Understanding the Xexibility of action–perception coupling

The idea that observing an action triggers an automatic and obligatory activation of an imitative action in the motor system of the observer has recently been questioned by studies examining complementary actions. Instead of a tendency for imitation, cooperative settings may facilitate the execution of dissimilar actions, resulting in a relative disadvantage for imitative actions. The present study aimed at clarifying the contribution of associative learning and interference of task representations to the reversal of congruency eVects. To distinguish between the two, an experiment was designed, in which we increased the eVects of associative learning and minimized the eVects of task interference. Participants completed a series of imitation and complementary action runs, in which they continuously imitated or complemented the actions of a virtual co-actor. Each run was alternated with a test run showing the same actions but including color-cues, and the participants were instructed to respond to color instead of the actor’s posture. Reaction times to test runs showed no reversal of facilitation eVects between the imitation and complementary action conditions. This result strongly argues that associative learning cannot adequately account for reversed facilitation eVects. Our study provides additional support for action–perception models that allow Xexible selection of action–perception coupling and challenges the existing models purely based on stimulus–response associations. A remarkable amount of empirical evidence has been reported in the literature so far suggesting a close relation between action observation and action execution. DiVerent studies show for instance that observing actions of other people stimulates motor areas in the observer’s brain (see Rizzolatti, Fogassi, & Gallese, 2001 for review). How exactly action observation and action execution relate to each other is still not fully understood. The dominant view in the literature is that observing an action triggers an automatic and obligatory activation of an imitative action in the motor system of the observer (e.g., Fadiga, Craighero, & Olivier, 2005). According to this view, activated motor representations map directly onto the observed actions, and a successful execution of an action that is diVerent from the one observed would require inhibition of the automatic tendency for imitation (e.g., Brass, Zysset, & von Cramon, 2001a). Recently, however, this direct and rigid view of action– perception coupling has been challenged by studies examining complementary actions. The basic idea is that, although imitation might be obligatorily introduced by action observation in some conditions, and although it might be beneWcial for social development of individuals (e.g., Williams, Whiten, Suddendorf, & Perrett, 2001), an automatic tendency to imitate may actually impede successful cooperation between individuals. When working together, like for instance in joint action tasks, co-actors would mostly be producing complementary or opposite actions rather then imitating each other in order to complete a task successfully and reach the joint goal (Sebanz, Bekkering, & Knoblich, 2006). It seems that, in complex social situations, action coordination relies more strongly on actions complementary to the actions of others than on imitative actions (Newman-Norlund, van Schie, van Zuijlen, & Bekkering, 2007; Shibata, Suzuki, & Gyoba, 2007). Emphasizing the role of complementary actions in action–perception domain, Graf, Schütz-Bosbach, and E. Poljac (&) · H. T. van Schie · H. Bekkering Donders Institute for Brain, Cognition and Behavior, Radboud University Nijmegen, P.O. Box 9104, 6500 HE Nijmegen, The Netherlands e-mail: [email protected] Psychological Research (2009) 73:578–586 579 123 Prinz (2009) have just recently suggested that complementary actions have a great functional importance for human interactions. Understanding social interactions might beneWt more from representations of complementary actions than previously believed. For instance, being able to predict someone’s action more precisely by watching the actions of another person involved in the same interaction might just make a diVerence between success and failure of that social interaction. Neri, Luu, and Levi (2006) have shown that visual discrimination of a human agent, presented as a masked point-light display, is inXuenced by the presence of a second agent. Namely, visual detection performance was better when the two agents were acting in a meaningful synchronized sequence, even though the temporal aspect of the stimuli was actually irrelevant for successful task execution. Neri et al. suggested that human implicit knowledge about the relations of actions between interacting agents generates the synchronicity eVect observed in their study. Additional evidence for the role of representations of complementary actions in human interactions was recently provided by van Schie, van Waterschoot, and Bekkering (2008). The authors demonstrated a reversal of automatic imitation eVect when executing actions in a cooperative setting, suggesting a Xexible action–perception coupling possibly inXuenced by task representations. In their study, participants either imitated (task A) or complemented (task B) a virtual co-actor’s grasp on a manipulandum. In both tasks, in 40% of all trials, a color-cue was presented, requiring the execution of a predeWned grasp independently of the observed action of the virtual actor. Responses in these color (test) trials were compared between congruent (same) and incongruent (opposite) actions of the participant and the co-actor to determine automatic response activation in this cooperative setting. They report that responses in congruent trials were faster than in incongruent trials when these colored trials were presented in the context of the imitation task, indicating a congruency eVect. This eVect, however, completely reversed in the color trials presented in the complementary-action task. The reversal of congruency eVects suggests that the cognitive system is able to circumvent the automatic tendency to copy directly actions or postures of others and to Xexibly set task-goal appropriate stimulus–response (S–R) associations to optimize performance. Van Schie et al. (2008) clearly demonstrate that the mapping between action observation and action execution is not obligatory or automatic but may reXect the nature