Salimi I, Hollender I, Frazier W, Gordon A M
Department of Biobehavioral Sciences, Teachers College, Columbia University, New York 10027, USA.
J Neurophysiol. 2000 Nov;84(5):2390-7. doi: 10.1152/jn.2000.84.5.2390.
The present study examines anticipatory control of fingertip forces during grasping based on the center of mass (CM) of a manipulated object. Subjects lifted an object using a precision grip while the fingertip forces and the angle about the vertical axis (roll) were measured. The object's CM could be shifted to the left or right of the object's center parallel to the grip axis without changing it's visual appearance. Subjects performed 20 lifts with the CM in the center, left, and right side of the object, respectively. Subjects were instructed to lift the object while preventing it from tilting. Within three to five lifts, subjects were able to asymmetrically partition the load force development before lift-off such that it was higher in the digit opposing the CM. This anticipatory load force partitioning prevented the object from rolling sideways at lift-off. To determine whether the internal representation underlying the anticipatory control is specific to the effectors used to form it, subjects performed five lifts with the right hand with the CM on one side. Following these lifts, they rotated the object 180 degrees around the vertical axis and performed one lift with the same hand or they translated the object to the left side of the body (with or without rotating it) and performed one lift with the left hand. Despite subjects' explicit knowledge of the new weight distribution, they were unable to appropriately scale the load forces at each digit, resulting in a subsequent large roll of the object. The findings suggest that within a few lifts subjects achieve a stable internal representation which accounts for the object's CM and is used to scale the fingertip forces in advance. They also suggest that this representation, which is used for anticipatory control of fingertip forces, is specific to the effectors used to form it. We propose that multiple internal representations may be used during the anticipatory control of grasping.
本研究基于被操作物体的质心（CM），考察了抓握过程中指尖力的预期控制。受试者使用精确抓握方式提起物体，同时测量指尖力和绕垂直轴的角度（滚动）。物体的质心可以在不改变其视觉外观的情况下，平行于抓握轴在物体中心的左侧或右侧移动。受试者分别在物体质心位于中心、左侧和右侧的情况下进行20次提起操作。受试者被要求提起物体时防止其倾斜。在三到五次提起操作内，受试者能够在离地前不对称地分配负载力的发展，使得在与质心相对的手指上负载力更高。这种预期的负载力分配防止了物体在离地时侧向滚动。为了确定预期控制背后的内部表征是否特定于用于形成它的效应器，受试者用右手在质心位于一侧的情况下进行五次提起操作。在这些提起操作之后，他们将物体绕垂直轴旋转180度，并用同一只手进行一次提起操作，或者将物体平移到身体左侧（无论是否旋转），并用左手进行一次提起操作。尽管受试者明确知道新的重量分布，但他们无法适当地调整每个手指上的负载力，导致物体随后大幅滚动。研究结果表明，在几次提起操作内，受试者获得了一种稳定的内部表征，该表征考虑了物体的质心，并用于提前调整指尖力。研究结果还表明，这种用于指尖力预期控制的表征特定于用于形成它的效应器。我们提出，在抓握的预期控制过程中可能会使用多种内部表征。
