Discounting of future rewards in behavioural variant frontotemporal dementia and Alzheimer's disease.

Objective: The clinical differential diagnosis of Alzheimer's disease (AD) and behavioral-variant frontotemporal dementia (bvFTD) can no longer rely only on episodic memory impairment or executive dysfunctions, as highlighted by recent findings showing that both diseases could present with similar impairments. Objective cognitive tests assessing specific symptoms, such as impulsivity in bvFTD, are thus crucially needed. The aim of this study was to evaluate the differences in impulsivity between bvFTD and AD using a delay-discounting paradigm.

Method: An ecological delay-discounting test was administrated to 70 participants including 30 ADs, 20 bvFTD and 20 controls. AD patients were divided according to the severity of the disease into mild or moderate group. The delay-discounting score, reflecting the total percentage of impulsive choice across the entire task, was analyzed for each group.

Results: This score showed that bvFTD patients were significantly more impulsive than controls and AD patients at mild or moderate stage. AD patients, regardless of disease stage, did not differ from controls. ROC analyses revealed high and significant area under the curve (AUC, 95% confidence interval) for this score to differentiate bvFTD from AD (0.704) or controls (0.904), or both group (AD + controls; AUC = 0.791).

Conclusion: The total delay-discounting score provided by our task showed that it could accurately differentiate bvFTD patients from AD and controls. These results support the relevancy of using tests inspired by experimental psychoeconomics and taping into reward processing to increase the distinction between both diseases.