**4.1 Difference thresholds for fine step-heights in active-touch discrimination task**

To measure the difference thresholds for fine step-heights in the active-touch experiments, five male subjects in their twenties of age touched and discriminated step pairs with the center of their index fingertips in active-touch manner. The subjects were allowed to touch step pairs in the 0-degree finger-motion direction as long as they wanted as choosing the motion velocity of their fingers arbitrarily. In the active-touch experiments, five step-heights of 10, 40, 70, 100 and 130 µm were used as the standard stimuli.

Table 1 shows the initial values of *δc*1, *R*1 and *Rmin* used in the PEST rules for each standard stimulus of *δs*. Each of the comparison step-heights of *δc*1 was the value presented in the first trial block of the discrimination tasks. Also the value of *P* was set at 0.75 to obtain the upper thresholds. During the trials the subjects were required to press the computer-mouse button to input the answers into the computer even if they could not judge the difference between the step pair. The step-heights of the continuous trials were calculated based on the PEST algorithm using the answers and finally the upper thresholds for each standard step-height were determined.


Table 1. Standard step-heights and the initial values used in the PEST rules.
