In this study, we used culture-specific, meaningful non-verbal hand gestures to investigate whether motor resonance during action observation is modulated by cultural factors. Indeed, the observation of actions performed by an individual of one's cultural and ethnic ingroup increases CSE, compared to observing actions performed by an outgroup member. While this modulation of CSE may be attributed to ethnic ingroup familiarity, the interaction between actor and emblem type cannot be accounted for by such familiarity. We propose that a plausible explanation of these findings is that unconscious motor resonance mechanisms are modulated by interacting biological and cultural factors.
While observing the actions of an ethnic and cultural ingroup member, we show stronger motor resonance. This novel result is interesting because it implicates one's own motor system in the perception of ingroup versus outgroup members, independent of observed motor actions. Our data showing increased CSE at the implicit, individual level, are in line with previously described effects at a more explicit and social level. Indeed, differential perception of ingroup versus outgroup members has been described extensively in the literature (for recent reviews see: 
). Persons tend to have higher empathy for ingroup members 
and favor them in reward allocations 
and in esteem
. Cognitively, people remember more detailed information about ingroup members than outgroup members 
. This bias leads people to encode the observed behaviors of ingroup and outgroup members at different levels of abstraction 
. For example, undesirable actions of outgroup members are presumed to be of intentional and dispositional origin (‘she is hostile’), compared to identical behaviors of ingroup members (‘she slapped the girl’). The converse is true for desirable actions, which are encoded at more concrete levels for outgroup members (‘she walked the old man across the street’) relative to the same behaviors in ingroup members (‘she is kind’) 
. Thus, it appears that neural systems supporting memory, empathy and general cognitions encode information related to ingroup versus outgroup members differently. One novel contribution of the current study is our finding that the human mirror neuron system specifically, is differentially sensitive to ingroup versus outgroup members.
This finding is particularly interesting in light of recent data implicating the fronto-parietal human mirror neuron system in self-other distinction 
. Based on recent findings, it has been proposed that a mechanism similar to that which enables the understanding of the actions of others also allows identification of other agents by mapping their physical characteristics onto one's own motor repertoire 
. Our data agree with this proposal, and provide additional evidence that a motor resonance mechanism mediates intersubjective communication and social communication in general.
In nature, as in our study, biological factors such as ethnic ingroup membership and cultural factors such as motor repertoire are inextricably linked, especially in investigations of highly culture-specific actions such as emblems 
. While this makes interpretation of data somewhat more complex, it does more accurately reflect what our brains process in the real world. Our current results show that ethnic ingroup membership and
a culturally learned motor repertoire influence the brain's responses to observed actions, specifically actions used in social communication. In functional terms at the neural level, the mirror neuron system is involved in predicting action goals 
and providing an ongoing simulation of the motoric complexity of observed actions 
while maintaining a representation of the intention behind those actions 
. The present data show that while this system for action representation is responding to observed actions, the response is modulated not only by the kind of action that is observed, but also by who is performing that action.
Our initial hypothesis, based on the neuroimaging literature on action perception, predicted that a shared motor repertoire leads to more effective communication. Thus, we predicted that our Euro-American participants would show facilitation of CSE during observation of the Euro-American actor due to a shared motor repertoire. This prediction was in fact borne out, as shown by our main effect of actor, however, the neural processes giving rise to this effect may not simply be due to the perception of familiar actions. Our results are more nuanced showing that the human mirror neuron system may identify elements in a shared motor repertoire, but it is also sensitive to ethnic group membership. This is evidenced in the performer by gesture interaction, showing that even if familiar actions are observed, it does not translate into stronger motor resonance, as indexed by an increase in CSE.
Additional support for this interaction of ethnicity and one's motor repertoire is the finding that the American emblems performed by the Nicaraguan actor did not lead to facilitation of CSE, but rather to a decrease. The decrease in CSE during observation of a Nicaraguan actor performing American emblems is likely due to a perceived incongruence between the actor and the action they are performing. Our American participants observing an ethnic outgroup member perform actions that the participants themselves know well, may trigger a ‘differentiation’ response rather than one of ‘identification’ with the actor. Such a response is likely due to an interaction of biological factors (ethnicity) and cultural factors (learned motor repertoire). Considering this finding another way, it is interesting to note that Nicaraguan emblems performed by the Nicaraguan actor did not lead to a decrease in CSE, and may indicate that the socially relevant nature of these gestures were evident to our participants (even without semantic comprehension), such that they may have tried to map these gestures onto their own motor repertoire.
This modulation of CSE while observing the Nicaraguan actor performing his own culturally learned emblems is intriguing, and suggests modulation of motor resonance mechanisms. This finding is similar to our recent data showing stronger recruitment of fronto-parietal mirror neuron regions during observation of complex hierarchical action sequences of increasing motoric complexity and increased reaction times during construction of such complex sequences 
. It suggests that motor resonance, while an implicit parameter of action recognition, is a nuanced one, conveying subtle learned differences in motor fluency.
Due to the close relationship of gesture and language
and the traditional view of the left hemisphere being language-dominant
it is important for us to consider the issue of laterality. In this study, we did not find any main effect or interaction with hemisphere. Consistent with our results, previous work examining the lateralization of the human mirror neuron system during hand action observation using TMS
has found that the system for action representation is on the whole bilateral. This was also the finding of the reanalysis of a large dataset of functional imaging studies (58 subjects) involving observation and imitation of simple finger movements
. A recent study of a split-brain patient assessed laterality of the mirror neuron system using TMS, and found that while the left hemisphere of the patient showed increased CSE during action observation, the right hemisphere did not
. However, a control group of normal subjects showed parallel increases in CSE in both hemispheres, indicating that in fact, action representation recruits both hemispheres.
We hope that this work will stimulate further experiments to investigate the effect of cultural learning on the motor system using participants from two different cultures. In fact, we also tried to recruit Nicaraguan participants for the present study from the Los Angeles area. However, due to the large variability in exposure to American culture, as well as varying degrees of assimilation and acculturation, it became evident that we would be unable to enroll participants that were equally naïve with respect to American gestures as the American participants were with the Nicaraguan gestures. This issue highlights the increasingly more relevant effects of globalization on research. A future experiment with participants from two cultures should help disentangle the effects of biological factors (ethnic ingroup membership) and cultural factors (motor repertoire) on the perception of action. A caveat with the current study is the limited number of participants, thus conclusions must be drawn carefully; however, several other TMS studies have also used eight or fewer participants to study cognitive phenomena 
In conclusion, our findings suggest that the neural substrates of action recognition and social communication may be tuned to both ethnic identity and cultural experience. We have shown that observing the actions of an individual who is an ethnic ingroup member and shares a culturally acquired motor repertoire yields higher motor resonance, compared to observing an individual who is an ethnic outgroup member and has an unfamiliar culturally acquired motor repertoire. Our findings suggest that the human mirror neuron system is implicated in distinguishing ingroup versus outgroup members, and this same neural mechanism is involved in representing culturally learned actions. These findings may have broad implications for motor skill and language learning, intergroup communication, as well as the study of intergroup attitudes and stereotyping.