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Necessary spatial resolution for realistic tactile feeling display


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In this paper, we show a hypothesis on the sensing mechanism in the human tactile organ and its resolution. The hypothesis is that human skin cannot resolve any finer pattern than the resolution suggested by the two-point-discrimination test, but that variety created by four kinds of signals from four kinds of mechano-receptors makes it possible to detect fine feature of texture. This means if we control stimulus to four kinds of mechanoreceptors individually, the realistic contact-feeling display will not need higher spatial resolution than suggested by the two-point discrimination threshold. We examine this hypothesis through psychophysical experiments.
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Necessary Spatial Resolution for Realistic Tactile Feeling Display
Naoya ASAMURA, Tomoyuki SHINOHARA, Yoshiharu TOJO, Nobuyoshi KOSHIDA
and Hiroyuki SHINODA
Department of electrical and Electronic Engineering,
Tokyo University of Agriculture and Technology
2-24-16 Koganei, Tokyo 184-8588 Japan (N. Asamura) (H. Shinoda)
In this paper, we show a hypothesis on the sensing
mechanism in the human tactile organ and its resolution.
The hypothesis is that human skin cannot resolve any
finer pattern than the resolution suggested by the
two-point-discrimination test, but that variety created
by four kinds of signals from four kinds of
mechano-receptors makes it possible to detect fine
feature of texture. This means if we control stimulus to
four kinds of mechanoreceptors individually, the
realistic contact-feeling display will not need higher
spatial resolution than suggested by the two-point
discrimination threshold. We examine this hypothesis
through psychophysical experiments.
Keywords; virtual reality, haptic interface, tactile
feeling display, spatial resolution, two-point
discrimination, teletaction
1. Introduction
The recent development of the Internet is motivating
a cutaneous display that makes people feel realistic
tactile feeling, for on-line shopping or amusements
[1,2]. For such a display not aiming some manipulation
tasks [ 3,4 ,5, 6,7,8 ,9], the display area should not
necessarily be focused on the finger tip having the
highest receptor density. Instead, high-fidelity of tactile
feeling becomes crucial.
In this paper, we discuss the minimum requirement
for the special resolution of such a tactile feeling
display. Our target area in this study is the palm
because it seems sufficiently sensitive for tactile feeling
transmission and also dull enough for technological
The two-pint discrimination threshold (TPDT) is one
classical and popular measure of the skin resolution.
This TPDT is the minimum distance with which we can
identify 2 points given in a simultaneous
two-point-contact. The TPDTs on a fingertip and a
palm are 2~3 mm and about 10 mm respectively [
However, the evaluation of the tactile resolution
includes some complex problems. Humans can identify
a very fine feature of objects less than the TPDT. For
example, our palms easily distinguish between the top
and the bottom of a pen though the both sizes are
smaller than the TPDT. Our skin is so sensitive that it
hardly feel the motion of pins arrayed on a device
create a realistic feeling of a cotton towel or a leather
In order to understand this paradoxical problem and
realize the realistic tactile display, we have to consider
both spatial resolution and skin receptor’s selective
sensitivity in parallel. In this paper, we propose a
hypothesis on the cutaneous sensing mechanism. The
scientific proof of the hypothesis will need other
researches including neurophysiological approaches
beyond the research in this paper. However, the
hypothesis suggests a base to understand the human
skin perception. In the following, we describe the
psychophysical experiments to examine the hypothesis,
and show that we can control various tactile feeling to
the palm by a low-resolution display device.
Fig. 1: Spatial resolution for displaying realistic tactile
feeling on the palm.
Proceedings of the 2001 IEEE
International Conference on Robotics & Automation
Seoul, Korea May 21-26, 2001
0-7803-6475-9/01/$10.00© 2001 IEEE
Fig. 2: Candidates for the four static stress bases in
hypothesis 2. The arrows represent the direction of the
applied force. The stress by σ
does not reach the deep
part of the skin.
2. A Hypothesis on human tactile
It is widely accepted that human glabrous skin has
four kinds of mechanical receptors. Though thermal
stimuli is also important for touch feeling [
11], we omit
that argument here. It is relatively easy to add thermal
controller to a mechanical stimulator because high
resolution and quick response are unnecessary.
