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. This study characterizes the scalp sensations of TMS by location (over modified BeamF3 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.

Study participants (n = 18) each provided ratings for 7 locations, with 3 frequencies per location. We adapted quantitative sensory testing method of limits, where participants identified the sensory threshold, the annoyance threshold, and the tolerability threshold, for each trial as TMS intensity increased.

For location, sites distal to the facial nerves and muscle (Knee, P3, M1, mPFC) were more tolerable, followed by BeamF3, 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.

The sensation and tolerability of TMS is dependent on stimulation location, and frequency, with increasing stimulation intensity.

From a research perspective, the tactile sensations can be a problematic confound, particularly when stimulation approaches an unpleasant or painful level. Clinically, providers need data on adjustments to stimulation parameters to improve tolerability and efficacy. This study provides TMS researchers and clinicians with empirical data to inform respective endeavors.