**5.1 Effects of touching manner of finger**

To evaluate the influence of the touching manner on the finger's fine step-height discrimination capability, the upper thresholds for the step-heights were measured in the active- and passive-touch experiments. Each human subject was tested twice for each standard step-height in the active- and passive-touch experiments and ten upper thresholds in total for each step-height were determined. Tables 3 and 4 show the upper thresholds in the active- and passive-touch experiments, respectively. At the bottoms of the tables the averages of the upper thresholds and the standard deviations are calculated.

Figure 9 describes the relationship among the touching manner, the upper threshold, and the standard step-height. The horizontal axis shows the standard step-height while the vertical axis shows the upper threshold. The threshold magnitudes of the active- and 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 the recognition modules for the step-heights larger than 50 µm.


SD: standard deviation

172 Advanced Topics in Measurements

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-

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

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-

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

To evaluate the influence of the touching manner on the finger's fine step-height discrimination capability, the upper thresholds for the step-heights were measured in the active- and passive-touch experiments. Each human subject was tested twice for each standard step-height in the active- and passive-touch experiments and ten upper thresholds in total for each step-height were determined. Tables 3 and 4 show the upper thresholds in the active- and passive-touch experiments, respectively. At the bottoms of the tables the

Figure 9 describes the relationship among the touching manner, the upper threshold, and the standard step-height. The horizontal axis shows the standard step-height while the vertical axis shows the upper threshold. The threshold magnitudes of the active- and

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

degree step of 10 µm and a 90-degree step were used as the standard stimuli.

averages of the upper thresholds and the standard deviations are calculated.

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

**various presentation angles** 

**various presentation angles** 

top of the subjects' fingertips.

**5. Experimental results and discussion 5.1 Effects of touching manner of finger** 

degree step were used as the standard stimuli.

the upper thresholds of the center of the subjects' fingertips.

Table 3. Upper thresholds for the 0-degree-presented standard step-heights discriminated using the center of an index fingertip of active-touch manner.


Table 4. Upper thresholds for the 0-degree-presented standard step-heights discriminated using the center of an index fingertip of passive-touch manner.

Measurement System of Fine Step-Height

moving in the motion direction of 90 degrees.

Human subject

fingertip at the presentation angles.

Human subject

L

M

at the presentation angles.

Discrimination Capability of Human Finger's Tactile Sense 175

are smaller than those of the center and that the upper threshold for the 90-degree step presented at the top is the smallest value. Therefore, it is found that the tactile sense of the top of a fingertip is highly sensitive to a 10 µm step-height as compared with that of the center. In addition, the results point out that you can make the most of the fingertip's discrimination ability when you touch a fine step-height with the top of your fingertip

<sup>F</sup>3.4 2.3 6.1

<sup>G</sup>1.2 1.6 2.7

<sup>H</sup>3.8 5.3 5.7

<sup>I</sup>3.4 3.4 3.4

<sup>J</sup>3.1 2.3 1.6

<sup>K</sup>3.1 3.1 3.4

Ave. [µm] 3.6 3.3 3.1 SD 1.2 0.75 2.3

Table 5. Upper thresholds for a 10 µm step-height presented to the center of an index

Ave. [µm] 2.2 1.3 SD 0.72 0.65 Table 6. Upper thresholds for a 10 µm step-height presented to the top of an index fingertip

Presentation angle [deg] 0 45 90 Upper threshold [µm]

6.4 4.2 1.2

4.9 3.4 1.9

3.8 3.4 0.1

4.2 3.8 3.8

3.8 3.4 4.9

2.3 3.4 2.3

Presentation angle [deg] 0 90 Upper threshold [µm]

1.9 0.8 2.6 1.9 2.6 1.5 1.1 1.1

2.3 0.0 1.1 2.3 1.9 1.1 3.4 1.5

Fig. 9. Relationship among the touching manner, the upper threshold, and the standard step-height.

#### **5.2 Effects of finger motion direction and fingertip region**

To evaluate the influences of the finger motion direction and the fingertip region on the fine step-height discrimination capability, the upper thresholds for a 10 µm step-height were determined when the subjects touched the step-height presented at the presentation angles of 0, 45 and 90 degrees using the top and center of their fingertips. Here the finger's motion direction can be defined as the step's presentation angle controlled by the presentation device since it was revealed that the fingertip's discrimination capability of the step-heights of about 10 µm does not depend on the active- and passive-touch manners (Kawamura et al., 1996).

