Neural basis of the undermining effect of monetary reward on intrinsic motivation

Abstract

Contrary to the widespread belief that people are positively motivated by reward incentives, some studies have shown that performance-based extrinsic reward can actually undermine a person's intrinsic motivation to engage in a task. This "undermining effect" has timely practical implications, given the burgeoning of performance-based incentive systems in contemporary society. It also presents a theoretical challenge for economic and reinforcement learning theories, which tend to assume that monetary incentives monotonically increase motivation. Despite the practical and theoretical importance of this provocative phenomenon, however, little is known about its neural basis. Herein we induced the behavioral undermining effect using a newly developed task, and we tracked its neural correlates using functional MRI. Our results show that performance-based monetary reward indeed undermines intrinsic motivation, as assessed by the number of voluntary engagements in the task. We found that activity in the anterior striatum and the prefrontal areas decreased along with this behavioral undermining effect. These findings suggest that the corticobasal ganglia valuation system underlies the undermining effect through the integration of extrinsic reward value and intrinsic task value.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.

Experimental protocol and behavioral results. (A) Illustration of SW and WS tasks. (B) Depiction of the experimental procedure. (C) Means and SEs of the number of times participants voluntarily played the SW and WS tasks during the first and the second free-choice periods. Performance-based reward undermined the intrinsic motivation for the SW task for both free-choice periods (Mann–Whitney U = 52.5 and 54.5; P < 0.05).

Fig. 2.

Bilateral striatum responses elicited by success trials relative to failure trials plotted for each session/group. Left: Activations superimposed on transaxial sections (P < 0.001, one-sample t test for display). Right: Mean contrast values and SEs of the bilateral striatum (averaged) activation are plotted. During the first session, significant bilateral striatum activation was observed in both groups, although the activation was significantly greater in the reward group than in the control group (two-sample t26 = 3.30, P < 0.01). In contrast, during the second session, whereas the control group sustained significant activity, the activation of the bilateral striatum in the reward group decreased significantly below that of the control group (two-sample t26 = 3.75, P < 0.01) and the activation was no longer significant. This striatal response pattern is in parallel with the behavioral undermining effect. Asterisks represent the statistical significance of one-sample/two-sample t tests (**P < 0.01).

Fig. 3.

Midbrain activation (peak at −9, −7, −11) detected in the session-by-group interaction during the feedback period (success trials minus failure trials; P < 0.05, small-volume-corrected; the image is shown at P < 0.001, uncorrected). Neural responses are displayed in sagittal and transaxial formats. The midbrain was activated when performance-based monetary reward was expected (during the first session; two-sample t26 = 1.80, P < 0.10), but the activation decreased significantly below the control group in the second session (two-sample t26 = 2.63, P < 0.05). Asterisks represent the statistical significance of one-sample/two-sample t tests (+P < 0.10, *P < 0.05, **P < 0.01).

Fig. 4.

Right LPFC activation (peak at 39, 41, 40) detected in the session-by-group interaction during the task cue period (P < 0.05, small-volume-corrected; image is shown at P < 0.001, uncorrected for display). Neural responses are displayed in transaxial and coronal formats. The bar plot represents mean contrast values and SEs for each session/group. During the first session, the LPFC in the reward group showed significantly larger activation than that in the control group (two-sample t26 = 2.62, P < 0.05). However, the activation became significantly smaller in the reward group than in the control group during the second session (two-sample t26 = 2.27, P < 0.05).

Fig. 5.

Relationship between behavioral choice during the first free-choice period and the neural undermining index. Significant negative relationship was observed in the reward group (β = −0.49, P = 0.037, one-tailed), indicating that those who did not voluntarily try the SW task during the free-choice period showed a larger decrease in activation of the corticobasal ganglia network. The relationship in the control group was not significant (β = 0.09, P = 0.75). Blue triangles represent participants in the control group; red squares represent participants in the reward group.