Facilitation and interference in spatial and body reference frames.

The observation of someone else's action facilitates similar actions in the observer. Such priming effects can be driven by alignment between the observer and the observed in body-centred or spatial coordinates (or both). The separate and joint contributions of these sources of priming remain to be fully characterised. Here, we compare spatial and body priming effects across the whole body "space", by using hand and foot responses. This allows a clearer separation of body priming from spatial priming than available from previous studies. In addition, we demonstrate two further features of these action priming effects. First, there are general interference and facilitation effects when the layout of viewed displays matches the participant's body (e.g. hand above the foot). These effects have not been considered in previous studies. Second, by taking these layout effects into account, we identify the facilitation and interference components of spatial and body priming effects. Both types of priming effect are observed, and facilitation and interference effects are only observed when both body and spatial frames of reference are working in the same direction. These findings show that in action perception, the behaviours of others are processed simultaneously in multiple frames of reference that have complex, interacting effects--both facilitating and interfering--on the motor system of the observer.