ACQ Vol 12 no 1 2010

Motor speech disorders

Assessing motor speech disorders using transcranial magnetic stimulation Justine V. Goozée, Bruce E. Murdoch, David Lloyd, and Stephan Riek

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. T ranscranial 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,

Keywords ASSESSMENT

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 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 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

This article has been peer- reviewed CORTEX SPEECH TRANSCRANIAL MAGNETIC STIMULATION TREATMENT

Justine V. Goozée

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ACQ Volume 12, Number 1 2010

www.speechpathologyaustralia.org.au