NEURORADIOLOGICAL ACTIVATION STUDIES ON T HE CEREBRAL ORGANIZATION OFLANGUAGE CAPACITIES - A REVIEW

Models on the cerebral organisation of speech and language capacities are predominantly based on lesion studies. Neuroradiological activatio n methods using positron emission or magnetic resonance tomography pro vide a further means to investigate brain-behaviour relationships. The present paper reviews the available data obtained with functional ima ging during speech and language tasks. The classical connectionist mod el suggests several distinct language centres within the perisylvian a rea of the dominant hemisphere. Especially the processing of complex v erbal stimuli yields haemodynamic and metabolic reactions outside this area. At least partially, these extrasylvian foci might reflect paral inguistic functions such as motivational or attentional aspects of spe ech production, or the processing of emotions conveyed by verbal utter ances. As far as linguistic capacities are concerned, functional image ry has yielded two results which extend the classical connectionist mo del of speech and language functions: (a) the medial part of the occip ital lobe of the dominant hemisphere seems to comprise representations of visual word forms; (b) word generation yields activation of cerebe llar structures. In contrast to the classical connectionist model, rea ding did not cause reactions of the angular region of the left hemisph ere. Furthermore, phonological and semantic processing of verbal stimu li include the anterior perisylvian language zones. The interpretation of discrepancies between data derived from lesion studies and those o btained with functional imaging is still unsettled. First of all, rapi d cognitive processes do not necessarily give rise to a detectable sig nificant haemodynamic or metabolic response. Secondly, highly automati sed language processes such as inner speech are difficult to control.