Smith Allan M, Chapman C Elaine, Deslandes Mélanie, Langlais Jean-Sébastien, Thibodeau Marie-Pierre
Centre de Recherche en Sciences Neurologiques, Département de Physiologie, Université de Montréal, CP 6128 Succursale Centre ville, Montréal, Québec, Canada, H3C 3T8.
Exp Brain Res. 2002 May;144(2):211-23. doi: 10.1007/s00221-002-1015-y. Epub 2002 Mar 22.
The present study examined the contribution of normal (Fz) and tangential (Fx) forces, and their ratio, kinetic friction (Fx/Fz), to the subjective magnitude estimations of roughness. The results suggested that the rate of variation in tangential stroking force is a significant determinant of roughness perception. In the first experiment, six volunteer subjects scaled the roughness of eight surfaces explored with a single, active scan of the middle finger. The surfaces were 7.5x2.4-cm polymer strips embossed with truncated cones 1.8 mm high with a spatial period of 2.0 mm in the transverse direction and 1.5-8.5 mm in the longitudinal, scanning direction. The surfaces were mounted on a six-axis force and torque sensor that measured the perpendicular, contact force (normal to the skin surface) and the tangential force along the axis of stroking. The results confirmed the findings of an earlier study that magnitude estimates of perceived roughness increase approximately linearly up to a longitudinal spatial period of 8.5 mm. Across subjects, no consistent correlations were found between perceived roughness and either the mean normal or tangential force alone. Although significant positive correlations were found between roughness and mean kinetic friction for all subjects, they were not as consistently robust as one might have expected. Furthermore, instantaneous kinetic friction varied widely over the course of a single stroke because of within trial oscillations in the tangential force. The amplitude of these oscillations increased with the longitudinal spatial period and their frequency was determined by a combination of the spatial period and the stroking velocity. These oscillations were even more conspicuous in the first derivative or rate of change of the tangential force (dFx/d t), which was quantified as the root mean square (RMS) of the tangential force rate. The mean normalized RMS proved to be strongly correlated with subjective roughness, averaging 0.88 for all subjects. In order to dissociate the fluctuations in tangential force from both the surface structure and the mean kinetic friction, a second experiment was performed on six additional subjects who estimated the roughness of identical lubricated and unlubricated (dry) surfaces. Lubrication with liquid soap reduced the mean kinetic friction by approximately 40%, the RMS of the tangential force rate by slightly more than 21% and the subjective estimates of roughness by 16.4%. Taken together, the results suggest that in tactile exploration, the RMS of the tangential force rate may be an important determinant of subjective roughness.
本研究考察了法向力（Fz）和切向力（Fx）及其比值即动摩擦（Fx/Fz）对粗糙度主观大小估计的贡献。结果表明，切向抚摸力的变化率是粗糙度感知的一个重要决定因素。在第一个实验中，六名志愿者受试者对用中指单次主动扫描探索的八个表面的粗糙度进行了评分。这些表面是7.5×2.4厘米的聚合物条，上面压印有高1.8毫米的截头圆锥体，横向空间周期为2.0毫米，纵向扫描方向为1.5 - 8.5毫米。这些表面安装在一个六轴力和扭矩传感器上，该传感器测量垂直的接触力（垂直于皮肤表面）和沿抚摸轴的切向力。结果证实了早期一项研究的发现，即感知粗糙度的大小估计在纵向空间周期达到8.5毫米之前大致呈线性增加。在所有受试者中，单独的感知粗糙度与平均法向力或切向力之间均未发现一致的相关性。尽管在所有受试者中粗糙度与平均动摩擦之间均发现了显著的正相关，但它们并不像预期的那样一致且稳健。此外，由于单次抚摸过程中切向力的试验内振荡，瞬时动摩擦在单次抚摸过程中变化很大。这些振荡的幅度随着纵向空间周期的增加而增大，其频率由空间周期和抚摸速度共同决定。这些振荡在切向力的一阶导数或变化率（dFx/dt）中更为明显，该变化率被量化为切向力变化率的均方根（RMS）。平均归一化RMS被证明与主观粗糙度密切相关，所有受试者的平均值为0.88。为了将切向力的波动与表面结构和平均动摩擦区分开来，对另外六名受试者进行了第二个实验，他们对相同的润滑和未润滑（干燥）表面的粗糙度进行了估计。用液体肥皂润滑使平均动摩擦降低了约40%，切向力变化率的RMS降低了略超过21%，粗糙度的主观估计降低了16.4%。综合来看，结果表明在触觉探索中，切向力变化率的RMS可能是主观粗糙度的一个重要决定因素。
