BILATERAL TEMPORAL-LOBE PATHOLOGY WITH SPARING OF MEDIAL TEMPORAL-LOBE STRUCTURES - LESION PROFILE AND PATTERN OF MEMORY DISORDER

The lesion sustained by the amnesic patient H.M. consisted of bilatera l ablation of medial temporal lobe structures with relative sparing of more lateral white matter and neocortical structures. We present the first detailed report of a case where the reverse pattern of lesions p redominated, namely bilateral pathology of white matter and neocortica l temporal lobe structures, with spared medial temporal lobe structure s. This damage, which was particularly severe in anterior loci in the temporal lobes, was sustained as a result of radionecrosis. High-resol ution magnetic resonance imaging was carried out to document the disti nctive anatomical profile of our patient, and this profile was compare d to that reported for the patient H.M. At the anatomical level, there was an almost ''mirror image'' profile, with contrasting involvement of lateral and medial temporal lobe structures. At the neuropsychologi cal level, our patient was not amnesic but showed patchy impairment on traditional tests of anterograde memory functioning, in the context o f notable ''semantic'' memory loss for knowledge acquired before and a fter the onset of his illness. Our findings demonstrate that bilateral temporal lobe pathology by itself does not lead to a classical amnesi c syndrome, but may result in a significant but more subtle ''semantic '' memory loss. Our data highlight the distinctive and dissociable con tribution of lateral and medial temporal lobe structures to human memo ry processing, and suggest a major role for anterior-inferior neocorti cal temporal lobe mechanisms in aspects of knowledge acquisition, stor age and retrieval.