Neuromodulatory Enhancement of BCI Critical EEG Features: Site Specific iTBS Effects on Movement Related Cortical Activity

Abstract

Self-paced voluntary movement generation induces a complex cascade of activity in the brain to coordinate and execute the desired movement. This activity can be observed as the movement-related cortical potential (MRCP) in the time domain, and the event-related spectral perturbation (ERSP) in the time-frequency domain. The MRCP is generated primarily by the supplementary motor area (SMA) and primary motor cortex (M1) during self-paced movement. Similar networks drive the ERSP leading to event-related desynchronization (ERD) and synchronization (ERS) during movement preparation and execution. Brain computer interfaces (BCIs) leveraging the MRCP and ERSP have emerged as a therapeutic option for supporting neurorehabilitation. These devices rely on high detection accuracy to function effectively, a trait hampered by the inherently noisy data. While research has considered various methods to improve detection accuracy, limited work has investigated methods which improve the saliency of the signal itself. Transcranial magnetic stimulation offers non-invasive methods which transiently modulate the brain. Intermittent theta burst stimulation (iTBS) is one such method which increases cortical excitability in a targeted region of the cortex.

This thesis proposes the application of iTBS to enhance the neural correlates of movement preparation and execution to ultimately improve the signal-to-noise ratio within the MRCP and ERSP. Different locations of iTBS stimulation were investigated targeting the primary generators of these signals to maximize the induced change. Three experiments were conducted differentiated by the location of iTBS stimulation, over M1, SMA, and both M1 and SMA sequentially. In each experiment, participants performed a self-paced ballistic gripping task before and after receiving the intervention. The MRCP was analyzed as mean amplitudes of four components: the readiness potential (RP), negative slope (NS′), motor potential (MP), and peak negativity (PN). In the frequency domain, ERSP was investigated through mean amplitudes of ERD/ERS in alpha and beta bands both before and after movement onset. Time windows of interest in the pre-movement phase were selected to align with MRCP components. Post-movement ERS was quantified over two windows of time to reflect early and late ERS.

Results indicated that iTBS significantly modulated the MRCP and ERSP across the three experiments. Within the MRCP, the NS′ and MP were enhanced after sequential M1 + SMA iTBS and M1 iTBS, respectively. The difference between these outcomes suggests that sequential iTBS preferentially modulated intracortical connections between SMA and M1. Surprisingly, SMA iTBS suppressed the PN while not significantly modulating any other component. While the RP was not modulated across any of the experiments, beta ERD was amplified following SMA iTBS in this pre-movement time window. However, this effect was inverted after sequential iTBS which induced a reduction in beta ERD during the pre-movement period. Both alpha and beta bands following movement onset were facilitated by sequential iTBS while M1 iTBS trended towards facilitation of only beta band ERS. SNR of the MRCP was significantly enhanced by sequential iTBS but not following single site iTBS at either site. Conversely, alpha and beta SNR were not significantly adjusted by any of the interventions. Lastly, time to maximal effects differed between the experiments such that iTBS protocols involving SMA exhibited their strongest effects thirty minutes after stimulation compared to M1 iTBS which was most prominent ten minutes after stimulation.

The findings underscore the influence iTBS has upon the neural correlates of movement preparation. Single- and multi-site iTBS significantly increased the size of the various components of the MRCP and the ERSP. Importantly, the interventions involving SMA iTBS are capable of modulating early movement preparation processes, a necessary trait to benefit many neurorehabilitation approaches. The ability of iTBS to modulate neural signatures of motor planning positions itself as a promising avenue for improving signal salience and ultimately BCI performance. Interestingly, the findings suggest potential advantages to each iTBS protocol rather than a generalized “one-size-fits-all” protocol. For example, BCIs operating in the frequency domain may find superior performance with the early beta ERD facilitation induced by SMA iTBS. Conversely, sequential iTBS may benefit MRCP-based BCI due to robust facilitation of the NS′. By examining three novel applications of iTBS, this thesis demonstrates that iTBS can modulate neural mechanisms underlying movement preparation, thereby advancing understanding of its potential for BCI and, by extension, neurorehabilitation.