Vision of the hand and environmental context in human prehension

Abstract

Previous findings on the role of visual contact with the hand in the control of reaching and grasping have been contradictory. Some studies have shown that such contact is largely irrelevant, while more recent ones have emphasised its importance. In contrast, information arising from the surrounding environment has received relatively little attention in the study of prehensile actions. In order to identify the roles of both sources of information, we made kinematic comparisons between three conditions. In the first, reaching was performed in a dimly lit room and compared with a second condition in which reaches in the dark, but with the thumb and first finger illuminated, were made to a luminous object. This contrast allows the effects of environmental context to be identified. A comparison between the second and a third condition, in which both vision of the hand and the environment was removed, but the object was still visually available, enabled the assessment of how and when vision of the hand plays a role. Removing environmental cues had effects both early and late in the reach, while vision of the hand was only crucial in the period after peak deceleration. In addition, removal of both sources of information resulted in larger grip apertures. Differences and similarities between our findings and those of other studies are discussed, as is the ongoing debate about the relative importance of visual feedback of the hand in the control and co-ordination of prehensile actions. We conclude with suggestions for further research based on the set-up used in the present study.

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

“…Taken together, this would imply that contacting the object with the finger before the thumb represents a failure of collision avoidance, and our finding that finger-first contacts were generally 'hard' and imprecise is indicative of an error of this kind (see Melmoth et al, 2007). It would also explain a key finding in our glow-in-the-dark experiment; that subjects close both their digits more slowly (i.e., between peak grip and initial contact) when they cannot see their thumb -just as when visual feedback from the whole hand is unavailable (present results; Jakobson and Goodale 1991;Churchill et al 2000;Watt and Bradshaw 2000;Schettino et al 2003) -but make initial thumb contact with a wider grip aperture, because they are more cautiously closing just their finger when vision of this digit is selectively occluded.…”

“…The purpose of including the Full-light condition was to confirm that our subjects performed the task properly when allowed normal vision of their moving hand, the target and its immediate surroundings, and that removing this latter environmental information in the Whole Hand in-the-dark condition did not affect their performance in ways other than those expected from similar work (Churchill et al 2000). The mean kinematic data obtained (see Table 4) indicated that these expectations were met, as they replicated evidence that reducing the environment context in this latter condition resulted in slower movements (e.g., reduced peak reaching velocity; longer movement durations) and in larger grip sizes at peak and object contact, compared to normal performance in Full-light.…”

Lack of depth constancy for grasping movements in both virtual and real environments

“…Possibly, seeing the entire hand within a full cue environment yields a more confident movement, which resulted in the adoption of a smaller margin of safety. In the same vein, movements performed with all cues available were much faster than those performed in the dark, indicating that participants achieved more confidence when able to see the object and the surrounding setting (Churchill et al 2000). In particular, by segmenting the movement in all its phases, we found that the effect of additional depth cues on the movement speed increased from the earlier to the later phases to be largest in the second part of the movement, during the deceleration phase.…”

“…Undoubtedly, the availability of different sources of feedback at the end of an action, like the vision of the hand and the physical presence of a target, plays a crucial role in guiding the limb to the appropriate contact position on the object (Jakobson and Goodale 1991;Gentilucci et al 1994;Connolly and Goodale 1999;Churchill et al 2000). As largely demonstrated, the absence of such feedback can induce atypical patterns of movements (Wing et al 1986;Gentilucci et al 1994;Churchill et al 2000;Bingham et al 2007;Hibbard and Bradshaw 2003;Bozzacchi et al 2014). These findings are in agreement with a theoretical framework postulating a nonmetric (i.e., non-Euclidean) representation of 3D properties.…”

“…Although the mean peak velocity was higher in the full-cue condition than in the virtual and glowin-the-dark conditions, its scaling with viewing distance did not differ among the three conditions. Since peak velocity is an indirect measure of the object distance estimate during the planning phase of a movement (Jeannerod 1984;Glennester et al 1996;Carey et al 1998;Churchill et al 2000;Hibbard and Bradshaw 2003), the fact that in all conditions distance modulated peak velocity in the same way might suggest that the compression of visual space was the same in all conditions as well.…”