Cortical and subcortical mechanisms in persistent stuttering ...

Chapter 1 Introduction
al., 2004). A direct test of the role of right-hemispheric motor areas to proposed specific
behaviors is so far missing in stuttering research.
In the first study of the current dissertation TMS was used to induce a virtual lesion in the
dorsolateral premotor cortex (PMd) to test its role in movement timing in persons who stutter.
In healthy subjects it has been reported that the left PMd is crucially involved in the control of
paced finger movements (Pollok et al., 2008). It is unclear whether this cortical lateralization
of timing control holds true in persons who stutter. Supporting evidence for an imbalanced
functional lateralization of the control of finger tapping in stuttering is given by a recent fMRI
study (Morgan et al., 2008). While in healthy subjects finger tapping with the right hand
activated the contralateral motor and premotor cortex, in persons who stutter the precentral
gyrus of either hemisphere was activated. In the study presented here we tested whether the
right premotor cortex is indeed functionally involved in a paced finger tapping task in persons
who stutter.
The second study included in this dissertation took aim at the neurophysiological mechanisms
in the primary motor tongue representation. From a neurophysiological point of view, the
right hemispheric hyperactivity of the primary motor cortex during symptom production
(Braun et al., 1997; Fox et al., 1996; Fox et al., 2000) has been interpreted as increased
cortical excitability (Ludlow and Loucks, 2003). By applying TMS it is possible to determine
cortical excitability (see Appendix D). Although TMS is well established and a widely used
technique there are only two reports on cortical excitability in stuttering research preceding
this dissertation (Sommer et al., 2009a; Sommer et al., 2003). The objective of the most recent
study (Sommer et al., 2009a) is to elucidate transcallosal interactions between the motor
cortices in adults who stutter. The interplay between hemispheres which is operationalized
with measures of transcallosal inhibition and ipsilateral silent period was normal in the
cortical hand representation in stuttering, not indicating that this interplay between motor
cortices is likely to play a decisive role in stuttering. The earlier study (Sommer et al., 2003)
ascertains the intracortical excitability of the cortical representation of a right hand. The
critical parameters are intracortical inhibition which is likely mediated by inhibitory motor
cortical interneurons (Hallett, 2000), and intracortical facilitation which is hypothesized to be
a net facilitation consisting of prevailing facilitation and weaker inhibition mediated, among
other mechanisms, by glutamatergic N-methyl-D-aspartate (NMDA) receptors and γ-
Aminobutyric acid (GABA)ergic receptors (Hanajima and Ugawa, 2008; Paulus et al., 2008).
Again, intracortical inhibition and intracortical facilitation were found to be normal in the
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