Influence of action execution on reachability and distance perception.

• ¹ Université catholique de Louvain, IPSY, Belgium

In the current literature, it is thought that the behaviours of reachability and distance perception rely on parietal lobe and motor system processes. For example, Coello et al. (2008; see also Coello & Delevoye-Turrell 2007) demonstrated that disruption of primary hand motor cortex moderated the decision of whether an object was reachable of not. This suggests that the perception of reachability or reach space appeared to rely on body representations and action simulation cognition. Similarly, other studies have demonstrated that distance perception can be moderated by the manipulated state of the body; for example by wearing a backpack (Proffitt, 2006), by having sport expertise (Witt, 2011) or by having the use of a tool (Witt, Proffitt & Epstein, 2005; Witt & Proffitt, 2008). These results suggest that the perception of space is derived from an embodied representation, as the environment seems to be perceived through the ability to act in it. Although we suggest here that reachability and distance perception might rely on similar cognitive representations, it remains unclear what differences exist between the two different behaviours, and moreover, whether there is a single common cognitive process underlying the behaviours. To investigate this, we measured spatial perception using reachability judgement and distance magnitude estimation tasks. The reachability judgement task measured participant’s judgements of whether a stimulus was within reach or not, and two distance magnitude estimation tasks measured the estimated distance between the participant and a stimulus (egocentric distance) or between two stimuli (allocentric distance). For the reachability judgement and egocentric distance magnitude estimation tasks, the same stimuli positions were used to allow for a direct comparison between the tasks. A further manipulation involved the participants making concurrent actions during the three tasks. These included hand actions of soft-ball grip squeezing, arm actions of weight lifting or the baseline comparison of no movements. We predicted that if action processes are needed for both the reachability and distance magnitude tasks, then performing an action at the same time might moderate the participant’s responses, measured by the reaction time and accuracy of response. Moreover, the aim was to investigate the differential impact of the motor disruption in allocentric and egocentric distance perception. Results showed that the motor disruption conditions had a significant impact on participant’s reaction time, with the grasping movements slowing the participant responses in the reachability judgement task and in the egocentric distance estimation task. However, the motor disruption conditions had no impact in the allocentric distance estimation task. None of the tasks showed any significant effects of motor disruption on accuracy. These findings confirm the existing literature as they show that executing concomitant actions moderated the perceptual tasks, suggesting that motor processes were necessary for the space cognition processes. Moreover, our data suggest that evaluating the distance separating two stimuli do not require action processes. The data are discussed in terms of how spatial cognition involves motor cognition that is body dependent.