The controlled imitation task: a new paradigm for studying self-other control

Introduction

In a popular version of the automatic imitation task (AIT), participants respond to a symbolic cue (usually a number) that instructs them to lift either their index finger or their middle finger. The symbolic cue is overlaid on a video showing another individual lifting their index or middle finger. When the cue and the video are incongruent, there is a reaction time (RT) cost to produce the cued movement compared to when the cue and the video are congruent. This cost, termed “interference”, is thought to be due to automatic activation of motor representations matching the observed action. Numerous control studies and conditions have been employed to demonstrate that the task really does seem to isolate automatic imitative tendencies and not simply spatial compatibility or other confounding processes (Brass et al., 2000; see Heyes, 2011 for a review), though the extent to which they are etiologically similar is a matter of open debate (Bertenthal & Scheutz, 2013; Cooper, Catmur & Heyes, 2012).

In essence, the AIT assesses the degree to which an observer’s motor system ‘resonates’ with an observed action of another individual. Successful task performance in the incongruent condition specifically involves suppressing the automatic tendency to imitate another individual, in favor of making the cued response (Heyes, 2011; Santiesteban et al., 2012b). The difference between incongruent and congruent trial RTs is robust even when participants expect the observed action to contradict their response, and thus the paradigm can be considered within the set of cognitive tasks that are generally accepted as reflecting automatic processes, such as the Stroop task (Hogeveen & Obhi, 2013; Stroop, 1935; Tzelgov, Henik & Berger, 1992). Notably, however, the neurocognitive bases for automatic imitation have been dissociated from Stroop interference, as neuroimaging and lesion data have shown that the two tasks are associated with activity in largely different brain regions (Brass, Derrfuss & von Cramon, 2005; Brass, Zysset & von Cramon, 2001; Spengler, von Cramon & Brass, 2010).

At the neural level, the automatic tendency to imitate the actions of another agent is thought to be due to mirror activity which induces motor resonance - the specific activation of motor representations that would be active if the observer themselves performed the action they are watching (Hogeveen & Obhi, 2011). This idea is bolstered by evidence from transcranial magnetic stimulation (TMS) studies which have consistently found an increase in motor cortical excitability when a participant watches an action, that is specific to the motor representation that would be involved in the performance of the action (e.g., Fadiga et al., 1995).

Over recent years the AIT, in one form or another, has been heavily employed by researchers interested in social information processing in normal participants and in participants with a range of disorders, but most notably autism spectrum disorders (ASD) (Cook & Bird, 2012; Hogeveen & Obhi, 2011; Spengler, Bird & Brass, 2010; Wang, Ramsey & Hamilton, 2011). A strong suggestion has been made that a key process involved in the AIT is the process of controlling self-related and other-related representations (Brass, Ruby & Spengler, 2009; Obhi et al., 2013; Santiesteban et al., 2012a; Spengler, Bird & Brass, 2010). Indeed, in a recent study, Santiesteban and colleagues (2012b) asked a group of participants to perform the opposite movement from the model onscreen for an extensive training period (counter-imitation training). The next day, they found a downstream improvement in both (i) their performance on incongruent trials of the automatic imitation task, and (ii) their ability to take someone else’s visual perspective. In the perspective taking task participants were required to move objects to locations within a bookshelf as requested by another person on the other side of the bookshelf, counter-imitation training was found to improve participants’ ability to suppress their own perspective and take the perspective of the other. On the one hand, reduced interference in the AIT corresponds to suppressing influence from another agent in favor of prioritizing a more task relevant cued response. On the other hand, taking the perspective of another person involves suppressing the self-related representation, in favor of the other perspective. In this light, both tasks can be said to require control of self-related and other-related representations.

