Cortical and subcortical mechanisms in persistent stuttering ...

Chapter 1 Introduction
1 Introduction
Stuttering is a speech disorder that occurs without known origin between 3 and 8 years of age
and often remits before puberty. When it persists after puberty it becomes a chronic adult
speech disorder throughout the lifespan (Andrews et al., 1983). The advances in neuroimaging
promoted insights into the highly distributed system of speech production and its alterations
in adulthood stuttering. One important motivation in stuttering research is to separate
neurobiological correlates of the core symptoms of stuttering from neurobiological correlates
associated with compensation, attempts to avoid stuttering. Results so far indicate a variety of
complex dysfunctional systems and it appeared problematic to distinguish between
mechanisms responsible for speech dysfluencies and those connected to compensation in the
adult system (Ludlow, 2000). The first study of this dissertation will tie in with this problem.
Another important aspect is again motivated by neuroimaging studies that stress irregularities
in the activation of the primary motor cortex during different functional states associated with
speech production. The regulation of blood supply which is the basis of functional magnetic
resonance imaging (fMRI) is correlated with summed neuronal activity but conclusions about
states of cortical excitability and thus neurophysiological mechanisms are indirect. The
modulation of cortical excitability is an inherent principle in the encoding of output signal by
the primary motor cortex and the method of choice to non-invasively study this in humans is
transcranial magnetic stimulation (TMS). This method is well established in Professor Paulus’
Department of Clinical Neurophysiology, for instance to investigate neuromodulation in the
primary motor cortex representation of small hand muscles. One aim of the dissertation was to
establish recordings of TMS induced motor evoked potentials (MEPs) from facial muscles.
This enabled me to conduct the first study of the intracortical excitability of the primary
tongue motor representation in adults who stutter.
A seminal work motivated the third study included in this dissertation: diffusion tensor
imaging (DTI) identified reduced white matter integrity in fibres connecting frontal, temporal
and parietal speech related areas (Sommer et al., 2002a). Affected are possibly fibres
mediating the mapping of speech sounds to articulation. The interaction of speech production
and speech perception and the interference of stuttering with this interaction is the topic of the
third study reported in this dissertation.
During the course of my doctoral studies I furthermore contributed to a longitudinal DTI
study aiming at identifying a biological marker of stuttering persistency at stuttering onset. In
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