According to this account, the same adjustments should apply to nonspeech target sounds following nonspeech precursors. This mechanism would thus influence the interpretation of speech sounds on the basis of spectral information in the preceding sentence (or extrinsic context). On the basis of this long-term average, the perceptual impact of acoustic energy in certain frequency regions of a subsequent target sound becomes attenuated. One account of this process proposes that it is the result of a general auditory mechanism that normalizes perception of any acoustic input by constructing a long-term average of the distribution of frequencies of a sound source (Kiefte & Kluender, 2008 Watkins & Makin, 1996). For example, the interpretation of vowels depends on the first- and second-formant characteristics of the speaker who utters those vowels (Ladefoged & Broadbent, 1957). Context-dependent interpretation helps listeners resolve speech sound ambiguities such as those that arise from speaker differences. Our interpretation of auditory events is dependent on the context in which they occur. Extrinsic normalization of vowels is due, at least in part, to an auditory process that may require familiarity with the spectrotemporal characteristics of speech. It appears that normalization is not restricted to speech but that the nature of the preceding sounds does matter. Additional experiments investigated the roles of pitch movement, amplitude variation, formant location, and the stimuli's perceived similarity to speech. None occurred, however, with nonspeech stimuli that were less speechlike, even though precursor–target LTAS relations were equivalent to those used earlier. Spectrally rotated nonspeech versions of these materials produced similar normalization. Targets on a – (low–high F1) continuum were labeled as more after high- F1 than after low- F1 precursors. Previous findings were replicated with first-formant ( F1) manipulations of speech. If so, normalization should apply to nonspeech stimuli. We asked here whether this process could be based on compensation for the long-term average spectrum (LTAS) of preceding sounds and whether the mechanisms responsible for normalization are indifferent to the nature of those sounds. Listeners tune in to talkers’ vowels through extrinsic normalization.
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