Tetsu Miyaoka

Mechanisms of fine-surface-texture discrimination in human tactile sensation

The purpose of this study was to evaluate the ability of touch to discriminate fine-surface textures and to suggest possible mechanisms of the discriminations. Two experiments were performed. In experiment 1, aluminum-oxide abrasive papers were adopted as stimuli, and psychometric functions and difference thresholds were determined in fine-surface-texture discrimination tasks. The grit values of abrasive papers were 400, 600, 1200, 2000, 3000, 4000, and 8000; corresponding average particle sizes were 40, 30, 12, 9, 5, 3, and 1 micron, respectively. Ten subjects participated in experiment 1. The difference thresholds obtained in experiment 1 were between 2.4 and 3.3 microns. In experiment 2, the tasks were discriminations of ridge height. The cross sections of the etched ridges were rectangular and the ridge heights were 6.3, 7.0, 8.6, 10.8, 12.3, 18.5, and 25.0 microns. Six subjects participated in experiment 2. The difference thresholds in experiment 2 were between 0.95 and 2.0 microns. It was reasoned, based on the Weber fraction values calculated from the difference thresholds and on the limit of neural information-processing ability of humans, that the subjects discriminate fine roughness only from the amplitude information presented in surface unevenness.

Figure and texture presentation capabilities of a tactile mouse equipped with a display pad of stimulus pins

To obtain specifications for a tactile display that would be effective in virtual reality and tele-existence systems, we have developed two types of matrix-type experimental tactile displays. One is for virtual figures (display A) and the other is for virtual textures (display B). Display As pad has a 4 × 6 array of stimulus pins, each 0.8 mm in diameter. Three pad configurations, in which distances between any two adjacent pins (pin pitch) are 1.2, 1.9, or 2.5 mm, were developed to examine the influence of distance on a human operators determination of virtual figures. Display B has an 8 × 8 array of stimulus pins, each 0.3 mm in diameter and with 1-or 1.8-mm pin pitch, because presentation of virtual textures was presumed to require a higher pin density. To establish a design method for these matrix-type tactile displays, we performed a series of psychophysical experiments using displays A and B. By evaluating variations in the correct answer percentage and threshold caused by different pin arrays and different pin strokes, we determined under what conditions the operator could best feel the virtual figures and textures. The results revealed that the two-point threshold should be adopted as the pitch between pins in the design of the tactile display, that a pin stroke should exceed 0.25 mm, and that the adjustment method is the most appropriate to evaluate the capabilities of tactile displays. Finally, when we compared the virtual texture with the real texture, we found that the threshold for the real texture is almost 1/3rd that of the virtual texture. This result implies that it is effective to present variations in patterns caused by rotation and variation in shearing force, itself produced by relative motion between the finger surface and object surface.

Experiments on Stochastic Resonance Toward Human Mimetic Tactile Data Processing

In the present research, human tactile stochastic resonance (SR) capable of enhancing sensitivity by superimposing proper noise upon undetectable weak signals is utilized to enhance the tactile processing method for social robotics. We develop an experimental apparatus composed of a piezoelectric actuator and its controller, and generate a step several microns high mixed with noise to perform a series of psychophysical experiments. Since psychophysical experiments are conducted based on the Parameter Estimation by Sequential Testing (PEST) method, we produce a PEST program that generates a stimuli sequence based on PEST. The experimental result shows that variation in the difference threshold (Difference Limen; DL) has a local minimum point in the relationship between DL and noise. Therefore, the tactile sensation’s just noticeable difference (JND) is decreased by appropriate external noise. Since JND denotes the scale divisions of sensation in the human mind, the present result shows that precise tactile sensations are enhanced by the appropriate external noise. Finally, we introduce a neural network model composed of nonlinear neurons with the bi-stable equilibrium condition to explain this result. Although original sensor data do not represent the morphology of the fine texture, the neural network model extracts the morphology and distinguishes the wave amplitude of the fine texture.

A Tactile Display Presenting Pressure Distribution and Slippage Force

In previous haptic presentation devices, the combination effects of distributed pressure and slippage force sensations have not been investigated in spite of their possibilities for virtual reality systems. In this study, we developed a mouse capable of presenting combined stimulation to discuss the combination effects on virtual reality. The mouse was equipped with a bimorph-piezoelectric-actuator-aray and a two-dimensional electro-magnetic-linear-motor to present pressure and slippage force, respectively. In order to evaluate the presentation accuracy of the mouse, we performed a series of tracing experiments of virtual figure contours. Since the deviation errors of the combination presentation were smaller than that of pressure presentation only, the combination presentation was effective for virtual reality because of the edge tracing easiness induced by the slippage force used on the edges. Finally, we present a new psychophysical methodology to examine the combination presentation with adjustment of comparison virtual edges inclination to coincide with standard one.

Effectiveness of Distributed Pressure and Slippage Force Presentation in Tactile Virtual Reality

In pervious virtual reality systems, haptic presentation devices have already been developed and their usage discussed. However, the combination effects of distributed pressure and slippage force sensations have not been investigated in spite of their possibilities for virtual reality systems. In this study, we developed a mouse capable of presenting combined stimulation to discuss the combination effects on virtual reality. The mouse was equipped with a bimorph-piezoelectric-actuator-array and a two-dimensional electro-magnetic-linear-motor to present pressure and slippage force, respectively. In order to evaluate the presentation accuracy of the mouse, we performed a series of tracing experiments of virtual figure contours. Since the deviation errors of the combination presentation were smaller than that of pressure presentation only, the combination presentation was effective for virtual reality because of the edge tracing easiness induced by the slippage force used on the edges.

