Academic journal article Attention, Perception and Psychophysics

Listeners' Perception of Compensatory Shortening

Academic journal article Attention, Perception and Psychophysics

Listeners' Perception of Compensatory Shortening

Article excerpt

English exhibits compensatory shortening, whereby a stressed syllable followed by an unstressed syllable is measured to be shorter than the same stressed syllable alone. This anticipatory shortening is much greater than backward shortening, whereby an unstressed syllable is measured to shorten a following stressed syllable. We speculated that measured shortening reflects not true shortening, but coarticulatory hiding. Hence, we asked whether listeners are sensitive to parts of stressed syllables hidden by following or preceding unstressed syllables. In two experiments (Experiments 1A and 1B), we found the point of subjective equality-that is, the durational difference between a stressed syllable in isolation and one followed by an unstressed syllable-at which listeners cannot tell which is longer. In a third experiment (Experiment 2), we found the point of subjective equality for stressed monosyllables and disyllables with a weak-strong stress pattern. In all of the experiments, the points of subjective equality occurred when stressed syllables in disyllables were measured to be shorter than those in monosyllables, as if the listeners heard the coarticulatory onset or the continuation of a stressed syllable within unstressed syllables.

Coarticulation has many definitions. For the present purposes, it refers to temporal overlap among gestures for serially nearby phonetic segments; it does not refer, for example, to adjustments in production of one segment to accommodate it to requirements of another. Temporal overlap of gestures ensures that the acoustic speech signal is highly context sensitive, because it is caused nearly everywhere by gestures for more than one consonant or vowel. Our interest in the present study is in the perception of coarticulated speech.

Compensation for Coarticulation and the Companion Finding

Listeners compensate for coarticulation of many kinds. That is, they respond to speech signals as if they have adjusted for the context sensitivity caused by coarticulation. For example, Mann (1980) found that listeners identified ambiguous members of a /da/-/ga/ continuum differently, depending on whether a precursor syllable was /al/ or /ar/. In particular, they identified more continuum members as /ga/ if the precursor was /al/ than if it was /ar/. Mann identified two possible accounts of the finding. One is that listeners track coarticulation. Productions of /l/ and /r/ persist after the onset of /da/ or /ga/ production (carryover coarticulation). A consequence of perseveration of /l/ may be that the persisting tongue-tip constriction or its gradual release causes the tongue dorsum constriction during /g/ to be pulled forward. In contrast, persistence or gradual release of the pharyngeal constriction for /r/ during /da/ production might pull the alveolar constriction for /da/ back. When listeners hear ambiguous continuum members, they hear a consonant that is rather far forward for /g/ and rather far back for /d/. If they are tracking coarticulation, and the precursor syllable is /al/, they may hear the ambiguous continuum member as /ga/, pulled forward by /l/; if the precursor is /r/, they may hear the continuum member as /da/, pulled back by /r/. An alternative account of the finding is that it reflects spectral contrast. The highending F3 of /l/ causes the onset F3s of continuum members to sound lower than they are and so more like /g/; the low-ending F3 of /r/ has the opposite effect.

These accounts of compensation for coarticulation have been intensely debated (see, e.g., Fowler, 2006; Lotto & Holt, 2006). In our view, the debate is coming to a resolution that clearly favors Mann's (1980) first account (see Viswanathan, Fowler, & Magnuson, 2009; Viswanathan, Magnuson, & Fowler, in press). The debate has some relevance to the research that we report here, in which durational contrast offers an alternative account of our findings, and so we will return to the debate in the General Discussion section. …

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