Sensitivity to shifts of a point stimulus: an instance of tactile hyperacuity

A biomimetic elastomeric robot skin using electrical impedance and acoustic tomography for tactile sensing

Human skin perceives physical stimuli applied to the body and mitigates the risk of physical interaction through its soft and resilient mechanical properties. Social robots would benefit from whole-body robotic skin (or tactile sensors) resembling human skin in realizing a safe, intuitive, and contact-rich human-robot interaction. However, existing soft tactile sensors show several drawbacks (complex structure, poor scalability, and fragility), which limit their application in whole-body robotic skin. Here, we introduce biomimetic robotic skin based on hydrogel-elastomer hybrids and tomographic imaging. The developed skin consists of a tough hydrogel and a silicone elastomer forming a skin-inspired multilayer structure, achieving sufficient softness and resilience for protection. The sensor structure can also be easily repaired with adhesives even after severe damage (incision). For multimodal tactile sensation, electrodes and microphones are deployed in the sensor structure to measure local resistance changes and vibration due to touch. The ionic hydrogel layer is deformed owing to an external force, and the resulting local conductivity changes are measured via electrodes. The microphones also detect the vibration generated from touch to determine the location and type of dynamic tactile stimuli. The measurement data are then converted into multimodal tactile information through tomographic imaging and deep neural networks. We further implement a sensorized cosmetic prosthesis, demonstrating that our design could be used to implement deformable or complex-shaped robotic skin.

Deformation Matching: Force Computation Based on Deformation Optimization

The tactile information to be presented to a user during interaction with a virtual object is calculated by simulating the contact between the object model and user model. In the simulation, a distributed force is applied to the contact area on the skin tissue of users' hands and results in deformation of the skin tissue. The skin deformation caused by the distributed force is the target contact state that should be presented by the device. However, most multipoint haptic displays do not have sufficient degrees of freedom (DoF) to represent the target contact state. This paper presents the concept and formulation of "deformation matching," whereby the output force is calculated to minimize the error between the target skin deformation and skin deformation that can be realized by the limited DoF device's output force. For comparison, the conventional concept of "force matching" was also formulated. The difference in human perception between these two concepts in the expression of friction was investigated through experiments using a pin-array tactile display capable of stimulating 128 points. It was demonstrated that the perception of the friction coefficient was more sensitive and the perception of the friction direction was more accurate in deformation matching than in force matching.

Fast and accurate edge orientation processing during object manipulation

Quickly and accurately extracting information about a touched object’s orientation is a critical aspect of dexterous object manipulation. However, the speed and acuity of tactile edge orientation processing with respect to the fingertips as reported in previous perceptual studies appear inadequate in these respects. Here we directly establish the tactile system’s capacity to process edge-orientation information during dexterous manipulation. Participants extracted tactile information about edge orientation very quickly, using it within 200 ms of first touching the object. Participants were also strikingly accurate. With edges spanning the entire fingertip, edge-orientation resolution was better than 3° in our object manipulation task, which is several times better than reported in previous perceptual studies. Performance remained impressive even with edges as short as 2 mm, consistent with our ability to precisely manipulate very small objects. Taken together, our results radically redefine the spatial processing capacity of the tactile system.

Putting on a necklace requires using your fingertips to hold open a clasp, which you then insert into a small ring. For you to do this, your nervous system must first work out which way the clasp and the ring are facing relative to one another. It then uses that information to coordinate the movements of your fingertips. If you fasten the necklace behind your head, your nervous system must perform these tasks without information from your eyes. Instead, it must use the way in which the edges of the clasp and the ring indent the skin on your fingertips to work out their orientation.

Earlier studies have examined this process by asking healthy volunteers to judge the orientation of objects – or more precisely edges – that an experimenter has pressed against their fingertips. But people perform worse than expected on this task given their manual dexterity. Pruszynski et al. wondered whether the task might underestimate the abilities of the volunteers because it involves passively perceiving objects, rather than actively manipulating them.

To test this idea, Pruszynski et al. designed a new experiment. Healthy volunteers were asked to use a fingertip to rotate a pointer on a dial to a target location. The participants could not see the dial, and so they had to use touch alone to determine which way the pointer was facing. They performed the task faster and more accurately than volunteers in the earlier passive experiments. Indeed, when the pointer was longer than a fingertip, the volunteers performed almost as well using touch alone as when allowed to look at the dial. Speed and accuracy remained impressive even when the pointer was only 2mm long.

