Dissociable Motivational Deficits in Pre-manifest Huntington’s Disease

Abstract

Summary 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.

Heightened effort discounting is a common feature of both apathy and fatigue

“…To derive the best estimates of each individual’s willingness to invest effort, we fit participants’ choices to three functions that are commonly used to model effort discounting—linear, parabolic, and hyperbolic. These model comparisons revealed that participants’ choices were best fit by a parabolic pattern of effort discounting (AIC: parabolic = 4544, linear = 4787, hyperbolic = 5329; BIC: parabolic = 5042, linear = 5285, hyperbolic = 5826), which is consistent with previous work 36 , 52 , 53 , 64 , 69 , 70 . We then extracted the k -values for each participant from the winning model, and correlated them with ratings on the apathy and fatigue inventories.…”

“…This was a purposeful element of our design to eliminate the possibility of choices being confounded by the accumulation of short-term fatigue, which is particularly important given the unclear relationship between fatigue as a trait, and shorter term fatiguability 15 , 56 , 57 . We note that previous studies have demonstrated the sensitivity of hypothetical decisions to quantifying effort discounting 23 , 36 , 52 , 58 , 59 and other reward-based behaviour 58 , 60 – 63 . The choice phase was presented in Presentation software (Neurobehavioral Systems), and participants made choices using the left and right arrow keys.…”

“…Participants completed only one of the three studies. A subset of these data ( n = 20) have been previously published 36 . These pooled data therefore represent a convenience sample, with post-hoc power analyses indicating an estimated power of at least 0.95 to detect a significant correlation between measures with an r of 0.4 at a threshold of p < 0.05.…”

“…Each of the three studies involved a physical effort discounting task, which required individuals to decide how much physical effort to trade off in return for reward (similar to previously published paradigms 36 , 52 , 53 ) (Fig. 1 ).…”

Choosing increases the value of non-instrumental information

“…These findings also leave open further questions about whether subjective changes in reward value and probability influence the value of non-instrumental information about those rewards. One example of this is effort discounting, in which the value of a reward is reduced when it is earned through effortful, as opposed to non-effortful means 51 – 55 . While evidence for the IOC has not been demonstrated in neural indices of reward 31 , the modulation of reward signals has been demonstrated in tasks requiring effort 53 , 54 .…”