The hypothesis we should examine here is:
A half of TPDT is the sufficient resolution (interval of
stimulation) for displaying any fine tactile feeling if we
individually control stimulus to the four kinds of hand
The hypothesis is based on an analogy of the human
visual system. It says we do not need much finer
resolution than the TPDT even if we should display
very fine texture, as long as we stimulate the four kinds
of the mechanoreceptors selectively. It is well known
that human skin can distinguish very fine features of
the texture [12,13]. The hypothesis insists that the
human skin should perceive these fine features from the
4-D vector detected by the four kinds of receptors with
sampling intervals 1/2
× TPDT. If the four kinds have
different spatial responses, they can detect such features,
just as our three kinds of RGB visual receptors identify
On the palm, it is said that the density of innervation
of each types of receptors is 10~30 units/cm
Among the four types, the two kinds of the superficial
receptors (Meissner corpuscle and Merkel cell) are
located more densely than the others, and their densities
are especially high on the fingertip [15]. The other two
receptors (Rufini ending and Pacinian corpuscle) are
located in the deep part of the skin. The temporal and
spatial responses are various among the four types [16],
but there is a certain temporal frequency range in which
all the types respond well.
6 mm
5 mm
1 mm
Acrylic resin
0.5 mm
in diameter
Fig. 3: Selective stimulator to a human palm. The S1
applies uniform normal stress (σ
) while the S2 applies
concentrated normal stress (σ
3. Examination of the hypothesis
Let (x,y) be the spatial coordinate system on a hand.
Suppose stress distribution given on the hand is written
as σ
σ(x,y,t) = (σ
(x,y,t)). Next we
describe the subjective feeling to a stress distribution σ
as p(σ
σ). A equation p(σ
) = p(σ
) means that the
tactile feeling to σ
is identical to (indistinguishable
from) that to σ
For a point Q(a,b) on the hand, we define Φ(Q) as a
subspace of stress distribution in which the stress is
zero outside the circle round the center Q with a radius
of TPDT/2. That is
Φ(Q (a,b))
= {σ
σ; σ
σ(x,y,t) = 0 where |(x-a, y-b)| > TPDT/2}.
Here we rewrite the hypothesis in a different manner.
Hypothesis 2
We can find four static stress bases σ
, and σ
Φ(Q) that realize P’ = P where
P = {p(σ
σ); σ
σ σ
P’ = {p(σ
σ); σ
σ σ
σ = α
This hypothesis means we can find four basic
components of stress pattern whose summation can
display any tactile feeling caused by any stress
distribution given around Q. If it is true, an array of the
stimulators giving the four bases with intervals of
TPDT/2 will be able to produce any tactile feeling.
The equivalence of the hypothesis 2 to the original
hypothesis is not very obvious. But from now we
examine the hypothesis 2 instead of the original one
that was a hint to come up with the second one.
The candidates we imagine now for the four bases
are 1) smooth distribution (around Q) of x-directional
stress, 2) y-directional stress, 3) z-directional stress
(vertical to the skin), and 4) concentrated vertical stress
that does not reach the deep part of the skin [17]. See
Fig. 2. These candidates were obtained by the
following logic. First, since the skin can discriminate
among σ
, and σ
, the bases should include them.
And because the human skin has an excellent ability to
detect sharp edge, we added one more basis σ
to them.
We guess the σ
is detected by the combination of
Meissner corpuscle and Merkel cell [18].
In this paper we report results of examining
hypothesis 2 for a subspace Φ
(Q) included in Φ(Q)
where Φ
(Q) consists of stress distributions having no
lateral components (no shearing stress). Then the two
bases σ
and σ
in Fig. 2 should display any tactile
feeling for any σ
σ in Φ
(Q). In order to examine this, we
prepared an apparatus as shown in Fig. 3 that stimulates
the skin with S1 and S2.
S1) Smooth normal stress distribution (σ
) by a moving
cylinder with a diameter of 1/2
S2) Concentrated normal stress distribution (σ
approximately) by a needle through the S1 cylinder.