The results of Pruszynski et al. show that we judge orientation more accurately when we manipulate objects than when we passively perceive them. In other words, we do better when we perform tasks in which being aware of orientation is vital. The results also suggest that the nervous system processes sensory information in different ways when it uses sensations to help control objects as opposed to just perceiving them. This could influence the development of new technology that aims to use brain activity to control computers or robotic limbs.

Human detection and discrimination of tactile repeatability, mechanical backlash, and temporal delay in a combined tactile-kinesthetic haptic display system

Many of the devices used in haptics research are over-engineered for the task and are designed with capabilities that go far beyond human perception levels. Designing devices that more closely match the limits of human perception will make them smaller, less expensive, and more useful. However, many device-centric perception thresholds have yet to be evaluated. To this end, three experiments were conducted, using one degree-of-freedom contact location feedback device in combination with a kinesthetic display, to provide a more explicit set of specifications for similar tactile-kinesthetic haptic devices. The first of these experiments evaluated the ability of humans to repeatedly localize tactile cues across the fingerpad. Subjects could localize cues to within 1.3 mm and showed bias toward the center of the fingerpad. The second experiment evaluated the minimum perceptible difference of backlash at the tactile element. Subjects were able to discriminate device backlash in excess of 0.46 mm on low-curvature models and 0.93 mm on high-curvature models. The last experiment evaluated the minimum perceptible difference of system delay between user action and device reaction. Subjects were able to discriminate delays in excess of 61 ms. The results from these studies can serve as the maximum (i.e., most demanding) device specifications for most tactile-kinesthetic haptic systems.

Perception of Direction for Applied Tangential Skin Displacement: Effects of Speed, Displacement, and Repetition

A variety of tasks could benefit from the availability of direction cues that do not rely on vision or sound. The application of tangential skin displacement at the fingertip has been found to be a reliable means of communicating direction and has potential to be rendered by a compact device. Our lab has conducted experiments exploring the use of this type of tactile stimulus to communicate direction. Each subject pressed his/her right index fingertip against a 7 mm rounded rubber cylinder that moved at constant speed, applying shear force to deform the skin of the fingerpad. A range of displacements (0.05-1 mm) and speeds (0.5-4 mm/s) were tested. Subjects were asked to respond with the direction of the skin stretch, choosing from four directions, each separated by 90 degrees. Direction detection accuracy was found to depend upon both the speed and total displacement of the stimulus, with higher speeds and larger displacements resulting in greater accuracy. Accuracy rates greater than 95 percent were observed with as little as 0.2 mm of tangential displacement and at speeds as slow as 1 mm/s. Results were analyzed for direction dependence and temporal trends. Subjects responded most accurately to stimuli in the proximal and distal directions, and least accurately to stimuli in the ulnar direction. Subject performance decreased slightly with prolonged testing but there was no statistically significant learning trend. A second experiment was conducted to evaluate priming effects and the benefit of repeated stimuli. It was found that repeated stimuli do not improve direction communication, but subject responses were found to have a priming effect on future performance. This preliminary information will inform the design and use of a tactile display suitable for use in hand-held electronics.

Tactile hyperacuity thresholds correlate with finger maps in primary somatosensory cortex (S1)

Behavioral tactile discrimination thresholds were compared with functional magnetic resonance imaging measurements of cortical finger representations within primary somatosensory cortex (S1) for 10 human subjects to determine whether cortical magnification in S1 could account for the variation in tactile hyperacuity thresholds of the fingers. Across 10 subjects, the increase in tactile thresholds from the index finger to the little finger correlated with the decrease in cortical representation across fingers in S1. Additionally, representations of the fingers within S1, in Brodmann areas 3b and 1, were also correlated with the thresholds. These results suggest that tactile hyperacuity is largely determined by the cortical representation of the fingers in S1.

Epidermal keratinocytes as the forefront of the sensory system

Various sensors that respond to physical or chemical environmental factors have been identified in the peripheral nervous system. Some of them, which respond to mechanical stress, osmotic pressure, temperature and chemical stimuli (such as pH), are also expressed in epidermal keratinocytes. Neurotransmitters and their receptors, as well as receptors that regulate the neuroendocrine system of the skin, are also present in keratinocytes. Thus, broadly speaking, epidermal keratinocytes appear to be equipped with sensing systems similar to those of the peripheral and central nervous systems. It had long been considered that only nerve C-terminals in the epidermis play a role in skin surface perception. However, building on earlier work on skin receptors and new findings introduced here, we present in this review a novel hypothesis of skin sensory perception, i.e. first, keratinocytes recognize various environmental factors, and then the information is processed and conveyed to the nervous system.

