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Abstract:
Sinewave analogs of speech are created using tones that
follow the center frequency of vocal tract resonances. They lack
acoustic attributes of actual vocal sounds and can be perceived as
speech or as nonspeech, depending on expectancy. We sought to
identify neural correlates of this perceptual shift using
functional magnetic resonance imaging. In an auditory task, 16
subjects determined whether a tone (the second formant analog,
presented alone) was included in a following three-tone complex.
The single tone was either aligned with the other tones in the
complex (phonetic stimulus) or temporally reversed (acoustic
stimulus). Halfway through the task, subjects were informed about
the stimuli and trained to recognize phonemes in the three-tone
complexes. Auditory task performance did not change following
training. In a subsequent phonetic task using acoustically
identical stimuli, subjects identified three-tone complexes
containing the phoneme /p/. ANOVA revealed no main effect of
stimulus type (phonetic vs. acoustic) in any brain areas. During
the acoustic task, activation was reduced in left Heschl's gyrus in
the informed relative to the naive condition. Activation increased
in the phonetic task relative to the acoustic task in left inferior
frontal gyrus (LIFG, Brodmann areas 44 and 45). These results
implicate the LIFG in phonetic perception. Phonetic training alone
did not result in increased brain activation during a purely
acoustic task.
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