Moving higher and higher: imitators’ movements are sensitive to observed trajectories regardless of action rationality

Abstract

Humans sometimes perform actions which, at least superficially, appear suboptimal to the goal they are trying to achieve. Despite being able to identify these irrational actions from an early age, humans display a curious tendency to copy them. The current study recorded participants’ movements during an established imitation task and manipulated the rationality of the observed action in two ways. Participants observed videos of a model point to a series of targets with either a low, high or ‘superhigh’ trajectory either in the presence or absence of obstacles between her targets. The participants’ task was to watch which targets the model pointed to and then point to the same targets on the table in front of them. There were no obstacles between the participants’ targets. Firstly, we found that the peak height of participants’ movements between their targets was sensitive to the height of the model’s movements, even in the ‘superhigh’ condition where the model’s action was rated as irrational. Secondly, participants showed obstacle priming—the peak height of participants’ movements was higher after having observed the model move over obstacles to reach her targets, compared to when there were no obstacles between her targets. This suggests that participants code the environment of co-actors into their own motor programs, even when this compromises the efficiency of their own movements. We discuss the implications of our findings in terms of theories of imitation and obstacle priming.

Repetitive transcranial magnetic stimulation reveals a role for the left inferior parietal lobule in matching observed kinematics during imitation

“…Whilst meaningless action imitation may be more reliant on kinematic processing (Buxbaum et al ., ), kinematic information might still be relevant to the way in which one must replicate a meaningful action. To take an extreme example, the imitator is not likely to ignore explicit but irrelevant kinematics, such as a particularly slow or rapid action which does not assist in the development of the final posture (see Forbes & Hamilton, , for evidence that possibly supports this).…”

Co-actors represent the order of each other’s actions

“…The fact that similar effects as in Experiment 1 were observed when co-actors could not see each other’s actions is crucial in that it distinguishes the present findings from earlier findings on automatic imitation of action sequences (e.g., Forbes and Hamilton, 2017 , Pan and Hamilton, 2015 ; for a recent meta-analysis on automatic imitation, see Cracco et al, 2018 ). In the latter studies, it was tested whether participants’ own performance of an action sequence is affected by prior observation of a (virtual) actor performing the same or a different action sequence.…”

“…For example, participants first watched an actor in a video perform three consecutive taps on three different drums; then participants tapped their own three drums either in the same or in a different order as the actor ( Pan & Hamilton, 2015 ). In another study ( Forbes & Hamilton, 2017 ), participants first observed an actor perform a series of three target-directed pointing movements in the presence or absence of obstacles between the targets. Afterwards, participants pointed to the same targets as the actor but there were no obstacles between the targets.…”

“…Afterwards, participants pointed to the same targets as the actor but there were no obstacles between the targets. Results showed that participants performed their actions faster when tapping the drums in the same order as the actor ( Pan & Hamilton, 2015 ) and that they performed their own movements higher after having observed the actor move over obstacles compared to when there were no obstacles in the actor’s movement path ( Forbes & Hamilton, 2017 ), respectively. These findings have been interpreted in the theoretical framework of automatic imitation which relies on the assumption that observing the movements of a human-like actor will prime the observer’s own motor system, resulting in, for example, faster performance for movements that are anatomically congruent to the observed movements, or in an assimilation of the observer’s movement trajectory to the observed trajectory.…”

A kinematic examination of dual-route processing for action imitation

“…Whilst previous work has quantitatively examined the kinematic elements of imitative behavior in both healthy people (e.g., Braadbaart et al, 2012;Campione & Gentilucci, 2011;Era et al, 2018;Forbes & Hamilton, 2017;Gold et al, 2008;Hayes et al, 2016;Krüger et al, 2014;Pan & Hamilton, 2015;Reader & Holmes, 2015;Reader et al, 2018;Sacheli et al, 2012;Sacheli et al, 2013, Sacheli, Christensen, et al, 2015Wild et al, 2010;Williams et al, 2013) and brain-damaged patients (e.g., Candidi et al, 2018;Hermsdörfer et al, 1996), as far as we are aware no previous experiments have looked at so many components of the velocity profile in order to compare the coarse-grained (i.e., wrist) kinematic approach to meaningful and meaningless action imitation. Much informative work has been done to assess action performance in meaningful and meaningless action imitation (e.g., Buxbaum et al, 2014;Goldenberg & Hagmann, 1997;Mengotti et al, 2013), but frequently using only subjective rating measures.…”