Mapping the brain atrophy mediating increased impatience for reward in frontotemporal dementia

Choices involving trade-offs between larger later (LL) and smaller sooner (SS) rewards-known as intertemporal preferences-are altered in many psychiatric and neurodegenerative conditions, leading to a preference for immediate rewards. Behavioural variant frontotemporal dementia (bvFTD), a neurodegenerative disorder characterised by high impulsivity and atrophies in brain systems relevant for decision-making, provides a neuropathological model to investigate structural networks linked to higher impatience for reward. We studied 22 bvFTD patients and 17 controls, using two intertemporal choice (ITC) tasks involving (1) monetary and (2) food rewards. We compared outcomes of these tasks (discount rate and sensitivity to LL reward amounts) between groups and examined correlations with bvFTD symptoms. We applied whole-brain mediation analysis to participants' structural MRI data to identify neural mediators of higher impatience for reward in bvFTD. BvFTD patients showed higher discount rates and lower sensitivity to LL reward for both money and food. These ITC outcomes for money (but not food) were related to inhibition deficits and lower executive functions among patients. Reduced grey matter density in the medial pulvinar and parahippocampal cortex mediated bvFTD's alteration of ITC outcomes. Lesions of these structures involved in emotional salience and projection may constitute neural markers of impatience for reward.