence of the F0 cue did not mediate the effect of prosodic boundaries on phonetic categorization. This suggests that a perception shift in phonetic categorization stems primarily from the listeners' adjustment to temporal variation, though its source is different from the speaking rate. The results are considered in terms of two possible accounts: one that takes both the boundary-induced and the speaking rate-induced effects as listeners' adjustments to low-level temporal variation, and the other that separates them by taking the boundary-induced effects to arise with computation of higher-level prosodic structure, given that the source of the localized slowing down effect is a prosodic boundary.