The AIT, then, is an example of a task requiring self-other control processes and in particular, the control requirement in the critical incongruent trials is to suppress the automatic imitative response, and enforce, or prioritize the cued response. In the present study, we introduce a complementary paradigm for the study of self-other control which has essentially the opposite control requirements to the AIT. We term the paradigm the “controlled imitation task” (CIT) so as to contrast it explicitly with the “automatic imitation task”. In the CIT, as in the AIT, participants are exposed to a picture of a neutral hand upon which a symbolic cue in the form of a numeral is superimposed. Also akin to the AIT, the participant must lift their index finger in response to the number 1 and must lift their middle finger in response to the number 2. Critically, however, both the temporal evolution of a given trial, and the task instructions in the CIT clearly dissociate it from the AIT. With respect to trial timing, whereas the AIT cue and movement appear simultaneously, in the CIT the cue is presented before the onset of any observed movement. Then, within 100 ms of the cued response, the hand upon which the symbolic cue was presented makes either a congruent or an incongruent action, such that half the time, participants see an index finger lift when they are preparing an index finger lift (congruent trial), or they see a middle finger lift when they are preparing an index finger lift (incongruent trial), and vice versa for cued middle finger lifts. With respect to task instructions, whereas in the AIT participants are asked to ignore the observed movement and enforce the cued response, in the CIT if the onscreen finger moves participants are instructed to respond immediately with an identical finger movement. In essence, on incongruent trials of the CIT participants must suppress their own cued action preparation, in favor of producing the imitative response – which constitutes the opposite control requirement compared to the standard AIT. Crucially, these congruent and incongruent trials only occur on half of the total trials in a block, and in the other half of trials, the neutral hand on the screen does not move and participants simply execute the cued response. Thus, across the block, participants expect to respond to the cue, rather than simply waiting for an imperative movement to occur.

Using this paradigm, we predicted that the RT to make the controlled imitative response would be greater on incongruent trials compared to congruent trials. However, in contrast to the RT cost in the AIT, which is driven by the need to suppress imitation, the RT cost in the CIT would reflect the time required to suppress the cued response, in favor of producing an imitative response. If the congruence effects we predict are found, we suggest that this task could be a useful paradigm for the study of self-other control processes. In particular, the use of the CIT in conjunction with the AIT, in experiments where self-other control is manipulated in the context of imitation (Santiesteban et al., 2012a; Santiesteban et al., 2012b) would allow researchers to be confident that what they are manipulated is indeed control of co-activated self- and other-generated motor representations. Beyond this, we suggest that the CIT could be a useful paradigm for the more general study of processing relating to self and other.

Experiment 1

The aim of experiment 1 was to provide a proof of concept that an activated motor plan interferes with one’s ability to imitate an incongruent observed action. To that end, participants planned to make simple index and middle finger lifts in response to the number 1 or 2, respectively. During the response preparation period (50, 70, or 90 ms after cue-onset), an onscreen hand was most likely to remain still (baseline condition), or might have depicted an index or middle finger lift. On one subset of trials, the depicted action was congruent with the symbolic cue, and was therefore congruent with the participants’ motor plan. However, on another set of trials, the depicted action was incongruent with the symbolic cue, and participants had to modify their plan to prioritize execution of an imitative response. The magnitude of the performance difference between the congruent and incongruent conditions could be informative about the degree to which a symbolically cued motor plan can interfere with the execution of an imitative action.

Methods

Participants

Twelve participants (9 female) between the ages of 19 and 33 (M = 23.33, SD = 3.55) completed the study. Three of the 12 participants were left-handed, and all had normal or corrected-to-normal vision. The study conformed to local ethical guidelines and informed consent was obtained from all participants.

Stimuli & apparatus

The experiment was programmed using Superlab v.4.5 (Cedrus Corporation) and run on a Lenovo desktop computer (Lenovo Group Limited). Experimental stimuli were adapted from previous automatic imitation studies (Brass et al., 2000; Cook & Bird, 2012; Hogeveen & Obhi, 2013). The stimuli contained a number cue, followed by an image series depicting either an index or middle finger movement (congruent and incongruent conditions), or a still image of a hand (baseline; Fig. 1). Baseline trials made up 50% of the total trials, to ensure that participants were expecting to make the cued response and not simply waiting for the movement of the onscreen hand. The images were rotated orthogonally to the participants’ hand to mitigate the influence of spatial compatibility between the observed action and the appropriate response. On baseline trials, the hand stayed in the same position for the duration of the trial (568 ms). On congruent and incongruent trials, the still hand was replaced by a number cue, which was then replaced by the first stage (34 ms), second stage (34 ms), and final stage (500 ms) of an index or middle finger lift. The final event on each trial was a blue screen, which was presented for 3000 ms to allow for delayed responses and provide participants with enough time to get situated for the next trial.

Procedure

The experiment took place in a dimly lit cubicle. Participants were seated at the computer, told they would be holding down the v and b keys of the keyboard with their right index and middle fingers throughout the experiment, and would need to execute an index or middle finger lift as fast and accurately as possible when the number 1 or 2 was presented onscreen, respectively. Responses were made by releasing the v or b keys on a standard Lenovo keyboard. Further, participants were told: “if the hand onscreen moves, cancel your response and imitate what you see regardless of the movement you were planning to make”.

