Optimum stimulus parameters for lateralized suppression of speech with magnetic brain stimulation

Rapid-rate transcranial magnetic brain stimulation produces lateralized suppression of speech output over the frontal lobe, consistent with cerebral dominance for language. But the sensitivity of magnetic speech localization has been limited, and reports are imprecise concerning the amount of discomfort involved. Using a focal magnetic coil, we evaluated the effectiveness and pain of stimulation at different intensities, orientations, and repetition rates (2 to 32 Hz) in six normal volunteers. We obtained complete and clearly lateralized speech arrest in all subjects. The best ratio of efficacy to pain occurred using slower repetition rates of 4 to 8 Hz with a horizontal alignment of the induced electric field. Lower stimulation frequency also allowed clearer distinction between speech arrest and dysarthria from tonic contraction of cranial muscles. The relative comfort and safety of stimulation at 4 Hz should allow more widespread use of magnetic speech localization in clinical and research applications.