Neural coding of the location and direction of a moving object by a spatially distributed population of mechanoreceptors

A neural code for the location and direction of an object moving over the fingerpad was constructed from the responses of a population of rapidly adapting type I (RAs) and slowly adapting type I (SAs) mechanoreceptive nerve fibers. The object was either a sphere with a radius of 5 mm or a toroid with radii of 5 mm on the major axis and either 1 or 3 mm on the minor axis. The object was stroked under constant velocity and contact force along eight different linear trajectories. The spatial locations of the centers of activity of the population responses (PLs) were determined from nonsimultaneously recorded responses of 99 RAs and 97 SAs with receptive fields spatially distributed over the fingerpad of the anesthetized monkey. The PL at each moment during each stroke was used as a neural code of object location. The angle between the direction of the trajectory of the PL and mediolateral axis was used to represent the direction of motion of the object. The location of contact between the object and skin was better represented in SA than in RA PLs, regardless of stroke direction or object curvature. The PL representation of stroke direction was linearly related to the actual direction of the object for both RAs and SAs but was less variable for SAs than for RAs. Both the SA and RA populations coded spatial position and direction of motion at acuities similar to those obtained in psychophysical studies in humans.

Effects of nonuniform fiber sensitivity, innervation geometry, and noise on information relayed by a population of slowly adapting type I primary afferents from the fingerpad

The capacity of a population of primary afferent fibers to signal information about a sphere indenting the fingerpad is limited by factors such as the inhomogeneity of sensitivity among the afferents, the pattern and density of innervation, and the effects of noise (response variability). Using experimental data recorded from single slowly adapting type I afferents (SAIs), we simulated the response of the SAI population to such a stimulus. The human ability to discriminate stimulus curvature, location, and force has been quantified previously. We devised three neural measures, treating them as surrogates for the real neural measures underlying human performance, and explored how population parameters usually overlooked in neural coding studies affect such measures. Variation in sensitivity among SAIs is large; this distorts population response profiles markedly but has no significant impact on the neural measures. Two classes of noise were introduced, one dependent on and the other independent of the level of neural activity. Resolution of the model was compared with discrimination in humans. Correlation of noise among neurons had different effects for the different measures. An increase in correlation decreased resolution in the measure for force but improved resolution in the measure for position. Increasing innervation density (1) always increased resolution for position and (2) increased resolution for force if noise was uncorrelated but had diminishing effects as correlation increased. Correlation and innervation density had complex effects on the measure for curvature, depending on the class of noise. Nonuniformity in the pattern of innervation had negligible effects on resolution.

Directional sensitivity to a tactile point stimulus moving across the fingerpad

The ability of subjects to discriminate between directions of a point contact moving across the fingerpad was examined. Subjects were required to report, using an adaptive two-interval, two-alternative forced-choice procedure, whether in two sequential stimuli the direction of motion changed in a clockwise or counterclockwise direction. The overall mean orientation-change threshold across eight stimulus orientations was approximately 14 degrees, with the lowest threshold for the point motion toward the wrist. This observed lower threshold in the distal-to-proximal direction is thought to be due to stretching of the skin at the tip of the fingernail, to which one may be particularly sensitive. For all orientations, thresholds were generally more uniform and higher than those reported on vibrotactile linear contactor arrays for horizontal and vertical orientations.

Gender-, side- and site-dependent variations in human perioral spatial resolution

Twenty-eight right-handed, young adults participated in a sensory testing experiment to evaluate spatial resolution at 10 positionally matched sites on the right- and left-hand sides of the face. An adaptive psychophysical (i.e. tracking) procedure was used to estimate the threshold spatial separation for perceiving two points of contact at each site. Estimates of the threshold at one site on both sides of the face were also obtained with a method-of-limits procedure similar to that employed for clinical evaluation of patients. In addition, each individual was asked to rate (i) his(her) overall facial sensitivity to touch and (ii) the degree to which he(she) could discern subtle changes in lip, cheek and chin position during speech, chewing and facial expression. Analysis of the estimates of the threshold separation obtained with the tracking procedure revealed a significant effect of gender (p < 0.04) and of site (p < 0.001). Females were more spatially sensitive than males: average threshold separations were 1.55 mm less. Most notably, the threshold increased ninefold with distance posterolaterally from the oral opening. The vermilion of the upper lip was the most spatially sensitive site (population geometric mean = 2.4 mm) and the preauricular skin the least spatially sensitive site (20.9 mm). Significant effects of side and of interactions among gender, side and site were not observed. The estimates obtained with the method-of-limits procedure were very similar to those obtained with the tracking procedure: the latter were 0.67 mm less on the average. Individuals' ratings of overall facial sensitivity to touch were similar for males and females (p > 0.70). Females, however, reported greater ability to discern subtle changes in lip, cheek and chin position than males (p < 0.03). The ratings of this sensory function correlated negatively with the estimates of the threshold separation on the vermilion of the upper lip (p < 0.03).

