Discussion
Our goal was to assess whether the positions of obstacles in the surrounding are constantly updated when moving to intercept a target. To do this, we designed a task in which participants had to respond to a sudden change in the target’s position and assessed whether the size of this response was dependent upon the gap size between two obstacles through which participants must pass. In the critical condition, the gap size changed at the same time as the target jumped. The response to the target jump was modulated by the size of the gap between the obstacles: the magnitude of the response to the target jump was smaller when the gap between the obstacles was smaller. This provides evidence that obstacle positions can be updated and incorporated into the adjustment of an ongoing reaching movement.Before discussing this interpretation further, we should rule out alternative explanations. The first is that the modulation of responses by changes in gap size have nothing to do with considering the latest positions of the obstacles, but are automatic responses to visual motion, as was suggested by Aivar et al. (We designed the study in a way that balanced the motion of nearby items to minimise the potential effect of any automatic responses to background motion. However, the finger moved further to the right when the red items were irrelevant, rather than moving straight between the items (Fig. In the early conditions, when the gap changed at the time of the target appearance, the obstacle positions could be incorporated into the initial movement plan. The fact that the response to the target jump considered the position of the obstacles in the early condition is in line with an earlier study showing that obstacles affect arm movement adjustments (Chapman and Goodale The response modulation revealed in the obstacles group appears to be stronger when the gap changed size earlier. This is not the result of moving in a manner that increases the possible vigour when one knows the gap size in advance. We interleaved all the conditions, but participants could have responded to early motion of the obstacles by adjusting how they moved towards the target. However, it is evident from Fig. Here, we perturbed the target to enable us to assess whether the response to such a perturbation was modulated by the position of the obstacles. Other research has used mechanical perturbations whereby the arm is perturbed by applying a mechanical force to assess the effect of obstacles on adjustments to the ongoing movements. De Comite et al. (