Tactile Acuity Research Articles

Edge orientation perception during active touch.

Previous studies investigating the perceptual attributes of tactile edge orientation processing have applied their stimuli to an immobilized fingertip. Here we tested the perceptual attributes of edge orientation processing when participants actively touched the stimulus. Our participants moved their finger over two pairs of edges, one pair parallel and the other nonparallel to varying degrees, and were asked to identify which of the two pairs was nonparallel. In addition to the psychophysical estimates of edge orientation acuity, we measured the speed at which participants moved their finger and the forces they exerted when moving their finger over the stimulus. We report four main findings. First, edge orientation acuity during active touch averaged 12.4°, similar to that previously reported during passive touch. Second, on average, participants moved their finger over the stimuli at ~20 mm/s and exerted contact forces of ~0.3 N. Third, there was no clear relationship between how people moved their finger or how they pressed on the stimulus and their edge orientation acuity. Fourth, consistent with previous work testing tactile spatial acuity, we found a significant correlation between fingertip size and orientation acuity such that people with smaller fingertips tended to have better orientation acuity. NEW & NOTEWORTHY Edge orientation acuity expressed by the motor system during manipulation is many times better than edge orientation acuity assessed in psychophysical studies where stimuli are applied to a passive fingertip. Here we show that this advantage is not because of movement per se because edge orientation acuity assessed in a psychophysical task, where participants actively move their finger over the stimuli, yields results similar to previous passive psychophysical studies.

The somatosensory temporal discrimination threshold changes after a placebo procedure.

In a recent study, we showed that tactile perception can be enhanced by applying a placebo manipulation consisting of verbal suggestion and conditioning (Fiorio et al., Neuroscience 217:96-104, 2012). Whether this change in perception is related to a better tactile functioning is still unknown. Aim of this study is to investigate whether placebo-induced enhancement of tactile perception results in better somatosensory temporal discrimination threshold (STDT), as a proxy of tactile acuity. To this purpose, a group of subjects (experimental group) was verbally influenced and conditioned about the effect of an inert cream in enhancing tactile perception, while a control group was informed about the real nature of the cream. In both groups, we measured STDT before and after cream application, by means of pairs of electrical stimuli delivered on the index fingertip and separated by ascending inter-stimulus intervals. STDT was defined as the shortest time interval at which the two stimuli were perceived as separated. Results revealed an increase in subjective perception of stimulus intensity and a reduction of STDT only in the experimental group. This study proves that a placebo procedure, consisting of verbal suggestion and a short conditioning, can reduce the temporal discrimination threshold.

Tactile learning transfer from the hand to the face but not to the forearm implies a special hand-face relationship

In the primary somatosensory cortex, large-scale cortical and perceptual changes have been demonstrated following input deprivation. Recently, we found that the cortical and perceptual changes induced by repetitive somatosensory stimulation (RSS) at a finger transfer to the face. However, whether such cross-border changes are specific to the face remains elusive. Here, we investigated whether RSS-induced acuity changes at the finger can also transfer to the forearm, which is the body part represented on the other side of the hand representation. Our results confirmed the transfer of tactile learning from the stimulated finger to the lip, but no significant changes were observed at the forearm. A second experiment revealed that the same regions on the forearm exhibited improved tactile acuity when RSS was applied there, excluding the possibility of low plastic ability at the arm representation. This provides also the first evidence that RSS can be efficient on body parts other than the hand. These results suggest that RSS-induced tactile learning transfers preferentially from the hand to the face rather than to the forearm. This specificity could arise from a stronger functional connectivity between the cortical hand and face representations, reflecting a fundamental coupling between these body parts.

Sensitivity to Conflict between Visual Touch and Tactile Touch.

The two point discrimination test is a well known means of assessing tactile sensory acuity. In this paper, we describe a similar test wherein one point of stimulation is tactile and the other one is visual. We propose this test to measure how vision and taction combine to form a unified touch percept, in the presence of sensory conflict. We perform the test at the leg above the knee, using virtual reality to render the visual "touch" while the real touch occurs by means of a monofilament. The results are compared with those obtained from a traditional two point discrimination test performed at the same location. This information can be used to understand how to allocate tactors in a haptic feedback array as well as to better inform the design of future virtual reality accessories.

