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Mark Mon-Williams, Geoffrey P. Bingham; Task constraints alter prehension movements qualitatively and quantitatively. Journal of Vision 2005;5(8):124. doi: https://doi.org/10.1167/5.8.124.
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A fundamental skill in everyday living involves reaching out and picking up an object (prehension). A fundamental issue in cognitive neuroscience is how movements vary in response to task requirements. It is important to address: (i) what lawful relations exist between task constraints and behaviour; (ii) which relations are stable across tasks; (iii) does behaviour change lawfully with task alteration? We asked participants to reach-and-grasp objects under different experimental conditions (n = 6 per condition). The participant's preferred hand moved to objects of different grip surface size (1, 2, 3cm) and object width (3, 5 and 7cm) placed at different distances (20, 30, 40cm). In condition 1, participants were allowed to touch the table off which the target objects were to be lifted. Movements showed the ‘classic’ prehension pattern and object distance was the only variable that predicted duration. In condition 2, participants were not allowed to touch the table whilst lifting the object and were required to generate fast, normal and slow movements. Object distance and grasp surface size predicted movement time whilst the spatial and temporal characteristics of the movement could be predicted from task constraints that affected the need for and use of visual feedback. In condition 3, participants had to secure a grasp without touching the table or moving the object when moving fast, normal and slow. In condition 3, prehension was found to consist of two components: (i) an initial component (IC) during which the hand reached toward the target while forming an appropriate grip aperture, stopping at (but not touching) the target object; (ii) a completion component (CC) during which the finger and thumb closed on the target. Across tasks the qualitative and quantitative aspects of prehension were governed by the task constraints that affected the need for and the use of visual feedback.
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