A common language network for comprehension and production: a contribution to the definition of language epicenters with PET

In this paper, we report on a PET activation study designed to assess whether functional neuroimaging would help to uncover essential language areas in normal volunteers and to provide a more accurate definition of their localization. Regional cerebral blood flow was repeatedly monitored in eight right-handed male volunteers, while performing a language comprehension task (listening to factual stories) and a language production task (covert generation of verbs semantically related to heard nouns), using silent resting as a control condition. The conjunction analysis, conducted with SPM, was used to uncover the network of activations common to both task that included three left hemisphere areas, namely (1) the pars opercularis and triangularis of the inferior frontal gyrus, (2) the posterior part of the superior temporal cortex centered around the superior temporal sulcus, extending to the planum temporale posterior part but sparing the supramarginalis and angular gyri, and (3) the most anterior part of the left inferior temporal gyrus at the junction with the anterior fusiform gyrus. The inferior and lateral parts of the right cerebellar cortex were also included in the conjunction network. Each of the three cortical areas, when they are site of lesion or electrical stimulation, elicit impairment in both language comprehension and production and can thus be considered as essential to language. Accordingly, the present results provide conservative anatomofunctional definitions of the Broca, Wernicke, and basal language areas. Interestingly, contralateral homologues of Broca's and Wernicke's areas also lighted up in the conjunction analysis that could be related to the interindividual variability of hemispheric language dominance.