The stimulator S1 represents an object with very
small curvature, while the S2 very large curvature. We
examine whether the combination of the S1 and S2 can
create tactile feeling of an intermediate curvature of an
object, and whether the curvature can be controlled
continuously from a sharp tip to a smooth surface.
If this is true, the hypothesis 2 for the subspace
(Q) sounds very reasonable because the contact in
general can be assumed as a combination of multiple
contacts with various curvature surfaces.
Fig. 4 shows the apparatus for experiments. The
diameter of S1 is 5 mm, a half of TPDT 10 mm on palm,
while the inside diameter of S1 is 1 mm. The S2 is a pin
with diameter of 0.5mm. Each stimulator moves
independently in vertical direction. Subjects put the
hand on a flat panel, and we apply the S1 and S2
through a hole in the panel. The examined part is the
thenar for the easiness of the experiment.
The S1 and S2 are actuated by ultrasonic motors with
displacement-resolution 0.002 mm/pulse.
(b) (c)
Fig. 4: Photographs of the apparatus. (a): The structure.
(b): The stimulators S1 and S2. (c): A view of the
experiment stimulating a thenar.
Fig. 5: Experiment I. Displacement patterns of the
stimulator S1 and S2.
Display surface
Skin (palm)
Steel s
Diameter :1mm
Fig. 6: Tactile feeling comparison between synthesized
stimuli and real sphere surfaces.
4. Experiment I
When the S1 and S2 were driven by a signal pattern
as shown in Fig. 5 (they are at the same level at the
beginning), the subjects did not feel a projection on a
flat surface but an intermediate curvature surface. Then
we examined if we could produce tactile feeling
identical to a round object with an intermediate
curvature by using S1 and S2. The hand is fixed and
passive to stimuli.
We prepare the best driving pattern to create the
feeling of the sphere 3 mm in diameter. In that best
signal, the maximum displacements of S1 and S2 are
0.8 mm and 1.2 mm, respectively. We name this
stimulus “Virtual.” Another stimulus is a contact with a
real 3 mm-diameter sphere moving in the S2 pattern in
Fig. 5. We name this “Real.” In addition to these, we
prepare one more similar stimulus “V2” for a reference,
in which S1 does not move (displacement zero) while
S2 follows the S2 pattern in Fig. 5. Using these signals,
we test if the subject can discriminate between the
synthesized feeling by S1 and S2 and the real spherical
We give the subject two stimuli sequentially at a 4
second interval. The one stimulus is the “Real” arising
twice at a 1 second interval. The other is one of three
kinds of stimuli “Virtual,” “Real,” and “V2” that also
arise twice at a 1 second interval. Then the subject
replies either “yes” or “no” to the question “Is the
second stimulation (after the 4 second interval)
identical with the first one?” See Fig. 6. The choice of
the stimulus combination and the order are random, and
each subject answers fifteen times to a series of trials.
The subjects were six males in their twenties with
eye-masks and headphones.
Fig. 7 shows the percentage that the subjects
answered “identical” in response to the sequentially
given 2 stimuli. “Real-Real” means the case the tester
touched the identical sphere twice, and “Real-Virtual”
means stimulus “Real” and “Virtual” were given
Even for “Real-Real,” the subjects sometimes felt
they were not identical. The result shows the stimulus
“Virtual” felt so similar to the “Real” that they missed
the difference once in twice even if they concentrated
on that.
Real-Real Real-V2 Real-Virtual
Stimulus pair
Percentage of yes [%]
0 %
Fig. 7: Results of discrimination test. Percentage that
the six subjects answered that the two stimuli given
sequentially felt identical. TheReal-Virtual” means the
case the stimulus “Real” and “Virtual” were given
5. Experiment II
In this experiment we examine how the perceived
curvature changes when we change the maximum
displacement of S1 in Fig. 5.
Also in this experiment, the S2 is driven in the S2
pattern in Fig. 5. And the Subjects are given seven
types of stimuli in which the S1 reaches the maximum
displacement in seven manners. The times to reach the
top and to start going down are common while the top
displacements are (A) 1.2mm, (B) 1.1 mm, (C) 1.0 mm,
(D) 0.9 mm, (E) 0.8 mm, (F) 0.7 mm, and (G) 0.6 mm,
respectively. In stimulus (A) the subjects felt a smooth
surface because the top displacements of S1 was equal
to that of S2. In stimulus (G) they felt a sharp object
because the projection of S2 was large.
