Abstract
To dissociate a choice from its antecedent neural states, motivation associated with the expected outcome must be captured in the absence of choice. Yet, the neural mechanisms that mediate behavioral idiosyncrasies in motivation, particularly with regard to complex economic preferences, are rarely examined in situations without overt decisions. We employed functional magnetic resonance imaging in a large sample of participants while they anticipated earning rewards from two different modalities: monetary and candy rewards. An index for relative motivation toward different reward types was constructed using reaction times to the target for earning rewards. Activation in the nucleus accumbens (NAcc) and anterior insula (aINS) predicted individual variation in relative motivation between our reward modalities. NAcc activation, however, mediated the effects of aINS, indicating the NAcc is the likely source of this relative weighting. These results demonstrate that neural idiosyncrasies in reward efficacy exist even in the absence of explicit choices, and extend the role of NAcc as a critical brain region for such choice-free motivation.
Highlights
Introduction “Motivation,” taken literally, describes an impulse to movement – as when the expectation of a desired outcome mobilizes someone into action
Some theories have emphasized a dissociable role for nucleus accumbens (NAcc) and anterior insular cortex (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)
All coordinates listed are in MNI space. Discussion both NAcc and aINS have been typically linked through a common role in anticipatory motivation (Knutson and Greer, 2008), our findings indicate that those brain regions make distinct contributions to the relative motivation associated with different reward modalities
Summary
Introduction “Motivation,” taken literally, describes an impulse to movement – as when the expectation of a desired outcome mobilizes someone into action. While multiple theories of motivation exist, most 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). 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) 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). Based on its convergent inputs from multiple rewardprocessing regions (Haber and Knutson, 2010), we conjectured that NAcc – but not aINS – would play the primary role in translating differences in motivation into behavior
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