Effects of combined transcranial stimulation interventions of the prefrontal cortex: a factorial, randomized, double-blind, sham-controlled trial

Background: Non-invasive brain stimulation (NIBS) approaches have been increasingly used to target the prefrontal cortex (PFC), with mixed results in the fields of cognitive and behavioral neuroscience. Recent studies suggest that the combination of distinct NIBS techniques may maximize required changes in brain activity. However, there is insufficient scientific knowledge regarding the combination of different NIBS targeting the PFC. Aim: To evaluate the application of standalone and combined protocols of transcranial direct current stimulation (tDCS) and intermittent theta-burst stimulation (iTBS) over the left dorsolateral PFC (DLPFC) of healthy volunteers through neuroimaging and cognitive measures. The brain regions of interest for the neuroimaging outcome were the PFC and its sub-regions, the cingulate cortex, insula and the temporal cortex. Methods: A factorial, randomized, double-blind, sham-controlled design was conducted. Participants received four different stimulation protocols (tDCS, iTBS, Combined Interventions and Placebo), once a week, over the DLPFC located by structural neuronavigation. TDCS was applied with a current of 2mA for 20 minutes and iTBS with 1620 pulses for 9 minutes. TDCS was applied alone for eleven minutes and concomitantly applied with iTBS for the remaining nine minutes. A radiopharmaceutical (99mTc-ethyl cysteinate dimer - 99mTC-ECD) was administered immediately after the start of the iTBS protocol. An adverse effects scale and the working memory test (nback) were applied right after the end of the neurostimulation session, while the Single Photon Emission Computed Tomography (SPECT) was performed within 45 minutes after the end of the neurostimulation session. Results: Twenty-five adults with a mean age of 28.6 years (standard deviation (SD) = 7) were included. Of those, 23 underwent 4 NIBS sessions while 2 of them underwent only the first session, with a total of 94 sessions being performed. The neuroimaging findings showed that the combined intervention significantly modulated deeper regions of the PFC, such as the left anterior (p=0.03) and the right posterior cingulate cortex (p=0.02), while the tDCS protocol increased the blood flow of the orbitofrontal right (p=0.02) and left cortex (p=0.04) in comparison with the placebo protocol. The comparison among active protocols showed that tDCS increased the activation in the right orbitofrontal cortex only over iTBS (p=0.02). In comparison to the tDCS protocol, iTBS increased blood flow in the superior medial frontal cortex (p=0.006) and both iTBS and combined interventions increase activation in the left insula (p=0.03). Finally, combined interventions were superior to the iTBS in the left temporal cortex (p=0.04). The cognition findings were conducted only with the subset of the first session and showed significant reaction time among protocols (p=0.02). Due to the small sample size, multiple comparisons analysis was not conducted. However, factorial analysis showed iTBS main effects, but not the tDCS main effects, significantly influencing the reaction time. Finally, all active protocols were similarly tolerable. Conclusion: The findings of this study encourage future studies: 1) for the application of the combined protocol in phase-II studies with a psychiatric population; 2) broader investigation of the effects of the iTBS protocol on cognitive performance; 3) conducting studies exploring the variability of tDCS response (AU)