Smith Allan M, Gosselin Geneviève, Houde Bryan
Centre de Recherche en Sciences Neurologiques, Département de Physiologie, Université de Montréal, C.P. 6128 Succursale Centre Ville, Montréal, Québec H3C 3T8, Canada.
Exp Brain Res. 2002 Nov;147(2):209-18. doi: 10.1007/s00221-002-1240-4. Epub 2002 Sep 20.
The purpose of this study was to examine how contact forces normal to the skin surface and shear forces tangential to the skin surface are deployed during tactile exploration of a smooth surface in search of a tactile target. Six naive subjects participated in two experiments. In the first experiment, the subjects were asked to explore a series of unseen smooth plastic surfaces by using the index finger to search for either a raised or recessed target. The raised targets were squares with a height of 280 micro m above the background surface and that varied in side lengths from 0.2 mm to 8.0 mm. A second series of smooth plastic surfaces consisted of small recessed squares (side lengths: 2.0, 3.0, 4.0 and 8.0 mm) that were etched to a depth of 620 micro m. Although made of an identical material, the plastic substrate had a lower coefficient of friction against the skin because only the recessed square had been subjected to the electrolytic etching process. The surfaces were mounted on a six-axes force and torque sensor connected to a laboratory computer. From the three axes of linear force, the computer was able to calculate the instantaneous position of the index finger and the instantaneous tangential force throughout the exploratory period. When exploring for the raised squares, the subjects maintained a relatively constant, average normal force of about 0.49 N with an average exploration speed of 8.6 cm/s. In contrast, all subjects used a significantly higher average normal force (0.64 N) and slightly slower mean exploration speed (7.67 cm/s) when searching for the small recessed squares. This appeared to be an attempt to maximize the amount of skin penetrating the recessed squares to improve the probability of target detection. In a second experiment, subjects were requested to search for an identical set of raised squares but with the fingertip having been coated with sucrose to impede the scanning movement by increasing the friction. Overall, the subjects maintained the same constant normal force that they used on the uncoated surface. However, they increased the tangential force significantly. The similarity of the search strategy employed by all subjects supports the hypothesis that shear forces on the skin provide a significant stimulus to mechanoreceptors in the skin during tactile exploration. Taken together, these data suggest that, in active tactile exploration with the fingertip, the tangential finger speed, the normal contact force, and the tangential shear force are adjusted optimally depending on the surface friction and whether the target is a raised asperity or a recessed indentation.
本研究的目的是考察在对光滑表面进行触觉探索以寻找触觉目标的过程中，垂直于皮肤表面的接触力和平行于皮肤表面的剪切力是如何分布的。六名未经训练的受试者参与了两项实验。在第一个实验中，受试者被要求用食指探索一系列看不见的光滑塑料表面，以寻找凸起或凹陷的目标。凸起目标是高度比背景表面高280微米的正方形，边长从0.2毫米到8.0毫米不等。第二组光滑塑料表面由小的凹陷正方形（边长为2.0、3.0、4.0和8.0毫米）组成，蚀刻深度为620微米。尽管由相同材料制成，但塑料基底与皮肤的摩擦系数较低，因为只有凹陷正方形经过了电解蚀刻过程。这些表面安装在连接到实验室计算机的六轴力和扭矩传感器上。计算机能够从线性力的三个轴上计算出食指在整个探索过程中的瞬时位置和瞬时切向力。在探索凸起正方形时，受试者保持相对恒定的平均法向力约为0.49牛，平均探索速度为8.6厘米/秒。相比之下，所有受试者在寻找小凹陷正方形时使用的平均法向力显著更高（0.64牛），平均探索速度略慢（7.67厘米/秒）。这似乎是为了最大限度地增加皮肤穿透凹陷正方形的量，以提高目标检测的概率。在第二个实验中，受试者被要求寻找一组相同的凸起正方形，但指尖涂上了蔗糖，以通过增加摩擦力来阻碍扫描运动。总体而言，受试者保持了与未涂覆表面相同的恒定法向力。然而，他们显著增加了切向力。所有受试者采用的搜索策略的相似性支持了这样一种假设，即在触觉探索过程中，皮肤表面的剪切力对皮肤中的机械感受器提供了重要刺激。综上所述，这些数据表明，在用指尖进行主动触觉探索时，手指的切向速度、法向接触力和切向剪切力会根据表面摩擦力以及目标是凸起的粗糙物还是凹陷的凹痕而进行最佳调整。
