Motor threshold is the measurement that sets the dose for your TMS treatment. Before your first session, the technician positions the coil over the part of your motor cortex that controls the hand, usually the thumb or index finger, and delivers single pulses at gradually increasing intensity. The lowest intensity that produces a visible twitch in about half of the attempts is your motor threshold. It is expressed as a percentage of the machine's maximum output and varies from person to person depending on skull thickness, anatomy and medications.
Once your threshold is known, treatment intensity is set relative to it. For depression protocols, stimulation is typically delivered at 120 percent of motor threshold. Because the dose is anchored to your own physiology rather than a fixed number, two people treated at the same percentage may receive different absolute intensities. The measurement is repeated periodically during a course, often at the start and again a few weeks in, because thresholds can shift as the brain adapts or if your medications change. Some medications, including certain benzodiazepines and anticonvulsants, raise the threshold, which is one reason your psychiatrist reviews your full medication list before TMS begins. Finding the threshold takes about 10 to 15 minutes and feels like a series of taps on the scalp. It is a normal part of the first appointment at our Palm Beach Gardens TMS office.