A physical constraint on perceptual learning: tactile spatial acuity improves with training to a limit set by finger size

Transfer of Tactile Learning to Untrained Body Parts: Emerging Cortical Mechanisms

Pioneering investigations in the mid-19th century revealed that the perception of tactile cues presented to the surface of the skin improves with training, which is referred to as tactile learning. Surprisingly, tactile learning also occurs for body parts and skin locations that are not physically involved in the training. For example, after training of a finger, tactile learning transfers to adjacent untrained fingers. This suggests that the transfer of tactile learning follows a somatotopic pattern and involves brain regions such as the primary somatosensory cortex (S1), in which the trained and untrained body parts and skin locations are represented close to each other. However, other results showed that transfer occurs between body parts that are not represented close to each other in S1-for example, between the hand and the foot. These and similar findings have led to the suggestion of additional cortical mechanisms to explain the transfer of tactile learning. Here, different mechanisms are reviewed, and the extent to which they can explain the transfer of tactile learning is discussed. What all of these mechanisms have in common is that they assume a representational or functional relationship between the trained and untrained body parts and skin locations. However, none of these mechanisms alone can explain the complex pattern of transfer results, and it is likely that different mechanisms interact to enable transfer, perhaps in concert with higher somatosensory and decision-making areas.

Surgical experience and different glove wearing conditions affect tactile sensibility

Background

The fingers' tactile sensibility is essential in surgery, especially in microsurgery. Therefore, surgeons seeking to improve their performance often prefer certain glove brands and wearing habits. There is the need of objectively testing these glove wearing conditions and determine the effect of surgical experience with regard to tactile sensibility by comparing surgeons with non-surgeons.

Methods

This cross-sectional single-center pilot-study was conducted between June and August 2021. Two groups of 27 surgeons and 27 non-surgeons underwent two-point-discrimination (2PD) and Semmes-Weinstein monofilament testing (SWMT) of both index fingers with bare hands and with wearing six different brands of surgical gloves. Different wearing conditions, such as single-gloving, double-gloving, well-fitted, under- and oversized gloves, were evaluated within and between the groups.

Results

Most glove types decreased tactile sensibility (2PD and SWMT) of surgeons and non-surgeons. Interestingly, the thinnest gloves showed similar 2PD values to bare hands in both groups. Double-gloving negatively impacted SWMT, without influencing 2PD. Undersized gloves showed better 2PD and SWMT than well-fitted gloves, while oversized gloves showed no tactile drawbacks. With bare hands and certain glove conditions, the surgeons' 2PD and SWMT was significantly better than the non-surgeons', indicating a positive effect of surgical experience on tactile sensibility.

Conclusion

Our study demonstrated the positive impact of surgical experience on tactile sensibility, as demonstrated by the surgeons. The sensibility of the gloved hand varies on the surgical glove type, but favors thinner gloves, single gloving (rather than double gloving) and undersized or well-fitted gloves.

Expansion and contraction of resource allocation in sensory bottlenecks

Topographic sensory representations often do not scale proportionally to the size of their input regions, with some expanded and others contracted. In vision, the foveal representation is magnified cortically, as are the fingertips in touch. What principles drive this allocation, and how should receptor density, for example, the high innervation of the fovea or the fingertips, and stimulus statistics, for example, the higher contact frequencies on the fingertips, contribute? Building on work in efficient coding, we address this problem using linear models that optimally decorrelate the sensory signals. We introduce a sensory bottleneck to impose constraints on resource allocation and derive the optimal neural allocation. We find that bottleneck width is a crucial factor in resource allocation, inducing either expansion or contraction. Both receptor density and stimulus statistics affect allocation and jointly determine convergence for wider bottlenecks. Furthermore, we show a close match between the predicted and empirical cortical allocations in a well-studied model system, the star-nosed mole. Overall, our results suggest that the strength of cortical magnification depends on resource limits.

