Effort discounting in human nucleus accumbens

Abstract

A great deal of behavioral and economic research suggests that the value attached to a reward stands in inverse relation to the amount of effort required to obtain it, a principle known as effort discounting. In the present article, we present the first direct evidence for a neural analogue of effort discounting. We used fMRI to measure neural responses to monetary rewards in the human nucleus accumbens (NAcc), a structure previously demonstrated to encode reference-dependent reward information. The magnitude of accumbens activation was found to vary with both reward outcome and the degree of mental effort demanded to obtain individual rewards. For a fixed level of reward, the NAcc was less strongly activated following a high-demand for effort than following a low demand. The magnitude of this effect was noted to correlate with preceding activation in the dorsal anterior cingulate cortex, a region that has been proposed to monitor information-processing demands and to mediate in the subjective experience of effort.

Figure 1

Two diagrammatic representations of effort discounting, one from animal behavior research and one from behavioral economics. Top: Based on choice behavior in animals, Phillips et al. (2007) proposed that the subjective value (net utility) of a fixed reward varies inversely with the effort required to obtain it (response cost). They review work suggesting that the strength of this discounting effect varies with concentrations of dopamine (DA) within the NAcc (Redrawn from Figure 2 of Phillips et al., 2007). Bottom: According to prospect theory (Tversky & Kahneman, 1992), the relationship between a reward’s magnitude and its subjective value is characterized by a curvilinear value function. Kivetz (2003) proposed that this function shifts to the right as the effort demanded to obtain the reward increases. The figure shows two value curves, one relevant to rewards requiring a fixed amount of effort (right), the other to rewards requiring no effort (left; horizontal axis: reward magnitude including both wins and losses, with zero at the point labeled r=0; vertical axis: subjective value, with the horizontal axis crossing at zero. From Figure 1 of Kivetz, 2003).

Figure 2

The sequence of events within low demand (left) and high demand (right) blocks. Each block included ten numerals, and could end with either a $ or an X.

Figure 3

(A) Top left: Coronal section showing the location of the NAcc ROI. NAcc: Mean parameter estimates for NAcc activation in response to $ and X cues occurring at the completion of high demand and low demand blocks, after subtraction of corresponding parameter estimates from control condition. Bars indicate standard error. OFC: Orbitofrontal cortex. MFC: Medial prefrontal cortex. AMG: Amygdala. ACC: Anterior cingulate cortex. (B) Top: Coronal section showing the location of the ACC (green) and DLPFC (blue) regions of interest. Bottom: Regression coefficients for each participant in the experimental group, relating reward-cue responses in NAcc to task-related activity in ACC and DLPFC. Position along the horizontal axis reflects the relation between ACC activation during task performance and the NAcc response to the subsequent reward cue. Position along the vertical axis reflects the relation between DLPFC activation during task performance and the NAcc response to the subsequent reward cue. A more consistent relationship was observed between ACC and NAcc than between DLPFC and NAcc. (C) Mean NAcc responses to the visual cues occurring at the beginning of high and low demand blocks, for experimental and control groups. Bars indicate standard error.