Upper cervical two-point discrimination thresholds in migraine patients and headache-free controls

BackgroundChronic pain including migraine is associated with structural and functional changes in the somatosensory cortex. Previous reports proposed two-point discrimination (TPD) as a measurement for cortical alterations. Limited evidence exists for tactile acuity in the neck and no data is available for migraine.MethodsTo introduce a standardized protocol for the measurement of TPD in the upper cervical spine, 51 healthy participants were investigated with a newly developed paradigm which was evaluated for intra-rater reliability. The same protocol was applied by two further examiners to 28 migraine patients and 21 age-, and gender-matched healthy controls to investigate inter-rater reliability and between group differences.ResultsResults indicated excellent intra-rater (right ICC(2,4) = 0.82, left ICC(2,4) = 0.83) and good inter-rater reliability (right ICC(2,4) = 0.70, left ICC(2,4) = 0.75). Migraine patients had larger TPD thresholds (26.86 ± 7.21) than healthy controls (23.30 ± 6.17) but these became only statistically significant for the right side of the neck (p = 0.02). There was a significant, moderate association with age for the right side (r = 0.42 p = 0.002, n = 51), and less strong association for the left side (r = 0.34, p = 0.14) in healthy individuals. TPD did not correlate with headache days per month or the dominant headache side in migraine patients.ConclusionsSurprisingly, migraine patients showed increased TPD thresholds in the upper cervical spine interictally. Although a body of evidence supports that hypersensitivity is part of the migraine attack, the current report indicates that interictally, migraine patients showed worse tactile acuity similar to other chronic pain populations. This has been hypothesized to indicate structural and functional re-organisation of the somatosensory cortex.

Impact of transcranial direct current stimulation on structural plasticity of the somatosensory system.

While there is a growing body of evidence regarding the behavioral and neurofunctional changes in response to the longitudinal delivery of transcranial direct current stimulation (tDCS), there is limited evidence regarding its structural effects. Therefore, the present study was intended to investigate the effect of repeatedly applied anodal tDCS over the primary somatosensory cortex on the gray matter (GM) and white matter (WM) compartment of the brain. Structural tDCS effects were, moreover, related to effects evidenced by functional imaging and behavioral assessment. tDCS was applied over the course of 5 days in 25 subjects with concomitant assessment of tactile acuity of the right and left index finger as well as imaging at baseline, after the last delivery of tDCS and at follow-up 4 weeks thereafter. Irrespective of the stimulation condition (anodal vs. sham), voxel-based morphometry revealed a behaviorally relevant decrease of GM in the precuneus co-localized with a functional change of its activity. Moreover, there was a decrease in GM of the bilateral lingual gyrus and the right cerebellum. Diffusion tensor imaging analysis showed an increase of fractional anisotropy exclusively in the tDCSanodal condition in the left frontal cortex affecting the final stretch of a somatosensory decision making network comprising the middle and superior frontal gyrus as well as regions adjacent to the genu of the corpus callosum. Thus, this is the first study in humans to identify structural plasticity in the GM compartment and tDCS-specific changes in the WM compartment in response to somatosensory learning.

Motor imagery performance and tactile acuity in patients with complaints of arms, neck and shoulder.

This study aims to gain more knowledge of the sensorimotor incongruence in patients with chronic nonspecific complaints of arm, neck and shoulder. Seven patients and seven healthy controls performed a left/right judgment task, and tactile acuity was assessed by the two-point discrimination threshold at fingers and shoulders. The results suggest a decreased tactile acuity in patients with chronic nonspecific complaints of arm, neck and shoulder and a faster reaction time at the painful arm, which might imply disturbed information processing of sensory and motor feedback. Due to the small sample size and low scores on the pain and disability questionnaires, these conclusions should be interpreted with care. Further research is recommended.

