Diverse and distributed haemodynamic effects of theta burst stimulation in the prefrontal cortex.

Background: Theta Burst Stimulation (TBS) is a form of non-invasive brain stimulation that can induce neuroplastic changes in the underlying intracortical areas. It has significant potential in clinical and research settings for modulating cognitive and motor performance. Little is known about how TBS affects oxygenations levels within and across brain hemispheres during stimulation of the Dorsolateral Prefrontal Cortex (DLPFC). This study aimed to investigate blood oxygenation levels in the DLPFC during TBS, using concurrent functional Near-Infrared Spectroscopy (fNIRS).

Methods: 44 young adults completed within-subjects 2 × 2 design with 4 conditions that included intermittent TBS (iTBS), and continuous TBS (cTBS) stimulation applied to the left and right DLPFC. FNIRS was recorded concurrently, with 12 optode channels spanning across the left, medial and right prefrontal cortex.

Results: Findings focused on corrected significant effects that revealed clear neurovascular coupling during stimulation. Right hemisphere iTBS stimulation on the DLPFC resulted in excitation within and between hemispheres as expected, however left hemisphere stimulation decreased oxygenation levels both ipsilaterally and contralaterally. CTBS on the right and left hemisphere revealed reductions in HbO as expected in support of previous literature and potential LTD-like effects.

Conclusion: This is the first study to show the extent and dispersion of blood-oxygenation changes in the ipsilateral and contralateral hemispheres during excitatory and inhibitory TBS applied to the DLPFC. The findings demonstrate that TMS stimulation may originate from more global and interhemispheric effects, but that iTBS on the left-DLPFC induces decreases in oxygenated haemoglobin (HbO) providing the potential links for beneficial effects in cognition.