Neuroradiological activation studies on the cerebral organisation of language capacities - A review.

Models on the cerebral organisation of speech and language capacities are predominantly based on lesion studies. Neuroradiological activation methods using positron emission or magnetic resonance tomography provide a further means to investigate brain-behaviour relationships. The present paper reviews the available data obtained with functional imaging during speech and language tasks. The classical connectionist model suggests several distinct language centres within the perisylvian area of the dominant hemisphere. Especially the processing of complex verbal stimuli yields haemodynamic and metabolic reactions outside this area. At least partially, these extrasylvian foci might reflect paralinguistic functions such as motivational or attentional aspects of speech production, or the processing of emotions conveyed by verbal utterances. As far as linguistic capacities are concerned, functional imagery has yielded two results which extend the classical connectionist model of speech and language functions: (a) the medial part of the occipital lobe of the dominant hemisphere seems to comprise representations of visual word forms; (b) word generation yields activation of cerebellar structures. In contrast to the classical connectionist model, reading did not cause reactions of the angular region of the left hemisphere. Furthermore, phonological and semantic processing of verbal stimuli include the anterior perisylvian language zones. The interpretation of discrepancies between data derived from lesion studies and those obtained with functional imaging is still unsettled. First of all, rapid cognitive processes do not necessarily give rise to a detectable significant haemodynamic or metabolic response. Secondly, highly automatised language processes such as inner speech are difficult to control.