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Session: Neurophysiology: Clinical Neurophysiology
Session starts: Thursday 24 January, 13:30
Presentation starts: 14:30
Room: Lamoraalzaal

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Arno Janssen (Radboud University Nijmegen Medical Centre, Donders Institute for Brain, Cognition and Behaviour, Department of Neurology and Clinical Neurophysiology, Nijmegen, the Netherlands)
Dick Stegeman (Radboud University Nijmegen Medical Centre, Donders Institute for Brain, Cognition and Behaviour, Department of Neurology and Clinical Neurophysiology, Nijmegen, the Netherlands)
Thom Oostendorp (Radboud University Nijmegen Medical Centre, Donders Institute for Brain, Cognition and Behaviour, Department of Cognitive Neuroscience, Nijmegen, the Netherlands)

Abstract:
Transcranial magnetic stimulation (TMS) is a non-invasive technique that is used in a wide range of neurophysiologic and clinical studies to measure or change the excitability of specific brain areas. A very high and brief current is send through a coil that causes a time-varying magnetic field. This time-varying magnetic field in its turn induces an electric field in the human head as described with Faradays law of induction, which may generate neural excitation. Many cortical areas have been studied with TMS, but most of the protocols are developed for the motor cortex (M1). When M1 is stimulated a motor evoked potential (MEP) can be measured with the use of electromyography (EMG). The minimal intensity needed to evoke an MEP is called the motor threshold (MT). Most of the cortical areas outside M1 do not have a similar outcome measure like the MEP. The stimulation intensity used for these areas is commonly based on the MT in M1. There are, however, inter-individual differences in brain anatomy and physiology between cortical areas. Therefore, the intensities used for the cortical areas outside M1 are probably sub-optimal. To optimize the TMS induced effects, the inter-individual differences between M1 and other cortical areas should probably be included in the determination of the stimulation intensity. A way to verify if these inter-individual differences have an effect on the electric field is by using TMS simulations. In this study the finite element method (FEM) was used to calculate the induced electric field in a realistic head model based on geometry and conductivity, acquired from Magnetic Resonance Imaging (MRI) and Diffusion Tensor Imaging (DTI) measurements. The electric field was calculated for 15 cortical locations, which were determined based on multiple experimental studies. The results show that the magnitude of the induced electric field differs between cortical locations, as expected. The distance between the cortical location and the coil has the most prominent effect on the electric field magnitude. In addition the local anatomy and conductivity have an influence. The results indicate that an increase in intensity for cortical areas more distant from the coil (for example cerebellum) is needed to induce a similar electric field magnitude as over M1. However, a decrease in stimulation intensity for cortical areas closer to the coil would not be advisable.