Automatic imitation in rhythmical actions: kinematic fidelity and the effects of compatibility, delay, and visual monitoring

Abstract

We demonstrate that observation of everyday rhythmical actions biases subsequent motor execution of the same and of different actions, using a paradigm where the observed actions were irrelevant for action execution. The cycle time of the distractor actions was subtly manipulated across trials, and the cycle time of motor responses served as the main dependent measure. Although distractor frequencies reliably biased response cycle times, this imitation bias was only a small fraction of the modulations in distractor speed, as well as of the modulations produced when participants intentionally imitated the observed rhythms. Importantly, this bias was not only present for compatible actions, but was also found, though numerically reduced, when distractor and executed actions were different (e.g., tooth brushing vs. window wiping), or when the dominant plane of movement was different (horizontal vs. vertical). In addition, these effects were equally pronounced for execution at 0, 4, and 8 s after action observation, a finding that contrasts with the more short-lived effects reported in earlier studies. The imitation bias was also unaffected when vision of the hand was occluded during execution, indicating that this effect most likely resulted from visuomotor interactions during distractor observation, rather than from visual monitoring and guidance during execution. Finally, when the distractor was incompatible in both dimensions (action type and plane) the imitation bias was not reduced further, in an additive way, relative to the single-incompatible conditions. This points to a mechanism whereby the observed action's impact on motor processing is generally reduced whenever this is not useful for motor planning. We interpret these findings in the framework of biased competition, where intended and distractor actions can be represented as competing and quasi-encapsulated sensorimotor streams.

Figure 1. Imperative action stimuli with the factors plane, habitual speed, and orientation.

Figure 2. Sequence of events in the automatic imitation experiment.

(A) A green circle appeared when participants placed their fingers in the start location. (B) Then a picture of the to-be-pantomimed (imperative) action was shown, followed by (C) a distractor movie of the model pantomiming either the same or a different action. (D) In blocks with delayed execution, a red circle was shown for either 4 or 8 s. (E) Execution of the imperative action was cued by display of a neutral, light-grey background, which appeared at the offset of either the distractor movie or the red circle.

Figure 3. Automatic imitation experiment: Cycle times (ms).

Mean cycle times for the factors habitual speed, distractor speed, and the availability of vision of the hand. The error bars show the standard error of the mean.

Figure 4. Automatic imitation experiment: Cycle time ratios (%).

Mean cycle time ratios (with standard error of the mean) for the factors delay, action type compatibility, and plane compatibility (SA = Same Action; DA = Different Action; SP = Same Plane; DP = Different Plane). The cycle time ratio in the distractor actions was 150%.

Figure 5. Intentional imitation experiment: Cycle times (ms).

Mean cycle times (with standard error of the mean) for the factors habitual speed, distractor speed, and the availability of vision of the hand, displayed relative to the actual distractor speeds.

Figure 6. Automatic vs. intentional imitation: Cycle time ratios (%).

Mean cycle time ratios (with standard error of the mean) for the factors intention, action type compatibility, and plane compatibility (SA = Same Action; DA = Different Action; SP = Same Plane; DP = Different Plane). The cycle time ratio in the distractor actions was 150%.

Figure 7. Hypothetical early-visual and sensorimotor processes during the three main events of an automatic imitation trial (for further details, see text).

Event 1: Although the imperative picture did not specify execution speed, it is likely that the sensorimotor representation of the to-be-performed action included the habitual rhythm (cycle time A). Event 2: During presentation of the distractor movie, instructed and distractor actions are represented as two parallel and potentially competing sensorimotor streams. Event 3: The cycle time C during motor execution reflects the result of the biased competition for those two representations in Event 2. The dotted route indicates an alternative explanation of the imitation bias, via visual monitoring and related corrections during execution, for which no evidence was found in the present study.