Dimensions of spatial acuity in the touch sense: changes over the life span

Spatial acuity of the touch sense and its variation in aging came under psychophysical scrutiny at the fingertip and control body sites. Acuity is viewed as encompassing the discrimination of four features of simple stimulus configurations: (11) discontinuity (gaps in lines or disks), (2) locus on the skin, (3) length (or area), and (4) orientation (e.g., along or across the finger). Each of these dimensions of acuity serves uniquely in tactile perception, as illustrated in the structure of braille. For their measurement, psychophysical tests were developed and refined. These were aimed at freedom from bias, rapid estimation of acuity thresholds in hundreds of subjects, and eventual applicability to the whole body surface. Some 14 versions of the tests were administered in three experiments, yielding 1478 individual thresholds. Experiment I (15 young and 15 elderly subjects) and Experiment II (131 subjects, ages 18 to 87 years) shed light on the nature of discrimination of discontiniuty and orientation. These mainly concern pitfalls of measurement and influence of exact stimulus configuration. Experiment III (115 subjects, ages 8 to 86 years) examined refined versions of tests for all four dimensions of acuity. Four principal findings emerged: (1) At all ages, thresholds for the four dimensions of acuity differ from one another in size--in order from smallest to largest: length, locus, orientation, and discontinuity. Exact sizes differ for transverse and longitudinal stimulus alignment. (2) All four acuity dimensions deteriorate with age, to a first approximation manifesting a constant increase in threshold of approximately 1% per annum between ages 20 and 80 years. That similar rates of deterioration characterize all four dimensions in the fingertip suggests a common mechanism, possibly thinning of the same mediating receptor network. (3) Acuity at more central sites (forearm, lip) deteriorates more slowly than at the fingertip. (4) Individual differences in acuity abound, even after the effects of aging are discounted.

Spatial cues serving the tactile directional sensibility of the human forearm

1. Tactile directional sensibility is considered to rely on the parallel processing of direction-contingent sensory data that depend on skin stretching caused by friction, and spatial cues that vary with time. A temperature-controlled airstream stimulus that prevented the activation of stretch receptors was used to investigate directional sensibility for the skin of the forearm. 2. The dependence on contact load and distance of movement was determined for normal subjects with a two-alternative forced-choice method. Testing was performed under two conditions, elbow bent or straight. Bracing the skin by straightening the arm did not alter the accuracy of the directional sensibility, in contrast to previous findings with stimuli that caused friction. 3. The accuracy of directional sensibility was correlated linearly to the logarithm of the distance of movement of the air jet. No correlation was found between accuracy and contact load, unlike findings with stimuli that cause friction. 4. Measurements were made with different subjects to determine the threshold distance at constant load. On average, subjects were able to distinguish direction with movements of < or = 8 mm. This acuity is sharper than has been reported with static stimuli. There was no correlation between subjects' threshold distances for judging direction and spatial acuity measured with absolute point localization. 5. The ability to distinguish direction was poor for the airstream stimulus compared with stimuli causing frictional contact with hairy skin. Nevertheless, the present findings are consistent with the suggestion that cutaneous spatial acuity is better for dynamic than for static stimuli.

The relationship between skin compliance, age, gender, and tactile discriminative thresholds in humans

Earlier research has suggested that tactile sensitivity, like visual and auditory acuity, may decrease with increasing age. But are decrements in tactile sensitivity attributable to changes in the nervous system, or simply to alterations in the mechanical properties of the skin? In the present study, skin compliance and discriminative thresholds for two-point and gap stimuli were measured on the pad of the left index finger of 102 persons ranging in age from 18 to 84 years. For both types of stimuli, age was found to be a significant predictor of tactile sensitivity, even when skin compliance and gender were controlled. The relationship between increasing age and decrements in tactile discrimination is apparently not attributable to changes in the mechanical properties of the skin, but to other factors, which may include changes in the nervous system affecting the speed, quantity, or quality of information processing.

