The target article by Cook et al. proposes that the mirror neuron system (MNS) becomes functionally specialized through sensorimotor experience during development, and is founded upon domain-general processes similar to that of Pavlovian and instrumental conditioning. The authors acknowledge that a difference in mirror neuron (MN) responses can be influenced by the subjective value of an observed action; however, they put little emphasis on the possible role of reward in MN activity. This associative learning account particularly lends itself to the suggestion that the perceived subjective value associated with both executed and observed actions may potentially hinder or facilitate the development of the MNS and its general functioning. The differences in the subjective value associated with others' actions, just as the processing of reward is dependent on the context in which the reward is presented (Nieuwenhuis et al. Reference Nieuwenhuis, Heslenfeld, von Geusau, Mars, Holroyd and Yeung2005), may also determine the degree to which MN areas are activated, and, consequently, may temper action perception.

The neural correlates of “social rewards” are beginning to be revealed, as studies are confirming the involvement of reward-related areas in certain social scenarios (Behrens et al. Reference Behrens, Hunt, Woolrich and Rushworth2008; Rushworth et al. Reference Rushworth, Mars and Sallet2013). It is known that the neural coding of reward is crucially involved in action selection and is therefore also intrinsic to goal-directed behavior (Schultz Reference Schultz2000). As the activity in the MNS has been shown to be specific only to observed actions that are goal-directed (Rizzolatti et al. Reference Rizzolatti, Fadiga, Gallese and Fogassi1996), it follows that reward and punishment are likely to have reciprocal effects on the neural activity associated with action observation.

Several studies have shown that the presence or absence of reward can influence the excitability of the motor cortices during both action execution and action observation, which has also been linked to motor learning (Hosp et al. Reference Hosp, Pekanovic, Rioult-Pedotti and Luft2011; Wickens et al. Reference Wickens, Reynolds and Hyland2003). Reward can change the size of motor evoked potentials (MEP) as a result of inhibition of primary motor cortex (Kapogiannis et al. Reference Kapogiannis, Campion, Grafman and Wassermann2008; Thabit et al. Reference Thabit, Nakatsuka, Koganemaru, Fawi, Fukuyama and Mima2011), and motor skill learning can be improved if rewarding feedback is given (Sugawara et al. Reference Sugawara, Tanaka, Okazaki, Watanabe and Sadato2012). Spontaneous mimicry of facial expressions can also be enhanced when the emotional face being mimicked has been associated with a high reward (Sims et al. Reference Sims, Van Reekum, Johnstone and Chakrabarti2012). We have also recently demonstrated that rewarding actions produced the greatest suppression in the EEG mu rhythm, an index of MNS activity in motor areas (Brown et al. Reference Brown, Wiersema, Pourtois and Brüne2013). These studies resonate with early work on social leaning by Bandura (Reference Bandura1965) and others such as Liebert and Fernandez (Reference Liebert and Fernandez1970a; Reference Liebert and Fernandez1970b), in which vicarious reward and punishment influenced the degree to which children tended to imitate the observed actions of adults.

There is much evidence showing that activity in the motor-related areas of the MNS can be affected by the context and “meaning” of the observed action, and here we propose that some of these findings could be accounted for by underlying differences in the subjective value associated with the seen actions. Studies demonstrating modulation of the mu suppression in the observation of painful versus non-painful action-related stimuli (Cheng et al. Reference Cheng, Yang, Lin, Lee and Decety2008; Perry et al. Reference Perry, Bentin, Bartal, Lamm and Decety2010) provide one clue to the influence of reward on the MNS, as pain and reward-processing are closely linked (Leknes & Tracey Reference Leknes and Tracey2008). In terms of differences in social context, actions performed in a social interaction produce greater mu rhythm suppression than actions performed outside of an interaction (Perry et al. Reference Perry, Stein and Bentin2011). The mu rhythm can also be modulated by the social relevance of the observed action (Kilner et al. Reference Kilner, Marchant and Frith2006; Oberman et al. Reference Oberman, Pineda and Ramachandran2007). Furthermore, the interpersonal liking between individuals can modulate MNS areas, as differences in premotor cortex activation were found when observing in-group versus out-group members' actions (Sobhani et al. Reference Sobhani, Fox, Kaplan and Aziz-Zadeh2012). Some have suggested that being in a social interaction, in itself, can be rewarding (Krach et al. Reference Krach, Paulus, Bodden and Kircher2010). If this is the case, then it would be plausible to argue that differences in motor activity seen in studies comparing social and non-social settings or stimuli may be confounded by differences in reward-processing.

The role of reward in vicarious motor cortex activity may also relate to previous studies investigating MNS function in psychopathologies that exhibit both deficits in social cognition and abnormalities in reward-processing (Gold et al. Reference Gold, Waltz, Prentice, Morris and Heerey2008; Penn et al. Reference Penn, Sanna and Roberts2008). It has been proposed that a dysfunction in the MNS may help to explain deficits in social cognition in schizophrenia and autism spectrum disorders (ASDs) (Arbib & Mundhenk Reference Arbib and Mundhenk2005; Williams et al. Reference Williams, Whiten, Suddendorf and Perrett2001). Some studies have found abnormal mu rhythm suppression in people with ASDs when observing actions (Bernier et al. Reference Bernier, Dawson, Webb and Murias2007; Oberman et al. Reference Oberman, Hubbard, McCleery, Altschuler, Ramachandran and Pineda2005; Reference Oberman, Ramachandran and Pineda2008), whereas others have not (Fan et al. Reference Fan, Decety, Yang, Liu and Cheng2010; Raymaekers et al. Reference Raymaekers, Wiersema and Roeyers2009). The findings in schizophrenia are also mixed, as one study from McCormick and colleagues (McCormick et al. Reference McCormick, Brumm, Beadle, Paradiso, Yamada and Andreasen2012) found greater mu suppression in psychotic patients, whereas another found lower mu suppression (Singh et al. Reference Singh, Pineda and Cadenhead2011), with both finding some relationship between mu suppression and psychotic symptoms. We suggest that these inconsistent results may have resulted from a more elementary pathological deficit in reward-processing. As reward has a substantial influence on vicarious motor activity and motor learning, then it is likely that abnormalities in the processing of reward could also have a detrimental impact on the development of the MNS, consequently leading to the apparent impairments in social cognition seen in autism and schizophrenia. This proposal is further supported by emerging evidence suggesting that problems in social functioning seen in ASDs may be founded upon an impaired response to social rewards (Dichter & Adolphs Reference Dichter and Adolphs2012).

To sum up, it does appear that reward has an influence on the motor-related areas of the MNS, which may also partially account for some of the previous findings demonstrating context-related modulation of motor cortex and MNS activity, and may help to explain some of the inconsistencies found in studies investigating MNS function in pathological groups. This proposal is well in line with the associative learning account of the MNS. Future studies looking at MNS activity may benefit from taking the potential interaction effects of reward into account.