The brain correlates of the effects of monetary and verbal rewards on intrinsic motivation

Abstract

Apart from everyday duties, such as doing the laundry or cleaning the house, there are tasks we do for pleasure and enjoyment. We do such tasks, like solving crossword puzzles or reading novels, without any external pressure or force; instead, we are intrinsically motivated: we do the tasks because we enjoy doing them. Previous studies suggest that external rewards, i.e., rewards from the outside, affect the intrinsic motivation to engage in a task: while performance-based monetary rewards are perceived as controlling and induce a business-contract framing, verbal rewards praising one's competence can enhance the perceived self-determination. Accordingly, the former have been shown to decrease intrinsic motivation, whereas the latter have been shown to increase intrinsic motivation. The present study investigated the neural processes underlying the effects of monetary and verbal rewards on intrinsic motivation in a group of 64 subjects applying functional magnetic resonance imaging (fMRI). We found that, when participants received positive performance feedback, activation in the anterior striatum and midbrain was affected by the nature of the reward; compared to a non-rewarded control group, activation was higher while monetary rewards were administered. However, we did not find a decrease in activation after reward withdrawal. In contrast, we found an increase in activation for verbal rewards: after verbal rewards had been withdrawn, participants showed a higher activation in the aforementioned brain areas when they received success compared to failure feedback. We further found that, while participants worked on the task, activation in the lateral prefrontal cortex was enhanced after the verbal rewards were administered and withdrawn.

Keywords: crowding-out; fMRI; intrinsic motivation; monetary rewards; verbal rewards.

Figure 1

Example trial. Two picture stimuli are presented simultaneously. The subjects have to find the number of differences between the stimuli. There are two differences between the original picture stimulus (top panel, left) and the manipulated version (top panel, right) in this example, in which a red and a black bar are missing. Feedback for a correct solution is displayed as given in the control group and in period 1 and 3 in the reward groups (leftmost bottom panel). The two alternative feedback panels display the feedback given in the monetary and verbal reward groups in period 2 (middle and rightmost bottom panels, respectively).

Figure 2

Performance increased from period 1 to period 2 in all groups, but from period 1 to 3 in the verbal reward group only.

Figure 3

(A) Activation for success—failure feedback was significantly higher in the right anterior striatum (left panel) and midbrain (right panel) in the monetary reward group compared to the control group in period 2 (whole brain uncorrected, p < 0.001, only clusters with a minimum of 10 activated voxels are shown; small volume corrected, p_FWE < 0.05). Activation clusters without baseline correction are shown in green, clusters with baseline correction are shown in red, activation overlaps are shown in yellow. (B) Parameter estimates are displayed for illustration.

Figure 4

(A) Activation for success—failure feedback was significantly higher in the right anterior striatum (left panel) and midbrain (right panel) in the verbal reward group compared to the control group in period 3 (whole brain uncorrected, p < 0.001, only clusters with a minimum of 10 activated voxels are shown; small volume corrected, p_FWE < 0.05). Activation clusters without baseline correction are shown in green, clusters with baseline correction are shown in red, activation overlaps are shown in yellow. (B) Parameter estimates are displayed for illustration.

Figure 5

(A) Activation for picture presentation was significantly higher in the right lateral prefrontal cortex in the monetary reward group compared to the control group in period 3 (whole brain uncorrected, p < 0.001, only clusters with a minimum of 10 activated voxels are shown; small volume corrected, p_FWE < 0.05). Activation clusters without baseline correction are shown in green, clusters with baseline correction are shown in red, activation overlaps are shown in yellow. (B) Parameter estimates are displayed for illustration.

Figure 6

(A) Activation for picture presentation was significantly higher in the right lateral prefrontal cortex in the verbal reward group compared to the control group in period 3 (whole brain uncorrected, p < 0.001, only clusters with a minimum of 10 activated voxels are shown). Activation clusters without baseline correction are shown in green, clusters with baseline correction are shown in red, activation overlaps are shown in yellow. (B) Parameter estimates are displayed for illustration.