UNILATERAL MEDIAL TEMPORAL-LOBE MEMORY IMPAIRMENT - TYPE DEFICIT, FUNCTION DEFICIT, OR BOTH

Previous research has characterized memory deficits resulting from uni lateral hippocampal system damage as 'material specific', suggesting t hat left damage results in verbal memory impairment with preservation of visuospatial function and the converse with right damage. Implicit within this hypothesis are the assumptions that the systems are indepe ndent and memory is lateralized for each type of material. To test the verbal component of this hypothesis, unilateral hippocampal lesion an d commissurotomy patients were compared with controls on a multiple-li st free-recall task. The material specific hypothesis predicts severe impairment only with left lesions; right lesions and commissurotomy pa tients should be only minimally impaired. However, secondary memory wa s compromised at immediate recall for all patient groups, with both un ilateral groups showing comparable and severe verbal episodic memory d eficits. Final testing across all lists also revealed severe impairmen t in commissurotomy patients. Finding both unilateral groups to be sim ilarly impaired for verbal material is taken as evidence against a mat erial specific deficit during this verbal episodic memory task. Althou gh previous data suggest that left patients are considerably more impa ired during some verbal tasks, this may not be specific to the materia l, but rather the combination of material and task demands. Implicatio ns for the material specific hypothesis are discussed. (C) 1998 Elsevi er Science Ltd. All rights reserved.