of the task goal (e.g., imitation or complementary action). It seems again that the coupling between action and perception is more Xexible than typically assumed in the imitation literature. Yet, the exact nature of this Xexible action– perception coupling needs further speciWcation. A possible explanation is that prolonged practice of speciWc S–R associations determines the way the congruency eVect behaves. According to the associative sequence learning (ASL) theory (Heyes, 2001), the cortical connections mediating motor activation through action observation arise primarily through correlated experience of observing and executing the same actions. In other words, the beneWt observed in congruent trials arises due to a lifetime of correlated experience between executed actions and perceptual consequences. Consistent with this view, Heyes, Bird, Johnson, and Haggard (2005) showed that practicing incongruent associations modulates automatic imitation. SpeciWcally, the imitation eVect was abolished after incompatible training. Furthermore, Catmur, Walsh, and Heyes (2007) demonstrated that training participants to perform a diVerent action than the one observed reversed the imitation eVect. Catmur et al. suggest that both imitation eVect and its reversal are based on sensorimotor learning through social interactions with others. Both Heyes et al. and Catmur et al. demonstrate that associative learning could account for automatic imitation (through previously established S–R associations) as well as for its context dependent abolition or reversal (through recently established S–R association). It seems thus possible that the reversal of the congruency eVect as reported by van Schie et al. was caused by associative learning. The study included a considerable amount of trials (120), in which participants executed and trained the complementary action task. In this way, these new S–R associations established in the complementary context might have caused the reversal of the congruency eVect. Although not denying the possible inXuences of learned S–R associations and experience (ASL), van Schie et al. (2008) argued for an approach less dependent on associative learning and more related to the goal level of tasks. The idea they put forward is that the successful reversal of congruency eVect might be due to task interference eVects arising in paradigms involving diVerent tasks, one of which van Schie et al. applied in their study. Previous research using task-switching paradigms, in which Xexible responding to stimulus features is required according to taskspeciWc S–R mappings, has stressed the importance of the task history and the interference that might arise between the competing tasks (e.g., Allport, Styles, & Hsieh, 1994). Interference may occur both at the level of task representations and at the level of speciWc stimuli (e.g., Koch & Allport, 2006; Koch, Prinz, & Allport, 2005; Waszak, Hommel, & Allport, 2003, 2004, 2005; Wylie & Allport, 2000). For instance, Koch and Allport demonstrated stimulus-based priming of tasks in a task-switching paradigm. Their study showed that the imperative stimuli themselves could act as task cues and activate speciWc task representations reXecting the previous task context in which these stimuli occurred. It is thus well possible that, in the study of 580 Psychological Research (2009) 73:578–586 123 van Schie et al., task-speciWc representations were activated when stimuli that were needed for the primary task (i.e., imitation or complementary action) were included in a secondary task (i.e. responding to the color of the same stimuli). In this way, the imitation task may have facilitated the processing of congruent actions in intermixed test trials, whereas the same stimuli speeded up the selection of incongruent actions in the context of a complementary-action task. The current study investigated in more detail the two possible theoretical explanations for the reported reversed congruency eVects. We based our study on the study conducted by van Schie et al. (2008) and aimed at clarifying the contribution of exhaustive training of stimulus– response (S–R) mappings on the one hand and the contribution of task representation inXuences on the other to the reversal of congruency eVects. To distinguish between the two, an experiment was designed, in which we (a) increased associative learning eVects and (b) minimized task interference eVects between the primary and the secondary task. As a result, Wnding a reversed congruency eVect would only be possible to explain in terms of associative learning. On the other hand, a failure to replicate the reversal in congruency between the two conditions (imitation and complementary action) would point toward the implication of task-interference in the original study. We increased associative learning simply by increasing the number of trials of the primary task in which the participants executed and trained the speciWc S–R associations. While van Schie et al. (2008) used 120 trials, we included 192 trials for imitation and complementary action each. Interference between the primary and the secondary task, on the other hand, was minimized by dividing test (color) trials and normal (imitation and complementary action) trials in separate runs. Just recently, Dreisbach and Haider (2008, 2009) have shown that specifying task representations, that is, the task rule that connects the stimuli on a more global level, allows for focusing of attention on task relevant stimulus features and prevents other possibly interfering stimulus features from being processed. In order to further maximize separation between task blocks, the task rules were explicitly introduced on screen before each run and the participants were instructed to actively prepare the upcoming task. Our analysis focused on the test trials, comparing the congruent (executed action is the same as the observed action) and the incongruent (executed action is opposite to the observed action) trials in the imitation task context and the complementary-action task context. If action–perception coupling is sensitive to the activation of multiple interfering task rules that need to be kept online, separating the two tasks in diVerent blocks should lead to a disappearance of reversed congruency eVects. If, however, extensive training of speciWc S–R associations is the main determinant of action–perception coupling, we should Wnd a similar reversal of the congruency eVect as in van Schie et al. (2008).