Nonspeech materials are widely used to identify basic mechanisms underlying speech perception. For instance, they have been used to examine the origin of compensation for coarticulation, the observation that listeners' categorization of phonetic segments depends on neighboring segments (Mann, 1980). Specifically, nonspeech precursors matched to critical formant frequencies of speech precursors have been shown to produce similar categorization shifts as speech contexts. This observation has been interpreted to mean that spectrally contrastive frequency relations between neighboring segments underlie the categorization shifts observed after speech, as well as nonspeech precursors (Lotto & Kluender, 1998). From the gestural perspective, however, categorization shifts in speech contexts occur because of listeners' sensitivity to acoustic information for coarticulatory gestural overlap in production; in nonspeech contexts, this occurs because of energetic masking of acoustic information for gestures. In 2 experiments, we distinguish the energetic masking and spectral contrast accounts. In Experiment 1, we investigated the effects of varying precursor tone frequency on speech categorization. Consistent only with the masking account, tonal effects were greater for frequencies close enough to those in the target syllables for masking to occur. In Experiment 2, we filtered the target stimuli to simulate effects of masking and obtained behavioral outcomes that closely resemble those with nonspeech tones. We conclude that masking provides the more plausible account of nonspeech context effects. More generally, we suggest that similar results from the use of speech and nonspeech materials do not automatically imply identical origins and that the use of nonspeech in speech studies entails careful examination of the nature of information in the nonspeech materials.
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