The Effects of Transcranial Magnetic Stimulation (TMS) on the Neural Activity of Awake Non-Human Primates

Transcranial magnetic stimulation, or TMS, is a non-invasive stimulation method which induces an electric field in the brain. For the past two decades it has been used extensively in clinical and research settings for basic research and treatment studies of neurological disorders such as depression. Despite its widespread use and established safety, the mechanism of effect for the stimulation is still poorly understood. The goal of this project is to study the effect of single pulse TMS on single neurons in awake rhesus macaques. By using the modified electronics developed by Mueller et al. (2014), we were able to minimize the duration of the stimulus artifact in the recordings down to a few milliseconds, allowing us to capture and characterize the neural activity in the frontal eye field (FEF) and primary motor cortex (M1) immediately following a TMS pulse. We found that the intracranial electric field induced by TMS has a variety of effects on individual neurons but a distinct pattern of effect on the population activity: a short-latency excitation (<20ms latency) followed by a longer-lasting inhibition (for ~100 ms). These effects were absent in Sham TMS treatments. Our single pulse TMS protocol caused no long term effects on neural activity, but repetitive TMS (rTMS) protocols of 1 and 5 Hz changed spontaneous firing rates. The outcome of this work is to demonstrate empirically how TMS affects neurons in the primate brain.