LETTER TO THE EDITOR Probabilistic map of language regions: challenge and implication

Sir, We have read the article entitled ‘Probabilistic map of critical functional regions of the human cerebral cortex: Broca’s area revisited’ by Tate et al. (2014) with great interest. In that article, the authors presented a bilateral probabilistic map for the essential functions by integrating the data from the ‘gold standard’ for mapping brain functions (direct cortical electrostimulation) in a large set of patients. In particular, the authors’ findings challenged two aspects of the classical theories of brain organization. First, their data added new and more direct evidence for the theory that the ventral premotor cortex (PMC), not the classical Broca’s area (Bizzi et al., 2012), is the speech output region. Second, these authors reported that the crucial epicentres for speech output were not lateralized to the dominant hemispheres. A bilateral cortical requirement for speech output was demonstrated in this article. These results have major implications for brain surgery, and this conclusion has the potential to change neurosurgeons’ traditional surgical philosophy. Tate et al. (2014) observed that the region of interest-based probability of anarthria/arrest (speech output) in the ventral premotor cortex of the left hemisphere is 83%, whereas this probability is only 4% for the pars opercularis. Thus, the authors concluded that the bilateral ventral PMC served as the final common pathway for speech output and that Broca’s area was only involved in the higher-order aspects of language. This result is slightly different from our personal experience; therefore, we applied a similar method to perform a probability analysis for the ventral PMC and pars opercularis using our data from 69 Chinese-speaking patients. The only methodological distinction is that we used a combination of navigation and intraoperative imaging to project the stimulation sites to the standard brain instead of using landmarks only. Our probability calculations suggested a 79% probability of anarthria/arrest with stimulation of the left ventral PMC, which is similar to the results (83%) obtained by Tate et al. (2014). However, the probability of speech arrest with the left pars opercularis is clearly higher in our series (32% compared with 4%) than that in Tate et al. (2014), indicating that the left pars opercularis still has important functions for speech production. In a recent study with a larger sample size (250 patients), Sanai et al. (2008) reported that the largest percentage of total stimulations at the sites in the pars opercularis is 26.7% (the percentage in the ventral PMC is 30.3%), further supporting our hypothesis. In addition, arcuate fascicle fibre tracking data indicated that the majority of streamlines of the arcuate fasciculus projected into the ventral PMC and that a minority of streamlines reached the pars opercularis (Henning Stieglitz et al., 2012); these observations provided the structural basis for our inference because the arcuate fasciculus and the superior longitudinal fasciculus were suggested to support sensory-motor mapping of sound-to-articulation (Hickok and Poeppel, 2007). To estimate the reliability of the structure basis, we also reconstructed the arcuate fascicle/superior longitudinal fasciculus (Catani et al., 2005) in 18 right-handed healthy subjects and in 34 right-handed patients (from the above-mentioned 69 patients) with intraoperative electrostimulation of the ventral PMC and the pars opercularis. In 14 (78%) of the healthy subjects, the frontal terminal territories of the arcuate doi:10.1093/brain/awu247 Brain 2015: 138; 1–3 | e337