ACQ Vol 12 no 1 2010

Figure 1. a) Photograph of a figure-of-eight TMS coil being held against the head over the motor cortex; b) Illustration of the TMS coil, activation of the corticohypoglossal pathway, and the resultant motor evoked potential (MEP) recorded in the tongue using electromyography (EMG)

measures of the latency and size of the motor evoked response (MEP) in the muscle of interest. Prolonged latencies may be indicative of demyelination of the pathways, whereas reduced amplitude responses may be suggestive of a loss of neurons or axons (Kobayashi & Pascual-Leone, 2003). The corticohypoglossal pathways of healthy speakers and speakers with disorders including stroke, amyotrophic lateral sclerosis, myotonic dystrophy, Guillian-Barré syndrome, and brainstem lesions, have been examined through measures of the size and latency of the MEP (e.g., Muellbacher, Mathis, & Hess, 1994; Urban, Hopf, Fleischer, Zorowka, & Müller-Forell, 1997). A noted benefit of TMS has been its utility in identifying early, subclinical upper motor neuron deficits (Pouget et al., 2000). Determining levels of cortical excitability Various measures can be used to determine cortical excitability or responsiveness to stimulation. • Motor threshold: lowest intensity of TMS stimulation required to produce a consistent motor evoked potential (MEP) in the muscle of interest. An increased motor threshold for a given individual indicates reduced excitability (i.e., greater stimulation required to activate the cortical region), whereas a decreased motor threshold indicates increased excitability. • Motor evoked potential (MEP) amplitudes: the size of the electrical potential (in microvolts) recorded in the muscle of interest when the corticobulbar/ spinal pathways are stimulated. Increased amplitudes indicate increased excitability. • Input–output responsiveness curves: This procedure involves taking a set of MEP amplitude recordings (output) at different levels of input (i.e., different voluntary contraction or TMS stimulation levels). Regression curves are applied to the MEP amplitudes, with measures of y-intercept and slope derived, representing sensitivity and gain (i.e., degree of facilitation associated with increased inputs), respectively. TMS has been used to examine tongue motor cortex excitability in healthy speakers on the basis of MEP amplitudes (Fadiga, Craighero, Buccino, & Rizzolatti, 2002) and motor thresholds (Muellbacher, Boroojerdi, Ziemann, & Hallett, 2001). Investigating inhibitory and facilitatory intracortical circuits Intracortical inhibition and facilitation can be studied using TMS presented as a paired-pulse and with silent periods.

parameters. TMS has been described as “a sensitive technique for investigating the corticobulbar tract, which is difficult to study by other methods” (Pouget, Trefouret, & Attarian, 2000, p. 182). It has an advantage over neuroimaging techniques in that it can directly interact with the brain and has a high degree of temporal precision. Neuroimaging techniques are based on measures of regional cerebral blood flow and glucose metabolism and, therefore, can only provide insights into the neural activity that is correlated with a given behaviour but can give no indication of whether the activity is excitatory or inhibitory in nature. Observational neuroimaging techniques also do not provide a means by which alterations in brain activity can be modulated like with TMS. TMS analyses and parameters The types of evaluations and functions that can be performed with TMS are introduced below, together with a review of how TMS has been applied in the study of speech and motor speech disorders to date, with a focus on tongue function. Mapping cortical regions, function and plasticity Single pulse TMS can be used to produce an anatomical map of the motor cortex by recording the sites over the scalp (and underlying cortex), which, when stimulated with TMS, activate the muscles of interest. By mapping the size of the stimulation sites at different time intervals, cortical plasticity associated with the learning of motor skills, disease progression, and/or recovery of function following injury can be examined (George et al., 1999; Pascual-Leone, Grafman, Cohen, Roth, & Hallett, 1997). The site of the tongue motor cortex has been mapped (Rödel, Laskawi, & Markus, 2003) and plasticity of the tongue motor cortex region induced by tongue training tasks in healthy speakers has been investigated (Svensson, Romaniello, Arendt-Neilson, & Sessle, 2003), as have changes in corti- cobulbar pathway organisation and tongue cortical motor maps associated with disease (e.g., unilateral peripheral facial paralysis, Rödel, Tergau, Markus, & Laskawi, 2004) and recovery following stroke (Muellbacher, Artner, & Mamoli, 1999). Evaluating the integrity of the corticobulbar pathways Testing whether the corticobulbar pathways are intact or damaged can be achieved with single pulse TMS through

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ACQ Volume 12, Number 1 2010

ACQ uiring knowledge in speech, language and hearing