**4.2 Difference thresholds for fine step-heights in passive-touch discrimination task**

To measure the difference thresholds for fine step-heights in the passive-touch experiments, the six male subjects that had participated in the above-mentioned experiments touched and discriminated step pairs with the center of their index fingertips in passive-touch manner. The steps were moved at the reciprocating velocity of 25 mm/s and the 0-degree presentation angle using the presentation device and the subjects were allowed to touch them through the hole of the cover plate with their fingers as long as they wanted. In the passive-touch experiment, five step-heights of 10, 30, 50, 70 and 100 µm were used as the standard stimuli.

Table 2 shows the initial values used in the PEST rules for each standard stimulus. Also the value of *P* was set at 0.75 to obtain the upper thresholds. The discrimination tasks in the passive-touch experiment were conducted and as a result, the PEST algorithm determined the upper thresholds.


Table 2. Standard step-heights and the initial values used in the PEST rules.

Measurement System of Fine Step-Height

Human subject

SD: standard deviation

Human subject

the recognition modules for the step-heights larger than 50 µm.

using the center of an index fingertip of active-touch manner.

using the center of an index fingertip of passive-touch manner.

Discrimination Capability of Human Finger's Tactile Sense 173

passive-touch discrimination tasks become larger as the magnitude of standard step-height increases in the range of 10 to 100 µm. It is also noticed that the threshold magnitudes for each of the step-heights are almost equal for variations of the standard step-height smaller than approximately 40 µm and that the thresholds of active-touch tasks are smaller than those of passive-touch tasks for variations of the standard step-height greater than 50 µm. The results suggest that the fingertip's tactile sense can increase the sensitivity to the stepheights by touching in active-touch manner. In addition, it could be considered that the tactile recognition module that recognizes fine step-heights of about 10 µm is different from

<sup>A</sup>2.7 9.2 1.8 5.9 9.4

<sup>B</sup>1.2 5.8 1.8 9.7 7.1

<sup>C</sup>4.6 4.7 13.8 13.4 7.1

<sup>D</sup>2.7 3.6 10.8 26.6 20.6

<sup>E</sup>2.7 8.1 19.8 30.9 9.4

Ave. [µm] 3.0 6.5 8.6 13.3 11.8 SD 0.97 2.0 5.7 8.4 5.6

Table 3. Upper thresholds for the 0-degree-presented standard step-heights discriminated

<sup>A</sup>2.3 9.1 12.6 30.2 17.2

<sup>B</sup>0.8 6.9 8.1 4.8 9.7

<sup>C</sup>3.1 7.6 6.9 10.8 9.7

<sup>D</sup>3.8 14.4 20.4 9.3 24.7

<sup>E</sup>3.8 5.4 21.6 12.3 15.3

Ave. [µm] 2.2 6.3 11.4 13.5 15.5 SD 1.3 3.4 5.4 7.1 6.0 Table 4. Upper thresholds for the 0-degree-presented standard step-heights discriminated

Standard step-height [µm] 10 40 70 100 130

3.1 6.9 10.8 11.6 2.6

1.9 5.8 1.8 4.1 11.6

2.7 4.7 7.8 5.9 13.9

4.2 10.3 12.3 13.4 16.1

3.8 5.8 4.8 11.6 20.6

Standard step-height [µm] 10 30 50 70 100

1.9 5.4 10.3 16.8 11.6

0.1 3.9 5.8 15.3 11.6

1.2 2.1 4.7 3.3 9.7

1.6 4.6 11.4 16.8 26.6

3.8 3.1 12.6 15.3 19.1

#### **4.3 Difference thresholds for a 10 µm step presented to the center of a fingertip at various presentation angles**

To measure the difference thresholds for a 10 µm step presented to the center of a fingertip at various presentation angles, six male subjects in their twenties of age touched and discriminated step pairs with the center of their index fingertips in passive-touch manner. The magnitude of standard stimulus is 10 µm step-height and the step pairs of the standard and comparison stimuli were presented at the presentation angles of 0, 45 or 90 degrees. The steps were moved at the reciprocating velocity of 30 mm/s by the presentation device and the subjects were allowed to touch the step-heights through the hole as long as they wanted. In the passive-touch experiments, a 0-degree step of 10 µm, a 45-degree step, and a 90 degree step were used as the standard stimuli.

In the experiments, the initial values of *δc*1, *R*1 and *Rmin* used in the PEST rules were 20 µm, 3 µm and 0.3 µm, respectively. The value of *P* was set at 0.75 to obtain the upper thresholds. The discrimination tasks were conducted and as a result, the PEST algorithm determined the upper thresholds of the center of the subjects' fingertips.

#### **4.4 Difference thresholds for a 10 µm step presented to the top of a fingertip at various presentation angles**

To measure the difference thresholds for a 10 µm step presented to the top of a fingertip at various presentation angles, two male subjects in their twenties of age touched and discriminated step pairs with the top of their index fingertips in passive-touch manner. The step pairs of the standard stimulus of a 10 µm step and the comparison stimuli were presented at the presentation angles of 0 or 90 degrees. The steps were moved at the reciprocating velocity of 30 mm/s using the presentation device and the subjects were allowed to touch the step-heights through the hole as long as they wanted. In the passive-touch experiments, a 0 degree step of 10 µm and a 90-degree step were used as the standard stimuli.

In the experiments, the initial values of *δc*1, *R*1 and *Rmin* used in the PEST rules were 20 µm, 3 µm and 0.3 µm, respectively. The value of *P* was set at 0.75 to obtain the upper thresholds. As a result of the experiments, the PEST algorithm determined the upper thresholds of the top of the subjects' fingertips.
