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Front. Hum. Neurosci. | doi: 10.3389/fnhum.2018.00069

The Effect of Speech Repetition Rate on Neural Activation in Healthy Adults: Implications for Treatment of Aphasia and Other Fluency Disorders

  • 1Neurology, Beth Israel Deaconess Medical Center, Harvard Medical School, United States

Functional imaging studies have provided insight into the effect of rate on production of syllables, pseudowords, and naturalistic speech, but the influence of rate on repetition of commonly-used words/phrases suitable for therapeutic use merits closer examination.
Aims: To identify speech-motor regions responsive to rate and test the hypothesis that those regions would provide greater support as rates increase, we used an overt speech repetition task and functional magnetic resonance imaging (fMRI) to capture rate-modulated activation within speech-motor regions and determine whether modulations occur linearly and/or show hemispheric preference.
Methods: Twelve healthy, right-handed adults participated in an fMRI task requiring overt repetition of commonly-used words/phrases at rates of 1, 2, or 3 syllables/second (syll./sec.).
Results: Across all rates, bilateral activation was found both in ventral portions of primary sensorimotor cortex and middle and superior temporal regions. A repeated measures ANOVA with pairwise comparisons revealed an overall difference between rates in temporal lobe regions of interest (ROIs) bilaterally (p<0.001); all six comparisons reached significance (p<0.05). Five of the six were highly significant (p<0.008), while the left-hemisphere 2- versus 3-syll./sec. comparison, though still significant, was less robust (p=0.037). Temporal ROI mean beta-values increased linearly across the three rates bilaterally. Significant rate effects observed in the temporal lobes were slightly more pronounced in the right hemisphere. No significant overall rate differences were seen in sensorimotor ROIs, nor was there a clear hemispheric effect.
Conclusions: Linear effects in superior temporal ROIs suggest that sensory feedback corresponds directly to task demands. The lesser degree of significance in left-hemisphere activation at the faster, closer-to-normal rate may represent an increase in neural efficiency (and therefore, decreased demand) when the task so closely approximates a highly-practiced function. The presence of significant bilateral activation during overt repetition of words/phrases at all three rates suggests that repetition-based speech production may draw support from either or both hemispheres. This bihemispheric redundancy in regions associated with speech-motor control and their sensitivity to changes in rate may play an important role in interventions for nonfluent aphasia and other fluency disorders, particularly when right-hemisphere structures are the sole remaining pathway for production of meaningful speech.

Keywords: speech rate, overt repetition, fMRI, bilateral activation, temporal lobes, right-hemisphere language networks, fluency, speech-motor function

Received: 02 Nov 2017; Accepted: 07 Feb 2018.

Edited by:

Peter Sörös, University of Oldenburg, Germany

Reviewed by:

Fabricio F. Oliveira, Escola Paulista de Medicina, Universidade Federal de Sao Paulo, Brazil
Anna Maria Alexandrou, Aalto University, Finland  

Copyright: © 2018 Marchina, Norton, Kumar and Schlaug. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

* Correspondence: MD, PhD. Gottfried Schlaug, Beth Israel Deaconess Medical Center, Harvard Medical School, Neurology, 330 Brookline Avenue, Neurology - Palmer 127, Boston, 02215, MA, United States, gschlaug@bidmc.harvard.edu