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Ryan W. Langridge, Jonathan J. Marotta; 2D and 3D Stimuli Both Generate Stable Digit Placement During a Manipulation Task. Journal of Vision 2020;20(11):940. https://doi.org/10.1167/jov.20.11.940.
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Previous research has demonstrated similarities in the locations people view and place their digits when ‘grasping’ a 2D target or a similarly shaped 3D object. A critical difference between these two actions is the ability to manipulate the 3D object once grasped. This lack of affordance when grasping 2D stimuli may lead to different visuomotor strategies. Using their index finger and thumb, participants grasped either 2D square shapes presented on a computer screen, or identical 3D objects attached to a presentation board. These shapes were located at either central (center of the screen/board) or non-central (leftward or rightward) positions. Participants were instructed to simply grasp the square (Grasp Only), or grasp and slide it to another location (Manipulate). An Optotrak Certus was used to measure placement of the digits, and MotionMonitor software was used to generate the manipulatable on-screen targets. Comparison of the 2D and 3D stimuli revealed similar digit placement – aligned with the square’s horizontal midline – when the square was located in the center of the screen/board. When grasping non-central 2D targets, participants favoured ‘convenient’ grasp locations, closer to the approaching arm. In contrast, when grasping 3D objects, grasps were positioned closer to the horizontal midline – prioritizing stability over convenience. However, the type of task had a similar influence on both 2D and 3D stimuli; when grasping only, digit placement was closer to the nearest edge (biased toward the approaching arm) of non-centrally located squares compared to more stable digit placement during manipulation. Despite the convenient digit placement when grasping 2D targets, the similar visuomotor behaviours observed when grasping and manipulating both types of stimuli suggest participants were treating the 2D targets as 3D objects. Stability was prioritized when manipulation was involved, despite the 2D targets lacking the physical properties requiring them to do so.
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