Academic journal article Cognitive, Affective and Behavioral Neuroscience

Effort Discounting in Human Nucleus Accumbens

Academic journal article Cognitive, Affective and Behavioral Neuroscience

Effort Discounting in Human Nucleus Accumbens

Article excerpt

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.

At the level of everyday commonsense knowledge, there is a close relationship between reward and effort. This is evident, for example, when a potential payoff is judged not to be worth the work it would require, or when a level of reward for some effort is considered to be unfair. The same direct connection between reward and effort is also found in formal theories, including behavioral and economic accounts of decision making (see Kivetz, 2003; Walton, Kennerley, Bannerman, Phillips, & Rushworth, 2006), social theories of equity (Walster, Walster, & Berscheid, 1978), and legal theories of distributive justice (Locke, 1690/1987). In these and other contexts, a common proposition is that effort carries a negative value or cost, sometimes referred to as the disutility of effort, and that this cost provides a reference against which earned rewards are evaluated (Figure 1). According to this basic principle, referred to in some contexts as effort discounting, a reward carries a higher net value if it is easily obtained than if it is obtained only through great effort (Kivetz, 2003; Phillips, Walton, & Jhou, 2007; Rudebeck, Walton, Smyth, Bannerman, & Rushworth, 2006). In effect, effort sets in place a reference point against which rewards are measured.

Effort discounting and the close relationship between reward and effort that underlies it clearly represent more than mere cultural convention. Rodents, birds, and nonhuman primates have been shown to weigh effort against reward in decision making (Phillips et al., 2007; Salamone, Cousins, & Bucher, 1994; Stevens, Rosati, Ross, & Hauser, 2005; Tsunematsu, 2001; Walton, Bannerman, Alterescu, & Rushworth, 2003; Walton et al., 2006; some contrary conclusions from the same literature are discussed below), and capuchin monkeys have been shown to reject rewards smaller than those received by conspecifics for an equal expenditure of effort (Brosnan & de Waal, 2003). Psychopharmacological interventions and lesions to specific brain structures have been observed to alter the relative weighting of effort and reward information in decision making (Denk et al., 2005; Floresco & Ghods- Sharifi, 2007; Salamone, Correa, Mingote, & Weber, 2003; Salamone et al., 1994; Walton et al., 2003). Furthermore, effort discounting relates closely to another form of discounting, delay discounting, for which specific neural substrates have been identified (see, e.g., Roesch, Taylor, & Schoenbaum, 2006; Rudebeck et al., 2006). Given such findings, it seems plausible that the tight relationship between reward and effort that holds at the behavioral level may reflect the operation of basic neural mechanisms.

In the present experiment, we used fMRI to investigate the integration of reward and effort information in the human brain. Our specific objective was to test for a neural correlate of effort discounting. The experiment focused on the nucleus accumbens (NAcc), a basal ganglia structure that has been found in numerous studies to respond to reward outcomes, often in a reference-dependent fashion (Breiter, Aharon, Kahneman, Dale, & Shizgal, 2001; Delgado, Nystrom, Fissell, Noll, & Fiez, 2000; Elliott, Friston, & Dolan, 2000), and has also been heavily implicated in effort-based decision making (Salamone, Correa, Farrar, & Mingote, 2007; Salamone et al. …

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