Workshop on basic research with Transcranial Magnetic Stimulation (TMS) Macquarie University, Sydney 30th June - 1st July, 2011 ABSTRACTS

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1 Workshop on basic research with Transcranial Magnetic Stimulation (TMS) Macquarie University, Sydney 30th June - 1st July, 2011 TMS: What is it good for? Paul Sowman, Macquarie University ABSTRACTS Transcranial Magnetic Stimulation (TMS) is a neurostimulation method that has been used in research for the past 30 years. During that time a number of different stimulation techniques have been developed that mean TMS has applications for understanding a number of different neuronal processes. This talk will briefly introduce the use of TMS in the study of corticospinal excitability and inhibition, disruption of neuronal processing and virtual lesions, and the induction of plasticity. The behavioural and neurophysiological basis of human theta-burst stimulation Chris Chambers, Cardiff University Human theta-burst stimulation (TBS) is a powerful new protocol for inducing sustained changes in cortical excitability, with an aftereffect lasting minutes. In this talk I will discuss summarise recent applications of TBS in cognitive neuroscience and the current opinion concerning its physiological basis, which appears to lie in modulation of GABAergic and glutamatergic systems. I will also focus on one of our recent studies at Cardiff where we combined TBS with GABAoptimised magnetic resonance spectroscopy (MRS) and magnetoencephalography (MEG) in order to understand a seemingly paradoxical finding: that inhibitory TBS of the visual cortex enhances visual awareness. Improvements in visual sensitivity induced by subthreshold TMS Justin Harris, Colin Clifford, Ehsan Arabzadeh, & Arman Abrahamyan University of Sydney Transcranial magnetic stimulation (TMS) delivered to visual cortex can elicit a fleeting visual percept known as a phosphene. TMS, delivered at intensities above the threshold to induce phosphenes, impairs the detection of visual stimuli. We investigated how the detection of a simple visual stimulus is affected by TMS applied to visual cortex at or below the phosphene threshold. Participants performed a task to detect a plaid stimulus, made from a vertical and a horizontal sinusoidal grating. The contrast of the plaid was varied from trial to trial according to an adaptive staircase procedure. Detection of the plaid was enhanced when a single pulse of TMS was delivered to the contralateral visual cortex 100 or 120 ms after stimulus onset at intensities just below the phosphene threshold. No improvement in visual sensitivity was observed when TMS was applied to the visual cortex in the opposite hemisphere (ipsilateral to the visual stimulus). We conclude that TMS-induced neuronal activity can sum with stimulus-evoked activity to augment visual perception.

2 Accurate and Rapid Estimation of Phosphene Thresholds (REPT) Arman Abrahamyan, University of Sydney To calibrate the intensity of transcranial magnetic stimulation (TMS) at the occipital pole, the phosphene threshold is used as a measure of cortical excitability. The phosphene threshold (PT) refers to the intensity of magnetic stimulation that induces illusory flashes of light (phosphenes) on a proportion of trials. The existing PT estimation procedures lack the accuracy and mathematical rigour of modern threshold estimation methods. We present an improved and automatic procedure for estimating the PT which is based on the well-established Ψ Bayesian adaptive staircase approach. To validate the new procedure, we compared it with another commonly used procedure for estimating the PT. We found that our procedure is more accurate, reliable, and rapid when compared with an existing PT measurement procedure. The new procedure is implemented in Matlab and works automatically with the Magstim Rapid 2 stimulator using a convenient graphical user interface. The Matlab program is freely available for download. Investigating the influence of selective attention on brain plasticity induced by transcranial magnetic stimulation Marc Kamke, University of Queensland Brain plasticity plays a fundamental role in allowing us to adapt to new environments, to learn new skills and to recover from injury to the central nervous system. In the human motor cortex, transcranial magnetic stimulation (TMS) can be used to not only measure plasticity, but also to induce short-term changes in neural excitability that mimic plasticity effects. In this talk I will present research we have undertaken using TMS to investigate the role of selective attention in brain plasticity. We induced plasticity in the hand area of the human motor cortex using TMS and the paired associative stimulation (PAS) procedure, which causes a lasting increase in neural excitability. We found that both the availability of attentional resources and the spatial focus of attention influenced the PASinduced effects. These results suggest that attention is a potent modulator of brain plasticity. The use of TMS to assist in probing functional and structural changes in human cortex following a motor training paradigm. Martin Sale, University of Queensland Several weeks of motor training can cause profound functional and structural changes in human cortex. These changes are thought to reflect the highly plastic nature of the human brain. Several stimulation paradigms have been developed that mimic this use-dependent plasticity. Interestingly, it has recently been shown that the ability to induce plasticity with one such paradigm is influenced by the time of day when stimulation occurs - the brain seems to be more plastic in the evening. I have recently begun a study to determine whether the brain's response to a 4-week motor training paradigm is similarly influenced by the time of day when the training occurs. I will present some preliminary data in which I have used TMS, as well as fmri and diffusion imaging to probe changes in brain function and structure following the training.

