An Encyclopaedia of Language_05

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An Encyclopaedia of Language_05222 LANGUAGE IN THE BRAINalongside it a thin sheet of grey matter, the claustrum. These are all behind the folded-in part of the cortex behind thetemporal lobe which is called the insula. Of all these structures the ones which have been most studied in respect of languageare the thalamus and the lenticular nucleus. All the evidence concerning the role of these structures in language inevitably comes from brain-damaged patientsincluding ones undergoing electrophysiological stimulation prior to surgery. That some of these structures play a role in themotor production of speech has been known for some time, but the idea that damage to them might produce aphasia (althoughmilder and less longer-lasting than cortical aphasias) has been revived relatively recently. Studies have generallydistinguished between damage to the basal ganglia and damage to the thalamus (Wallesch and Wyke 1983). Damage to the basal ganglia accompanying Parkinson’s disease has been reported to result in language difficulties as wellas motor speech disorders. Lees and Smith (1983) describe naming difficulties in this condition. Tanridag and Kirshner(1987) have reviewed a number of studies which describe language disorders after strokes in the left internal capsule andstriatal regions. Particular attention has been paid to the lenticular nucleus, and aphasic symptoms have been described aftereither putamenal lesions or lesions to the globus pallidus. Haemorrhage frequently occurs in the region of the putamen, andNauser, Alexander, Helm-Estabrooks, Levin, Laughlin, and Geschwind (1982) have suggested that the pattern of aphasiadiffers according to whether the damage is anterior or posterior. Although these subcortical aphasias are most commonlylinked in type with the transcortical aphasias (Wallesch 1985), since the ability to repeat is generally preserved, patterns distinctfrom those of cortical aphasias have been described e.g. the occurrence of articulatory difficulty with jargon. Aphasia after damage to the thalamus has been studied in rather more detail (Ojemann 1982; Mateer and Ojemann 1983; Mohr1983; Lhermitte 1984). Word-finding difficulties are greater and may be accompanied by perseveration and lack of insight.Language difficulties, however, fluctuate, a feature not seen in cortical aphasias, and the perseverations may be intrusions ofirrelevant words. ESB, instead of blocking language, may result in the production of these perseverative words. Perseverationseems to be associated particularly with the medial central portion of the ventral lateral thalamus, which Ojemann interpretsas a site of interaction between language and motor speech functions. The ventrolateral part of the thalamus is said to includealerting circuits which are involved in short-term memory as well as in naming. Stimulation here can have an effect on wordretrieval which may last as long as a week, suggesting that it participates in long-term memory as well. Crosson, Parker, Kim,Warren, Kepes, and Tully (1986), however, consider that that part of the thalamus known as the pulvinar is the critical zone,as deduced from a post-mortem study of an 82-year-old man, whose thalamic lesion had resulted in a fluent aphasia withsemantic paraphasias. These authors hold that the thalamus maintains the tone of cortical language mechanisms and releasesmonitored language for its motor programming. Bechtereva, Bundzen, Gogolitsin, Malyshev, and Perepelkin (1979) have alsosuggested that subcortical structures have a pace-maker mechanism which controls and reorganises the brain for themaintenance of mental activity. Specifying in more detail what role subcortical structures play in language will require thetracing of cortical-subcortical circuits, such as those proposed by Lamendella (1977). Wallesch and Wyke (1983) haveproposed three parallel anatomical pathways: firstly a cortical-subcortical (basal ganglia and thalamus) loop; secondlyreciprocal cortical-thalamic-cortical connections and thirdly the ascending reticular-thalamic-cortical activation system.Crosson (1985) has advanced a more elaborate model in which he has incorporated some features of the classical corticalmodel (e.g. that the posterior zone performs phonological verification and the anterior zone motor programming) withinhibitory circuitry through the caudate nucleus from the anterior zone, and inhibitory links with the posterior zone throughthe lenticular nucleus and thalamus. In this model subcortical structures inhibit motor output, while the cortex exercises anediting and checking function on the planned language. This could perhaps account for the reportedly frequent occurrence ofsemantic paraphasias after subcortical damage. Crosson’s model is reviewed by Murdoch (in press). A scheme of subcortical aphasias has been set out by Alexander, Naeser and Palumbo (1987), based empirically on th ...