The Influence of Perceptual Categories on Auditory Feedback Control
Caroline Niziolek1, Frank Guenther12
1Harvard-MIT Division of Health Sciences and Technology, Cambridge, MA/2Department of Cognitive and Neural Systems, Boston University, Boston, MA
Introduction:Speakers use auditory feedback to monitor their own speech, ensuring that the observed output matches the intended output. By altering the speech feedback signal before it reaches the ear, we can induce perceived errors and observe both the acoustic and neural consequences. This studyinvestigates the neural mechanisms responsible for the detection and consequent correction of auditory errors, as well as the influence of phonetic categories on this auditory feedback control. Because subjects’ perceptual boundaries for vowels are often asymmetric around the vowel production target, within- and across-category perturbations of the same magnitude can be contrasted within a single subject. Our aim is to compare the neural activation due to a phonetic change — for example, from the word “bed” to the word “bad” — to that due to a non-linguistic auditory change — for example, from a prototypical example of “bed” to a different version of the same word.
Methods:For each subject, vowel production data were collected for each of the six vowels [i, ɪ, ɛ, æ, ɑ, u]. Perceptual boundaries between pairs of vowels were assayed using a word categorization task with formant-shifted stimuli. Subjects assigned phonetic labels to vowel tokens generated from their own productions, shifted in the first and second formant frequencies (F1 and F2). Functional magnetic resonance imaging was then employed to measure neural responses to subjects’ speech with and without auditory perturbation. During each trial, subjects were presented with a word to read aloud or with a control stimulus. On one out of every four test trials, F1 and F2were perturbed in real time in one of two directions. This altered speech was fed back to subjects through headphones, creating a sudden, unexpected mismatch between the vowel target and the perceived realization.
Results:Psychophysical results show a compensatory shift of the first two formants during the perturbed conditions. Preliminary fMRI data from three subjects show increased activation of the inferior frontal gyrus for cross-category shifts than for within-category shifts. Shifted conditions show more cortical activation in STG than unshifted conditions. In general, cortical activation is greater in extent for shifts that cross a category boundary than for those that do not, even though these shifts are of the same magnitude.
Conclusions:Vowel category boundaries, as assayed by a perceptual categorization test, were found to be asymmetric within subjects and variable across subjects. This variability enables a direct contrast between cross- and within-category perturbationsin a single subject: a constant shift magnitude (in the perceptually-derived melspace) can elicit different phonetic percepts, depending on the directionof the shift in formant space. Auditory (non-phonetic) shifts and phonetic shifts seem to evoke activity from two types of error correction cells in auditory cortex: one type that occurs during an auditory difference between intended and perceived sounds (within-category shift), and another that is specific to a phonetic difference (between-category shift).
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