Functional localization of the frontal language area using speech arrest induced by low intensity transcranial magnetic stimulation

Objective:To investigate a modified method of speech arrest (SA) induced by transcranial magnetic stimulation (TMS) for non-invasive functional localization of the language-related area in the inferior frontal gyrus (IFG).Methods:Instead of the conventional overlearned speech production, we used reading of an unfamiliar manuscript as the speech task. While participants were reading the manuscript aloud, 5-Hz TMS pulse sequences were applied for 2 seconds at random times. The stimulation site was monitored using a real-time navigation system with the participants’ pre-obtained structural MRI images.Results:We identified a SA hot-spot in the left IFG (Broca's area) in 24 out of 28 subjects with a lower TMS intensity (88% of active motor threshold) than that used in previous reports. In 5 subjects, right IFG stimulation also induced speech distortion. Conclusion:Using a speech task involving reading an unfamiliar manuscript, we successfully induced SA with a lower TMS intensity than those previously reported. This low-intensity stimulation enabled a reduction of not only participant discomfort, but also the contaminative effects of stimulation.