Idiosyncratic selection of active touch for shape perception

Hand movements are essential for tactile perception of objects. However, the specific functions served by active touch strategies, and their dependence on physiological parameters, are unclear and understudied. Focusing on planar shape perception, we tracked at high resolution the hands of 11 participants during shape recognition task. Two dominant hand movement strategies were identified: contour following and scanning. Contour following movements were either tangential to the contour or oscillating perpendicular to it. Scanning movements crossed between distant parts of the shapes’ contour. Both strategies exhibited non-uniform coverage of the shapes’ contours. Idiosyncratic movement patterns were specific to the sensed object. In a second experiment, we have measured the participants’ spatial and temporal tactile thresholds. Significant portions of the variations in hand speed and in oscillation patterns could be explained by the idiosyncratic thresholds. Using data-driven simulations, we show how specific strategy choices may affect receptors activation. These results suggest that motion strategies of active touch adapt to both the sensed object and to the perceiver’s physiological parameters.

No evidence for sex differences in tactile distance anisotropy

Perceptual illusions of the distance between two touches have been used to study mental representations of the body since E. H. Weber's classic studies in the nineteenth century. For example, on many body parts tactile distance is anisotropic, with distances aligned with body width being perceived as larger than distances aligned with body length on several skin regions. Recent work has demonstrated sex differences in other distortions of mental body representations, such as proprioceptive hand maps. Given such findings, I analysed the results of 24 experiments, conducted by myself and my colleagues, measuring tactile distance anisotropy on the hand dorsum in both women and men. The results showed clear, and highly consistent anisotropy in both women and men, with no evidence for any sex difference.

Grating orientation task trial numbers for short- and long-term tactile discrimination learning

Perceptual learning generally improves with training intensity, but the number of training trials sufficient for transient and long-term improvement in tactile grating orientation task (GOT) discrimination has not been systematically studied. To define reliable trial numbers for tactile discrimination learning, we compared tactile orientation discrimination performance of the right-finger following 200 and 400 training trials. Fifty-one neurologically healthy subjects were recruited. Tactile spatial acuity for orientation (parallel or orthogonal to the long axis of the finger) across different grating frequencies was assessed before, immediately after, 30 min after, and 24 h after sessions consisting of 200 training trials (50/block × 4 blocks), 400 training trials (50/block × 8 blocks), or no training (sham control). Both the 200- and 400-trial training conditions reduced the grating orientation discrimination threshold at 24 h after training. In contrast, the control condition had no effect on the grating orientation discrimination threshold. There was a negative correlation between the baseline grating orientation discrimination threshold and training-induced change in threshold (improvement) following both 200 and 400 trials. Fewer GOT trials (200) substantially prolong tactile discrimination learning, presumably by promoting the consolidation of the underlying neuroplastic mechanisms. In this widely used perceptual learning paradigm, 200 and 400 training trials appear effective for inducing short-term and long-term perceptual memory.

Perceptual learning evidence for supramodal representation of stimulus orientation at a conceptual level

When stimulus inputs from different senses are integrated to form a coherent percept, inputs from a more precise sense are typically more dominant than those from a less precise sense. Furthermore, we hypothesized that some basic stimulus features, such as orientation, can be supramodal-represented at a conceptual level that is independent of the original modality precision. This hypothesis was tested with perceptual learning experiments. Specifically, participants practiced coarser tactile orientation discrimination, which initially had little impact on finer visual orientation discrimination (tactile vs. visual orientation thresholds = 3:1). However, if participants also practiced a functionally orthogonal visual contrast discrimination task in a double training design, their visual orientation performance was improved at both tactile-trained and untrained orientations, as much as through direct visual orientation training. The complete tactile-to-visual learning transfer is consistent with a conceptual supramodal representation of orientation unconstrained by original modality precision, likely through certain forms of input standardization. Moreover, this conceptual supramodal representation, when improved through perceptual learning in one sense, can in turn facilitate orientation discrimination in an untrained sense.

