Neural effects of theta-burst Transcranial Magnetic Stimulation on single neurons in macaque parietal cortex

Theta-burst transcranial magnetic stimulation (TBS) has recently received much interest because it can be used to up- or downregulate brain activity non-invasively for up to an hour, by a mechanism thought to rely on synaptic plasticity. However, a major issue with TBS is that its behavioral effects are highly variable. A better understanding of TBS effects on different populations of neurons is currently needed to improve its reproducibility. Recently we have recorded single-unit activity in parietal cortex during and after single-pulse transcranial magnetic stimulation (TMS, Romero et al. Soc Neurosci abstract 2016) and found that single-pulse TMS (120% of resting motor threshold) induces a short-latency burst of action potentials in parietal neurons lasting on average 80 ms. Here, we measured the after-effects of continuous TBS (cTBS, 300 pulses) on the TMS-induced burst and spontaneous neuronal activity. Guided by anatomical MRI, we recorded single-cell activity in parietal area PFG in one macaque monkey during passive fixation of real-world objects. We compared trials with and without cTBS (60% of resting motor threshold). In each recording session, we first searched for well isolated single neurons, then ensured we could record stable TMS-evoked bursts before cTBS. If a TMS-evoked response was found we proceeded with applying cTBS, after which we immediately resumed the single-pulse TMS protocol to assess changes in excitability up to 50 minutes after the start of cTBS. On average, cTBS caused a robust reduction (29%, p30 min). In addition, we found a significant reduction in the average spontaneous activity (31 %, p