Developing a Novel and Objective Behavioral Index of Motivational Anhedonia

Motivational anhedonia is a debilitating feature of many neurological and psychiatric disorders. Motivational anhedonia, lack of motivation for reward, can occur to varying degrees in the general population (Ang et al. 2017, PLoS One). To understand underlying biological mechanisms and facilitate treatment development, the Effort Expenditure for Rewards Task (EEfRT) was developed to objectively assess motivational anhedonia. However, the EEfRT presents several confounds - specifically delayed discounting, probabilistic discounting, constrained decision time, and fatigue. To address these issues, the PEDM task was developed. Computational models were applied to characterize subjective devaluation of reward by physical effort.
The PEDM was administered to 49 individuals, compromised of 21 young adults, 15 older adults, and 10 from a mental health outpatient clinic. It was administered with three questionnaires, the Apathy Motivation Index (AMI), Snaith-Hamilton Pleasure Scale (SHAPS), and the Quality of Life and Enjoyment Scale Short Form (Q-LES-Q-LES). It was predicted that there would be significantly higher levels of anhedonia in the mental health population, a significant relationship of the PEDM and the behavioral activation component of the AMI, the ability to self initiate goal-directed behavior (Ang, Lockwood, Apps, Muhammed, Husain 2017, Plos One), no significant relationship to the SHAPS, since it is a measure of consummatory pleasure rather than the ability to expend effort for rewards, and that the mental health group would have greater self-reported anhedonia and apathy levels. 
Analysis of relationships between "k" value and dimensions of apathy and anhedonia revealed a strong correlation with behavioral motivation in healthy students, while findings were not significant for older adults and the mental health group. There were no significant correlations between consummatory anhedonia and the PEDM. The mental health group had significantly higher scores for anhedonia but not apathy. Lastly, anhedonia levels from the PEDM were similar in the young adult and mental health group. These findings suggest that the PEDM is a promising measure of motivational anhedonia in healthy populations and clinical populations, but needs future testing to truly elucidate and support the findings.