The Neural Basis of Retrograde Memory: Evidence From Positron Emission Tomography for the Role of Non-mesial Temporal Lobe Structures

Abstract

Lesion studies have suggested that retrieval of old, factual, concrete knowledge (retrograde memory) and learning of new, factual, concrete knowledge (anterograde memory) depend on distinct systems in the temporal lobe. Here we report a case study using serial neuropsychological assessments and serial structural and functional imaging studies which provides further support for this notion. A 70-year-old right-handed woman with limbic encephalitis attributed to thymoma was evaluated 4 months after diagnosis. Severe multimodal anterograde and retrograde memory defects were found. An [¹⁸F] fluorodeoxyglucose (FDG) resting positron emission tomography (PET) scan showed diffuse cortical, but especially right anterolateral temporal hypometabolism. Two years later, there was marked resolution of the retrograde memory defect, with persistent anterograde memory Impairment. A repeat FDG resting PET showed improved metabolism in the anterolateral temporal cortex and in other cortical regions. Metabolism in both mesial temporal regions declined markedly. Striking recovery of retrograde memory, but not anterograde memory, occurred in conjunction with the improvement in anterolateral temporal metabolism and despite reduction in mesial temporal metabolism. These findings provide further support for the hypotheses that 1) mesial temporal structures are not the repository of retrograde factual knowledge, and 2) anterolateral temporal lobe structures, especially on the right, may be critical for retrieval of retrograde factual knowledge.