Dissociable Motivational Deficits in Pre-manifest Huntington's Disease

Abstract

Motivation is characterized by a willingness to overcome both cognitive and physical effort costs. Impairments in motivation are common in striatal disorders, such as Huntington's disease (HD), but whether these impairments are isolated to particular domains of behavior is controversial. We ask whether HD differentially affects the willingness of individuals to overcome cognitive versus physical effort. We tested 20 individuals with pre-manifest HD and compared their behavior to 20 controls. Across separate trials, participants made choices about how much cognitive or physical effort they were willing to invest for reward. Our key results were that individuals with pre-manifest HD were less willing than controls to invest cognitive effort but were no different in their overall preference for physical effort. These results cannot be explained by group differences in neuropsychological or psychiatric profiles. This dissociation of cognitive- and physical-effort-based decisions provides important evidence for separable, domain-specific mechanisms of motivation.

Keywords: Huntington’s disease; apathy; cognitive effort; decision making; effort discounting; motivation; physical effort; reward; striatum.

Figure 1

Task Design Participants were first trained on (A and B) a cognitively effortful task and (C and D) a physically effortful task before (E) indicating their preference for investing effort for reward. (A) The cognitive effort task required participants to monitor one to six RSVP streams for a target letter (“T”). (B) Each trial began with a blue pie chart indicating the number of streams they had to monitor on that trial. After completing each effort level, participants received feedback on their performance. Each trial lasted 10 s. (C) The physical effort task required participants to sustain variable amounts of force on a hand-held dynamometer, with the target levels of force defined as a function of each individual’s maximum voluntary contraction (MVC) (4%, 12%, 20%, 28%, 36%, and 44%). (D) Each trial began with a red pie chart indicating the amount of force they had to apply on that trial. Trial durations were identical to those for the cognitive effort task (10 s). At the conclusion of each trial, participants received feedback on their performance. (E) The choice phase required participants to decide how much effort they were willing to invest for reward. The choice was always between a fixed baseline option (the lowest level of effort for the lowest reward; one point) and a variable high-effort/high-reward offer (higher levels of effort; rewards of two to ten points). Separate choices were made for cognitive and physical effort.

Figure 2

Performance in the Cognitive and Physical Effort Tasks (Mean ± 1 SEM) (A) In the cognitive effort task, target detection sensitivity (left panel) and reinforcement rates (right panel) were identical across groups. Controls are shown in black and pre-manifest HD in blue. (B) In the physical effort task, pre-manifest HD and controls did not differ in the proportion of time they were able to maintain their grip over the target effort level (left panel), which was reflected in identical reinforcement rates between groups (right panel). Controls are shown in black and pre-manifest HD in red.

Figure 3

Choices in the Cognitive and Physical Effort Tasks (Mean ± 1 SEM) Acceptance rates for the higher-effort/higher-reward offer are plotted as a function of effort (left column) and reward (center column). Difference plots illustrate choice differences between controls and pre-manifest HD across the two-dimensional effort-reward space (right column). Red indicates greater motivation in controls than pre-manifest HD. (A) For cognitive-effort-based choices, pre-manifest HD was less willing to accept the higher-effort/higher-reward offers. (B) For the physical-effort-based choices, decisions were similar between groups. See also Figures S1 and S2.