ASYMMETRIC FUNCTION OF THE THALAMUS IN MAN *

The suggestion that the right and left human thalamus may not be functionally equivalent is largely the result of observations of language deficits associated with left, and not right, thalamic lesions. These observations have been made both with spontaneous thalamic lesions, particularly hemorrhages,l, 2 and with surgical lesions.3.5 These language deficits can occur with lesions confined only t o left thalamus, indeed just t o ventrolateral n u ~ l e u s . ~ ? ~ On the other hand, language disturbances d o not invariably follow either spontaneous or surgical lesions of the left thalamus.63 And when language deficits appear acutely, they generally clear with time. Only in a n occasional case does the deficit persist for long period^.^? 8, The nature of this language deficit after left thalamic lesions, when present, does not seem t o be identical with any of the described language deficits after cortical lesions of the language-dominant hemisphere. There is generally a fluent dysnomia with frequent per~evera t ion .~ The ability to repeat from dictation and comprehension are usually intact. Language function may fluctuate widely, f rom nearly normal, t o a silence resembling sleep, depending on the degree of environmental stimulation.2 Other functions have also been identified that are altered differentially by lesions of the right or left human thalamus. Deficits in short-term verbal memory appear after left, but not right, thalamic lesions.9,10~ 11 Deficits in face-matching have been described following right and not left ventrolateral thalamic surgical lesions,l2 and deficits in Porteus-maze performance have occurred following right subthalamic lesions and not left.13 Electrical stimulation of the thalamus at the time of stereotaxic operations for the treatment of dyskinesias provides another opportunity for investigating asymmetrical thalamic functions. We have utilized this technique t o study changes in autonomic function,14 language performance as measured by objectnaming,15-18 short-term verbal mernory,17-21 mental arithmetic,21 and shortterm memory for figures that have few verbal associations (nonverbal information).18>22 This paper discusses the asymmetrical effects of thalamic stimulation reported in these studies, and in some of my more recent and 'not previously published material.