Measurement of Human Tactile Sentation Capability to Discriminate Fine Surface Textures Using a Variable Step-height Presentation System.

Several papers report that there are four types of mechanoreceptive units in glabrous skins of human hand, and that two of them, which are called FA I and FA II, detect vibrotactile sentation. In order to elucidate functions of these units for surface texture recognition, the present paper evaluated the human tactile sensation capability to descriminate fine step heights of 0 to 30 μm in height using a variable step-height presentation device mounted on an X-table. Human subjects touched the fine step-heights arising between two aluminum plates mounted on the step-height device. One of the plates was driven by a piezoelectric ceramic actuator; the other was fixed. A computer changed the step-height by adjusting the input voltage of the actuator on the basis of a PEST (Parameter Estimaton by Sequential Testing) trial sequence. In experiments, five human subjects judged which step height presented at random order in a step of PEST's sequence was stronger. In Experiment A, the human subjects actively touched the aluminum plates of 37°C by moving the finger. In Experiment B, they passively touched the aluminum plates of 37°C driven linearly by the X-table with cyclic movement. In Experiment C, they actively touched the plates of 15°C. As a result of these phychophysical testings, human fingers are found to have the difference thresholds between 2 μm and 3 μm, in spite of variating the experimental conditions. These results indicate that human beings make a descrimination without the information of the finger movement, and that FA I unit plays an important role in the sensibility to discriminate between the step-heights.