In the discrimination tasks using the center of a fingertip, each subject was tested twice for each of the standard stimuli presented at 0, 45 or 90 degrees and twelve upper thresholds in total for each presentation angle were determined, on the other hand, in the discrimination tasks using the top of a fingertip, each subject was tested four times for each of the standard stimuli presented at 0 or 90 degrees and eight upper thresholds in total for each presentation angle were determined. Tables 5 and 6 show the upper thresholds for a 10 µm step-height presented to the center and top of the subjects' fingertips, respectively. At the bottoms of the tables the averages of the upper thresholds and the standard deviations are calculated.

Figure 10 describes the relationship among the fingertip region, the upper threshold, and the presentation angle. The horizontal axis shows the presentation angle while the vertical axis shows the upper threshold. The magnitude of upper threshold measured at the center of the fingertips almost stays constant or decreases slightly for variations of the presentation angle in the range of 0 to 90 degrees. On the other hand, the magnitude of upper threshold measured at the top of the fingertips becomes smaller as the presentation angle changes from 0 to 90 degrees. It is also noticed that the upper thresholds of the top of the fingertips

●: passive-touch manner ○: active-touch manner

Fig. 9. Relationship among the touching manner, the upper threshold, and the standard

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140

Standard step-height, *δs* µm

To evaluate the influences of the finger motion direction and the fingertip region on the fine step-height discrimination capability, the upper thresholds for a 10 µm step-height were determined when the subjects touched the step-height presented at the presentation angles of 0, 45 and 90 degrees using the top and center of their fingertips. Here the finger's motion direction can be defined as the step's presentation angle controlled by the presentation device since it was revealed that the fingertip's discrimination capability of the step-heights of about 10 µm does not depend on the active- and passive-touch manners (Kawamura et al., 1996).

In the discrimination tasks using the center of a fingertip, each subject was tested twice for each of the standard stimuli presented at 0, 45 or 90 degrees and twelve upper thresholds in total for each presentation angle were determined, on the other hand, in the discrimination tasks using the top of a fingertip, each subject was tested four times for each of the standard stimuli presented at 0 or 90 degrees and eight upper thresholds in total for each presentation angle were determined. Tables 5 and 6 show the upper thresholds for a 10 µm step-height presented to the center and top of the subjects' fingertips, respectively. At the bottoms of the tables the averages of the upper thresholds and the standard deviations are calculated.

Figure 10 describes the relationship among the fingertip region, the upper threshold, and the presentation angle. The horizontal axis shows the presentation angle while the vertical axis shows the upper threshold. The magnitude of upper threshold measured at the center of the fingertips almost stays constant or decreases slightly for variations of the presentation angle in the range of 0 to 90 degrees. On the other hand, the magnitude of upper threshold measured at the top of the fingertips becomes smaller as the presentation angle changes from 0 to 90 degrees. It is also noticed that the upper thresholds of the top of the fingertips

**5.2 Effects of finger motion direction and fingertip region** 

0

5

10

Upper threshold for step-height,

15

20

25

*ΔU* µm

step-height.

are smaller than those of the center and that the upper threshold for the 90-degree step presented at the top is the smallest value. Therefore, it is found that the tactile sense of the top of a fingertip is highly sensitive to a 10 µm step-height as compared with that of the center. In addition, the results point out that you can make the most of the fingertip's discrimination ability when you touch a fine step-height with the top of your fingertip moving in the motion direction of 90 degrees.


Table 5. Upper thresholds for a 10 µm step-height presented to the center of an index fingertip at the presentation angles.


Table 6. Upper thresholds for a 10 µm step-height presented to the top of an index fingertip at the presentation angles.

Measurement System of Fine Step-Height

compared with that of the center.

**7. References** 

561.

998-1003.

264-267.

Pt. 2, p. 2771.

Singapore.

Discrimination Capability of Human Finger's Tactile Sense 177

Next, to investigate the effects of the finger's motion direction and fingertip region in recognizing fine step-heights, the upper thresholds for a 10 µm step-height were determined when the human subjects discriminated the pairs of step-heights presented at various presentation angles using the top and center of their fingertips. When the presentation angle of a step-height to a fingertip changed from 0 to 90 degrees, although the thresholds of the center of the fingertips almost stayed constant, the threshold for the step-height presented to the top of the fingertips at 90 degrees became the smallest value. Therefore, it was found that the tactile sense of the top of a fingertip is highly sensitive to the step-height as

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Fig. 10. Relationship among the fingertip region, the upper threshold for a 10 µm stepheight, and the presentation angle.
