Evaluation Experiment

The first experiments performed with the Shear-Force Display address basic psychophysical questions. They where conducted in collaboration with the Max Planck Institute for Biological Cybernetics in T¨ubingen. A key question was whether the stimuli produced by the Shear-Force Display are appropriate for human perception, which is directly related to the usability of the Shear-Force Display for further investigations. The presentation here focuses on the results relevant to this question. The complete experiments, including detailed discussions, are presented in [44].

In Experiment 1, the discrimination performance for different directions of single-pin movement was studied. To our knowledge, there is just a single study on human direction discrimination on the finger tip [105]. In Experiment 2, the integration of parallel move- ment of multiple pins was studied. From well-established theories on sensory integration in humans, we expected that discrimination performance should profit from multi- as com- pared to single-pin stimuli - at least if the brain processes movements displayed by the single pins independently from another [46].

Experiment 1

Methods_{. 14 right-handed participants (9 females, age range 21 to 40 years, average age} 26) took part for pay. None of them had any known tactile deficit.

The participants sat in a quiet room in front of a table with their left elbow resting comfortably on a custom-made support. By using robust tape, their left hand was attached to the Shear-Force Display in a way that the tip of their left index finger was reliably centered on the midpoint of the pin display. We used the Shear-Force Display with a single pin only (for this experiment the other three pins were removed) and a particular modified base plate (Fig. 3.10 right, upper) that allowed for individual adaptation of gap size and,

thus, maximal spread of skin stretch. A blind prevented participants from seeing the display, and white noise displayed via headphones masked the sounds of the display during pin movement. A custom-made program on a PC controlled the stimulus presentation and collected the responses, using a basic input device (switch).

Figure 3.10: Setup in Experiment 1.

Tactile stimuli were unidirectional single strokes of 1 mm length (velocity 1 cm/s) starting at the midpoint of the display. For the strokes, we defined eight standard directions with respect to the finger (separated by 45◦_{, Fig. 3.10 left). For each standard stroke, a set of}

19 comparison strokes was chosen, the directions of which were distributed in 10◦ _steps

around the corresponding standard within an area of ± 90◦ _{(Fig. 3.10 right, lower). In}

each single trial, the participants successively felt a standard and a comparison stroke and then had to decide by a button press which of the two strokes had been oriented more in clock-wise direction. Between the strokes, participants had to lift their finger to allow for moving the pin back to the midpoint without evoking tactile stimulation. Required finger movements were signaled by different sounds on the headphones.

Each pair of standard and comparison stroke was presented twelve times during the experiment. The order of standard and comparison was balanced across repetitions, and the order of pairs was completely random. For each individual participant, the experiment lasted about four hours, performed within two sessions (including three breaks) on different days.

Using the psignifit toolbox for Matlab [211, 212], we fitted individual psychometric functions (cumulative Gaussians; Maximum Likelihood procedure) to the proportion of trials in which the comparison stroke was perceived as more clockwise-oriented than the standard stroke, plotted against the comparison direction. From the fits, we obtained individual 84%-discrimination thresholds per standard.

Results and Discussion_{. After the experiment, all participants reported that they} had felt pin translation and most that they had also perceived skin stretch. Figure 3.11

Figure 3.11: Medians and quartiles of 84%-thresholds by standard direction.

depicts medians and quartiles of the individual discrimination thresholds by standard di- rection. Median thresholds ranged from 23◦ _{to 35}◦_{, thresholds quartiles ranged from a}

maximal 75% -percentile of 54◦ _{down to a minimal 25% -percentile of 12}◦_{. Further, the}

individual thresholds per standard direction were subjected to a Friedman test [51, 52]. The test reached significance, χ2_{(7) = 20.7, p < .01, indicating a perceptual anisotropy,}

i.e., participants discriminated better between movements in up-direction as compared to the other directions.

Most importantly here, threshold magnitudes demonstrate that humans can well dis- criminate the directions of the movements displayed. It is also important to note that the technical directional resolution of the shear-force device clearly exceeds the perceptual thresholds we observed. Thus, the Shear-Force Display produces tactile stimuli that are appropriate for human perception, and shear forces seem to be usable to obtain a differen- tiated impression of at least one environmental aspect. Consequently, the next experiment explored to which extent shear forces displayed by more than one pin are integrated in perception.

Experiment 2

Methods_{. Twelve paid right-handed participants without known tactile deficits took} part in Experiment 2 (6 females, age range 19 to 40 years, average age 27). The same setup, procedure and stimuli as in Experiment 1 were used. However, just two standard directions were included (up and down, with respect to the finger ), and 19 corresponding comparison directions. Directions were displayed either with one, two or four pins. The

Figure 3.12: Setup in Experiment 2.

other pins were removed (see Fig. 3.12 for directions and pin configuration). In the multi- pin conditions, all pins moved simultaneously and with identical velocity and direction; the pins were separated by 3 mm (center-to-center). Trials with one, two or four pins were performed in three sessions spread over different days. The order of sessions was balanced between participants according to a Latin square design [15]. The whole experiment lasted about three hours.

Results and Discussion_{. Fig. 3.13 depicts medians and quartiles of the individ-} ual discrimination thresholds by standard direction and pin number. Median thresholds ranged from 19◦ _{to 23}◦_{, threshold quartiles ranged from a maximal 75% -percentile of 33}◦

down to a minimal 25% -percentile of 16◦_{. The individual thresholds per standard direction}

and pin number were subjected to a Friedman test. The test clearly failed to reach signif- icance, χ2_{(5) = 6.7, p > .20, indicating that the participants’ discrimination performance}

depended neither on movement direction nor on the number of pins.

In general, the magnitude of discrimination thresholds in Experiment 2 confirmed that humans well discriminate between the directions of the movements displayed and that the perceptual resolution did not exceed the technical resolution. Thresholds in this experi- ment tended to be slightly lower than in Experiment 1. However, given that there were substantially less different standard directions compared to the last experiment, memory effects may have played a role. Surprisingly, we were not able to replicate the advantage of up-directions observed in Experiment 1.

Most importantly, we did not find any effect of pin number on discrimination perfor- mance. Well-established models on human signal processing state that the human brain integrates independent redundant signals on the same physical property to a combined estimate, which is more reliable than the single signals [46]. The lack of an integration

Figure 3.13: Medians and quartiles of 84% -thresholds by standard direction and pin number.

benefit in the present study may indicate that the movements of the different pins of the shear force device were not independently processed in the perceptual system.