The study began with a practice session containing 24 trials, which were observed by the experimenter to ensure that participants understood the task. If the participant made more than 5 mistakes, the practice section was re-run, which only occurred for one participant in the sample. The experiment proper was divided into six blocks of 32 trials, each containing four congruent index finger trials, four congruent middle finger trials, four incongruent index finger trials, four incongruent middle finger trials, eight baseline index finger trials, and eight baseline middle finger trials. Each trial began with a still image of the hand on screen (300, 400, 500, 600, 700, or 800 ms), then the response cue was presented, and after a delay period (50, 70, or 90 ms) the hand either moved or stayed still (see Stimuli & Apparatus for a description of the hand movement).

Results

Data exclusion

As described in the Methods section, the baseline condition was in place purely to ensure that participants actually prepared the cued response on each trial, and was excluded from further analysis. Reaction time data for index and middle finger actions were collapsed and responses that were 3 SD’s above or below the mean within each experimental condition (i.e., congruent, or incongruent) were excluded from statistical analysis, resulting in the removal of 0.39% of the data. The remaining congruent and incongruent trial data was then subjected to inferential statistical analysis.

Reaction time (RT)

Each participant’s average RT for correct responses was entered into a repeated-measures ANOVA with one factor (congruence: congruent, or incongruent). The effect of congruence was highly significant (F_1,11 = 103.06, p < .001, η² = .90). Participants were faster on congruent trials (M = 434, SE = 9) than they were on incongruent trials (M = 553, SE = 14; t₁₁ = −10.15, p < .001, η² = .68; Fig. 2A).

Proportion of errors (PE)

Errors and missed responses (3.95% of total data) were summed and divided by the number of trials, yielding the proportion of errors (PE) for each participant for each condition. This data was entered into a repeated-measures ANOVA akin to the RT data, and the main effect of congruence was highly significant (F_1,11 = 38.11, p < .001, η² = .78). Participants made a substantially smaller number of errors on congruent (M = 0.01, SE = 0.01) trials compared to incongruent trials (M = 0.14, SE = 0.02; t₁₁ = −6.17, p < .001, η² = .46; Fig. 2B).

Experiment 2

In experiment 2, we sought to determine whether the CIT indexes self-other control in an imitation context, or simply quantifies the cost associated with modifying a planned response. To this end, participants performed the same task as described in experiment 1, but with the addition of a moving dot condition (spatial control; Cross et al., 2013; Fig. 3). The dot cues were superimposed over the hand stimuli from experiment 1, and on CIT blocks participants were instructed to match what they saw onscreen whenever one of the dots, or one of the fingers, moved during their response preparation period. The difference between ‘finger incongruent-congruent’ interference and ‘dot incongruent-congruent’ interference provides an approximation of the cost associated with cancelling a self-related motor plan in favor of an imitative response. Admittedly, if no interaction between stimulus type and congruence were present in Experiment 2, the matching RT functions could still be driven by distinct underlying mechanisms, but such subtraction logic is often an inherent limitation of purely behavioral data (Rumelhart & McClelland, 1986). Experiment 2 also included a set of AIT blocks.

General Discussion

In two experiments, we report a novel task requiring the online control of competing self- and other-generated motor representations. We have called this task the controlled imitation task (CIT), and suggest that it is a useful complementary task to the well-studied AIT. On each experimental trial, participants had to prepare a cued response, but on 50% of trials they were required to suppress this response and instead prioritize the production of an imitative response that was either congruent or incongruent with the cue. The robust result is that participants were significantly better at imitating an observed movement when it matched the response cue, relative to when it did not match the cue (experiment 1), and that this effect was reliable and of a much larger magnitude than the traditional AIT (experiment 2).

We suggest the CIT provides a valuable method for examining the influence of the self with respect to self-other control in the imitative context. To the extent that the production of a symbolically-cued response reflects a task specific self-activated process, the incongruent condition in the CIT can be thought of as requiring suppression of this process in favor of producing an imitative response. Thus, the present results nicely complement typical AIT data: in the AIT interference effects reflect the cost of inhibiting an other-activated motor representation in favor of a cued response, whereas in the CIT interference effects reflect the cost of inhibiting a self-activated motor representation in favor of imitation.

