AMNESIAS FOLLOWING LIMITED THALAMIC LESIONS

In man, the localized thalamic lesions giving rise to memory disorders have generally a vascular origin (infarcts). They very often affect several nuclei or associated tracts, which makes the pathophysiologic interpretations more difficult. Disorders are furthermore partially regressive with time, in their severity but also in their quality, which explains why some of the classical dissociations in amnesia are only observed at the late stage. Memory disorders are variable according to the lesion site and their uni or bilateral character. The “purest” and most severe amnesia are observed in bilateral injury of the anterior or antero-internal structures, which is much more rare in unilateral ones. Severe deficits in “declarative” learning tasks and in long term memory are then observed, which predominate in free recall. Short-term memory is often deficient at the initial and secondary stages, even when late evaluation may find it to be relatively unimpaired. Retrograde amnesia is frequent, but less severe than anterograde amnesia and the temporal gradient, frequently described by the patients, is difficult to assess with the classical tests (questionnaires). These patients have important and often lasting impairment in the control of time, which would contribute to the explanation of their amnesia (source amnesia). In the case of paramedian and moreover subthalamic extension of the lesions, patients frequently have at the initial stage vigilance disorders and then attention impairment and cognitive slowing. Their short-term memory and retrograde memory disorders could be more severe. One of the other factors which may contribute to the severity of amnesia is the extension of the lesions in the more external structures, dorsomedial and lateral nuclei: patients then present supplementary cognitive disorders in language (left lesions) or treatment of spatial information (right lesions). At the late stage, a memory disorder corresponding to the main initial deficit may persist. This phenomenon, related to the hemispheric specialization, is more easily observed in unilateral lesions. Amnesia could also be more severe in the case of left thalamic injury, and this could be related to the “declarative” character of most of the tests used.