Faculty of Arts

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Browsing Faculty of Arts by Author "Abbas, Hana H"

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    Eye-hand Coordination in Reaching and Grasping Vertically Translating Targets
    (2019) Thulasiram, Matsya R; Langridge, Ryan W; Abbas, Hana H; Marotta, Jonathan J
    Previous research in our lab has revealed a strong visual bias toward the eventual index finger contact location when grasping stationary or horizontally moving targets. However, the unique properties of the thumb may have an increased role when grasping vertically moving targets. Using their index finger and thumb, right-handed individuals reached for and grasped 2-D rectangular targets moving upward or downward on a vertically oriented computer screen. It was expected that fixations would be directed toward the index finger when grasping upward moving targets, and to the thumb when grasping downward moving targets. In trials involving upward moving targets, initial fixations were positioned well above the target, in anticipation of the eventual movement, and remained above the target even after the target started to move. Gaze shifted to the leading (upper) edge of the target when participants initiated their reaching movement, suggesting that participants were primarily attending to the eventual contact location of the index finger. Initial fixations toward downward moving targets were positioned well below its bottom edge, once again anticipating eventual movement. When the targets started to move, gaze shifted upward to a position just below the target’s centre of mass (COM) and remained there during reach onset. Final fixations for both upward and downward moving targets were positioned slightly above the target’s COM, a location that would allow the monitoring of both index and thumb when grasping. These results suggest that while index finger placement is prioritized when grasping upward moving targets, visual attention is directed toward more central locations when grasping downward moving targets, thus allowing both contact points to be monitored simultaneously. This is suggestive of an increased importance of the thumb, in addition to the index finger placement, not shown previously when grasping stationary or horizontally translating targets.
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    Grasping in a Cluttered Environment: Avoiding Obstacles Under Memory Guidance
    (2019) Abbas, Hana H; Marotta, Jonathan J
    Humans often reach to remembered objects, such as when picking up a coffee cup from behind our morning paper. When reaching to previously seen, now out-of-view objects, we rely on our perceptual memory of the scene, to guide our actions (Milner & Goodale, 1995). Based in relative coordinates, encoded perceptual representations may likely exaggerate the risk associated with nearby obstacles. For instance, a cereal bowl next to our coffee cup may be judged as larger than it really is under memory-guided conditions, resulting in a more cautious obstacle avoidance approach to best prevent a messy collision. In contrast, when visual information is available up to the point when a reach is initiated, the precise positions of objects relative to the self are likely to be computed and incorporated into a motor plan, allowing for finely tuned eye-hand maneuvers around positioned obstacles. The objective of this study was to examine obstacle avoidance during memory-guided grasping. Eye-hand coordination was monitored as subjects had to reach through a pair of obstacles in order to grasp a 3D target. The availability of visual information underwent a between-subjects manipulation, such that reaches occurred either with continuous visual information (visually-guided condition), immediately in the absence of visual feedback (memory-guided no-delay condition), or after a 2-s delay in the absence of visual feedback (memory-guided delay condition). The positions and widths of obstacles were manipulated, though their inner edges remained a constant distance apart. We expected the memory-guided delay group to exhibit exaggerated avoidance strategies, particularly around wider obstacles. Results revealed subjects were able to effectively avoid obstacles in the visually-guided and memory-guided no-delay conditions, though overall performance was poorer in the no-delay group, resulting from the inability to use visual information for the online control of action. Still, subjects in these groups consistently altered the paths of the index finger and wrist and adjusted the index finger position on the target object to accommodate obstacles that obstructed the reach path to different degrees. Contrary to expectation, the memory-guided delay group resorted to a more moderate strategy, with fewer instances of altered index finger and wrist paths or adjusted index finger positions on the target object in response to positioned obstacles, though successful grasps were still seen. In other words, subjects reaching to remembered objects tended to use a “good enough” approach for avoiding obstacles. In conclusion, obstacle avoidance behaviour, driven by our stored perceptual representations of a scene, appears to adopt a more moderate, rather than exaggerative, strategy. This work was funded by Research Manitoba, NSERC CGSM, and NSERC Discovery Grant.