Getting a grip: different actions and visual guidance of the thumb and finger in precision grasping

Abstract

This is the unspecified version of the paper.This version of the publication may differ from the final published version. Permanent repository link AbstractWe manipulated the visual information available for grasping to examine what is visually guided when subjects get a precision grip on a common class of object (upright cylinders). In Experiment 1, objects (2 sizes) were placed at different eccentricities to vary the relative proximity to the participant's (n=6) body of their thumb and finger contact positions in the final grip orientations, with vision available throughout or only for movement programming.Thumb trajectories were straighter and less variable than finger paths, and the thumb normally made initial contact with the objects at a relatively invariant landing site, but consistent thumbfirst contacts were disrupted without visual guidance. Finger deviations were more affected by the object's properties and increased when vision was unavailable after movement onset. In Experiment 2, participants (n=12) grasped 'glow-in-the-dark' objects wearing different luminous gloves in which the whole hand was visible or the thumb or the index finger were selectively occluded. Grip closure times were prolonged and thumb-first contacts disrupted when subjects could not see their thumb, whereas occluding the finger resulted in wider grips at contact because this digit remained distant from the object. Results were together consistent with visual feedback guiding the thumb in the period just prior to contacting the object, with the finger more involved in opening the grip and avoiding collision with the opposite contact surface. As people can overtly fixate only one object contact-point at a time, we suggest that selecting one digit for on-line guidance represents an optimal strategy for initial grip placement.

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

“…Why? We and others have shown (Schlicht and Schrater 2007;Melmoth and Grant 2012) that outward movement of the finger almost entirely accounts for the width of the peak grip aperture when subjects are preparing to precision grasp isolated table-top objects. We suggest that the smaller and later peak grips along with their altered end-point orientations in the presence of the finger-side obstacle resulted from a plan to maintain this digit at a safe distance throughout the movement, an unnecessary precaution for the thumb-side obstacle location due to the much straighter path typically adopted by the thumb in moving to its usual contact site (Wing and Fraser 1983;Haggard and Wing 1997;Melmoth and Grant 2012).…”

“…These locations were centered 6.5 cm (~9 o ) to the left ('thumb side') of the target; 13 cm (~18 o ) to the right ('finger side') of the target; or 10 cm directly behind it. Additional to our review of previous literature, we chose to double the relative separations between the target and the left/thumb-side versus right/finger-side obstacle locations, to allow sufficient space for the maximal deviations of each digit away from their contact sites to occur at peak grip and which are typically double the relative distance for the finger compared to the thumb (see Melmoth and Grant, 2012). Consistent trial-to-trial object and obstacle placements were achieved by aligning their lower centres with 0.5 cm diameter stickers attached to the upper plinth surface.…”

“…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.…”

“…Our present study, as before (Melmoth and Grant 2012), used a cylindrical table-top target which was usually contacted first by the subject's thumb. With normal binocular viewing, our current participant's gaze clearly shifted horizontally as their hand movement progressed such that, on average, they were often looking towards the left/thumb grasp-side of the target just before and at initial object contact.…”

“…And do they use different gaze/fixation strategies in the presence of obstacles at different locations relative to the moving hand? These questions were prompted, in part, by our recent evidence (Melmoth and Grant 2012) supporting a long-standing suggestion that the thumb is employed as an online guide to initial contact when precision grasping isolated (cylindrical) table-top objects (Wing and Fraser 1983;Haggard and Wing 1997). Movement of the finger, by contrast, appeared mainly responsible for opening the initial grip aperture, while also avoiding a subsequent collision with the target's opposite, less visible, surface, both findings being in line with other recent evidence that collision avoidance is prioritized during single-object grasping too (Verheij et al 2012(Verheij et al , 2014a.…”