The subjects answer the perceived curvature
comparing with reference objects of metal sphere with
diameters of 1, 3, and 5 mm, moving in the S2 pattern
in Fig. 5.
A subject receives one pattern of seven kinds of
stimuli A, B, - - F, and G from the S1-S2 stimulator,
and memorize the feeling especially paying attention to
the curvature. Next, the subjects touch the three
reference objects coming sequentially at constant
intervals of 1 second, and they answer the comparison
of the curvature between the S1-S2 stimulus and the
reference objects. The answers are classified into the
seven categories and give those categories points from
0 to 6 as shown in Tabl e 1. For instance, if it felt as the
same as the sphere with diameter of 3 mm, we give it 3
points. (Though feeling of the S1-S2 stimulator was not
always identical with that of reference objects,
comparison was possible.)
The experiment was done 3 times for each signal.
During the experiment, the subjects wore headphones
and eye-masks not to obtain any cues from the sound
and sight. The subjects were five males in their twenties
with eye-masks and headphones.
Tabl e 1 : Assigning points to the perceived curvature
categories by S1-S2 stimulus.
of sphere
x [mm]
x < 1
1 < x < 3
3 < x < 5
x > 5
Point 0 1 2 3 4 5 6
Fig. 8: Histogram of perceived curvature for various
intensities of S1-S2. The maximum displacements of
S1 were (A) 1.2, (B) 1.1, (C) 1.0, (D) 0.9, (E) 0.8, (F)
0.7 and (G) 0.6 mm, while S2 always moved in the S2
pattern in Fig. 5.
Tabl e 2 and Fig. 8 show the perceived curvature versus
stimulus A, B, - - F, and G. Fig. 8 is a histogram of the
perceived diameter classified following Table 1. The
perceived curvature changed by the maximum
displacement of S1. When maximum displacement of
S1 decreased, subjects felt higher curvature. The
average points for the stimulus A, B, - - F, and G are
summarized in Table 2. And its graphical plots are
shown in Fig. 9. For stimuli B - G, subjects felt finer
object than the cylinder of S1 though the surface of S1
always touched the skin. The results showing
continuous change of perceived curvature along the S2
projection change, is consistent with our hypothesis.
Tabl e 2 : Subjective curvature versus the maximum
displacements of S1. The averaged points defined in
Table 1 are shown.
Stimulus A B C D E F G
5.4 4.7 4.1 3.4 3.0 2.1 1.3
0.6 0.8 1 1.2
Maximum displacement of S1 [mm]
Perceived diameter [mm]
Fig. 9: Plots of Table 2. The perceived diameter is the
average point in Table 2.
6. Summary and discussions
We proposed a hypothesis on realistic tactile display
and its spatial resolution. It suggests that 4-D
stimulators arrayed at intervals of TPDT/2 can produce
any tactile feeling. The 4-D stimulator means a
stimulator to the skin applying four bases of local stress
distribution and their summation. And we showed the
explicit forms of the four bases.
In experiments, we examined the hypothesis for a
subspace Φ
of tactile stimulation in which the stress
has no lateral components. In this case the hypothesis
tells us that the number of the bases to span all tactile
feelings becomes two from four. Then one of the two
bases is a local but smooth normal-stress distribution
S1, and the other is a concentrated distribution S2.
In Experiment I, we examined whether the
combination of the S1 and S2 can create tactile feeling
of an intermediate curvature of an object. And we
obtained a driving pattern of S1 and S2 in which the
subjects felt once in twice that the stimulus was
identical to that of an intermediate curvature of an
In Experiment II we confirmed the perceived
curvature could be controlled continuously from a
sharp tip to a smooth surface.
These results for the subspace Φ
supported the
hypothesis because the contact in general can be
assumed as a combination of multiple contacts with
various curvature surfaces.
However, we have to add a remark that it is not
straightforward to display wide range of tactile feeling
with a simple array of the cylinders and pins as is
shown in Fig. 3 because we easily feel the shapes of the
cylinder and the pin by an active movement of the hand.
