To make a decision, an agent needs to predict the affective consequences of each available option. We find that expectations of hedonic (Sharot, De Martino & Dolan, 2009) and aversive (Sharot, Shiner & Dolan, 2010) outcomes are represented in the striatum, modulated by dopamine (Sharot et al., 2009), and most importantly, predict the choices people make at a later time. Interestingly, this striatal activity not only predicts choice, but is also subsequently altered by choice, as are predictions. After a stimulus is chosen, people indicate that they expect to derive greater pleasure from the chosen stimulus (Sharot, De Martino & Dolan, 2009; Sharot, Velasquez & Dolan 2010) and that their expectations of aversive outcomes are decreased (Sharot, Shiner & Dolan, 2010).The effects are long lasting (Sharot, Fleming et al., 2012). Striatal activity mirrors this behavioural effect: after a decision is made, activity associated with positive expectations is enhanced for stimuli that are selected and decreased for stimuli that are rejected.
These findings support a notion long held by psychologists that predictions of affective outcome not only determine our choices, but are also altered by them (Brehm, 1956; Festinger, 1957). It has been shown that after making a difficult decision people subsequently value the selected alternative more strongly than they had initially, and the discarded one less so. Nonhuman animals also exhibit this effect (Egan, Santos & Bloom, 2007). Our findings suggest that making a choice involves a rapid update of the striatal signal that represents expected value, a process that may enhance confidence in the decision taken.