Abstract
Transcranial Magnetic Stimulation (TMS) is a noninvasive neuromodulation technique commonly utilized in brain research and clinical applications. However, it has been debated that some of the TMS evoked responses can be the result of peripheral effects rather than direct neural activation. To this end, we recorded brain activity from two anesthetized monkeys (subjects W+H) with three implanted electrodes projecting from visual area to frontal eye field, auditory and temporal regions in the left hemisphere. Biphasic single-pulse TMS was delivered over the left hemisphere to four (in W) and five (in H) locations. We stimulated a lateral location on the right hemisphere as the location control. Control protocols for auditory click (in W+H), auditory masking and somatosensory electric stimulation (in H) were also executed. Stimulation intensities were set to 10, 25, 50, 70 and 90% for W+H with additional power mode setting at 126% of the maximum stimulator output for H. We preprocessed the neural data by removing noisy channels, TMS and muscle artifacts, interpolating, down-sampling to 1 kHz and bandpass filtering at 1-50 Hz. For analysis, we performed time-locked averaging across trials. We observed a negative deflection occurring 50 ms after TMS delivery (N50) in local field potentials (LFPs). This component showed a dose-dependency in several stimulation locations in W+H with lateral stimulation showing a logistic response in the tested range of intensities. Stimulation of frontal locations in W+H showed an inverse relationship of N50 with coil-contact distance (ρ W =0.81, ρ H =0.52). LFPs from auditory control revealed responses confined to auditory region. LFPs from somatosensory control did not elicit a response comparable to active TMS. Auditory masking did not alter the response in non-auditory contacts significantly. However, decreased N50 response was observed in auditory contacts. In conclusion, we provide evidence that direct and peripheral effects are distinct.