Two is better than one: physical interactions improve motor performance in humans

Abstract

How do physical interactions with others change our own motor behavior? Utilizing a novel motor learning paradigm in which the hands of two - individuals are physically connected without their conscious awareness, we investigated how the interaction forces from a partner adapt the motor behavior in physically interacting humans. We observed the motor adaptations during physical interactions to be mutually beneficial such that both the worse and better of the interacting partners improve motor performance during and after interactive practice. We show that these benefits cannot be explained by multi-sensory integration by an individual, but require physical interaction with a reactive partner. Furthermore, the benefits are determined by both the interacting partner's performance and similarity of the partner's behavior to one's own. Our results demonstrate the fundamental neural processes underlying human physical interactions and suggest advantages of interactive paradigms for sport-training and physical rehabilitation.

Figure 1. (A) Setup: The figure shows a cartoon of the setup used for the experiment.

Subjects worked in dyads, each holding a handle of the robot interface. They tracked a moving red target on their respective monitor with a cursor (white dot) representing their hand position. They had no visual information of their arm which was covered by the table. The target movement was the same for both subjects in the dyad. In the dual trials, their hands were connected by a virtual elastic band (represented in brown) such that each subject was pulled towards the hand position of the partner. The subjects could see their partner but could not see the partner's hand or monitor. (B) Improvement during interaction: The improvement in task performance in each subject for each dual trial was plotted against the relative performance of their partner. The dual trial improvement was measured by the change in tracking error by a subject during a single trial compared to his individual tracking error in the immmediatly preceding dual trial. It is observed that when connected to a better performing partner (+abcissa), an individual's performance improved. Interestingly, the individual performance improved even when connected to a partner with inferior performance in the task (−abcissa). (C) Learning during interaction: The improvement in task performace (relative to the first trial) across the single trials of the interacting subjects (green trace) in the visuomotor learning sessions was compared to improvement of task performance by solo subjects (red trace) who never interacted with a partner. Note that the trace combines single trials across subjects such that each data point and error bar represents single trials made by ten of the (total of 20) subjects. Intermittent interaction enabled significatly higher motor learning in individuals.

Figure 2. Influence of interaction characteristics.

The data from the main interaction experiment (black trace), force-playback control (green trace), trajectory-playback control (orange trace), target-connect (purple trace) and expert-connect (blue trace) were plotted in the same format as Fig. 1b. The positive- abcissa plots an individual's performance improvement when connected to a partner with superior performance in the task. The negative abcissa plots an individual's performance improvement when connected to a partner with inferior performance in the task. We compared the overlapping abscissa between any two partner conditions and observed the behavior in the control experiments (green and orange traces) to be significantly different from the interaction experiment. While connection to the target (purple) or an expert human (blue) were qualitatively similar to connection to a novice human (black trace), they were still quantitatively different, indicating that both the performance and nature of the partner are important determinants of interaction performance in humans.