Two-point Threshold Research Articles

Human Perception of Pneumatic Tactile Cues

The ability to combine tactile, on-the-skin, sensory cues with the kinesthetic ones that widely available force-feedback devices are able to produce is a desirable one, opening the door to the production of more realistic, compelling virtual environments. Pneumatic air-jet displays can be easily mounted on existing force-feedback devices and we believe have the potential to provide useful complimentary tactile information. However, there is little concrete psychophysical data relating to pneumatic displays — a fact that hinders their adoption. This paper addresses this challenge, and presents brief psychophysical studies examining localization rate, the two-point threshold and the stimulus intensity of cues produced by pneumatic air jets. Using insights gained from these studies, we also present a more concrete application-focused investigation where we evaluate the effect of combining tactile and force-feedback cues in a simple object manipulation task in a virtual environment. We show that task completion times are significantly improved with the addition of tactile information, validating our ideas and suggesting this topic warrants further attention.

Functional Outcomes of Cleft Lip Surgery. Part IV: Between- and Within-Participant Variables Affecting Lip Vermilion Sensory Thresholds

Compare neurosensory assessments for participants with and without a cleft lip; identify between- and within-participant variables affecting sensory thresholds on the vermilion of participants with cleft lip. A parallel group, nonrandomized clinical trial. There were 56 participants with cleft lip and 37 noncleft participants. Two-point perception and warmth and cool detection thresholds were measured on the right and left sides of the upper and lower vermilion. A cotton-tip stick, stroked across the skin, was used to identify altered sensation. Linear mixed effects modeling was used to examine the effects of between- and within-participant variables on the thresholds. Threshold values on the upper and lower vermilion were similar for cleft and noncleft participants and were unaffected by the presence of a cleft on the side tested. Participants with cleft lip who reported hyposensitive altered sensations had higher two-point thresholds on the upper lip than those who reported hypersensitivity. Participants with cleft lip who reported altered midface sensation had lower warmth detection, but higher cool detection thresholds, on the lower vermilion than participants with cleft lip who did not report altered sensation. Participants with bilateral cleft lip had lower warmth detection thresholds on the upper vermilion than participants with unilateral cleft lip. Although participants with cleft lip and noncleft participants exhibit similar thermal and two-point discrimination, on average, differences exist among subgroups of participants with cleft lip that may reflect central disturbances in the processing of somatosensory stimuli.

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

SUMMARYTo 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 A's 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 operator's 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.

Virtual Lattice Texture Presentation Using a Tactile Mouse Featuring a Fine Pin Array (2nd Report, Examination on Pin Pitch, Texture Density and Ridge Height)

In the previous study, we developed a tactile mouse featuring a fine pin array, the purpose of which is to be effective in virtual reality and tele-existence systems. It features an 8-by-8 array of stimulus pins each measuring 0.3mm in diameter and with a 1.0-or 1.8-mm pin pitch. Since the stimulus pins within the display's pads can be made to protrude above the pad's surface and their motions are controlled by piezoelectric actuators, the operator can feel various combinations of pins protruding according to the surface texture of the virtual object. In order to establish a design method for the tactile displays, we performed a series of psychophysical experiments using the display. By evaluating variation in threshold for discriminating crossed angles of virtual lattice textures under different pin arrays, texture density, and pin strokes, we determined suitable pin pitch, presentable density and ridge height of the virtual texture. It was found that the two-point threshold should be adopted as the pin pitch between pins in the design of the tactile display. The presentable density and ridge height of the virtual textures were N=50 and δ=0.3mm. Finally, we compared the discrimination precision of virtual textures with those of real textures to evaluate the present tactile display's presentation quality. Since the difference threshold for the real texture was 1/3 that of the virtual texture, the present tactile display should be improved with respect to shearing force presentation to simulate relative motion feeling between the finger surface and virtual object surfaces.