Fine surface‐texture discrimination ability depends on the number of mechanoreceptors participating in the discrimination task

The purpose of this study was to investigate the relation between fine surface‐texture discrimination ability in tactile sensation and the number of mechanoreceptors participating in the discrimination task. Two experiments were performed. In experiment 1 the spatial‐summation effect was measured at the fingertips using aluminum oxide abrasive papers as stimuli, with grit values between 600 and 8000. Difference thresholds could not be determined when the area of the papers was 25 mm2. Thresholds were between 11.3 and 13.5 μm at 100 mm2, and they were between 3.14 and 5.58 μm at 400 mm2. In experiment 2 the difference thresholds of the texture‐discrimination tasks were measured at the fingertips and the thenar eminence using large‐sized (2500 mm2) abrasive papers with grit values between 400 and 8000. The difference thresholds were smaller at the thenar eminence than at the fingertips because the stimulated area at the thenar eminence was eight times larger than the area at the fingertips. From the results of experiments 1 and 2, it was concluded that the larger the number of mechanoreceptors which participate in the discrimination tasks, the smaller the difference thresholds. [Work supported by the Ministry of Education, Science and Culture, No. 07610093 and Mikiya Science and Technology Foundation.]

Human Tactile Stochastic Resonance Affected by Stimulus Direction

Tactile sensors and their data processing are very important in enabling robots to handle objects. With information processing by biological organisms, stochastic resonance (SR) can enhance sensitivity by superimposing proper noise upon undetectable weak signals to detect the target signal. To investigate the SR of human tactile sensation and elucidate the mechanism of tactile SR for the development of a new tactile sensor, we performed a series of psychophysical experiments using tangential vibration with 2.5-mm and 8-mm-sized stimulus points. We examined the difference threshold (difference limen; DL) variation obtained from these experiments to clarify which conditions of vibration direction and stimulus size cause the strongest SR. The experimental results show that neither normal nor tangential DL is significantly affected by stimulus point size. Moreover, tactile sensing precision is enhanced by appropriate noise. The characteristics of SR, with normal vibration obtained from our prior study is quite different from that with tangential vibration.

Estimation of vibration stimulus threshold for inducing kinesthetic illusion

In order to utilize the phenomenon of kinesthetic illusion as a human interface capable of presenting kinesthetic sense, it is necessary to define specification of the vibrator required to elicit kinesthetic illusion. In this study, the stimulation thresholds of the illusion have been investigated using the staircase method, which is an adaptive psychometric method. The result of the experiment reveals that the kinesthetic illusion can be elicited with an acceleration of about 40 [m/s2], with vibratory stimuli of 50 to 90 [Hz]. Furthermore, at the highest frequency of 120 [Hz] the illusion cannot be elicited unless the acceleration is increased to 60 [m/s2].

Stochastic resonance occurring in tactile sensation of human finger

In this paper, we review the influence of external noise on human tactile sensation as outlined in prior and current studies. In the last few decades, researchers have found that, although noise is always considered detrimental, it provides the benefit of stochastic resonance (SR) phenomena. Based on previous studies, we investigate the effect of the SR phenomenon on human tactile sensation. In this context, we developed a system consisting of an experimental apparatus and a computer program, and performed a series of psychophysical experiments using different types of stimulus with normal vibration. The experimental results show that tactile sensation precision can be enhanced by an appropriate level of external noise. Furthermore, we introduce a neural network model composed of nonlinear neurons with a bi-stable equilibrium condition to clarify the result. Finally, we perform a sequence of psychophysical experiments with tangential vibration to clarify which conditions of vibration direction and stimulus size cause the strongest SR. The results show that the normal difference limen (DL) is significantly affected by stimulus point size. On the other hand, neither normal nor tangential DL is significantly affected by stimulus point size. Moreover, the characteristics of SR with normal vibration are quite different from those with tangential vibration.

Investigation of conditions generating velvet hand illusion toward tactile displays

We have investigated several characteristics of one kind of tactile illusion, called the Velvet Hand Illusion (VHI), to utilize the experimental results to generate virtual feeling of a material. In VHI, a human subject gently rubs his/her hands on both sides of a wire grid strung through a frame. The sensation produced on his/her hands is very smooth and slippery, like velvet. We focused on the VHI mechanism for new tactile displays in the virtual reality field because such tactile illusions play a useful role in deceiving the brain so that operators believe a virtual sensation is real. VHI characteristics are obtained from accomplishing a series of psychophysical experiments using Thurstones method of paired comparison. In the experiment, the stroke movement distance of wires, r, is varied under constant wire spacing, D; the velocity of wire movement is varied, with both wire spacing and stroke movement distance of the wires held constant. It is found that the strongest VHI was obtained at r/D and that the strongest VHI occurred at a specific velocity generating tangential vibration of around 50 Hz. Since VHI requires both compressive stress and tangential stimulus, it is caused by not just one of the four varieties of mechanoreceptive units, but two or more of them. This finding confirms that VHI does not occur in the mechanoreceptive units themselves but in the brain.