Neural effects of continuous theta-burst stimulation in macaque parietal neurons

Theta-burst transcranial magnetic stimulation (TBS) has become a standard non-invasive technique to induce offline changes in cortical excitability in human volunteers. Yet, TBS suffers from a high variability across subjects. A better knowledge about how TBS affects neural activity in vivo could uncover its mechanisms of action and ultimately allow its mainstream use in basic science and clinical applications. To address this issue, we applied continuous TBS (cTBS, 300 pulses) in awake behaving rhesus monkeys and quantified its after-effects on neuronal activity. Overall, we observed a pronounced, long-lasting and highly reproducible reduction in neuronal excitability after cTBS in individual parietal neurons, with some neurons also exhibiting periods of hyperexcitability during the recovery phase. These results provide the first experimental evidence of the effects of cTBS on single neurons in awake behaving monkeys, shedding new light on the reasons underlying cTBS variability.

Methods
We recorded single-unit activity in PFG using tungsten microelectrodes (impedance: 1 MΩ at 1 kHz; FHC) inserted through the dura by means of a 23-gauge stainless steel guide tube and a hydraulic microdrive (FHC, USA). We combined these recordings with Transcranial Magnetic Stimulation (TMS) using two different protocols: single-pulse TMS (sTMS) and continuous Theta-Burst Stimulation (cTBS). We performed extracellular recordings before and after cTBS to investigate the changes in neuronal excitability, assessed with sTMS during passive fixation.