Using the grip strength effort task to measure reward processing dysfunction in schizophrenia and major depressive disorder

The aims of the Reward Task Optimisation Consortium (RTOC) study (Bilderbeck et al., 2020) were to explore the validity, reliability, and feasibility of a battery of reward processing tasks for the development of new treatments for anhedonia. We report our findings from the Grip Strength Effort Task (GSET), an effort-based decision-making task in which participants chose to either perform easy trials that required less physical effort for low monetary reward or hard trials that required more effort for potentially larger rewards. Thirtyseven participants with schizophrenia (SZ), 40 with major depressive disorder (MDD), and 59 age- and sexmatched healthy controls were administered the task across four European sites. 19% of participants (8.5% HC, 27% SZ, 27.5% MDD) were 'inflexible responders' who always chose hard trials irrespective of the reward amount. MDD participants showed less willingness to exert physical effort for high reward than controls, when inflexible responders were excluded, but no statistically significant differences were observed between SZ participants and controls. Across all participants, willingness to exert effort for high reward negatively correlated with measures of anhedonia. Inflexible responders exhibited higher depressive symptoms and higher anticipation of punishment than other participants. Forty-three participants performed the GSET again after 3-5 weeks and moderate-to-high test-retest reliability was observed. Minimal site effects confirmed operational feasibility of the task in multi-site studies. We conclude that the GSET can provide objective behavioural biomarkers of reward processing dysfunction, but further investigation is needed to understand inflexible responding and its implications on the task's design and interpretation.