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Joint action (JA) is a continuous process of motor co‐regulation based on the integration of contextual (top‐down) and kinematic (bottom‐up) cues from partners. The fine equilibrium between excitation and inhibition in sensorimotor circuits is, thus, central to such a dynamic process of action selection and execution. In a bimanual task adapted to become a unimanual JA task, the participant held a bottle (JA), while a confederate had to reach and unscrew either that bottle or another stabilized by a mechanical clamp (No_JA). Prior knowledge was manipulated in each trial such that the participant knew (K) or not (No_K) the target bottle in advance. Online transcranial magnetic stimulation (TMS) was administered at action‐relevant landmarks to explore corticospinal excitability (CSE) and inhibition (cortical silent period [cSP]). CSE was modulated early on before the action started if prior information was available. In contrast, cSP modulation emerged later during the reaching action, regardless of prior information. These two indexes could thus reflect the concurrent elaboration of contextual priors (top‐down) and the online sampling of partner's kinematic cues (bottom‐up). Furthermore, participants selected either one of two possible behavioural strategies, preferring early or late force exertion on the bottle. One translates into a reduced risk of motor coordination failure and the other into reduced metabolic expenditure. Each strategy was characterised by a specific excitatory/inhibitory profile. In conclusion, the study of excitatory/inhibitory balance paves the way for the neurophysiological determination of individual differences in the combination of top‐down and bottom‐up processing during JA coordination. [ABSTRACT FROM AUTHOR]

Abstract: Previous studies have shown a motor recruitment during the observation of graspable objects. This recruitment has been considered crucial in encoding the observed objects in terms of one or more potential motor acts. However, an agent can actually act upon an object only when the latter is close enough to be reached. Thus, the question we deal with in this paper is whether the motor system is always activated whenever a graspable object comes into view or whether it requires the object to be located within the reachable space of the perceiver. The left primary motor cortex was magnetically stimulated and motor evoked potentials (MEPs) were recorded while participants observed graspable and non graspable objects located within or outside their own reachable space. We found higher MEPs during the observation of graspable objects falling within the reachable space compared to the observation of either a non graspable object or a graspable object falling outside the reachable space. Our results shed new light on the functional role of the motor system in encoding visually presented objects. Indeed, they clearly indicate that its recruitment is spatially constrained, as it depends on whether the object falls within the actual reaching space of the onlooker. [Copyright &y& Elsevier]