Nucleus accumbens mediates relative motivation for rewar in the absence of choice ds

(Rangel et al., 2008; Balleine and O’Doherty, 2010) implies that idiosyncrasies in withinor across-individual motivation stemming from variable anticipatory responses may lead to subsequent variability in choice-related responses. Two regions – the nucleus accumbens (NAcc, a subregion of ventral striatum) and the anterior insular cortex (aINS) – have been considered core contributors to idiosyncrasies in reward anticipation (Knutson and Greer, 2008). NAcc and aINS are also both frequently included in neural models of motivation (Delgado, 2007; Craig, 2009; Naqvi and Bechara, 2009), based in part of evidence from non-human primate (Schultz et al., 1992) and rodent (Roitman et al., 2005) studies. However, these regions’ anatomical connectivity suggests that they could contribute to different aspects of motivation: aINS is positioned to integrate interoceptive information, while NAcc receives input from both dopaminergic neurons and from multiple cortical regions (Haber and Knutson, 2010). Some theories have emphasized a dissociable role for NAcc and aINS in processing appetitive compared to aversive rewards (Knutson and Greer, 2008), whereas others have posited a dissociation between motivational and reward-processing in these two regions (Craig, 2009). Further, preference-related signals have been found in both NAcc (O’Doherty et al., 2006; Knutson et al., 2007) and aINS (Kim et al., 2011). Strikingly, however, a comprehensive literature review of reward anticipation studies (see Appendix) found no human neuroscience study that dissociated the unique contributions of these two regions to motivation in the absence of choice. Here, working with expressed instead of inferred preference, we elicited individual differences in relative motivation for two appetitive reward modalities, independent of choices between them. Participants performed a reaction-time task to obtain two rewarding goods: money and candy. The task required participants IntroductIon “Motivation,” taken literally, describes an impulse to movement – as when the expectation of a desired outcome mobilizes someone into action. Motivation can incorporate a variety of drives or internal states that direct behaviors or decisions, ranging from basic homeostatic imbalance correction (Cannon, 1932) to psychosocial needs (Maslow, 1943). While multiple theories of motivation exist, nearly all posit that organisms translate the anticipated hedonic reward for potential outcomes (Berridge, 2004) into the force behind their effort to obtain a reward (Niv et al., 2007; Salamone et al., 2009). Individuals differ in their preferences for rewards, often exhibiting greater motivation in response to some reward modalities compared to others (e.g., striving for income versus personal fulfillment). This heterogeneity in preferences not only impacts choices between rewards, but also shapes, sometimes independently, how much effort will be exerted to receive a single reward in the absence of choice. As most real-world environments involve multiple kinds of rewards, a core challenge for understanding goal-directed behavior lies in understanding how individuals calibrate their expressed effort across rewards of different modalities. Many neural studies of economic behavior downplay the contributions of motivation to adaptive decision-making. Typical paradigms focus on inferred value signals that predict observable, individual choices between rewards (Platt and Huettel, 2008; Rangel et al., 2008), de-emphasizing the role of motivation for expected outcomes (Kahneman et al., 1997). Nevertheless, both behavioral neuroscience (Berridge, 2004) and behavioral economics (Loewenstein and Lerner, 2003) have highlighted the role that anticipated affect plays in shaping behaviors and preferences (Knutson and Greer, 2008). Although different phases of rewardprocessing, such as anticipation and valuation, are distinct in terms of cognition, the significant overlap in their neural substrates Nucleus accumbens mediates relative motivation for rewar in the absence of choice ds