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Robert Volcic, Fulvio Domini; Sequential dependencies in grasping movements. Journal of Vision 2015;15(12):1148. doi: 10.1167/15.12.1148.
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© ARVO (1962-2015); The Authors (2016-present)
Responses to a stimulus reflect the properties of the given stimulus (direct effects). More often than not, however, the responses also depend on the previously presented stimulus (sequential effects). These sequential effects might reflect learning and calibration processes, or simply interference. Here, we investigate the direct and sequential effects in a task that involves grasping movements. To properly address this question, we build fully counterbalanced ordered sets of stimuli in which every stimulus type was equally likely to be preceded by any other stimulus type. These sets were based on the de Bruijn sequence of order 2, the shortest sequence of stimuli that ensures that all possible stimuli combinations of length 2 are presented. Subjects performed grasping movements in a virtual environment along the depth axis of a spherical object located at different distances. Both visual feedback of the hand (rendered virtual spheres representing the subject's thumb and index fingertips) and haptic feedback were provided. Despite the availability of these feedback and the fact that the subjects always grasped the same object, object distance had a direct effect on grip aperture profiles, resembling the lack of depth constancy found in perceptual tasks. Importantly, we found clear sequential effects, i.e., the preceding grasping action affected specific aspects of reach and grasp dynamics along the movement trajectory of the subsequent action. Systematic changes were found in early phases (wrist velocity) as well as in late phases of the movement (grip aperture). These findings draw attention to an important source of systematic movement variability and highlight the intricate role played by feedforward and feedback control mechanisms in planning and guiding grasping behavior, and possibly actions in general.
Meeting abstract presented at VSS 2015
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