The shared circuits model (SCM) presented in Susan Hurley's target article provides a useful framework for understanding imitation, deliberation, and mindreading. The shared informational dynamics of perception/action and self/other not only enable copying and understanding others' actions, but also enable interpersonal synchrony. The temporal dynamics of perception and action are essential in interpersonal synchrony, and incorporating these aspects could further elucidate two key facets of the SCM: prediction and the self/other distinction. The temporal precision of predictive simulations, integration, and mirroring enables interpersonal synchrony. Tightly coupled interpersonal synchrony can blur the self/other distinction and potentially increase interpersonal empathy.
Interpersonal synchrony and imitation are examples of social coordination, but differ in temporal aspects. Synchrony, by definition, occurs in shared time, whereas imitation occurs after some delay. The time course in imitation has been investigated. Meltzoff (Reference Meltzoff1988b), for example, employed deferred imitation as a measure of memory in infants. In order to investigate perception/action links more directly in imitation, Iacoboni et al. (Reference Iacoboni, Woods, Brass, Bekkering, Mazziotta and Rizzolatti1999), for example, utilized immediate observation-execution, and thereby mitigated intermediary processes such as memory and interpretation. The delay can and must be excised completely in joint action and synchrony tasks that rely on prediction.
In order to synchronize actions with another person, one cannot simply react to their partner's actions; instead, one must predict what the other will do and then plan and execute accordingly (Sebanz et al. Reference Sebanz, Bekkering and Knoblich2006). In a joint task requiring precise temporal coordination, Knoblich and Jordan (Reference Knoblich and Jordan2003) had partners track a circle on the screen with each partner controlling one tracking direction. Successful tracking required anticipatory coordination. The results showed that anticipatory coordination can become as effective with timing feedback from a partner's action, as when performing the task alone. Action planning was based on the prediction of the joint effects of self and other. This supports the notion of shared representations for self and other and that predictive simulations of others' actions are integrated with temporal precision.
Similar predictive mechanisms enable interpersonal synchrony in ensemble music performance. Musical synchrony is extremely precise despite the common expressive timing deviations from isochrony (e.g., Rasch Reference Rasch and Sloboda1988). Keller et al. (Reference Keller, Knoblich and Repp2007) suggest that such precision is possible by predictively simulating the actions of others. In their study, pianists synchronized more precisely with recordings of themselves after a delay of several months than with recordings of others. Presumably, the predictive simulations were more accurate for self-generated performance because they were carried out on the same perception/action system (with all its idiosyncratic constraints). As Hurley writes, “I perceive your action by means that engage my capacity for similar action” (sect. 4.1.2, para. 4, point 2); and when I perceive my own action, this resonance would be strongest.
The tendency to synchronize with others is well established (e.g., Schmidt et al. Reference Schmidt, Carello and Turvey1990). A recent electroencephalographic (EEG) experiment provided evidence for mirror system involvement in interpersonal synchrony (Tognoli et al. Reference Tognoli, Lagarde, DeGuzman and Kelso2007). In this interpersonal finger-tapping study, two EEG oscillatory components, whose topographies were consistent with the mirror system, distinguished coordinated from uncoordinated tapping. The authors suggest that the EEG component during phase-locked coordination could be associated with mirror system enhancement, whereas the component during uncoordinated tapping could be associated with mirror system inhibition. Although the movements and visual input were the same in both cases, the purported mirror system rhythm emerged only when self/other activity was coupled in time.
With shared circuits for perception and action and for self and other, we must somehow distinguish self-produced from other-produced action. However, in interpersonal synchrony, this distinction can become difficult. One way to distinguish self from other is based on the predicted efference copy (SCM's layer 2): If predicted and actual sensory consequences of an action closely correspond, then the action can be attributed to the self. Temporal correspondence is a key factor in attributing actions to oneself (Sato & Yasuda Reference Sato and Yasuda2005). But when another's movement is similar to one's own in both form and timing, sensory consequences from the other's movement overlap with one's own movement prediction and therefore can render self/other attributions ineffective. Another basis for distinguishing self-produced from other-produced actions is based on monitored output inhibition (SCM's layer 4). Observing another's action maps onto one's own action system and primes similar action, but motor output is inhibited, so the action is not overtly copied. This monitored inhibition during mirroring infers that the observed movement is externally generated, whereas lack of motor output inhibition infers that the movement is self-generated. However, during interpersonal synchrony, mirroring others' actions is not associated with motor output inhibition; hence, one may attribute others' actions to oneself. In interpersonal synchrony, these mechanisms for distinguishing self-generated from other-generated actions are less effective, which in essence blurs the self/other distinction.
Mirror systems and shared intersubjective information prior to distinguishing self and other provide a plausible neural basis for interpersonal empathy (Gallese Reference Gallese2001). Mimicry can lead to affiliation between people because of shared self/other codes as suggested by Chartrand and Bargh (Reference Chartrand and Bargh1999). By extension, interpersonal synchrony could produce even stronger affiliation effects because the representational overlap additionally incorporates temporal alignment. Recent data from an interpersonal finger-tapping study support this notion (Hove & Risen, submitted). The degree of temporal synchrony between co-actors predicted subsequent affiliation ratings. Similarly, ensemble musicians and coupled dancers often report affiliation and empathy with partners. Indeed, Walter Freeman (Reference Freeman, Wallin, Merker and Brown2000) proposed that music and dance evolved as a technology of social bonding. Shared representations, accurate predictions, and temporal alignment can lead to interpersonal empathy and understanding.
In summary, the temporal dynamics during interpersonal synchrony offer an avenue to elucidate the SCM's key aspects of predictive simulation and the self/other distinction. The precise time-course of predicting and integrating other's actions via mirror systems enables interpersonal synchrony. This synchrony can render self/other distinctions ineffective and thereby potentially increase interpersonal empathy. The inclusion of temporal aspects could make the SCM an even more inclusive explanatory framework.
