ACQ Vol 12 no 1 2010

fundamental frequency and voice intensity in 30 patients with Parkinson’s disease were noted after just a single session of rTMS (Dias et al., 2006). Ingham, Fox, Ingham, Collins, and Pridgen (2000) delivered slow rTMS (1Hz) to the right supplementary motor area (SMA) of five persons who stuttered for 20 minutes per day for 10 consecutive days to reduce the SMA overactivation that was observed by neuroimaging. One of the persons who stuttered showed a reduction in stuttering approximately one month following the TMS program, with this reduction sustained for at least five months. No behavioural changes were noted for the remaining persons who stuttered. Limitations of TMS and methodological considerations There are a number of limitations of TMS and methodological considerations that need to be considered when planning a TMS study. First, the magnetic field generated by TMS stimulates to a depth of only approximately 2cm below the cortical surface (George et al., 1999) limiting the brain regions that can be stimulated to those on the cortical surface, rather than deep brain structures. Second, TMS typically indirectly stimulates pyramidal neurons as it preferentially stimulates those neurons that are parallel to the cortical surface. These preferentially stimulated neurons are believed to be mostly interneurons, which indirectly and trans-synaptically activate the pyramidal neurons (Pascual- Leone et al., 1998). Thirdly, MEP recordings need to be made from stationary muscles and, hence, speech tasks cannot be utilised. Tasks that require participants to imagine speaking could be utilised instead, however, as neuroimaging has shown that brain activations during imagined speech resemble those during overt speech (Ingham et al., 2000). Finally, the optimal TMS parameter combinations to be used for neuromodulatory treatment (i.e., frequency and number of TMS pulses and sessions) are not yet well understood and require further study. Conclusion The present review illustrates how TMS can be used as an adjunct to neuroimaging and other neurophysiological techniques to investigate potential neural disturbances underlying motor speech disorders. By using TMS in future studies of motor speech disorders it is anticipated that not only will our understanding of the neurophysiological underpinnings be better informed, but the options and efficacy of treatment that can be offered will also improve. References Dias, A.E., Barbosa, E. R., Coracini, K., Maia, F., Marcolin, M. A., & Fregni, F. (2006). Effects of repetitive transcranial magnetic stimulation on voice and speech in Parkinson’s disease. Acta Neurologica Scandinavica , 113 , 92–99. Fadiga. L, Craighero, L., Buccino, G., & Rizzolatti, G. (2002). Speech listening specifically modulates the excitability of tongue muscles: A TMS study. European Journal of Neuroscience , 15 , 399–402. George, M. S., Lisanby, S. H., & Sackeim, H. A. (1999). Transcranial magnetic stimulation: Applications in neuropsychiatry. Archives of General Psychiatry , 56(4), 300–311. Hallett, M. (2000). Transcranial magnetic stimualation and the human brain. Nature , 406 , 147–150.

Paired-pulse TMS involves delivering a subthreshold conditioning TMS pulse that activates cortical neurons but that is too small to result in any descending spinal cord activation, followed by a second, test pulse (Hallett, 2000). The stimulus intensity and the interstimulus interval between conditioning and test pulses appear to affect which intracortical circuits are activated (i.e., facilitatory or inhibitory), which, in turn, affects the MEP amplitudes recorded following the test stimulus (Kobayashi & Pascual- Leone, 2003). Typically, interstimulus intervals of less than 4ms induce inhibition, while interstimulus intervals of 5ms to 30ms induce facilitation (George et al., 1999; Kobayashi & Pascual-Leone, 2003). Electromyographic silent periods refer to the period in which EMG activity is suppressed in a voluntarily contracted muscle following suprathreshold TMS stimulation. The initial portion of the silent period is believed to be due in part to spinal cord refractoriness, with the latter portion due to cortical inhibition (Hallett, 2000). Intracortical inhibition and facilitation of the tongue motor cortex has been studied using cortical silent periods (Katayama et al., 2001) and paired-pulse TMS (Muellbacher et al., 2001). Sommer, Wischer, Teragau, and Paulus (2003) used single pulse and paired-pulse TMS to investigate excitability and intracortical inhibition and facilitation of the dominant hand motor cortex in a group of 18 right-handed speakers with developmental stuttering. Their study was driven by a proposal that persistent developmental stuttering is a task-specific dystonia characterised by reduced intracortical inhibition. Interestingly, intracortical inhibition and facilitation were found to be normal, while motor thresholds were increased, suggestive of reduced motor cortical excitability. Inducing neuromodulation Repetitive TMS (rTMS) involves delivering trains of TMS pulses ( ≥ 1 Hz or equal to or greater than one per second), which summate to effect temporary neural modulation. This modulation can comprise inhibition or facilitation of cortical excitability depending on stimulation intensity, frequency, and duration (Pascual-Leone et al., 1998). Slow rTMS in the 1Hz frequency range has been found to transiently decrease excitability, while rapid rTMS, at frequencies of 5Hz and higher, looks to transiently increase excitability (Hallett, 2000; Kobayasi & Pascual-Leone, 2003). In addition to investigating behavioural effects (e.g., reduction or increase in stuttering) induced by TMS neuromodulation, rTMS can also be used to temporally disrupt neural activity, thereby creating a temporary virtual lesion, to evaluate a cortical region’s function (Hallett, 2000). TMS applications in motor speech treatment The application of TMS in the treatment of motor speech disorders is a growing, yet at present, a limited and under- developed field. Treatment protocols have involved the use of rTMS on the basis of its capability in effecting neuromodulation and have been found to be successful in regulating cortical function and normalising the balance of inter-hemispheric excitability with resultant changes in behaviour in a range of disorders, including depression (e.g., Pascual-Leone, Rubio, Pallardo, & Catalá, 1996), motor dysfunction and aphasia associated with stroke (e.g., Martin et al., 2004), and motor and vocal function in Parkinson’s disease (e.g., Dias et al., 2006). Significant improvements in

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ACQ Volume 12, Number 1 2010

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