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 bilateral 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 predominated, namely bilateral pathology of white matter and neocortical temporal lobe structures, with spared medial temporal lobe structures. This damage, which was particularly severe in anterior loci in the temporal lobes, was sustained as a result of radionecrosis. High-resolution magnetic resonance imaging was carried out to document the distinctive anatomical profile of our patient, and this profile was compared 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 neuropsychological level, our patient was not amnesic but showed patchy impairment on traditional tests of anterograde memory functioning, in the context of notable "semantic" memory loss for knowledge acquired before and after the onset of his illness. Our findings demonstrate that bilateral temporal lobe pathology by itself does not lead to a classical amnesic syndrome, but may result in a significant but more subtle "semantic" memory loss. Our data highlight the distinctive and dissociable contribution of lateral and medial temporal lobe structures to human memory processing, and suggest a major role for anterior-inferior neocortical temporal lobe mechanisms in aspects of knowledge acquisition, storage and retrieval.