In the experiments of this paper, the thenar was fixed
and stimulated through a hole to obtain perfect
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The latest wearable technologies demand more intuitive and sophisticated interfaces for communication, sensing, and feedback closer to the body. Evidently, such interfaces require flexibility and conformity without losing their functionality even on rigid surfaces. Although there have been various research efforts in creating tactile feedback to improve various haptic interfaces and master–slave manipulators, we are yet to see a comprehensive device that can both supply vibratory actuation and tactile sensing. This paper describes a soft pneumatic actuator (SPA)-based skin prototype that allows bidirectional tactile information transfer to facilitate simpler and responsive wearable interface. We describe the design and fabrication of a 1.4 mm-thick vibratory SPA – skin that is integrated with piezoelectric sensors. We examine in detail the mechanical performance compared to the SPA model and the sensitivity of the sensors for the application in vibrotactile feedback. Experimental findings show that this ultra-thin SPA and the unique integration process of the discrete lead zirconate titanate (PZT)-based piezoelectric sensors achieve high resolution of soft contact sensing as well as accurate control on vibrotactile feedback by closing the control loop. [Online:]
The word haptics, believed to be derived from the Greek word haptesthai, means related to the sense of touch. In the psychology and neuroscience literature, haptics is the study of human touch sensing, specifically via kinesthetic (force/position) and cutaneous (tactile) receptors, associated with perception and manipulation. In the robotics and virtual reality literature, haptics is broadly defined as real and simulated touch interactions between robots, humans, and real, remote, or simulated environments, in various combinations. This chapter focuses on the use of specialized robotic devices and their corresponding control, known as haptic interfaces, that allow human operators to experience the sense of touch in remote (teleoperated) or simulated (virtual) environments.
The development of new types of visual-aid tablet for visually impaired people requires the development of cheap, but still very effective photoactuating materials. This requirement can be satisfied by the use of new kind of elastomers filled by nanofillers, such as carbon nanotubes. Nanocomposites based on commercial ethylene vinyl-acetate (EVA) copolymer and multiwalled carbon nanotubes (MWCNT) were prepared by casting from solution. The non-covalent surface modification of MWCNT was carried out by special, newly synthesized compatibilizer cholesteryl 1-pyrenecarboxylate (PyChol). In order to mimic Braille character, special home-built silicone punch and die moulds were used. The Braille element based on EVA/MWCNT-PyChol composite displays reversible, multiple changes of dimension in the direction of the irradiation during/upon illumination by red and blue light-emitted diode (LED). Transmission electron microscopy (TEM) showed a good dispersion of the MWCNT-PyChol within the matrix. The Braille element behaviour under illumination was analysed by atomic force microscopy (AFM) and by nanoindentor. Nanoindentor, even if the purpose of its original use is different, can be effectively applied for the determination of the actuation stroke, the sample dimensional changes in the direction of irradiation.
A combined-type vibrotactile displayed with a horizontal fin actuator array and a vertical pin actuator array is stimulated. The former uses piezoelectric element to stimulate transverse vibration and the latter uses electromagnetic vibration exciter to stimulate longitudinal vibration, respectively. These two stimulation methods are uncoupled. This display can realize the representation of touch sense of various surface textures through the combination and control of vibration direction, amplitude and frequency of the vibrators. Surface roughness representation is researched by using this display, and the theory of grey color theory is introduced. The transverse vibration in the fin array is applied to stimulate the sense of smooth, and the longitudinal vibration in the pin array is used to stimulate the sense of roughness. These two types of vibration are interweaved together to produce some specific tactile sensations of surface roughness. The mechanical parameters quantification experiment test and the mental experiment both verify that this combination of various vibration modes of slices and needles is effective to the representation of various roughness textures.
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The human hand and the brain are close partners in two important and closely interconnected functions, i.e. to explore the physical world and to reshape selected segments of it according to man's intentions. Both these functions are highly dependent on accurate descriptions of mechanical events when objects are brought in contact with the hand. A key role in providing such information is played by the population of mechanoreceptive afferent units innervating the hairless skin of the volar aspect of the hand, i.e. the glabrous skin. Recently it became possible to explore the characteristics of these units in man and to elucidate their role in perception as well as in motor functions.
