Speech dynamics are coded in the left motor cortex

Speaking fluently can be challenging even for a political or mathematical genius such as Winston Churchill and Alan Turing, as recently illustrated for the latter in the movie „The Imitation Game“. It demonstrates obvious but temporary breakdowns in speech flow. New research has now shown that the way how speech motor areas of the brain prepare for intended speech is abnormal in adults suffering from persistent developmental stuttering, as reported in this month in Brain.

Original publication: Speech dynamics are coded in the left motor cortex in fluent speakers but not in adults who stutter. Neef NE, Hoang TN, Neef A, Paulus W, Sommer M. Brain. 2015 Mar;138 (Pt 3):712-25. PMID: 25595146

Moving the right leg or arm requires the activation of the motor area in the left hemisphere of the brain. Conversely, orofacial midline muscles such as tongue, lips, jaw, and vocal folds are bilaterally controlled – both hemispheres innervate both sides of the articulatory apparatus. For this reason it would be plausible to expect that operating neurons are similarly excited in both hemispheres during speaking. But this is not the case.

Drs. Nicole Neef and Martin Sommer from the University Medical Center Göttingen, together with Dr. Andreas Neef from the Max Planck Institute for Dynamics and Self-Organization, Göttingen, used a technique with high temporal resolution and a direct functional read-out: they stimulated the cortical area that controls the tongue with brief electromagnetical pulses while participating subjects were speaking. The stimulation efficiency was monitored with electrodes on the tongue. This allowed them, for the first time, to track changes in the local cortical excitability that accompanied the transition between speech gestures. In control subjects, excitability in the left hemisphere motor area increased during this transition, indicating a speech motor preparation confined to the left brain hemisphere. In adults who stutter, this pattern was lacking on the left side. The more severe individuals stuttered the more impaired was the speech motor preparation in the left hemisphere.

These results integrate structural and neurophysiological findings into a plausible model of speech pathophysiology in persistent developmental stuttering. They pinpoint the left primary motor cortex and its interconnected areas as key players in the generation of fluent speech, and will pave the way to directly modulating the excitability of these areas to influence speech fluency.

Universitätsmedizin Göttingen, Georg-August-Universität
Klinik für Klinische Neurophysiologie
Prof. Dr. Martin Sommer
Phone: +49 (0) 551 / 39-8463
Robert-Koch-Straße 40, 37075 Göttingen

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