Experiment 2 enables us to suggest not only that controlled and automatic imitation are more pronounced than general response modification and inhibition, respectively, but also that this biological preference is driven by recent experience. Typically, biological-specificity effects are present a priori (Kanwisher, McDermott & Chun, 1997; Kilner, Paulignan & Blakemore, 2003), but here preference for the imitative stimulus varied as a function of task order. Participants who engaged in the CIT initially, showed a finger-preference on the subsequent AIT, and participants who did the AIT first showed this preference on the subsequent CIT, but neither group showed a stimulus preference during the initial task. This suggests that inhibiting imitation at time 1 (AIT) makes it more difficult to perform imitation at time 2 (CIT), which was not true for the dot control condition. Conversely, performing imitation at time 1 (CIT) makes it harder to inhibit imitation at time 2 (AIT), which again was not the case for the dot control condition. Thus, it appears the shared self-other representational system that is invoked to explain the traditional AIT is also engaged by the CIT. Furthermore, this system seems to be strongly affected by recent experience (Catmur, Walsh & Heyes, 2007; Heyes et al., 2005; Hogeveen & Obhi, 2012).

Beyond the relationship between the CIT and AIT, there are several interesting issues and possibilities for future work that could be studied with the CIT. For example, for studies designed to manipulate self-other control, the CIT provides an excellent and quite well matched experimental paradigm with which to contrast AIT, and investigate task-appropriate shifts in processing towards self or other in the motor domain. A recent study by Santiesteban and colleagues (2012b) found that imitation-inhibition training on day 1 suppresses the automatic tendency to imitate on day 2, in order to facilitate task performance which necessitates production of the cued response. Whereas the size of the AIT interference effect was reduced after training, accuracy on a visual perspective-taking task was improved, suggesting that processing of the other person was enhanced or inhibited in a task-appropriate manner (Santiesteban et al., 2012b). Despite the impressive nature of this finding, the CIT provides a closer other-centric homologue for AIT than visual perspective taking. Thus, if counter-imitation training has a functional impact on task dependent control of self- and other-generated motor representations, we would expect to observe an increased ability to suppress the self-activated motor representation in the CIT. Seeing a pattern of results that is consistent with suppression of self-related activity in the CIT combined with suppression of other-related activity in the AIT would bolster the notion that the counter-imitation training improves task specific control of self-other processing.

In addition to comparing the AIT and CIT directly after a self-other control manipulation, the extant difference between the two tasks could be used to index individual differences in self-related bias. On their own, the size of the CIT or AIT interference effects may reflect self-other distinction, control, or both. However, by subtracting the AIT from the CIT, experimenters could quantify the extent to which self-other control is biased in favor of the self, which could then be related to personality variables hypothesized to relate to self-other control. It remains for future work to investigate this suggestion further.

In the non-social context, another very interesting aspect of the CIT is that it can be clearly contrasted with the truncation paradigm, in which internal generation of action is pitted against external cueing of action in non-imitative contexts (Obhi & Haggard, 2004; Obhi, Matkovich & Chen, 2009; Obhi, Matkovich & Gilbert, 2008). In particular, in the truncation paradigm participants begin the trial preparing to make an index finger movement at a time of their own choosing, but are interrupted on a portion of trials by a tone prompting production of the same movement. That is, in the truncation paradigm, participants prepare to make a self-paced action and are interrupted with a non-social external cue to produce the same action or a different action, whereas in the current study the external cue to action is a movement by another person. Given the dissociation between the ability to inhibit imitation in the AIT and the inhibition of other overlearned responses (Brass, Derrfuss & von Cramon, 2005; Spengler, von Cramon & Brass, 2010), it would be interesting to determine whether the CIT and action modification in the truncation paradigm depend on similar or specialized neural substrates. Implementation of this line of enquiry would necessitate modification to the CIT to include a self-paced action, as opposed to a cued action. Such experiments could be useful for understanding the domain-generality or domain-specificity of the functional mechanisms underlying self-other control.

In summary, in two experiments we introduce a new paradigm that we have termed the “controlled imitation task”. Using an identical stimulus set to the established AIT, the CIT yields robust interference and facilitation effects that appear to shed light on self-other control processes, and that specifically tap into the capacity to suppress, or harness self-activated motor representations to facilitate fast production of an other-activated response (cf. Brass, Ruby & Spengler, 2009). We have outlined just two possible areas of inquiry where the CIT could be a useful paradigm; self-other control, and the more general exploration of internally versus externally triggered action generation. It remains for future work to consider exactly how this paradigm can be employed to further understanding in these areas of study.