Mechanisms of tactile sensory deterioration amongst the elderly

It is known that roughness-smoothness, hardness-softness, stickiness-slipperiness and warm-cold are predominant perceptual dimensions in macro-, micro- and nano- texture perception. However, it is not clear to what extent active tactile texture discrimination remains intact with age. The general decrease in tactile ability induces physical and emotional dysfunction in elderly, and has increasing significance for an aging population. We report a method to quantify tactile acuity based on blinded active exploration of systematically varying micro-textured surfaces and a same-different paradigm. It reveals that elderly participants show significantly reduced fine texture discrimination ability. The elderly group also displays statistically lower finger friction coefficient, moisture and elasticity, suggesting a link. However, a subpopulation of the elderly retains discrimination ability irrespective of cutaneous condition and this can be related to a higher density of somatosensory receptors on the finger pads. Skin tribology is thus not the primary reason for decline of tactile discrimination with age. The remediation of cutaneous properties through rehydration, however leads to a significantly improved tactile acuity. This indicates unambiguously that neurological tactile loss can be temporarily compensated by restoring the cutaneous contact mechanics. Such mechanical restoration of tactile ability has the potential to increase the quality of life in elderly.

Short-term visual deprivation boosts the flexibility of body representation

Short-term visual deprivation by blindfolding influences tactile acuity and orientation in space and, on a neural level, leads to enhanced excitability of visual and motor cortices. However, to the best of our knowledge, the possible effects of short-term visual deprivation on body representation have not been examined. In the present study, we tested two groups of 30 healthy participants with the somatic rubber hand illusion, a well-established paradigm to probe the dynamic plasticity of body representation. Before the start of the procedure, the experimental group was blindfolded for 120 minutes, while the control group wore transparent goggles for the same amount of time. We found that although there was no difference in the subjective feeling of ownership of the rubber hand during the illusion, the blindfolded group showed a significantly larger recalibration of hand position sense towards the location of the rubber hand than the control group. This finding suggests that short-term visual deprivation boosts plasticity of body representation in terms of multisensory spatial recalibration of hand position sense.

Exploring Behavioral Correlates of Afferent Inhibition.

(1) Background: Afferent inhibition is the attenuation of the muscle response evoked from transcranial magnetic stimulation (TMS) by a prior conditioning electrical stimulus to a peripheral nerve. It is unclear whether the magnitude of afferent inhibition relates to sensation and movement; (2) Methods: 24 healthy, young adults were tested. Short-latency afferent inhibition (SAI) and long-latency afferent inhibition (LAI) were obtained following median and digital nerve stimulation. Temporal tactile acuity was assessed with a temporal order judgement (TOJ) task, spatial tactile acuity was assessed using a grating orientation task (GOT), and fine manual dexterity was assessed with the Pegboard task; (3) Results: Correlation analyses revealed no association between the magnitude of SAI or LAI with performance on the TOJ, GOT, or Pegboard tasks; (4) Conclusion: The magnitude of SAI and LAI does not relate to performance on the sensory and motor tasks tested. Future studies are needed to better understand whether the afferent inhibition phenomenon relates to human behavior.

Tactile-STAR: A Novel Tactile STimulator And Recorder System for Evaluating and Improving Tactile Perception.

Many neurological diseases impair the motor and somatosensory systems. While several different technologies are used in clinical practice to assess and improve motor functions, somatosensation is evaluated subjectively with qualitative clinical scales. Treatment of somatosensory deficits has received limited attention. To bridge the gap between the assessment and training of motor vs. somatosensory abilities, we designed, developed, and tested a novel, low-cost, two-component (bimanual) mechatronic system targeting tactile somatosensation: the Tactile-STAR—a tactile stimulator and recorder. The stimulator is an actuated pantograph structure driven by two servomotors, with an end-effector covered by a rubber material that can apply two different types of skin stimulation: brush and stretch. The stimulator has a modular design, and can be used to test the tactile perception in different parts of the body such as the hand, arm, leg, big toe, etc. The recorder is a passive pantograph that can measure hand motion using two potentiometers. The recorder can serve multiple purposes: participants can move its handle to match the direction and amplitude of the tactile stimulator, or they can use it as a master manipulator to control the tactile stimulator as a slave. Our ultimate goal is to assess and affect tactile acuity and somatosensory deficits. To demonstrate the feasibility of our novel system, we tested the Tactile-STAR with 16 healthy individuals and with three stroke survivors using the skin-brush stimulation. We verified that the system enables the mapping of tactile perception on the hand in both populations. We also tested the extent to which 30 min of training in healthy individuals led to an improvement of tactile perception. The results provide a first demonstration of the ability of this new system to characterize tactile perception in healthy individuals, as well as a quantification of the magnitude and pattern of tactile impairment in a small cohort of stroke survivors. The finding that short-term training with Tactile-STAR can improve the acuity of tactile perception in healthy individuals suggests that Tactile-STAR may have utility as a therapeutic intervention for somatosensory deficits.

