University of Twente Student Theses


Source localization of brain activity

Dijk, K. van (2012) Source localization of brain activity.

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Abstract:Parkinson's disease is a brain disease in which, in particular, a degeneration of neurons in the substantia nigra compacta occurs. The substantia nigra compacta is a brain region that is connected to the striatum, the main input area of the basal ganglia. A novel treatment method for Parkinson's disease is deep brain stimulation (DBS). An electrode which continuously emits high-frequency stimuli is placed within the subthalamic nucleus (STN), a sub-area of the basal ganglia. It is important that the stimulation electrode is positioned properly within the STN, otherwise side effects may occur. An improvement of the placement of the electrode may be done by means of an electrophysiological map. To be able to use a map for localization, it is necessary to find unique spatial electrophysiological properties within the area. In these electrophysiological signals, one can distinguish the high frequency single unit activity and the low frequency local field potentials (LFPs). In this research, we examined the potential of evoked LFPs in creating a map of the STN. We have done this by using a rat experiment. Previous research showed a relationship between the evoked LFPs and synchronous synaptic activity induced by postsynaptic neuronal populations. Therefore, we stimulated postsynaptic neuronal populations of the STN, the motor cortex and cingulate gyrus, and simultaneously measured the electrophysiological signals within the STN. The motor cortex was stimulated with two different intensities (300μA and 600 μA) while the cingulate gyrus was only stimulated with one intensity (600 μA). Through systematic measurements with a multi-channel electrode, it has been possible to measure the evoked LFPs within a 3D grid of 320 measurement points. The measured LFPs are used to reconstruct the ionic current flows within the extracellular medium induced by the evoked synaptic activity. We describe the ionic currents by the current source density (CSD), which we calculated with the inverse current source density method. The evoked activity within the rat STN after motor cortex stimulation resulted in a promising spatial and temporal behavior of the CSD distribution. However, the evoked activity after cingulate gyrus stimulation did not show this promising behavior. The evoked CSD after motor cortex stimulation shows a local sink around 10 ms post stimulation and a local source around 32 ms post stimulation. In addition, the source is located laterally to the sink. The displacement of the source in respect to the sink suggests that they are not caused by the same synaptic contacts. We believe the sink is evoked through the excitatory mono-synaptic pathway and the source by the inhibitory poly-synaptic pathway. The two stimulation intensities have a different effect on the source and sink. Namely, the strength of the sink was less affected by the reduction of stimulation intensity. This suggests that they are caused by different mechanisms and/or different neuronal populations. Due to the local behavior of the motor cortex evoked CSD distribution, we believe that in the future, the evoked CSD might be used as an electrophysiological map during DBS electrode placing.
Item Type:Essay (Master)
Faculty:EEMCS: Electrical Engineering, Mathematics and Computer Science
Subject:42 biology, 44 medicine
Programme:Biomedical Engineering MSc (66226)
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