McIsaac Tara L, Santello Marco, Johnston Jamie A, Zhang Wei, Gordon Andrew M
Department of Biobehavioral Sciences, Teachers College, Columbia University, 525 West 120th Street, Box 199, New York, NY, 10027, USA.
Exp Brain Res. 2009 Mar;194(1):79-90. doi: 10.1007/s00221-008-1671-7. Epub 2008 Dec 19.
During multi-digit grasping both local and non-local digit force responses occur in response to changes in texture at selected digits depending on the grasp configuration. However, the extent to which the specific patterns of force distribution depend on the requirement to hold the object against gravity remains to be determined. In the present study, we examined whether grasp force sharing patterns are invariant when the constraint of maintaining the object orientation vertical against gravity is removed. We used changes in object texture to elicit force changes at single digits during two grasping tasks with different behavioral contexts. One task entailed holding an object against gravity (object hold [OH]). A second (force production [FP]) task consisted of generating lifting forces on an object clamped to the tabletop that were matched to those used during OH. Unlike OH, the FP task lacks the behavioral consequences associated with erroneous sharing of normal and tangential digit forces, e.g., object tilt. Ten subjects lifted and simulated lifting an instrumented object measuring grasping normal and vertical tangential forces at all five digits when the textures were uniformly high-friction sandpaper or low-friction rayon and when one digit contacted a different frictional texture than the other four digits. We found that in both tasks texture changes at individual digits elicited force changes at that digit as well as other digits. However, the specific pattern of force distribution changes differed during OH compared to FP. While subjects modulate the normal and tangential digit forces to different degrees depending on object texture and the grasping task, they ignore the requirement of moment equilibrium when this has no consequences on object orientation (FP task). These findings indicate that multi-digit force responses to texture revealed by previous studies are not obligatory and suggest that the behavioral context of a task should be considered when inferring general principles of multi-digit force coordination.
在多位抓取过程中，根据抓握配置，局部和非局部手指力响应会因选定手指处纹理的变化而出现。然而，力分布的特定模式在多大程度上取决于对抗重力握持物体的要求仍有待确定。在本研究中，我们研究了去除保持物体垂直于重力方向的约束时，抓握力分配模式是否不变。我们利用物体纹理的变化，在两种具有不同行为背景的抓取任务中，引发单个手指的力变化。一项任务是对抗重力握持物体（物体握持[OH]）。第二项任务（力产生[FP]）是对夹在桌面上的物体产生提升力，使其与OH过程中使用的力相匹配。与OH不同，FP任务缺乏与法向和切向手指力错误分配相关的行为后果，例如物体倾斜。当纹理为均匀的高摩擦砂纸或低摩擦人造丝时，以及当一个手指接触的摩擦纹理与其他四个手指不同时，10名受试者抬起并模拟抬起一个装有测量所有五个手指抓握法向力和垂直切向力仪器的物体。我们发现，在两项任务中，单个手指的纹理变化都会引发该手指以及其他手指的力变化。然而，与FP相比，OH过程中力分布变化的具体模式有所不同。虽然受试者会根据物体纹理和抓取任务不同程度地调节法向和切向手指力，但当这对物体方向没有影响时（FP任务），他们会忽略力矩平衡的要求。这些发现表明，先前研究揭示的多位对纹理的力响应并非必然，并表明在推断多位力协调的一般原则时应考虑任务的行为背景。