Stimulus-dependent effects on tactile spatial acuity

BackgroundPrevious studies have shown that spatio-tactile acuity is influenced by the clarity of the cortical response in primary somatosensory cortex (SI). Stimulus characteristics such as frequency, amplitude, and location of tactile stimuli presented to the skin have been shown to have a significant effect on the response in SI. The present study observes the effect of changing stimulus parameters of 25 Hz sinusoidal vertical skin displacement stimulation ("flutter") on a human subject's ability to discriminate between two adjacent or near-adjacent skin sites. Based on results obtained from recent neurophysiological studies of the SI response to different conditions of vibrotactile stimulation, we predicted that the addition of 200 Hz vibration to the same site that a two-point flutter stimulus was delivered on the skin would improve a subject's spatio-tactile acuity over that measured with flutter alone. Additionally, similar neurophysiological studies predict that the presence of either a 25 Hz flutter or 200 Hz vibration stimulus on the unattended hand (on the opposite side of the body from the site of two-point limen testing – the condition of bilateral stimulation – which has been shown to evoke less SI cortical activity than the contralateral-only stimulus condition) would decrease a subject's ability to discriminate between two points on the skin.ResultsA Bekesy tracking method was employed to track a subject's ability to discriminate between two-point stimuli delivered to the skin. The distance between the two points of stimulation was varied on a trial-by-trial basis, and several different stimulus conditions were examined: (1) The "control" condition, in which 25 Hz flutter stimuli were delivered simultaneously to the two points on the skin of the attended hand, (2) the "complex" condition, in which a combination of 25 Hz flutter and 200 Hz vibration stimuli were delivered to the two points on the attended hand, and (3) a "bilateral" condition, in which 25 Hz flutter was delivered to the two points on the attended hand and a second stimulus (either flutter or vibration) was delivered to the unattended hand. The two-point limen was reduced (i.e., spatial acuity was improved) under the complex stimulus condition when compared to the control stimulus condition. Specifically, whereas adding vibration to the unilateral two-point flutter stimulus improved spatial acuity by 20 to 25%, the two-point limen was not significantly affected by substantial changes in stimulus amplitude (between 100 – 200 μm). In contrast, simultaneous stimulation of the unattended hand (contralateral to the attended site), impaired spatial acuity by 20% with flutter stimulation and by 30% with vibration stimulation.ConclusionIt was found that the addition of 200 Hz vibration to a two-point 25 Hz flutter stimulus significantly improved a subject's ability to discriminate between two points on the skin. Since previous studies showed that 200 Hz vibration preferentially evokes activity in cortical area SII and reduces or inhibits the spatial extent of activity in SI in the same hemisphere, the findings in this paper raise the possibility that although SI activity plays a major role in two-point discrimination on the skin, influences relayed to SI from SII in the same hemisphere may contribute importantly to SI's ability to differentially respond to stimuli applied to closely spaced skin points on the same side of the body midline.

A novel device for delivering two-site vibrotactile stimuli to the skin

Current methods for applying two-site vibration stimuli to the skin typically involve the use of two separate vibrotactile stimulators, which can lead to difficulty with positioning of stimuli and in ensuring that stimuli are delivered perfectly in phase at the same amplitude and frequency. This report describes the Two-Point Stimulator (TPS) that was developed in order to deliver two-point stimuli to the skin at variable distances between the sites of stimulation on a trial-by-trial basis. The apparatus attaches to a vibrotactile stimulator, modifying it from the standard single probe tip to two probe tips. Each of the two probe tips can be independently positioned to set the tip-to-tip spacing. Both points of the TPS are driven by the single vibrotactile stimulator and distances between the two sites can be varied on a trial-by-trial basis. To test the device, a modified Bekesy tracking method was developed and used for two-point limen testing under stimulus conditions of varying amplitude and frequency. Data collected were consistent with previously published reports, suggesting that one possible use of the device would be to provide a means for improved measures of spatio-tactile acuity.

IM-06 FIGURE AND TEXTURE PRESENTATION CAPABILITIES OF TACTILE DISPLAYS FEATURING AN ARRAY OF STIMULUS PINS DRIVEN BY PIEZOELECTRIC BIMORPH ACTUATORS

In order to establish a method for designing a tactile display, we developed two kinds of matrix-type tactile displays, and tested them in several experiments. In virtual figure presentations, a human subject felt virtual figures such as a circle, triangle, square, pentagon, and hexagon. In experiments on virtual texture presentations, the human subjects touched virtual textures composed of crossed axis patterns, and they judged variations in crossed-axis stripe angles. A two-point threshold was adopted as the pitch between stimulus pins in the design of the tactile display.

