Why does an obstacle just below the digits’ paths not influence a grasping movement while an obstacle to the side of their paths does?

Abstract

When we grasp objects in daily life, they are often surrounded by obstacles. To decrease the chance of colliding with an obstacle, people tend to move in a manner that does not bring body parts too near to the obstacle. However, in a previous study, when we compared moving above empty space and moving above an obstacle (a table), we did not find an effect of the obstacle on the height of the digit's paths despite the fact that the distance between the final positions of the digits and the obstacle was marginal. This lack of effect seems to be inconsistent with what we know about avoiding obstacles, because we would expect an increase in the height of the digits' paths when the obstacle is present. We consider four possible explanations for the lack of effect: that people changed movement speed rather than movement path, that the height component is not sensitive to obstacles that do not physically obstruct the movement, that obstacles below the starting position are not taken into account because the digits do not enter the space below the starting position, and that manipulable obstacles interfere with movement planning while a table does not. We found that from these four explanations only not taking obstacles placed below the starting position into account can be responsible for the lack of effect found in our previous study.

A review of grasping as the movements of digits in space

“…Since slower movements have less risk of hitting the target object accidentally, the approach can pass closer to the object's surface, so maximum grip aperture can be smaller (Smeets and Brenner 1999;Verheij et al 2012). No effect on maximum grip aperture is observed if a vertical curvature is induced without slowing down the movement (Verheij et al 2014b).…”

Gaze–grasp coordination in obstacle avoidance: differences between binocular and monocular viewing

“…Moreover, selection of the thumb or the finger as the guide to contact appears less dependent on whether their respective grasp-sites are visible and, hence, available for fixation (de Grave et al 2008;Melmoth and Grant, 2012;Voudouris et al 2012a), but more on the end-point grip orientation adopted, a postural constraint also known to be influenced by the presence of obstacles (Rosenbaum et al 2001;Voudouris et al 2012b;Verheij et al 2014b). In this context, Mon-Williams and McIntosh (2000) have speculated that choosing the thumb or finger as the guide for coordinating grasping in multiple-object scenes might depend on the relationship between obstacle proximity to each digit's preferred contact-site.…”

Binocular advantage for prehension movements performed in visually enriched environments requiring visual search

“…Most everyday prehension movements are performed in cluttered environments; however, only few researchers have examined prehension toward targets presented among other objects (Mon-Williams et al, 2001 ; Biegstraaten et al, 2003 ; Tresilian et al, 2005 ; Verheij et al, 2014 ). When participants were asked to reach for a block of wood with an obstacle placed at various distances from the target (3, 6, 9 cm), the influence of the obstacle depended on the target-to-obstacle distance (Mon-Williams et al, 2001 ).…”

“…A recent study by Verheij and colleagues demonstrated that obstacles placed underneath the movement path seem to have little effect on the kinematics compared to those that are to the side of the desired object. (Verheij et al, 2014 ) Therefore, obstacles change the kinematics of reaching and grasping, but the effect is dependent on the location of the obstacles.…”