Individual Performance in Compliance Discrimination is Constrained by Skin Mechanics but Improved under Active Control

Tactile acuity differs between individuals, likely a function of several interrelated factors. The extent of the impact of skin mechanics on individual differences is unclear. Herein, we investigate if differences in skin elasticity between individuals impact their ability to distinguish compliant spheres near limits of discriminability. After characterizing hyperelastic material properties of their skin in compression, the participants were asked to discriminate spheres varying in elasticity and curvature, which generate non-distinct cutaneous cues. Simultaneous biomechanical measurements were used to dissociate the relative contributions from skin mechanics and volitional movements in modulating individuals' tactile sensitivity. The results indicate that, in passive touch, individuals with softer skin exhibit larger gross contact areas and higher perceptual acuity. In contrast, in active touch, where exploratory movements are behaviorally controlled, individuals with harder skin evoke relatively larger gross contact areas, which improve and compensate for deficits in their acuity as observed in passive touch. Indeed, these participants exhibit active control of their fingertip movements that improves their acuity, amidst the inherent constraints of their less elastic finger pad skin.

Human perceptual learning is delayed by the N-methyl-D-aspartate receptor partial agonist D-cycloserine

BACKGROUND

The optimisation of learning has long been a focus of scientific research, particularly in relation to improving psychological treatment and recovery of brain function. Previously, partial N-methyl-D-aspartate agonists have been shown to augment reward learning, procedural learning and psychological therapy, but many studies also report no impact of these compounds on the same processes.

AIMS

Here we investigate whether administration of an N-methyl-D-aspartate partial agonist (D-cycloserine) modulates a previously unexplored process - tactile perceptual learning. Further, we use a longitudinal design to investigate whether N-methyl-D-aspartate-related learning effects vary with time, thereby providing a potentially simple explanation for apparent mixed effects in previous research.

METHODS

Thirty-four volunteers were randomised to receive one dose of 250 mg D-cycloserine or placebo 2 h before tactile sensitivity training. Tactile perception was measured using psychophysical methods before and after training, and 24/48 h later.

RESULTS

The placebo group showed immediate within-day tactile perception gains, but no further improvements between-days. In contrast, tactile perception remained at baseline on day one in the D-cycloserine group (no within-day learning), but showed significant overnight gains on day two. Both groups were equivalent in tactile perception by the final testing - indicating N-methyl-D-aspartate effects changed the timing, but not the overall amount of tactile learning.

CONCLUSIONS

In sum, we provide first evidence for modulation of perceptual learning by administration of a partial N-methyl-D-aspartate agonist. Resolving how the effects of such compounds become apparent over time will assist the optimisation of testing schedules, and may help resolve discrepancies across the learning and cognition domains.

Exploring the influence of reading medium on braille learning outcomes: A case series of six working-age and older adults

Tactile sensitivity is known to decline with age. Braille provides a vital method of reading and writing for working-age and older adults with acquired visual impairment. The proliferation of low-cost braille displays raises new possibilities for adult braille learners, with dots of greater height than standard paper braille, potentially benefitting older adults with reduced tactile sensitivity. This study explored the influence of reading medium (paper vs braille display) on the accuracy and speed of six working-age and older adult braille learners and examined differences when transitioning from one reading medium to another. Findings indicate that (1) learning letters on a braille display resulted in better speed and accuracy (time: M = 44.2, SD = 37.3, accuracy: M = 83%, SD = 24.8%) than on paper (time: M = 54.3, SD = 40.4, accuracy: M = 80.6%, SD = 28.1%); (2) transitioning from one medium to another generally resulted in the same or better performance (reading times decreased by 11.2% and accuracy improved by 2.4%); and (3) the advantage of the braille display appears to be greatest when reading letters in combination (reading times decreased by 26.8% and accuracy improved by 6.5% for letter-pairs vs a 1.9% reduction in speed and a 2% improvement in accuracy for single letters). The benefit of the braille display condition was most pronounced for participants with reduced tactile sensitivity. Although preliminary, these findings suggest that the use of braille displays in early braille instruction may decrease frustration for those with reduced tactile sensitivity and should not adversely affect the ability for learners to transition to standard paper braille, assuming that both formats are introduced and reinforced throughout training.

The Stiffness Comparison Test: A pilot study to determine inter-individual differences in palpatory skill related to gender, age, and occupation-related experience

BACKGROUND

Manual palpation is a core skill in physical examination. Assessing elastic properties such as tissue stiffness has the potential for being an important diagnostics tool in the detection of cancer and other diseases.

OBJECTIVE

The study describes the newly developed Stiffness Comparison Test (SCT). The aim of our study was to test the SCT as a tool to detect interindividual differences in palpation skill related to gender, age and occupational experience.