Impact of transcranial direct current stimulation on somatosensory transfer learning: When the secondary somatosensory cortex comes into play

Transfer learning is an immanent feature of perceptual learning. Yet, despite the increasingly widespread application of transcranial direct current stimulation (tDCS) to study learning, transfer effects in response to tDCS have not been studied. Therefore, the present study investigated the transfer of tactile acuity evoked by repeatedly applied anodal tDCS over the left primary somatosensory cortex (S1) over the course of five days from the dominant (right) to the non-dominant (left) index finger (IF). There was a complete transfer of improvement of the right IF to its contralateral homologue by follow-up four weeks later. Changes in tactile acuity of the left IF in the tDCSanodal condition were accompanied by a significant longitudinal change in functional connectivity between the left S1 and the right secondary somatosensory cortex (S2) assessed at day five of tDCS delivery and four week later. Moreover, we observed a close link between tactile acuity and (changes of) functional connectivity of the right S2 in the tDCSanodal condition identifying the S2 as neural correlate to mediate the transfer of tDCS effects in the somatosensory domain. These findings provide unprecedented evidence of transfer effects evoked by tDCS, implicate the S2 in somatosensory transfer learning and provide evidence in support of models of perceptual learning allocating learning to the reweighting of connections between different levels of processing.

AB098. Perceptual, motor and cognitive factors related to braille reading performance in aging: a scoping review

Background: A growing number of older adults with vision loss require vision rehabilitation services to address reading difficulties. Braille may be the most appropriate option for those with functional blindness, poor visual prognoses or dual sensory loss. While standardized braille assessment and training protocols are in place to guide interventions with children, there is a high degree of inconsistency and a lack of evidence-based knowledge about best practices to use with adults and seniors who require braille training. Age-related declines in tactile acuity, motor dexterity and cognition present unique barriers to braille training, but very little is known about the impact of aging on factors related to braille reading performance. The aim of this scoping review is to identify the perceptual, motor, and cognitive factors related to braille reading performance and to determine how these factors have been assessed or measured among blind adults and elderly individuals in prior studies.

Association Between Sensorimotor Impairments and Functional Brain Changes in Patients With Low Back Pain: A Critical Review.

Low back pain (LBP) coincides with sensorimotor impairments, for example, reduced lumbosacral tactile and proprioceptive acuity and postural control deficits. Recent functional magnetic resonance imaging studies suggest that sensorimotor impairments in LBP may be associated with brain changes. However, no consensus exists regarding the relationship between functional brain changes and sensorimotor behavior in LBP. Therefore, this review critically discusses the available functional magnetic resonance imaging studies on brain activation related to nonnociceptive somatosensory stimulation and motor performance in individuals with LBP. Four electronic databases were searched, yielding nine relevant studies. Patients with LBP showed reduced sensorimotor-related brain activation and a reorganized lumbar spine representation in higher-order (multi)sensory processing and motor regions, including primary and secondary somatosensory cortices, supplementary motor area, and superior temporal gyrus. These results may support behavioral findings of sensorimotor impairments in LBP. In addition, patients with LBP displayed widespread increased sensorimotor-evoked brain activation in regions often associated with abnormal pain processing. Overactivation in these regions could indicate an overresponsiveness to sensory inputs that signal potential harm to the spine, thereby inducing overgeneralized protective responses. Hence, functional brain changes could contribute to the development and recurrence of LBP. However, future studies investigating the causality between sensorimotor-related brain function and LBP are imperative.

Is tactile acuity altered in individuals with acute mechanical neck pain?

