Taking both sides: do unilateral anterior temporal lobe lesions disrupt semantic memory?

The most selective disorder of central conceptual knowledge arises in semantic dementia, a degenerative condition associatedwith bilateral atrophy of the inferior and polar regions of the temporal lobes. Likewise, semantic impairment in both herpessimplex virus encephalitis and Alzheimer’s disease is typically associated with bilateral, anterior temporal pathology. Thesefindings suggest that conceptual representations are supported via an interconnected, bilateral, anterior temporal networkand that it may take damage to both sides to produce an unequivocal deficit of central semantic memory. We tested andsupported this hypothesis by investigating a case series of 20 patients with unilateral temporal damage (following vascularaccident or resection for tumour or epilepsy), utilizing a test battery that is sensitive to semantic impairment in semanticdementia. Only 1/20 of the cases, with a unilateral left lesion, exhibited even a mild impairment on the receptive semanticmeasures. On the expressive semantic tests of naming and fluency, average performance was worse in the left- thanright-unilateral cases, but even in this domain, only one left-lesion case had scores consistently more than two standarddeviations below control means. These results fit with recent parallel explorations of semantic function using repetitivetranscranial magnetic stimulation as well as functional imaging in stroke aphasic and neurologically intactparticipants. The evidence suggests that both left and right anterior temporal lobe regions contribute to the representation ofsemantic memory and together may form a relatively damage-resistant, robust system for this critical aspect of higher cognition.