Division of Optometry and Visual Science, Henry Wellcome Laboratories for Visual Sciences, City University London, Northampton Square, London EC1V 0HB, UK.
Exp Brain Res. 2012 Oct;222(3):265-76. doi: 10.1007/s00221-012-3214-5. Epub 2012 Aug 17.
We manipulated the visual information available for grasping to examine what is visually guided when subjects get a precision grip on a common class of object (upright cylinders). In Experiment 1, objects (2 sizes) were placed at different eccentricities to vary the relative proximity to the participant's (n = 6) body of their thumb and finger contact positions in the final grip orientations, with vision available throughout or only for movement programming. Thumb trajectories were straighter and less variable than finger paths, and the thumb normally made initial contact with the objects at a relatively invariant landing site, but consistent thumb first-contacts were disrupted without visual guidance. Finger deviations were more affected by the object's properties and increased when vision was unavailable after movement onset. In Experiment 2, participants (n = 12) grasped 'glow-in-the-dark' objects wearing different luminous gloves in which the whole hand was visible or the thumb or the index finger was selectively occluded. Grip closure times were prolonged and thumb first-contacts disrupted when subjects could not see their thumb, whereas occluding the finger resulted in wider grips at contact because this digit remained distant from the object. Results were together consistent with visual feedback guiding the thumb in the period just prior to contacting the object, with the finger more involved in opening the grip and avoiding collision with the opposite contact surface. As people can overtly fixate only one object contact point at a time, we suggest that selecting one digit for online guidance represents an optimal strategy for initial grip placement. Other grasping tasks, in which the finger appears to be used for this purpose, are discussed.
我们操纵了可供抓握的视觉信息，以研究在主体对常见一类物体（直立圆柱体）进行精确抓握时，哪些是受视觉引导的。在实验 1 中，将物体（2 种大小）放置在不同的偏心率处，以改变其相对于主体（n=6）拇指和手指在最终抓握方向中的接触位置的相对接近度，视觉信息在整个过程中或仅在运动编程期间可用。拇指轨迹比手指路径更直，变化更小，拇指通常在相对不变的着陆点与物体最初接触，但在没有视觉引导的情况下，一致的拇指首次接触会被打乱。手指偏差受物体特性的影响更大，在运动开始后无法看到物体时会增加。在实验 2 中，参与者（n=12）戴着不同发光手套抓握“夜光”物体，这些手套可以完全看到整个手，或者拇指或食指被选择性遮挡。当主体无法看到拇指时，握持关闭时间延长，拇指首次接触被打乱，而遮挡手指会导致在接触时握持更宽，因为该手指仍远离物体。这些结果与视觉反馈在接触物体之前的时期引导拇指的结果一致，手指更多地参与了打开握持和避免与对面接触表面碰撞的过程。由于人们一次只能明显注视一个物体接触点，我们认为选择一个手指进行在线引导是初始握持位置的最佳策略。其他似乎为此目的而使用手指的抓握任务也将进行讨论。