METHODS

We used eight pairs of polyuterhane gel pads with the stiffness difference decreasing from the first to the last pair. Test subjects were asked to palpate each pair and determine stiffness differences.

PARTICIPANTS

We recruited 25 osteopaths, 48 other manual therapists and 50 participants from other non-manual professions.

RESULTS

As hypothesized there was no significant difference in SCT performance between the sexes (t(121) = 0.288, p = .774). To investigate if an age-related decline would have an effect on palpation skill, we carried out a linear regression. As hypothesized, the model did not predict any significant associations (F(1, 121) = 2.733, b = -0.149, p = .101, R² = 0.022). To compare the effect of occupational groups on SCT performance a one-way ANOVA was conducted. There were no statistically significant differences between group means (F(2, 120) = 0.598, p = .552).

CONCLUSIONS

The SCT can be used as simple and affordable tool for assessment, teaching and training in all disciplines of manual medicine. Further refinements of the tool are suggested to advance its discrimination power.

Comparing Tactile to Auditory Guidance for Blind Individuals

The ability to travel independently is crucial to an individual's quality of life but compromised by visual impairment. Several navigational aids have been developed for blind people to address this limitation. These devices typically employ auditory instructions to guide users to desired waypoints. Unfortunately, auditory instructions may interfere with users' awareness of environmental sounds that signal dangers or provide cues for spatial orientation. Accordingly, there is a need to explore the use of non-auditory modalities to convey information for safe and independent travel. Here, we explored the efficacy of a tactile navigational aid that provides turn signals via vibrations on a hip-worn belt. We compared the performance of 12 blind participants as they navigated a series of paths under the direction of the tactile belt or conventional auditory turn commands; furthermore, we assessed the effect of repeated testing, both in the presence and absence of simulated street sounds. A computer-controlled system triggered each turn command, measured participants' time-to-path-completion, and detected major navigational errors. When participants navigated in a silent environment, they performed somewhat worse with the tactile belt than the auditory device, taking longer to complete each trial and committing more errors. When participants navigated in the presence of simulated street noises, the difference in completion time between auditory and tactile navigation diminished. These results suggest that tactile navigation holds promise as an effective method in everyday environments characterized by ambient noise such as street sounds.

Tactile angle discriminability improvement: roles of training time intervals and different types of training tasks

Perceptual learning, which is not limited to sensory modalities such as vision and touch, emerges within a training session and between training sessions and is accompanied by the remodeling of neural connections in the cortex. However, limited knowledge exists regarding perceptual learning between training sessions. Although tactile studies have paid attention to between-session learning effects, there have been few studies asking fundamental questions regarding whether the time interval between training sessions affects tactile perceptual learning and generalization across tactile tasks. We investigated the effects of different training time intervals on the consecutive performance of a tactile angle discrimination (AD) task and a tactile orientation discrimination (OD) task training on tactile angle discriminability. The results indicated that in the short-interval training group, AD task performance significantly improved in the early stage of learning and nearly plateaued in the later stage, whereas in the long-interval training group, significant improvement was delayed and then also nearly plateaued in the later stage; additionally, improved OD task performance resulted in improved AD task performance. These findings suggest that training time interval affects the early stage of learning but not the later stage and that generalization occurs between different types of tactile tasks.

NEW & NOTEWORTHY Perceptual learning, which constitutes important foundations of complicated cognitive processes, is learning better perception skills. We demonstrate that training time interval can affect the early stage of learning but not the later stage. Moreover, a tactile orientation discrimination training task can also improve tactile angle discrimination performance. These findings may expand the characteristics of between-session learning and help understand the mechanism of the generalization across tactile tasks.

Postural stability in people with visual impairment

BACKGROUND

The visual system enables the brain to assess information regarding the position of the body in space. Congenital or acquired blindness leads to the development of abnormal sensory-motor interactions that results in development of typical musculoskeletal deformities and gait patterns that cause disability. Diabetes and related complications are expected to increase exponentially in the next 10 years; thus, the number of people with visual impairment is expected to increase. However, there have been few studies regarding etio-pathogenesis of postural alteration and balance impairment in people with visual impairment; moreover, no previous study has investigated postural stability in this population. This study aimed to assess the center of gravity (COG) velocity in subjects with visual impairment and compared with that in sighted subjects.