Human, tactile, directional sensibility and its peripheral origins

Tactile directional sensibility is probably functionally important and deserves attention as it is known to be sensitive to many different disturbances of the somatosensory system. Therefore, the ability of healthy adults to determine the direction of motion of a light tactile stimulus travelling proximally or distally along a straight line on depilated, hairy skin of the forearm was examined with two-alternative, forced-choice technique. The aim was to investigate the relative importance of different types of afferent information which may be used for this purpose. A test was started with the moving stimulus covering a distance of no less than 2.5 mm, which was subsequently increased until the subject could report the direction of motion reliably. Afterwards, the distance was decreased until the subject could no longer do so. Three different stimulation conditions were used and for a point stimulator touching the skin it was found that the necessary distance decreased to 2.5 mm after a moderate increase of the vertical contact load. No such decrease was found when a frictionless air-stream point stimulator was used instead. The distances which had to be covered by the point stimulator touching the skin increased to values which were comparable to those obtained with the air-stream stimulator after the lateral extensibility of the skin had been diminished. This was achieved by attaching a surgical sticky plaster around the stimulated skin area. The present findings consequently indicated that optimal, tactile, directional sensitivity depends on peripheral afferent messages which signal the direction of lateral stretching of the skin.

Electrotactile and vibrotactile displays for sensory substitution systems

Sensory substitution systems provide their users with environmental information through a human sensory channel (eye, ear, or skin) different from that normally used, or with the information processed in some useful way. We review the methods used to present visual, auditory, and modified tactile information to the skin. First, we discuss present and potential future applications of sensory substitution, including tactile vision substitution (TVS), tactile auditory substitution, and remote tactile sensing or feedback (teletouch). Next, we review the relevant sensory physiology of the skin, including both the mechanisms of normal touch and the mechanisms and sensations associated with electrical stimulation of the skin using surface electrodes (electrotactile (also called electrocutaneous) stimulation). We briefly summarize the information-processing ability of the tactile sense and its relevance to sensory substitution. Finally, we discuss the limitations of current tactile display technologies and suggest areas requiring further research for sensory substitution systems to become more practical.

Rhythm perception equipment for skin vibratory stimulation

The characteristics of vibrotactile stimulation and an experimental device for the transmission of rhythm information contained in music and similar sounds by means of vibrotactile stimulation are described. The actuator and input/output behavior of the device are examined. Measurement of the intensity discrimination threshold and beat frequency perception threshold is discussed. Application to music education in schools for the deaf is described, and results of a clinical test are reported.

Tactile guidance of movement

In some prototype mobility aids for the blind information about the environment is obtained with the aid of patterns within a matrix of tactile point stimuli. The aim of this report is to summarize some experiments on the possibilities of guiding movements in 3-D space by devices of this kind, the movements studied being: (1) batting a ball, (2) walking and pointing to a target, and (3) slalom walking. The results were that movements could be guided by such a matrix with reasonable precision and time consumption. There are many remaining problems, especially in a cluttered environment, but they can be expected to be decreased if we are able to increase our knowledge about how touch, or rather the haptic system, is functioning, and if we can utilize this knowledge in constructing more effective tactile displays.

Tactile vision substitution: past and future

Tactile sensory substitution for blind persons has been studied primarily in a laboratory setting. Although the studies demonstrated the feasibility of the approach, to date no practical systems are in use. In this paper, previous tactile vision substitution studies are described, and the reasons why practical systems have not yet been developed are discussed. The theoretical basis for sensory substitution is examined primarily in regard to the capacity of the somatosensory system to mediate high resolution "visual" information. Future developments that may lead to practical systems for blind persons are considered.

Tactile sensitivity to two types of stimulation: continuous and discrete shifting of a point stimulus

Tactile sensitivity to shifts of a point stimulus was determined at the forehead for spatially filled (continuous) and spatially separated (discrete) stimulation. The minimal limen was between 12 and 30 mm/sec. for continuous stimulation and at a 200-msec. interstimulus interval for discrete stimulation. The continuous thresholds were always lower than the discrete thresholds at the examined time rates. Furthermore, the continuous thresholds were slightly higher for oblique directions than for both vertical and horizontal directions, while the discrete thresholds were quite low for both vertical and horizontal directions.

Tactile pattern perception

Because efforts to use the cutaneous sense for conveying speech and visual information have met with only partial success, it would be useful to understand better the pattern-sensing capabilities of touch. This paper is an account of the sensory and perceptual factors known or hypothesized to limit the tactile perception of simple two-dimensional patterns, with special attention to the limited spatial bandwidth of touch.