University Medical Center Göttingen, Germany
Structural imaging data suggest a disconnection of speech relevant brain areas as an underlying neurological correlate of persistent developmental stuttering. We wondered how this translates into measurable dysfunctions of speech preparation. To study this, we used transcranial magnetic stimulation over the part of the motor cortex controlling the tongue, a key articulatory muscle. By activating the neurons steering the tongue, such an external magnetic field induces an electric current in the motor cortex some 1-2 cm below a shielded copper coil held onto the head, with the magnetic field elegantly bypassing the resistance of skull and skin. Such an induced current generates a subtle, barely perceivable jerk of the tongue, called a motor evoked potential. Its amplitude indicates the current state of excitability of the area of the brain controlling tongue movements. We interspersed single transcranial magnetic stimulation pulses in the transition period (lasting a few hundred milliseconds) between an initial and a following utterance. A group of fluent speaking adults showed an increase of the motor evoked potential amplitude in this transition period of about 200 milliseconds, indicating a gradual preparation of the primary motor cortex towards an imminent utterance. In a group of adults who stutter, this dynamic modulation of motor cortex excitability in preparation of speech was lacking to an extent correlating with speech dysfluency (Neef et al., 2015). We regard this as a pathophysiological marker on speech dysfluency, likely reflecting insufficiently timed input from orchestrating areas such as the supplementary motor or the basal ganglia through the thalamus. In a subsequent study, we tested whether this lack of coordinated dynamic excitability change of a primary motor cortex was present in more realistic speech settings as well, and this was essentially confirmed (Whillier et al., 2018). In the framework of the Packman and Attanasio model (Packman, 2012), the current findings substantiate an insufficient motor cortex preparation for speech as a core deficit for neural processing of spoken language in persistent developmental stuttering.
To summarize, the high time resolution of transcranial magnetic stimulation interspersed in a speech task has allowed revealing a physiological mechanism of impaired and dyscoordinated speech preparation in adults who stutter. It remains to be seen whether this is influenced by therapy, either behaviourally or through direct cortical stimulation.
Neef, Hoang, Neef, Paulus and Sommer (2015) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4408433/
Whillier, Hommel, Neef, Wolff, Paulus and Sommer (2018) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6179203/
Packman (2012) https://www.sciencedirect.com/science/article/pii/S0094730X12000642?via%3Dihub