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Right anterior temporal lobe dysfunction underlies theory of mind impairments in semantic dementia

Muireann Irish, John R. Hodges, Olivier Piguet
DOI: http://dx.doi.org/10.1093/brain/awu003 1241-1253 First published online: 12 February 2014

Summary

Semantic dementia is a progressive neurodegenerative disorder characterized by the amodal and profound loss of semantic knowledge attributable to the degeneration of the left anterior temporal lobe. Although traditionally conceptualized as a language disorder, patients with semantic dementia display significant alterations in behaviour and socioemotional functioning. Recent evidence points to an impaired capacity for theory of mind in predominantly left-lateralized cases of semantic dementia; however, it remains unclear to what extent semantic impairments contribute to these deficits. Further the neuroanatomical signature of such disturbance remains unknown. Here, we sought to determine the neural correlates of theory of mind performance in patients with left predominant semantic dementia (n = 11), in contrast with disease-matched cases with behavioural-variant frontotemporal dementia (n = 10) and Alzheimer’s disease (n = 10), and healthy older individuals (n = 14) as control participants. Participants completed a simple cartoons task, in which they were required to describe physical and theory of mind scenarios. Irrespective of subscale, patients with semantic dementia exhibited marked impairments relative to control subjects; however, only theory of mind deficits persisted when we covaried for semantic comprehension. Voxel-based morphometry analyses revealed that atrophy in right anterior temporal lobe structures, including the right temporal fusiform cortex, right inferior temporal gyrus, bilateral temporal poles and amygdalae, correlated significantly with theory of mind impairments in the semantic dementia group. Our results point to the marked disruption of cognitive functions beyond the language domain in semantic dementia, not exclusively attributable to semantic processing impairments. The significant involvement of right anterior temporal structures suggests that with disease evolution, the encroachment of pathology into the contralateral hemisphere heralds the onset of social cognitive deficits in this syndrome.

  • frontotemporal dementia
  • semantic memory
  • social cognition
  • anterior temporal lobe
  • theory of mind
  • Abbreviations
    ACE-R
    Addenbrooke’s Cognitive Examination-Revised
    FTD
    frontotemporal dementia
    ToM
    theory of mind
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