METHODS

Seventy male subjects, 20-40 years of age, participated in this study; they were divided into sighted (control) and visually impaired groups. COG velocity while standing on an unstable surface was measured using the NeuroCom^® Balance Master Version 8.5.0. For the sighted group, data were recorded with eyes open and then with eyes closed. For the visually impaired group, no instructions were given with respect to eyes during data collection.

RESULTS

Mean COG velocity was significantly higher in the visually impaired group than in the sighted group with eyes open. However, there was no difference in mean COG velocity between the visually impaired group and the sighted group with eyes closed. The difference in mean COG velocity between sighted subjects with eyes open and eyes closed was also significant. Mean COG velocity while standing on a foam surface varied among visually impaired subjects and sighted subjects with eyes open and closed.

CONCLUSION

This study showed that subjects with visual impairment, regardless of eye opening or closure, behave in the same manner as sighted subjects with eyes closed.

Sex differences in perceptual hand maps: A meta-analysis

A large body of research has suggested that localisation of the hand in external space relies on distorted representations of the hand. We developed a paradigm for measuring implicit perceptual maps of the hand (Longo & Haggard, 2010, Proc Natl Acad Sci USA, 107, 11727-11732), which show systematic deviation from actual hand shape, including overestimation of hand width and underestimation of finger length. Recently, Coelho and Gonzalez (in press, Psychol Res) reported sex differences in these perceptual hand maps, with women showing greater overestimation of hand width, but less underestimation of finger length than men. In the current study, I conducted a meta-analysis of 19 experiments using this paradigm by myself and my colleagues. The results replicated the sex differences reported by Coelho and Gonzalez. Importantly, however, these sex differences were not apparent when actual hand size was included as a covariate in analyses, suggesting that they may, at least in part, be due to women having smaller hands on average than men.

Does Studying Veterinary Medicine Improve Students' Haptic Perception Ability? A Pilot Study With Two Age-Groups

Haptic perception is an important tool for veterinarians. The present study analyzed the association between the haptic perception threshold of veterinary students and their palpatory experience. To approach this goal, 35 female students of veterinary medicine were divided into two groups with different levels of experience: (a) students with little practical experience, at the beginning of their studies (first year), and (b) students close to the end of their theoretical training (fourth year). To thoroughly evaluate the students' sense of touch, three different test procedures were used: the Haptic Threshold Test (HTT), the Haptic Figures Test (HFT), and tactile acuity. Contrary to our expectations, we found worse mean haptic perception thresholds (HTT) in the more experienced students than in the less experienced group. This effect was significantly correlated with age. Furthermore, we found that longer exploration times were not sufficient to compensate for shortcomings in haptic perception. We also found large interindividual differences. Future studies should investigate whether and to what extend these effects have an impact on students' palpation performance on simulators and live animals. Moreover, which beneficial effects may be achieved through an additional haptic training for students with inferior haptic thresholds should be investigated. Improving haptic perception abilities in veterinary students could be one important step toward achieving satisfactory Day One Competences in university graduates.

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.

Measuring dexterity in the podiatrist population: a cross-sectional comparison of novice students and experienced podiatrists

BACKGROUND

There is no 'gold-standard' for the evaluation of dexterity for the health professional or podiatrist populations. This has resulted in a broad array of generalised tests to evaluate dexterity. Thus, the aim was to determine which objective generalised dexterity tests are best suited to evaluating dexterity in a podiatry student population.

METHODS

A cohort of Novice podiatry students and Experienced podiatrists were recruited and evaluated on a battery of dexterity tests selected to evaluate a variety of different elements. Group differences were evaluated statistically and regression undertaken on significant test outcomes.

RESULTS

A total of 108 participants were recruited with 54 participants in each of the Novice and Experienced groups. Five of the eight tests were able to discriminate dexterous ability of participants in the Novice and Experienced groups. These included the Grip-lift task, GPT, P-MVC, G-MVC and the AsTex® sensory discrimination test. These tests comprised a total of 11 significant dependent variables (p <  0.05). From the test battery, outcomes were able to predict 79% of the group membership. Age and experience did not explain within-group variability for the Experienced group.