CHARACTERISTICS OF TACTILE SENSATION UNDER VIBRATORY PIN STIMULATION

This paper describes the characteristics of tactile sensation which were measured under vibratory-pin stimulation. A two-point diserimination and sensation scaling were investigated by using two vibratory pins that were driven at varied interval distances, frequencies and amplitudes. The results of psychophysical procedures indicated that the two-point threshold was smaller at 250 Hz stimulation than 50 Hz, and the number of discriminable steps of sensation intensity was larger at 250 Hz than that observed at 50 Hz. These results support to adopt 250 Hz for the frequency of driving tactile display.

Two-point discrimination thresholds and schizotypy: illuminating a somatosensory dysfunction

The current study sought to determine if somatosensory deviations, assessed by determining two-point discrimination thresholds, were associated with schizotypic features in non-psychotic subjects ( N=100). It was hypothesized that elevated two-point discrimination thresholds would be associated with an increased likelihood of higher levels of schizotypy features. The results of the study suggested that at the level of individual differences, higher two-point discrimination thresholds (ascending series assessments) were associated with higher levels of schizotypy as operationalized by several prominent measures of schizotypy. A deviant subgroup analysis clearly suggested that, for ascending series two-point threshold assessments, those persons requiring the greatest distance (i.e. highest thresholds) to detect two-point stimulation were substantially more schizotypic than a contrast group. Control analyses that focused on psychological state (depression, anxiety) and intellectual functioning variables revealed that these factors were, more or less, unrelated to two-point discrimination thresholds and they did not account for the observed significant relations between the schizotypy scales and two-point threshold values. The results of this study are discussed within a neuropsychological context implicating parietal cortex involvement with schizotypy and schizophrenia.

The Two-Point Threshold

The two-point threshold, or compass test, has long been used as a measure of tactile spatial resolution; however, since it was first developed, there have been problems associated with its use. Some of these problems include setting an appropriate criterion for responding “two,” extreme variability both within and between subjects, and the ability of subjects to discriminate two points from one at separations well below the two-point threshold. Recent neurophysiological results have clarified some of the neural mechanisms responsible for spatial resolution and demonstrated the inadequacy of the two-point threshold as a measure of spatial mechanisms. Several new methods may overcome these problems and provide a valid measure of spatial resolution and a reflection of neural mechanisms.

Echo-delay resolution in sonar images of the big brown bat, Eptesicus fuscus.

Echolocating big brown bats (Eptesicus fuscus) broadcast ultrasonic frequency-modulated (FM) biosonar sounds (20-100 kHz frequencies; 10-50 microseconds periods) and perceive target range from echo delay. Knowing the acuity for delay resolution is essential to understand how bats process echoes because they perceive target shape and texture from the delay separation of multiple reflections. Bats can separately perceive the delays of two concurrent electronically generated echoes arriving as little as 2 microseconds apart, thus resolving reflecting points as close together as 0.3 mm in range (two-point threshold). This two-point resolution is roughly five times smaller than the shortest periods in the bat's sounds. Because the bat's broadcasts are 2,000-4,500 microseconds long, the echoes themselves overlap and interfere with each other, to merge together into a single sound whose spectrum is shaped by their mutual interference depending on the size of the time separation. To separately perceive the delays of overlapping echoes, the bat has to recover information about their very small delay separation that was transferred into the spectrum when the two echoes interfered with each other, thus explicitly reconstructing the range profile of targets from the echo spectrum. However, the bat's 2-microseconds resolution limit is so short that the available spectral cues are extremely limited. Resolution of delay seems overly sharp just for interception of flying insects, which suggests that the bat's biosonar images are of higher quality to suit a wider variety of orientation tasks, and that biosonar echo processing is correspondingly more sophisticated than has been suspected.

