Tactile Perception and Tolerability Thresholds of TMS Characterized by Intensity across Locations and Frequencies

Transcranial Magnetic Stimulation (TMS) involves pulsed magnetic fields that pass through the scalp to stimulate the brain, with incidental stimulation to superficial nerves and muscles. From a research perspective, the tactile sensations can be a problematic confound, particularly when stimulation approaches an unpleasant or painful level. Additionally, tactile sensations contribute to difficult challenges in establishing an appropriate sham control condition. Clinically, some patients find stimulation uncomfortable or intolerable. Clinicians need data on adjustments to stimulation parameters to improve tolerability and efficacy. The primary objective of this study was to characterize the tolerability of TMS by location (over modified Beam F3 prefrontal, THREE-D prefrontal, right orbitofrontal, medial prefrontal, motor, and parietal cortical targets, as well as the knee) and by frequency (1 Hz, 10 Hz, or iTBS), with increasing levels of stimulation intensity. We also characterized sensory thresholds and qualitative aspects of stimulation across locations and frequencies. For location, sites distal to the facial nerves and muscle (Knee, P3, M1, mPFC) were more tolerable, followed by Beam F3, with the THREE-D and AF8 locations as least tolerable. For frequency, we found that 1 Hz was significantly more tolerable than 10 Hz and iTBS. iTBS was more annoying than 10 Hz but only marginally different in tolerability. TMS researchers and clinicians should understand the impact of sensation based on location and frequency, with increasing stimulation intensity. This is a single-session study in generally healthy individuals, and there is a need for additional data to further inform research and clinical practice.