Line-bisectioning and obstacle avoidance: evidence for separate strategies.

Previous studies have frequently applied a combination of line-bisection tasks (in which participants indicate the middle of a line) and obstacle avoidance tasks (in which participants move their hand between two obstacles) with the aim of revealing perception-action dissociations in certain neurological disorders, such as visual form agnosia and optic ataxia. However, valid conclusions about the underlying processing pathways can only be drawn if participants apply the same strategy in both tasks (i.e. finding the middle between the obstacles). Yet, this assumption has never been tested directly. In this experiment, we investigated whether participants perform obstacle avoidance and line-bisectioning using similar strategies by manipulating the position of the obstacles and the start position of the hand relative to the obstacles. Our results indicate that the lateral hand position during obstacle avoidance does not only vary as a function of obstacle location but also strongly depends on the start position. Moreover, participants showed increased sensitivity to obstacle shifts occurring closer to the hand's start position. In contrast, during line-bisectioning the sensitivity to obstacles shifts was unaffected by the hand's start position. The findings suggest that during obstacle-avoidance the need to keep a safe distance from the obstacles is balanced with the requirement to minimise energetic demands. In contrast, the main intention during line-bisectioning is to move to the perceived midpoint as accurately as possible. The fact that very different constraints underlie trajectory planning in both tasks implies that caution has to be taken when interpreting differences in performance levels.