Grasping an object comfortably: orientation information is held in memory.

It has been shown that memorized information can influence real-time visuomotor control. For instance, a previously seen object (prime) influences grasping movements toward a target object. In this study, we examined how general the priming effect is: does it depend on the orientation of the target object and the similarity between the prime and the target? To do so, we examined whether priming effects occured for different orientations of the prime and the target objects and for primes that were either identical to the target object or only half of the target object. We found that for orientations of the target object that did not require an awkward grasp, the orientation of the prime could influence the initiation time and the final grip orientation. The priming effects on initiation time were only found when the whole target object was presented as prime, but not when only half of the target object was presented. The results suggest that a memory effect on real-time control is constrained by end-state comfort and by the relevance of the prime for the grasping movement, which might mean that the interactions between the ventral and dorsal pathways are task specific.

Ultra-fast selection of grasping points.

To grasp an object one needs to determine suitable positions on its surface for placing the digits and to move the digits to those positions. If the object is displaced during a reach-to-grasp movement, the digit movements are quickly adjusted. Do these fast adjustments only guide the digits to previously chosen positions on the surface of the object, or is the choice of contact points also constantly reconsidered? Subjects grasped a ball or a cube that sometimes rotated briefly when the digits started moving. The digits followed the rotation within 115 ms. When the object was a ball, subjects quickly counteracted the initial following response by reconsidering their choice of grasping points so that the digits ended at different positions on the rotated surface of the ball, and the ball was grasped with the preferred orientation of the hand. When the object was a cube, subjects sometimes counteracted the initial following response to grasp the cube by a different pair of sides. This altered choice of grasping points was evident within ∼160 ms of rotation onset, which is shorter than regular reaction times.

Do humans prefer to see their grasping points?

To grasp an object the digits need to be placed at suitable positions on its surface. The selection of such grasping points depends on several factors. Here the authors examined whether being able to see 1 of the selected grasping points is such a factor. Subjects grasped large cylinders or oriented blocks that would normally be grasped with the thumb continuously visible and the final part of the index finger's trajectory occluded by the object in question. An opaque screen that hid the thumb's usual grasping point was used to examine whether individuals would choose a grip that was oriented differently to maintain vision of the thumb's grasping point. A transparent screen was used as a control. Occluding the thumb's grasping point made subjects move more carefully (adopting a larger grip aperture) and choose a slightly different grip orientation. However, the change in grip orientation was much too small to keep the thumb visible. The authors conclude that humans do not particularly aim for visible grasping points.

Do obstacles affect the selection of grasping points?

The selection of grasping points, the positions at which the digits make contact with an object's surface in order to pick it up, depends on several factors. In this study, we examined the influence of obstacles on the selection of grasping points. Subjects reached to grasp a sphere placed on a table. Obstacles were placed either near the anticipated grasping points or near the anticipated elbow position at the time of contact with the object. In all cases, subjects adjusted the way they moved when there was an obstacle nearby, but only obstacles near the thumb had a consistent influence across subjects. In general, the influence of the obstacle increased as it was placed closer to the digit or elbow, rather than the subject grasping in a manner that would be appropriate for all conditions. This suggests that under these circumstances the configuration of the arm and hand at the moment of contact was a critical factor when selecting at which points to grasp the objects.

Visual feedback training improves postural adjustments associated with moving obstacle avoidance in elderly women.

The study examined the impact of visually guided weight shifting (WS) practice on the postural adjustments evoked by elderly women when avoiding collision with a moving obstacle while standing. Fifty-six healthy elderly women (70.9+/-5.7 years, 87.5+/-9.6 kg) were randomly assigned into one of three groups: a group that completed 12 sessions (25 min, 3s/week) of WS practice in the Anterior/Posterior direction (A/P group, n=20), a group that performed the same practice in the medio/lateral direction (M/L group, n=20) and a control group (n=16). Pre- and post-training, participants were tested in a moving obstacle avoidance task. As a result of practice, postural response onset shifted closer to the time of collision with the obstacle. Side-to-side WS resulted in a reduction of the M/L sway amplitude and an increase of the trunk's velocity during avoidance. It is concluded that visually guided WS practice enhances elderly's ability for on-line visuo-motor processing when avoiding collision eliminating reliance on anticipatory scaling. Specifying the direction of WS seems to be critical for optimizing the transfer of training adaptations.