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Abstract:
Comparing speech to nonspeech perception usually prevents
having the acoustics be completely comparable. Earlier work found
areas selectively active for speech; acoustic differences, perhaps,
activated processing regions for extraneous reasons. Natural speech
or standard synthesis require acoustic differences, but
comparability exists in sinewave speech, which replaces speech
resonances with sinewaves. The result resembles, initially, weird
computer sounds, but once phonetically perceived, elicits all
phoneme types. Here, sinewave speech, and nonspeech organizations
of those tones, provided fMRI evidence that speech perception
occupies a neurological specialization regardless of acoustics.
Sinewave versions of nonwords used amplitude- and
frequency-modulated tones. Nonspeech versions combined tones from
different syllables, time-reversing the mid-frequency tone, and
swapping the halves of the lowest tone. Sentences and nonwords were
identified first; the fMRI test used passive listening. Test blocks
contained speech or nonspeech; control conditions included a silent
block and one containing musical chords. Nineteen right-handed
adults participated. Behaviorally, females typically failed to
identify the speech. Most males and one female performed
accurately. Brain maps, created separately for performers and
nonperformers, contrasted speech with nonspeech. Posterior STG was
active for both, while a parietal region and the parahippocampal
gyrus, only for performers. This suggests the parietal region
accompanied a conscious speech percept, while posterior STG
processes speech regardless.
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