Pre-TMS Phase and Power of Ongoing EEG Oscillations Modulates Cortical Activity Response at the Dorsolateral Prefrontal Cortex

Aiyush Bansal (1,2), Reza Zomorrodi (1), Itay Hadas (1), Willy Wong (2, 3), Robert Chen, Jonathan Downard (4), Pantelis Loumis (1), Daniel M. Blumberger (1, 4), Zafiris J. Daskalakis (1)

1) Temerty Centre for Therapeutic Brain Intervention, Centre for Addiction and Mental Health, 2) Institute of Biomaterials and Biomedical Engineering, University of Toronto, 3) Department of Electrical and Computer Engineering, University of Toronto, 4) Institute of Medical Sciences, University of Toronto

Background

The timing of transcranial magnetic stimulation (TMS) pulses can be locked to specific phases of cortical activity which may lead to more effective cortical activation. Precise physiological timing may facilitate plasticity and potentially improve clinical outcome with rTMS treatment. Previous studies from the motor cortex have shown that targeting the trough of the alpha phase with TMS pulses leads to higher motor-evoked potential (MEP) amplitudes than random phases. However, there are no similar studies evaluated at the M1 and dorsolateral prefrontal cortex (DLPFC) through TMS-EEG. This study endeavored to evaluate this relationship.

Objective/Hypothesis:

To establish the relationship between pre-TMS phase, power, and transcranial evoked potential (TEP) at the M1 and DLPFC. Our hypothesis is that TMS pulses during the peak or trough of different phase frequency bands will lead to a greater TEP amplitude compared to random phases.

Methods

Current Source Density (CSD) analysis of 19 healthy subjects that underwent single pulse TMS delivered at the M1 and DLPFC. Four frequency bands were analyzed - alpha (8-13 Hz), beta (14-30 Hz), theta (4-7 Hz), and delta (1-3 Hz). The effect of TEP when pre-TMS phase is at peak (90±15°) and trough (-90±15°) is compared to random phase while using pre-TMS power as a covariate.

Results

A strong positive correlation was found between pre-TMS power and TEP with the beta band showing the highest correlation with AUC25-250 (r>0.8, p<.005) at M1 and DLPFC. Further analysis showed that TMS pulses during the peak of beta phase, potentiated AUC25-250.

Conclusion

The results of this study validate that pre-TMS power has a strong correlation with TEP which may guide further research in development of closed-loop TMS models to effectively target the DLPFC.