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Recordings from single peripheral nerve fibres made it possible to analyse the functional properties of tactile afferent units supplying the glabrous skin of the human hand and to assess directly the relation between impulse discharge and perceptive experiences. The 17,000 tactile units in this skin area of the human hand are of four different types: two fast adapting types, FA I and FA II (formerly RA and PC), and two slowly adapting types, SA I and SA II. The receptive field characteristics and the densities in the skin of the type I units (FA I and SA I) indicate that these account for the detailed spatial resolution that is of paramount importance for the motor skill and the explorative role of the hand. The relationship between the stimulus amplitude and perceived intensity during sustained skin indentations did not match the corresponding stimulus response functions of SA units suggesting non-linear transformations within the central nervous system. These transformations, in turn, appear to vary between subjects. A single impulse in a single FA I unit may be felt when originating from the most important tactile regions of the hand, indicating that the psychophysical detection may be set by the threshold of the sense organs. Moreover, no significant noise seems to be superimposed in the respective central sensory pathways.
There are several tactile receptors in the tissue of human fingers. In this study we calculate in detail the deformation of finger tissue when a finger comes into contact with a rigid plate using a FE (finite element) model to clarify the reason for the precise location of the receptors. The FE model is constructed using measured geometry and material properties. As a result, we found that the strain energy is concentrated at the tactile receptor locations. When a frictional force is applied, the stress/strain is concentrated near the edge of the contact area. By calculating the stress/strain distribution using models with/without epidermal ridges/papillae, we found that the shape of the epidermal ridges/papillae influences the stress/strain distribution near the tactile receptors.
Conference Paper
The author describes a teleoperated hand system developed to study the role of tactile and fine-force sensing in telemanipulation. Both master and slave manipulators are two-fingered hands designed for precision tasks that humans typically execute with a pinch grasp. A direct drive parallel linkage configuration and brushless DC servomotors permit smooth, accurate control of contact forces and small motions, which is essential for effective tactile sensing and display. Initial experiments demonstrated that an operator can perform precision tasks using this system, and that the ability to convey small force levels to the operator required careful attention to the coupling between the operator's finger tips and the master manipulator. The system has been used to demonstrate that a tactile display can convey frictional information sensed at the slave manipulator
Conference Paper
This paper (Part II of II) surveys the existing touch display technologies in the literature. This survey indicates 5 main approaches to touch feedback, involving visual, pneumatic, vibro-tactile, electro-tactile and Neuromuscular stimulations. A pneumatics approach could use air-jets, air pockets or inflatable bladders to provide touch feedback cues to the operator. Similarly, the vibro-tactile approach could use vibrating pins, voice coils, or piezoelectric crystals to provide tickling sensation to the human operator's skin to signal the touch. The electro-tactile stimulation method can provide electric pulses, of appropriate width and frequency, to the skin while the neuromuscular stimulation approach provides the signals directly to the primary cortex of the operator's brain. With regard to this, seventeen (17) devices, most of whom were built for sensory substitution purposes, have been examined and compared for their suitability as touch feedback devices for dexterous tele-manipulation.
Human subjects were required to differentiate grating surfaces of alternating grooves and ridges by moving a finger back and forth across the surface. Their discriminative capacities were measured, as well as the movement and force profiles that they selected. To measure discrimination, a forced choice paradigm was used in which three surfaces were presented on each trial. Two surfaces were the same (standards) and the subject was required to indicate which of the three surfaces (the comparison) differed from the other two. Two series of surfaces were used with standards whose spatial periods were 770 and 1002 mu, respectively. Subjects were able to discriminate, at the 75% correct level, two gratings which differed in spatial period by the order of 5%. When tangential movement between the surface and the finger was eliminated, and only radial contact permitted, discrimination was degraded and the 75% correct levels increased to the order of 10%. Subjects were free to choose their own patterns of finger movement and of contact force between finger and surface. Movement was measured cinematographically. For all subjects movement patterns were close to sinusoidal, with frequencies in the range of 4.0 Hz and with mean velocities of the order of 160 mm/s. Patterns of contact force were measured by a force transducer. For all subjects the force varied rhythmically in synchrony with movement, but the patterns and magnitudes varied with the subject. Gratings were scaled for perceived roughness by a magnitude estimation technique: the relationship between perceived roughness and grating period was monotonic.
