ACQ Vol 12 no 1 2010

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Motor speech disorders
ACQ Volume 12, Number 1 2010 33
Justine V.
Goozée
This article
has been
peer-
reviewed
Keywords
ASSESSMENT
CORTEX
SPEECH
TRANSCRANIAL
MAGNETIC
STIMULATION
TREATMENT
2003). The magnetic field strength is typically about 2 Tesla
(dependent on the TMS system used), which is similar to the
strength of the magnetic field used in magnetic resonance
imaging (MRI).
When TMS is delivered to the motor cortex, the neural
pathways leading to various voluntary muscles can be
activated and the resultant muscle contraction, known as
a motor evoked potential (MEP), can be recorded using
electromyography (EMG, see figure 1). Various measures,
including the size of the MEP that varies with the level of
cortical excitability, and the latency of the MEP (i.e., time
from cortical stimulation to when the MEP occurs), which
reveals the integrity of the cortico-bulbar/spinal pathways,
can be taken. Two main types of TMS system are available:
single pulse and repetitive. The main differences refer to
the frequency with which the pulses are delivered and
the resultant effects rendered on the brain (for further
details, see Hallet, 2000). Single pulse TMS is considered
to be safe and free of side-effects (George et al., 1999;
Wassermann, 1998), whereas repetitive TMS has the
potential to induce protracted changes in brain function.
Adverse effects following the repetitive form of TMS are rare,
but may potentially include seizures, effects on cognition
and mood, transient auditory threshold shift, and headache
(Wassermann, 1998). Safety guidelines for using TMS have
been outlined in Wasserman (1998).
A range of measures detailing different aspects of
cortical and corticobulbar tract integrity and function can
be obtained by varying TMS stimulation and experimental
This paper introduces a neurophysiological
technique, called transcranial magnetic
stimulation (TMS), which can be used to
non-invasively stimulate the cortex. TMS
provides a means of examining and modifying
cortical function and the central motor
pathways. Operating principles, types of
analyses, and methodological considerations
will be discussed, together with a review of
TMS applications to the study and treatment
of motor speech disorders to date, with a
focus on tongue function.
Transcranial magnetic stimulation (TMS) provides
a unique means of examining the excitability and
integrity of the cortical regions and corticobulbar
tracts controlling the speech musculature. In other words,
it looks at how easily the cortical regions and pathways are
activated and whether they are intact or damaged (see box
1 for a glossary). As such, it has the potential to improve our
understanding of neurogenic disorders of speech, including
dysarthria, apraxia, and stuttering. Although the technique
has been utilised in neurological research for over 20 years,
its application to the investigation and treatment of speech
motor control and its disorders is only in its infancy. The
purpose of the present paper is to provide an introduction
to the technique, including how it works and the types of
analyses that can be performed. Brief reviews of the limited
speech motor control/disorder studies that have been
conducted to date are provided and references for further
reading are included.
Operating principles
TMS is now a common and widely accepted form of brain
stimulation. It utilises Faraday’s principle of electromagnetic
induction and involves holding a flat wire coil against the
head over the cortical region of interest (see figure 1). A brief,
high-intensity current is passed through the coil, creating a
transient magnetic field perpendicular to the plane of the coil.
The magnetic field is able to pass unimpeded and relatively
painlessly through the scalp and skull to the underlying
cortex. This magnetic flux induces a small, localised electric
current in the underlying cortical tissue and can depolarise
(stimulate) the neurons in the immediate vicinity (George,
Lisanby, & Sackeim, 1999; Kobayashi & Pascual-Leone,
Assessing motor speech
disorders using transcranial
magnetic stimulation
Justine V. Goozée, Bruce E. Murdoch, David Lloyd, and Stephan Riek
Box 1. Glossary
Electromyography (EMG): Technique that records
muscle activation
Excitability: How easily a brain region and neural
pathway can be activated
MEP latency: Time from cortical stimulation to when the
MEP occurs
Motor evoked potential (MEP): Muscle contraction
elicited following stimulation of the motor cortex;
recorded using EMG
Neuromodulation: Changing a brain region to make it
more or less excitable
Transcranial magnetic stimulation: Non-invasive
stimulation of the cortex by magnetic fields