TMS and EEG in the study of human brain dynamics

Abstract

The studies that comprise the basis of this Thesis intended to detect specific brain responses, known as event-related potentials (ERP), and changes in the cortical rhythms through the utilization of electroencephalography (EEG). In addition, transcranial magnetic stimulation (mTMS) was performed with a multichannel transducer to evaluate cortical excitability and connectivity for many locations and orientations. The results revealed novel aspects of the functioning network of the brain.

Study I proves that detection of deviant visual stimuli is possible based on the amplitude modulation of the α-band oscillations. This was manifested in deviant stimuli inducing stronger synchronization and desynchronization of the α-rhythm, as compared to standard stimuli.Study II presents the principles of the electric field (E-field) electronic rotation using an mTMS transducer. The study quantifies the role of the E-field orientation for neuronal activation through the changes of amplitude and latencies in motor evoked potentials (MEP) recorded from hand muscles.

The sensitivity of specific neuronal populations to the E-field orientation was demonstrated based on changes in inhibition and facilitation of the amplitude of MEP induced by different orientations of the stimulus during paired-pulse stimulation. Studies III and IV investigate the role of conditioning and test stimuli in short-interval intracortical inhibition (SICI), intracortical facilitation (ICF), and short-interval intracortical facilitation (SICF). The studies show that the sensitivity of these phenomena depends on mechanisms related to specific stimulus intensities, orientations, and interstimulus intervals.

Finally, Study V explores the role of the E-field orientation in generating mismatch negativity (MMN)-like responses. Stimulation of the same area of the motor cortex with slightly differently oriented stimuli resulted in stronger brain responses induced by rare deviant stimulus than by standard stimulus. The presence of TMS-induced MMN suggests that the brain has the ability to automatically detect deviant stimuli. Due to technical limitations, Study V has not been published.

This thesis introduces novel methods and instrumentation for human brain investigation. The results demonstrate that these methods can elucidate new aspects of brain functioning, allowing for their further application in contemporary scientific research and clinical practice.