, 2010; Palminteri et al., 2009a; Hare et al., 2008). Many anatomo-functional models of reward learning share the idea that reward prediction errors (obtained minus expected reward) are encoded in dopamine signals that reinforce corticostriatal synapses (Bar-Gad and Bergman, 2001; Frank et al., 2004; Doya, 2002). The same mechanism could account for punishment learning:
dips in dopamine release might weaken approach circuits and/or strengthen avoidance circuits. This is consistent with numerous studies showing that dopamine enhancers improve reward learning, but impair punishment learning in patients with Parkinson’s disease (Frank et al., 2004; Bódi et al., 2009; Palminteri et al., 2009b). It has been suggested that another neuromodulator, serotonin, could Ion Channel Ligand Library concentration play an opponent role: it would encode punishment prediction errors (obtained minus expected punishment) so as to reinforce the avoidance pathway (Daw et al., 2002). However, this hypothesis has been challenged by several empirical studies in monkeys and humans (McCabe et al., 2010; Palminteri et al., 2012; Miyazaki et al., 2011). Beyond neuromodulation, the existence of opponent regions, which would process punishments as the ventral Bioactive Compound Library prefrontal cortex and striatum process reward, remains controversial.
In humans, fMRI studies of reinforcement learning have yielded inconsistent results. At the cortical level, several candidates for an opponent punishment system have been MTMR9 suggested, among which the anterior insula emerged as particularly prominent. Indeed, the anterior insula was found to represent cues predicting primary punishments, such as electric shocks, fearful pictures, or bad tastes, and these punishments themselves (Büchel et al., 1998; Seymour et al., 2004; Nitschke et al., 2006). These findings have been later
extended to more abstract aversive events, such as financial loss or risk (Kuhnen and Knutson, 2005; Samanez-Larkin et al., 2008; Kim et al., 2011, 2006). However, some studies have also found insular activation linked to positive reinforcers and orbitofrontal activation linked to negative reinforcers (O’Doherty et al., 2001; Gottfried and Dolan, 2004; Kirsch et al., 2003). The functional opponency between ventral prefrontal cortex and anterior insula, in learning to predict reward versus punishment, is therefore far from established. At the striatal level, many fMRI studies have reported activations related to primary or secondary reinforcers during instrumental learning (O’Doherty et al., 2003; Galvan et al., 2005; Pessiglione et al., 2008; Daw et al., 2011). Again, some studies supported the idea that the same regions encode both reward and punishments cues or outcomes, whereas other studies argued for a functional dissociation between ventral and dorsal regions (Jensen et al., 2003; Delgado et al., 2000; O’Doherty et al., 2004; Seymour et al., 2007).