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Specific word category deficits after lesions in the right hemisphere
Specific word category deficits after lesions in the right hemisphere
We have proposed that language units are cortically processed by distributed cell assemblies including neurons in both hemispheres, transcortical cell assemblies (TCA, Mohr et al., 1994; PulvermŸller & Mohr, 1996; see also L4.4). These neurons may be left-lateralized in most individuals in the sense that they include more left- than right-hemispheric neurons, but the networks crucial for processing a word would still bridge the midline. The claim is based on associative memory theory and the fact that the perception of a spoken language unit, for example a word, always leads to correlated activity in both cortical hemispheres. To test the TCA model, we can look at the effect of focal lesions in the hemisphere not dominant for language.The TCA model suggests that a right hemispheric lesion can lead to similar, although less pronounced, category-specific deficits to those reported earlier following lesions to the left dominant hemisphere.
Category-specific deficits arising from lesions in the dominant hemisphere are well-known (Warrington & McCarthy, 1983; Warrington & Shallice, 1984). Lesions in the left temporal lobe are sometimes associated with selective difficulties in processing particular word categories, e.g. animal names, whereas left frontal lesions in many cases impair the processing of action verbs (Daniele et al., 1994). Together with Bettina Neininger, doctorate student at the University of Konstanz, Germany, we investigated whether focal lesions in the right non-dominant hemisphere can also lead to selective difficulties in processing particular types of words.
No overt language dysfunction was found in our population of 18 patients with focal ischemic lesions primarily affecting the right frontal or temporal lobes. Consistent with the model, however, a sensitive psycholinguistic test, a lexical decision task, revealed a double dissociation between word groups. Processing of action verbs was imparied after right-frontal lesions, whereas the processing of nouns primarily characterized by visual associations was degraded after lesion in right temporo-occipital areas (Neininger & PulvermŸller, in press). Neurological control patients did not show similar deficits, nor was there any impairment of the processing of nouns characterized by both strong action and visual associations. It was striking that that small focal lesions, for example in primary and premotor cortex, could lead to a marked behavioral impairment that showed up on the lexical decision task (Neininger & PulvermŸller, 2001). This supports the idea that the neuron networks necessary for word processing have a right-hemispheric component as well.
We plan to follow up on this work by searching for other word-category deficits accompanying specific focal right-hemispheric lesions. In this context, we will use TMS to cause temporary modification of the function of different right-hemispheric areas. The effect of such temporary functional modulation will be looked at with psycholinguistic experiments and the results will be used to further evaluate the TCA model.