DOUBLE DISSOCIATIONS OF MEMORY AND EXECUTIVE FUNCTIONS IN WORKING-MEMORY TASKS FOLLOWING FRONTAL-LOBE EXCISIONS, TEMPORAL-LOBE EXCISIONS ORAMYGDALO-HIPPOCAMPECTOMY IN MAN

Thirty-two neurosurgical patients with unilateral or bilateral frontal lobe excisions, 41 patients with unilateral temporal lobe lesions and 19 patients Who had undergone unilateral amygdalo-hippocampectomy wer e compared with matched controls on a computerized test of spatial wor king memory. A significant deficit was observed in the frontal lobe gr oup, even at the least challenging level of task difficulty and this i mpairment was found to relate to the inefficient use of a particular s earching strategy shown to improve performance on this task. In contra st deficits in the temporal lobe group and the amygdalo-hippocampectom y group were only observed Pt the most difficult level of the task and in neither group could the deficit be related to the inefficient use of any particular searching strategy. In a follow-tip study, the three patient groups were compared on analogous computerized tests of visua l-and verbal working memory. No deficits were observed in the frontal lobe group. By comparison, both the temporal lobe patients and the amy gdalo-hippocampectomy group were significantly impaired in the visual working memory condition but not in the verbal working memory conditio n. These deficits were clearly evident at all levels of task difficult y and were not related to any particular searching strategy The data a re discussed in terms of the relative contributions of 'executive' and 'mnemonic' mechanisms to the contrasting, material dependent deficits observed in the frontal and temporal lobe groups.