Choosing to make an effort: the role of striatum in signaling physical effort of a chosen action

Abstract

The possibility that we will have to invest effort influences our future choice behavior. Indeed deciding whether an action is actually worth taking is a key element in the expression of human apathy or inertia. There is a well developed literature on brain activity related to the anticipation of effort, but how effort affects actual choice is less well understood. Furthermore, prior work is largely restricted to mental as opposed to physical effort or has confounded temporal with effortful costs. Here we investigated choice behavior and brain activity, using functional magnetic resonance imaging, in a study where healthy participants are required to make decisions between effortful gripping, where the factors of force (high and low) and reward (high and low) were varied, and a choice of merely holding a grip device for minimal monetary reward. Behaviorally, we show that force level influences the likelihood of choosing an effortful grip. We observed greater activity in the putamen when participants opt to grip an option with low effort compared with when they opt to grip an option with high effort. The results suggest that, over and above a nonspecific role in movement anticipation and salience, the putamen plays a crucial role in computations for choice that involves effort costs.

Fig. 1.

Left top: grip and hold stimuli. Grip stimulus: a horizontal line indicated reward levels (in pence), a vertical line indicated effort levels (in % maximum grip). We added a random value to effort and reward levels of each grip stimulus; values in brackets show the averages. Hold stimulus: a horizontal line indicated a fixed reward value in pence. Middle: a schematic of the task. Choice period: in each choice trial, a fixation cross appeared, followed by a grip and a hold stimulus. Participants had to make a decision to grip or to hold. There were 12 choice trials; each grip stimulus was presented pseudorandomly. At the end of each choice period, the computer randomly selected 9 of 12 participants' choices from the preceding choice period to be executed. Execute period: immediately following the 12th choice trial, execute period comprised 9 trials; either grip or hold trial, is started. In the grip trials, a thermometer with a target level was displayed to guide squeezing the hand grip. In the hold trials, a frozen thermometer was presented. Each participant carried out 5 sets of choice and execute period in total. Bottom right: a thermometer stimulus is used to guide squeezing during execute period. The red “mercury” indicates current force level; yellow horizontal line indicates target level.

Fig. 2.

Behavioral choice, subjective rating, and response times (RTs). A: proportion of trials where participants chose to grip (dark shade) and their subjective rating (light shade) for each option. Participants chose to grip more often when the reward offered was high than when it was low and when the effort anticipated was low than when it was high. The interaction was nonsignificant. Liking (light shade) was higher for options with high reward than for options with low reward, higher for options with low effort than for options with high effort, and comparable between hold and low effort-low reward. The interaction was nonsignificant. B: the same liking data to A showing that on average, the order of rating from lowest to highest is high effort-low reward, low effort-low reward, high effort-high reward, and low effort-high reward. C: RTs were slower for choice to hold than for choice to grip. (mean ± SD).

Fig. 3.

Persistence, behavioral choice, and in dorsal anterior cingulate cortex signal. A: persistence is negatively correlated with the effect of effort on choice (n = 18). Regardless of reward, low persistence is associated with a higher preference for options with low effort, whereas high persistence is associated with indifference between options with low effort and options with high effort. B: activity in the dorsal anterior cingulate cortex when the rejected option entailed low effort is positively correlated with persistence (P < .001 uncorrected, 11 voxels; n = 17).

Fig. 4.

Activity in left putamen is higher when participants chose to grip an option which involved low effort than when they chose to grip an option which involved high effort (cluster-corrected family-wise error P = 0.01, 51 voxels). Bar graph depicts the parameter estimates for this contrast for visual illustration.

Fig. 5.

Reward level is positively correlated with activity in bilateral nucleus accumbens when participants chose to grip an option that involved high effort. Activation displayed in pink is thresholded at P < 0.005 (uncorrected, 5 voxels), activation displayed in yellow is thresholded at P < 0.05 (uncorrected, 582 voxels).