Grasping 2-D Targets in Motion: The Influence of a Preferable Central Grasp Location on Eye-Hand Coordination

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Langridge, Ryan W.
Marotta, Jonathan J.
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When using a precision grip to grasp a rectangular object (index finger on top and thumb on the bottom of the object), digit placement is usually close to the object’s horizontal center, ensuring stability. An object’s position will also influence convenient digit placement (e.g. the left side of a rightward positioned object). This research investigated the direction of gaze and index finger placement while visually pursing and grasping moving targets that either allowed for or discouraged a central placement of the index finger and thumb when grasping. Right-handed participants’ eye and hand movements were recorded while they reached for and ‘grasped’ 2-D computer generated targets translating horizontally across a computer screen. Reaches were cued at early, middle, and late stages of target travel. Control targets appeared as a 4 x 4 cm square. Experimental targets resembled Control targets, however the middle portion of the top and bottom edges were removed. At onset of target movement, gaze was directed toward its leading edge. During the reach itself, including at the time of contact, gaze and grasp positions shifted toward the trailing edge. This was especially true for grasps occurring at later stages of target travel, where deviations behind the midline were the largest. In general, gaze deviated further behind the Experimental targets’ midline compared to the Control targets. Overall, horizontal gaze and index finger placement were closer to the target’s midline when grasping Leftward moving targets. Though a misplaced grasp has no consequence when grasping 2-D targets, these results suggest participants favoured a safe, mechanically comfortable movement. In the absence of a preferable central grasp location, participants avoided the front of the object, potentially minimizing the risk of a collision with the leading edge. Increased accuracy when grasping leftward moving targets may reflect a compensatory strategy for cross-body reaching.
Eye-hand coordination, Reaching, Grasping, Fixations, Gaze, Grasp Stability, Convenience