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We investigated how two people learn to coordinate their movement to achieve a joint goal. Pairs of participants oscillated a joystick with their dominant hand whilst looking at a common feedback, a Lissajous figure, where each participant controlled either the vertical or horizontal coordinate of a moving dot. In the absence of specific instructions, inter-personal coordination was highly variable, punctuated by intermittent phase locking. When participants were required to produce a circular Lissajous figure, coordination variability decreased while accuracy, transfer entropy and the incidence of stable coordinative solutions (fixed points, including bi-stability) increased as a function of practice trials. When one partner closed his/her eyes, so that the other one received the full control of error correction, the stability and accuracy of coordination decreased. A questionnaire showed that partners experienced the feeling of we-control. The results were interpreted in terms of a disturbance ∼ correction challenge: joint action is enhanced by having a flexibly adjusting co-actor rather than a more predictable, but not adjusting, partner. At transfer, partners were able to produce a new, never-practiced Lissajous pattern, evidencing the generalisability of joint learning., (Copyright © 2018 Elsevier B.V. All rights reserved.)

This study investigates how motor coordination undergoes the passage from a discrete to a continuous movement régime. Participants repeated concatenated discrete movements with each hand such that one hand was lagging the other by a quarter of a cycle (i.e., with a 90° phase difference). As movement frequency increased, the tendency to persist in this relative phase competed with a progressive effect of the interlimb coupling favoring 0° and 180°. In 61% of the participants, a switch from a discrete to a continuous motion régime was accompanied by a shift toward the 0° or 180°. The 0° was more often favored than 180°. The remaining participants sustained a relative phase close to 90° even at the highest movement frequency and proved to be more accurate at the initial lowest frequency. These findings indicate that a priming effect may circumvent the tendency to produce preferred patterns and favor the production of nonpreferred patterns and that initial individual differences affect how motor coordination evolves with changing constraints.