Temporal lobe lesions and semantic impairment: a comparison of herpes simplex virus encephalitis and semantic dementia

Both herpes simplex virus encephalitis (HSVE) and semantic dementia (SD) typically affect anterior temporal lobe structures. Using voxel-based morphometry (VBM), this study compared the structural damage in four HSVE patients having a semantic deficit particularly affecting knowledge of living things and six SD patients with semantic impairment across all categories tested. Each patient was assessed relative to a group of control subjects. In both patient groups, left anterior temporal damage extended into the amygdala. In patients with HSVE, extensive grey matter loss was observed predominantly in the medial parts of the anterior temporal cortices bilaterally in SD patients the abnormalities extended more laterally and posteriorly in either the left, right or both temporal lobes. Based on a lesion deficit rationale and converging results from several other sources of evidence, we suggest that (i) antero-medial temporal cortex may be important for processing and differentiating between concepts that are 'tightly packed' in semantic space, such as living things, whereas (ii) inferolateral temporal cortex may play a more general role within the semantic system.