Studying cortical excitability with transcranial magnetic stimulation and electroencephalography

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School of Science | Licentiate thesis
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Lääketieteellinen tekniikka
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45 + [17]
Neurons in the human brain constitute a complex dynamic network with inhibitory and excitatory connections; the state of each neuron, characterized by its membrane potential, depends on the net input it receives. The state of a cortical neuronal population is reflected as cortical excitability, measuring the ability of the neurons to respond to stimuli. Transcranial magnetic stimulation (TMS) can be used to deliver external stimuli to the cortex and, combined with electrophysiological measurements of the evoked activity, to probe cortical excitability. Because stimulation of the primary motor cortex (M1) produces an easily measurable response in the peripheral electromyographic (EMG) signal measuring the electrical activity of muscles, TMS was initially mainly applied on M1. The evoked response seen in the EMG signal, called motor evoked potential (MEP), reflects the excitability of the tract leading from M1 through spinal cord to the motor neurons. Later, development of the technically challenging TMS-compatible electroencephalography (EEG) device has enabled studying the reactions of the brain to the stimulation more directly, without contribution of the spinal mechanisms and also following stimulation of other areas than M1, In addition, oscillations seen in the spontaneous EEG signal inform us about cortical excitability; the oscillations reflect alterations in membrane potentials of neuronal populations. Large-amplitude oscillations are generally associated with an idling state of a cortical region. Modulation of TMS-evoked MEPs is related to altered motor cortical excitability in a variety of conditions, such as during tasks occupying the motor system and as a result of neuronal diseases, as well as spontaneously, Because TMS-evoked EEG (TMS-EEG) provides a much more flexible tool for probing cortical excitability but is a relatively new method, it is important to know how cortical excitability as determined with TMS-EEG is related to the Ionger-standing method studying TMS-evoked MEPs. Also, to further clarify if and to which extent the different methods reflect excitability fluctuations in the same neuronal population, the relationship between TMS-evoked MEPs and spontaneous oscillations is of great interest. In this Thesis, the relationship of TMS-evoked MEPs with spontaneous and TMS-evoked EEG responses was studied in resting human subjects. The early components of TMS-evoked EEG responses are correlated with the amplitudes of MEPs indicating that these early components reflect cortical activity following directly the activation of the stimulated site. The observation encourages the use of TMS-EEG in cortical excitability probing. In addition, spontaneous EEG recorded above the stimulated motor cortex and above occipital sites is weakly related to motor cortical excitability as measured with MEP5. The results elucidate how the activity at the stimulated and interconnected cortical areas affects the different measures of cortical excitability and improves the understanding of which aspects of cortical excitability each of the methods reflect. In the future, these methods may prove useful in the diagnosis of some neurological diseases and in various new scientific studies.
Ilmoniemi, Risto
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Ilmoniemi, Risto
cortical excitability, aivokuoren reaktiivisuus, transcranial magnetic stimulation, transkraniaalinen magneettistimulaatio, electroencephalography, elektroenkefalografia
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