Spatial Acuity of Touch: Ubiquitous Decline with Aging Revealed by Repeated Threshold Testing

Spatial acuity of touch, like that of vision, tends to decline eventually in nearly everybody's lifetime. This has been revealed by more thorough than customary testing of individual young and elderly subjects. Three kinds of acuity threshold were assessed repeatedly in the index finger. These measured ability to discriminate tactile (1) gaps (by a refined version of two-point threshold), (2) orientation of lines (across vs. along the finger), and (3) length of lines. These acuities relate to prominent discriminatory features of braille, and have been shown earlier to average about 1% larger per annum over the adult life span from about 20 to 80 years. Although there were reliable differences among the elderly subjects in the present experiment, all of them tested consistently worse than the least acute young adult controls. The customary single brief threshold tests heretofore applied are inadequate to capture this ubiquitous but differential individual deficit in advanced age; however, the average of six 15- to 20-min tests spread over 3 days proved more than adequate. The method of repeated threshold testing--applied earlier to olfactory and gustatory sensitivity, and now to tactile acuity--serves to dispel the notion that incidence of sensory loss with aging is highly idiosyncratic.

Spatial acuity of the body surface over the life span.

Spatial acuity over 13 regions of the body was assessed cross-sectionally in 122 male and female subjects between 8 and 87 years of age. Of two measures, the primary one was a threshold for detecting a gap between two points (a refinement of the conventional two-point threshold). The secondary one was a threshold of point localization in 7 of these 13 body regions. The two measures yielded similar pictures of body acuity and age-related changes in acuity, and they agreed in essentials with an early acuity map dating back to Weber in 1835, as cited and confirmed experimentally by Weinstein (1968). To this acuity map, the present study added the dimension of age. The main finding was that aging is much harder on some body regions than on others. Declining acuity with age was found to characterize all regions to one degree or another, but the hands and feet turned out to be far more vulnerable than the more central regions, including the very acute lip and tongue. Deterioration of acuity in the great toe (averaging 400% between youth and advanced age) and fingertip (averaging 130%) may adversely affect such diverse activities as braille reading, grasping, and maintaining balance. The acuity map determined by gap discrimination was essentially the same for males and females; however, males gave significantly smaller localization thresholds than females. In two body regions tested (fingertip and upper lip), children significantly outperformed young adults at gap discrimination.

Phase discrimination in void fraction measurements via genetic algorithms

The paper deals with the difficulties of reconstructing a hydrodynamic structural function from a time-dependent raw signal produced by a sensor. In the presence of two fluid phases, the deformation of a phase interface at a sensor leads to a lag between the occurrence and the detection of the event, and the probe response characteristics may be altered with time for several reasons. A new phase discrimination technique is presented based on man–machine interaction and utilizing genetic optimization. To discriminate the phases, two-point threshold discrimination is proposed at first. In addition, a man–machine method is introduced which can mimic a human expert observation of a raw signal and his intuitive discrimination procedure. Finally, genetic algorithms are used to search for those threshold settings that produce solutions closest to those defined by an expert. The proposed new method is capable of adaptation to specific flow conditions.

Perceived roughness as a function of body locus

Twenty subjects made magnitude estimates of the roughness of grooved metallic surfaces applied to 10 body loci. To a first approximation, perceived roughness grew as a power function of groove width, in accordance with earlier studies. The exponents and intercepts (up-down position in log-log coordinates) of the power function turn out to depend strongly on body locus. The straight lines in log-log coordinates tend to diverge with groove width so that differences among body loci are especially pronounced at large groove widths. Sensitivity to roughness was greatest for the lips, fingers, and forearm, and least for the heel, back, and thigh. The rank order of the body loci in terms of roughness sensitivity closely parallels the rank order for punctate pressure sensitivity, as reported by von Frey in 1894, but apparently not for other measures of tactile sensitivity, such as vibration thresholds to various frequencies, two-point thresholds, and error of point localization.

Minimal two-point discrimination in human tongue and palate

The distribution of minimal two-point discrimination values on the tongue and soft palate was investigated in normal subjects using electric stimulation. It was performed using square pulses of 0.35 ms duration, a rate 10 Hz, a voltage between 3.5 and 7.0 with an average of 4.09 using two silver-silver chloride electrodes (tip diameter, 0.4 mm), and a strain-guage device to control crimping pressure. The spatial threshold was 1.650 ± 0.433 mm at the tongue tip and 2.650 ± 0.856 mm and 1.675 ± 0.269 mm on the anterior margin of each side of the tongue. The soft palate and lingual dorsum were almost completely insensitive. Electric stimulation is a convenient method of measuring two-point limen values.