Background: Tactile acuity measured by point discrimination (TPD) refers to the precision by which we can sense touch. An increase in TPD threshold (loss of tactile acuity) is considered suggestive of disruptions to S1 cortical maps of that specific body part. In some chronically painful conditions, reduced tactile acuity is a manifestation of Central sensitization (CS). The other symptoms include hyperalgesia and allodynia due to repeated activation of spinal nociceptors. A recent study has shown that tactile acuity is affected in individuals with chronic neck pain. While there seems to be adequate evidence stating that tactile acuity is reduced in individuals with chronic pain, CS may not be limited to chronic pain states. There is a paucity of literature with respect to the tactile acuity of a person with acute neck pain. A measurement of tactile acuity of the affected body area in acute pain, may suggest the extent of the altered threshold of sensory discriminative aspect of pain experience. Objectives: To compare the two-point discrimination over C7 spinous process between the symptomatic individuals with mechanical neck pain and age matched healthy controls. Methods: 30 individuals with mechanical neck pain & 30 age matched normals were assessed for two point discrimination using mechanical calipers, The two sharp points of the caliper were vertically placed against the skin surface over C7 spinous process, commencing with 5mm, which was stretched out till the subject appreciated the two points. Values were noted down in millimeters. Results: An independent t – test showed a significant difference in the two point discrimination between the 2 groups (P < 0.000). Conclusion: It can be concluded that individuals with acute mechanical neck pain demonstrated a change in tactile acuity.

Lumbar Tactile Acuity in Patients With Low Back Pain and Healthy Controls: Systematic Review and Meta-Analysis.

Diminished tactile acuity in chronic non-neuropathic pain syndromes has been attributed to central pain processing and cortical reorganization. The latter was recently targeted in clinical trials that demonstrated no clear advantages over traditional approaches for the reduction of nonspecific low back pain (LBP). The aim of this systematic review and meta-analysis was to summarize the current evidence on tactile acuity in LBP and pain-free controls. Six databases were independently searched by 2 researchers. Nineteen studies with either case-control, cross-sectional, or baseline lumbar tactile acuity data collected before an intervention were included in the qualitative and quantitative synthesis. All pooled analyses were based on random effects models. Risk of bias was assessed using the Downs and Black scale and selection criteria were verified independently by 2 assessors. Data on patients (n=547) and controls (n=346) were summarized. Studies on patients included data on nonspecific chronic LBP only; no data on acute LBP were identified. There was a significant mean difference between patients and controls for lumbar tactile acuity including (11.74 mm; 95% confidence interval, 8.61-14.87) and excluding (9.49 mm; 95% confidence interval, 3.64-15.34) higher risk of bias studies. A gap of knowledge regarding tactile acuity in populations with acute and chronic non-neuropathic LBP needs to be addressed in future research as this may significantly help the understanding of the causality of tactile acuity alterations.

Tactile acuity is reduced in people with chronic neck pain

BackgroundTactile acuity deficits have been demonstrated in a range of persistent pain conditions and may reflect underlying cortical re-organisation. ObjectiveThis study aimed to determine whether tactile acuity is impaired in people with chronic neck pain relative to controls, and whether deficits relate to pain location, duration and intensity. MethodsIn this cross-sectional study, 20 people with chronic neck pain (5 idiopathic neck pain; 15 whiplash-associated disorder) and 20 pain-free controls underwent two-point discrimination (TPD) testing at the neck, back and arm, and point-to-point (PTP) and graphesthesia tests of tactile acuity at the neck and arm. ResultsLinear mixed effects models demonstrated a significant group*body region interaction for TPD, Graphesthesia and PTP tests (Ps < 0.001), with post hoc tests showing impaired TPD in people with neck pain relative to controls at the neck, low back, and arm (P ≤ 0.001). Graphesthesia and PTP was also impaired at the neck (P < 0.001) but not the arm (P ≥ 0.48). TPD correlated with intensity and duration of pain (Pearson's r = 0.48, P < 0.05; Pearson's r = 0.77, P < 0.01). There was no sig difference between the two neck pain groups for any tactile acuity measure (TPD: P = 0.054; Graphesthesia; P = 0.67; Point to Point: P = 0.77), however, low power limited confidence in this comparison. ConclusionPeople with chronic neck pain demonstrated tactile acuity deficits in painful and non-painful regions when measured using the TPD test, with the magnitude of deficits appearing greatest at the neck. The study also revealed a positive relationship between TPD and pain intensity/duration, further supporting the main study finding.

