Covert Speech Arrest Induced by rTMS over Both Motor and Nonmotor Left Hemisphere Frontal Sites Lisa Aziz-Zadeh, Luigi Cattaneo, Magali Rochat, and Giacomo Rizzolatti Abstract & Blocking the capacity to speak aloud (overt speech arrest, SA) may be induced by repetitive transcranial magnetic stimulation (rTMS). The possibility, however, of blocking internal speech (covert SA) has not been explored. To investigate this issue, we conducted two rTMS experiments. In the first experiment, we stimulated two left frontal lobe sites. The first was a motor site (left posterior site) and the second was a nonmotor site located in correspondence to the posterior part of the inferior frontal gyrus (IFG) (left anterior site). The corresponding right hemisphere nonmotor SA site was stimulated as a control. In the second experiment, we focused on the right hemisphere and stimulated a right hemisphere motor site (right posterior site), and, as control sites, a right hemisphere nonmotor site corresponding to the IFG (right anterior site) and a left hemisphere anteromedial site (left control). For both experiments, participants per- formed a syllable counting task both covertly and overtly for each stimulation site. Longer latencies in this task imply the occurrence of an overt and/or covert SA. All participants showed significantly longer latencies when stimulation was either over the left posterior or the left anterior site, as compared with the right hemisphere site (Experiment 1). This result was observed for the overt and covert speech task alike. During stimulation of the posterior right hemisphere site, a dissociation for overt and covert speech was observed. An overt SA was observed but there was no evidence for a covert SA (Experiment 2). Taken together, the results show that rTMS can induce a covert SA when applied to areas over the brain that are pertinent to language. Furthermore, both the left posterior/motor site and the left anterior/IFG site appear to be essential to language elaboration even when motor output is not required. & INTRODUCTION Several laboratories have now documented overt speech arrests (SAs), as evoked by repetitive transcranial mag- netic stimulation (rTMS) (Stewart, Walsh, Frith, & Roth- well, 2001; Epstein, 1998; Pascual-Leone, Gates, & Dhuna, 1991). This phenomenon is most commonly described as a slowing and distortion of speech, an inability to get words out, and an inability to formulate words. It generally occurs for the time period of the stimulation (commonly a few seconds) and, subse- quently, speech is immediately recovered. There has been some discussion about whether the SA evoked by rTMS is related to an interference with language processing or is, in fact, due to motor interfer- ence determined by cortical stimulation of orofacial muscles. In a review by Epstein (1998), the need to distinguish between different types of SAs was discussed. It was noted that, at the time, the most common form of SA was a motor one induced by interfering with motor areas involved with muscular control of speech. Sup- porting this interpretation was the observation that SA could also be obtained with right hemisphere stimula- tion. However, on the basis of data obtained from stimulating the left and right hemisphere in epileptic patients for language dominant hemisphere assessment, Pascual-Leone et al. (1991) concluded that TMS-induced SAs may result from a disruption of language processing, and not only derive from motor interference. Since then, Stewart et al. (2001) reported two differ- ent types of overt SAs induced by rTMS over two different left hemisphere sites. The first type, called ‘‘motor’’ SA, was determined by stimulation of a ‘‘pos- terior’’ site located in the motor area of the frontal lobe, and was associated with EMG activity evoked in lower facial muscles, through activation of the cortico-bulbar pathway. The second one, called a ‘‘nonmotor’’ SA, was caused by stimulation of an ‘‘anterior’’ site located in the posterior part of the inferior frontal gyrus (IFG) and was not associated with EMG activity in orofacial muscles. The ‘‘nonmotor’’ SA was thought to result from stimu- lation of Broca’s area. The two SA sites were further distinguished by their differing abilities to produce an SA following stimulation of the right hemisphere: The site corresponding to the left hemisphere posterior site produced an SA, whereas the site corresponding to Universita ` di Parma, Italy D 2005 Massachusetts Institute of Technology Journal of Cognitive Neuroscience 17:6, pp. 928–938