Two-point discrimination test. A valuable part of hand surgical rehabilitation, e.g. in tetraplegia

In assessing the prospects for surgical and other kinds of rehabilitation in tetraplegia and stroke patients the two-point discrimination test, correctly performed by experienced examiners, is of great value. Valid and repeatable results depend on exact technique and proper tools. There is a firm correlation between the thresholds of two-point discrimination on the pulps of the fingers and the accuracy of position sense at the metacarpophalangeal and interphalangeal joints. Microneurography has shown that cutaneous receptors have proprioceptive as well as exteroceptive functions. Thus the two-point discrimination test can be used as a measure of proprioceptive function. The results can be expressed numerically. A two-point threshold less than 10 mm on the pulp is a valid measure of useful finger proprioception. It also shows tactile gnosis, necessary for precision sensory grips.

弛緩による身体感覚の変化 触ぞく二点間弁別いきの測定を用いて

The purpose of this study was to explore changes in body sensation through muscular relaxation by means of measurement of tactile two-point limen. This study comprised two experiments. The first experiment, in which twenty female college students participated, was carried out to examine the effect of relaxation on two-point limen, and the relationship between the acceptance of relaxation and two-point limen. The second experiment involved three physically handicapped children with cerebral palsy and hemiplegia. To determine whether they would show a decrease in two-point limen in the shoulders and forearms as a function of improvement of relaxation, they were given a week long intensive Motor Action Training, which was administered by means of the relaxation and movement training method established by Naruse. The results showed that the two-point limen decreased remarkably for both normal adult subjects and physically handicapped subjects as they relaxed muscular tension. Concerning the acceptance of relaxation, the high acceptance group showed a large decrease in the two-point limen, whereas the low acceptance group did not show any change.

Temperature and the two-point threshold.

Studies dating back to 1834 have shown that the temperature of objects contacting the skin can substantially intensify their apparent pressure on the skin. Later research demonstrated qualitatively that object temperature can also sharpen the spatial acuity of the skin as revealed by gap perception (two-point and two-edge thresholds). Pressure intensification and sharpening probably relate intimately. The present experiments sought to provide several more accurate and parametric extensions of thermal sharpening: (1) sharpening can improve tactile spatial acuity by as much as 60%, but the degree of sharpening is graded as a function of deviation of stimulator temperature from normal (neutral) skin temperature; (2) thermal sharpening seems to characterize the body surface since it takes place freely in forearm, forehead, and palm; local differences do, however, become apparent; (3) large thermal sharpening can even occur when one tip of the stimulator is warm, the other cold; and (4) thermal sharpening is easily captured by experiment and is basically the same in magnitude whether assessed by modern forced-choice procedure (controlled criterion) or by the more traditional procedures (uncontrolled criterion) used for more than a century before the advent of signal detection theory. Various arguments are put forth here and elsewhere to suggest that both thermal intensification of pressure sensation and thermal sharpening of gap perception result from direct thermal stimulation of mechanoreceptors and/or polymodal nociceptor networks; neither phenomenon yeilds readily to a "cognitive" interpretation.

Spatial organization of human perioral reflexes

Reflex responses recorded from the upper and lower divisions of the human orbicularis oris muscle were studied as a function of the site of stimulation. Stimuli were applied to 11 sites, ranging from the glabrous skin of the upper and lower lip vermilion borders to the hairy skin of the cheek. Highly localized, innocuous mechanical stimuli were created by displacing a servo-controlled probe over the surface of the perioral skin. Reflex response amplitude was strongly dependent on the site of stimulation. Stimulation of some sites, for example the ipsilateral corner of the mouth, the chin, and cheek, produced no responses, whereas stimulation of other sites, particularly the ipsilateral vermilion borders, produced large reflex responses. Changes in response amplitude as a function of stimulation site were the same for the upper lip and lower lip muscle recordings, with the largest responses at both recording sites produced by stimulation of the ipsilateral upper lip vermilion border. These results suggest that the upper and lower divisions of orbicularis oris share common synaptic drive, at least from inputs generated via reflex pathways, and that the upper vermilion border may be more densely innervated with mechanoreceptors than the lower. The latter hypothesis was supported by an additional experiment examining two-point discrimination thresholds for the glabrous skin of the upper and lower lips. Two-point thresholds were significantly smaller for the upper compared with the lower lip vermilion border.