Tactile acuity (dys)function in acute nociceptive low back pain: a double-blind experiment.

Research shows that chronic pain is related to cortical alterations that can be reflected in reduced tactile acuity, but whether acute pain perception influences tactile acuity has not been tested. Considering the biological role of nociception, it was hypothesized that nociceptive pain will lead to a rapid improvement in tactile acuity and that this effect is correlated with pain intensity and pain distribution. In this randomised double-blind controlled experiment (trial no. NCT03021278), healthy participants were exposed to 1 of 3 experimental conditions: acute, nociceptive low back pain induced by saline injection, a sham injection (without piercing the skin) potentially inducing nocebo pain, or no intervention. Tactile acuity was measured by a battery of tests, including two-point discrimination threshold (TPD), before, during the pain experience, and after it subsided. We found that TPD did not improve but deteriorated during pain induction in the experimental group compared with the control group (P < 0.001; η = 0.20) and changed from 56.94 mm (95% confidence interval: 53.43-60.44) at baseline to 64.22 mm (95% confidence interval: 60.42-68.02) during the pain experience. Maximum reported pain was a significant predictor (β = 0.55, P = 0.01) and accounted for 26% of the variance in TPD (P < 0.05). Other tests, point-to-point test and two-point estimation task, changed with a similar trend but did not reach significance. We concluded that acute, nociceptive pain does not improve but deteriorates tactile acuity linearly. The biological role of the observed phenomenon is unknown, and therefore, future studies should address this question.

Sensorimotor performance and function in people with osteoarthritis of the hand: A case-control comparison.

To determine whether hand left/right judgements, tactile acuity, and body perception are impaired in people with hand OA. To examine the relationships between left right judgements, tactile acuity and hand pain. To explore the relationships between sensorimotor measures (left/right judgements and tactile acuity) and measures of hand function in people with hand OA. Twenty patients with symptomatic hand OA and 19 healthy pain-free controls undertook a hand left/right judgment task, a control left/right judgement task, two-point discrimination (TPD) threshold testing (assessing tactile acuity), a neglect-like symptoms questionnaire (assessing body perception) and several established measures of hand function. Neglect-like symptoms were experienced more frequently in the hand OA group (P < 0.05). People with hand OA were slower (P < 0.05) and less accurate (P < 0.05) in the hand left/right judgement task when compared to healthy controls, with no significant difference in the control task. Significant associations were found between hand left/right judgement reaction time and pain intensity (P < 0.05) and accuracy and pain intensity (P < 0.05). TPD was not different between groups, and no correlation was found between TPD and left/right judgement performance. No association was found between left/right judgement performance and measures of hand function (all P > 0.05). However, TPD (tactile acuity) was related to several measures of hand function (all P < 0.05). People with hand OA had more frequent neglect-like symptoms and were slower and less accurate compared to healthy controls at hand left/right judgments, which was indicative of disrupted working body schema. Future studies may wish to examine whether interventions targeting sensorimotor dysfunction are effective at reducing pain and improving hand function and dexterity in people with hand OA.

Relative vibrotactile spatial acuity of the torso

While tactile acuity for pressure has been extensively investigated, far less is known about acuity for vibrotactile stimulation. Vibrotactile acuity is important however, as such stimulation is used in many applications, including sensory substitution devices. We tested discrimination of vibrotactile stimulation from eccentric rotating mass motors with in-plane vibration. In 3 experiments, we tested gradually decreasing center-to-center (c/c) distances from 30 mm (experiment 1) to 13 mm (experiment 3). Observers judged whether a second vibrating stimulator (‘tactor’) was to the left or right or in the same place as a first one that came on 250 ms before the onset of the second (with a 50-ms inter-stimulus interval). The results show that while accuracy tends to decrease the closer the tactors are, discrimination accuracy is still well above chance for the smallest distance, which places the threshold for vibrotactile stimulation well below 13 mm, which is lower than recent estimates. The results cast new light on vibrotactile sensitivity and can furthermore be of use in the design of devices that convey information through vibrotactile stimulation.