Straight Reaches are Due to Standard Visual Feedback

It is generally understood that reaching movements that aim to be efficient, for example by minimizing metabolic costs or mechanical power, will arc the hand along curved paths. Yet behavioral studies consistently find that subjects move the hand along a straight path. It has also been shown that visual feedback depicting hand and target locations influences how subjects move. Could this influence explain the lack of path curvature in experimental results? In particular, would reaches examined under different feedback conditions elicit curvature indicative of efficient movements? To address these questions, we reproduced an early and influential study (Morasso 1981) along with two variations that progressively reduced the extent of visual feedback of the hand and target locations in two experiments. As is often found, subjects moved their hands along relatively straight paths when provided with the standard visual feedback. In contrast, the two new conditions in the Experiment elicited curved paths (as quantified by perpendicular errors, angular errors, and path length). Notably, this curvature increased as the extent of visual feedback decreased. The changes in curvature could not be attributed to increases in task difficulty, but rather appear to reflect the motor system’s intentional strategies for reaching when the visual feedback of the task is diminished. Furthermore, in order to check if the observed curvature in the absence of visual feedback was actually a preference and not a mere chance or an artifact of unclear picture of accurate target location, we devised another experiment. Here subjects reached for the same targets, but the standard visual feedback was removed either in the first or the second half of the experiment. The results from this experiment while showcased a preference for curved trajectories in the absence of visual feedback irrespective of the order in which the feedback was removed, also displayed the strong influence of the standard visual feedback even after it was removed. Our results are further evidence that the straight reaches often observed may be due to a bias to move the displayed feedback along straight paths. Eliminating this visually-induced bias may expose innate reaching behaviors more appropriate for investigating the motor system, possibly revealing a preference for efficient reaches.