3 The importance of scalp-cortex distance in transcranial magnetic stimulation Chris Chambers, Cardiff University Accurate calibration of TMS intensity is crucial to ensure safe and effective levels of brain stimulation. The standard measure of calibrating TMS intensity is motor threshold (MT), determined as the output intensity necessary to induce a threshold-level corticospinal response in the primary motor cortex. However, conventional MT measures do not take into account a simple fact: that distance between scalp and cortex varies considerably between different brain regions. At the same time, TMS effects are strongly dependent on distance, with the induced electric field strength declining sharply according to an inverse square law. We have measured the effect of distance on TMS-induced brain activity by placing perspex spacers between the coil and scalp. From coil-scalp distances of 0-10mm we find that MT increases linearly and reliably by about 2.5% per mm. Measuring this distance-effect gradient allows us to then adjust TMS output in each individual according to the relative depth of different cortical areas. In subsequent experiments we have found comparable functional slopes in the visual and motor cortex; and mathematical modelling of the induced electric field strength shows that the linearity of our empirical data is due to sampling the linear extent of a broader nonlinear function. Overall, our results indicate that consideration of scalp-cortex is important to ensure safe and effective levels of TMS in experimental and clinical applications.

4 POSTERS Ready, Set, STOP Andrew Etchell, Macquarie University Response inhibition is the ability to stop behaviors that are irrelevant, dangerous or no longer appropriate. It is what enables us to stop ourselves from stepping into the path of an oncoming car or what allows us to stop speaking about a person as they enter the room. It has been said that preparation is the key to success. This is all the more important in the case of response inhibition, but it appears very few studies have yet been conducted that have directly investigated the neural strategies involved in preparation to stop. The proposed experiment will use TMS to investigate modulation of motor cortex excitability during response preparation in a modified stop signal task. In this task subjects were presented with a certain go signal which always required the execution of a response, an uncertain go trial that required the execution of a response, and a stop trial, where they were required to withhold a response. The difference in reaction time between he certain go trials and the uncertain go trials is a measure of the degree to which an individual is prepared to inhibit a response and is referred to as the preparation cost. It has been proposed that the optimal strategy to successfully inhibit a response is to prepare to do so. We hypothesise that subjects whose "preparation cost" is smallest will have the greatest suppression of cortical excitability in the early stages of response selection and conversely those that have the largest preparation cost will have the least suppression of cortical excitability in the early stages of response selection. The pattern of activation of the MEPs will elucidate whether individuals are using inhibitory processes to withhold their response or merely maintaining a low level of cortical excitability to facilitate the inhibition of a response. Comparing the MEP responses to the behavioural results will allow us to examine whether a particular neural strategy is associated with optimal behavioural performance. Furthermore, we also intend to replicate this experiment in the vocal domain comparing a population of people who stutter to people who don t stutter to examine the role that preparing to inhibit a response has in stuttering. Deconstructing the attentional tracking network Anina Rich, Todd Horowitz, Mark Williams Macquarie University & Harvard Medical School Howe et al. (2009) suggested that multiple object tracking is subserved by a brain network comprising five primary nodes: MT+, SPL, FEF, anterior IPS (aips), and posterior IPS (pips). In their model, pips represents the location of attended objects, while aips is repsonsible for updating the locations of moving objects. We propose to test this model using TMS. We will ask observers to track moving or stationary targets in either the right or the left hemifield, while we zap aips or pips in the left hemisphere. According to the model, tracking in the ipsilateral hemifield should be unaffected. Zapping pips should disrupt both tasks in the contralateral hemifield. Zapping aips should selectively disrupt tracking in the contralateral hemifield.

5 Neurophysiological effects of CBIT in Tourettes Syndrome Owen Tsao, University of New South Wales Past TMS studies have shown decreased inhibitory mechanisms in the motor cortex in Tourettes syndrome. We aim to correlate this phenomenon to tic severity, and consequently we hope to show that tic improvement through habit reversal therapy can be correlated to an improvement in inhibitory mechanisms in the motor cortex. TMS induced blindsight in normal participants: Reaching for unseen targets Genevieve Quek, Macquarie University Patients with lesions to V1 cortex exhibit residual ability to discriminate and localise stimuli they report no conscious visual perception of a phenomenon termed blindsight. Transcranial Magnetic Stimulation (TMS) can be used to transiently inhibit V1 processing in normal subjects, effectively simulating blindsight. The aim of the proposed project is to demonstrate functional dissociation between vision-for-perception and vision-for-action when TMS applied to the occipital pole prevents V1 processing entirely.