Psychophysical experiments were designed to assess the tactile discriminative abilities of human subjects when touching textured surfaces. Plastic strips were produced which had raised dots in a square arrangement (standard surface) or in one of a number of rectangular arrangements (modified surfaces) in which the spacing of the dots differed from the standard surface by some constant amount in one direction. Subjects were presented with pairs of surfaces and asked to discriminate whether each pair consisted of (a), two identical standard surfaces, or (b), a standard surface and a modified surface. Performance measurements were analysed using decision theory. When subjects moved their fingers over the surfaces (active touch) their responses were virtually unbiased, and there was a linear relationship between discriminative performance and the difference between the spacing of the dots on the two surfaces. At the 75% correct level, subjects could distinguish surfaces in which the period of the dots differed by only 2%. Performance was virtually independent of the method of movement used, despite large differences in the velocity profiles of the various movements. Experiments in which the surfaces were moved under the subject's stationary finger (passive touch) displayed the same linear relationship between performance and period difference as in the active-touch experiments. Furthermore, the discriminative performance levels were very similar in the two types of experiments. In the passive-touch experiments, subjects could distinguish smaller differences in period in the surface dimension parallel to (along) the direction of movement than they could distinguish in the dimension perpendicular to (across) the direction of movement. The hypothesis is advanced that normal active discrimination of surfaces is made possible by using similar movements in successive surface contacts and a relatively simple neural code.
Isolated responses were recorded from fibers in the median nerves of human subjects by using microneurography. Mechanoreceptive afferent fibers with receptive fields on the fingerpads were selected. The fingers were immobilized and spherical stimuli were applied passively to the receptive field with a contact force of 40-, 60-, or 80-g weight. The radii of the spheres were 1.92, 2.94, 5.81, or 12.4 mm or infinity (flat); the corresponding curvatures, given by the reciprocal of the radii, were 694, 340, 172, 80.6, or 0 m-1, respectively. When the spheres were applied to the receptive field center of slowly adapting type I afferents (SAIs), the response increased as the curvature of the sphere increased and also increased as the contact force increased. All SAIs behaved in the same way except for a scaling factor proportional to the sensitivity of the afferent. When a sphere was located at different positions in the receptive field, the shape of the resulting response profile reflected the shape of the sphere; for more curved spheres the profile was higher and narrower (increased peak and decreased width). Slowly adapting type II afferents (SAIIs) showed different response characteristics from the SAIs when spheres were applied to their receptive field centers. As the curvature of the stimulus increased from 80.6 to 172 m-1, the response increased. However, further increases in curvature did not result in further increases in response. An increase in contact force resulted in an increase in the response of SAIIs; this increase was proportionately greater than it was for SAIs. For SAIIs, the shape of the receptive field profile did not change when the curvature of the stimulus changed. For fast-adapting type I afferents (FAIs), the responses were small and did not change systematically with changes in curvature or contact force. Fast-adapting type II afferents (FAIIs) did not respond to our stimuli. Human SAIs, FAIs, and FAIIs behaved like monkey SAIs, FAIs, and FAIIs, respectively. The response of the SAI population contains accurate information about the shape of the sphere and its position of contact on the finger and also indicates contact force. Conversely, whereas SAIIs possess a greater capacity to encode changes in contact force, they provide only coarse information on local shape.
Conference Paper
A teletaction system uses a tactile display to present the user with information about texture, local shape, and/or local compliance. Current tactile displays are flat and rigid, and require precise machining and assembly of many parts. This paper describes the fabrication and performance of a one-piece pneumatically-actuated tactile display molded from silicone rubber. Tactor spacing is 2.5 mm with 1 mm diameter tactor elements. Tactile display compliance ensures contact between the finger and tactile display at all times. Unlike previous pneumatic tactile displays, there is no chamber leakage and no seal friction. A psychophysics experiment showed that a synthetic grating on the tactile display was perceived as well as a low-pass-filtered real contact