CONCLUSION

Whilst the Experienced group displayed superior performance in strength and speed, the Novice group showed superior coordination and sensory ability. From these findings, we would recommend that outcomes from the Grooved Pegboard Test, Grip-lift task, Grip Strength test and Pinch Grip strength test be used to evaluate elements of dexterity in this population.

Functional consequences of experience-dependent plasticity on tactile perception following perceptual learning

Continuous training enhances perceptual discrimination and promotes neural changes in areas encoding the experienced stimuli. This type of experience-dependent plasticity has been demonstrated in several sensory and motor systems. Particularly, non-human primates trained to detect consecutive tactile bar indentations across multiple digits showed expanded excitatory receptive fields (RFs) in somatosensory cortex. However, the perceptual implications of these anatomical changes remain undetermined. Here, we trained human participants for 9 days on a tactile task that promoted expansion of multi-digit RFs. Participants were required to detect consecutive indentations of bar stimuli spanning multiple digits. Throughout the training regime we tracked participants' discrimination thresholds on spatial (grating orientation) and temporal tasks on the trained and untrained hands in separate sessions. We hypothesized that training on the multi-digit task would decrease perceptual thresholds on tasks that require stimulus processing across multiple digits, while also increasing thresholds on tasks requiring discrimination on single digits. We observed an increase in orientation thresholds on a single digit. Importantly, this effect was selective for the stimulus orientation and hand used during multi-digit training. We also found that temporal acuity between digits improved across trained digits, suggesting that discriminating the temporal order of multi-digit stimuli can transfer to temporal discrimination of other tactile stimuli. These results suggest that experience-dependent plasticity following perceptual learning improves and interferes with tactile abilities in manners predictive of the task and stimulus features used during training.

Braille in the Sighted: Teaching Tactile Reading to Sighted Adults

Blind people are known to have superior perceptual abilities in their remaining senses. Several studies suggest that these enhancements are dependent on the specific experience of blind individuals, who use those remaining senses more than sighted subjects. In line with this view, sighted subjects, when trained, are able to significantly progress in relatively simple tactile tasks. However, the case of complex tactile tasks is less obvious, as some studies suggest that visual deprivation itself could confer large advantages in learning them. It remains unclear to what extent those complex skills, such as braille reading, can be learnt by sighted subjects. Here we enrolled twenty-nine sighted adults, mostly braille teachers and educators, in a 9-month braille reading course. At the beginning of the course, all subjects were naive in tactile braille reading. After the course, almost all were able to read whole braille words at a mean speed of 6 words-per-minute. Subjects with low tactile acuity did not differ significantly in braille reading speed from the rest of the group, indicating that low tactile acuity is not a limiting factor for learning braille, at least at this early stage of learning. Our study shows that most sighted adults can learn whole-word braille reading, given the right method and a considerable amount of motivation. The adult sensorimotor system can thus adapt, to some level, to very complex tactile tasks without visual deprivation. The pace of learning in our group was comparable to congenitally and early blind children learning braille in primary school, which suggests that the blind’s mastery of complex tactile tasks can, to a large extent, be explained by experience-dependent mechanisms.

Tactile orientation perception: an ideal observer analysis of human psychophysical performance in relation to macaque area 3b receptive fields

The ability to resolve the orientation of edges is crucial to daily tactile and sensorimotor function, yet the means by which edge perception occurs is not well understood. Primate cortical area 3b neurons have diverse receptive field (RF) spatial structures that may participate in edge orientation perception. We evaluated five candidate RF models for macaque area 3b neurons, previously recorded while an oriented bar contacted the monkey's fingertip. We used a Bayesian classifier to assign each neuron a best-fit RF structure. We generated predictions for human performance by implementing an ideal observer that optimally decoded stimulus-evoked spike counts in the model neurons. The ideal observer predicted a saturating reduction in bar orientation discrimination threshold with increasing bar length. We tested 24 humans on an automated, precision-controlled bar orientation discrimination task and observed performance consistent with that predicted. We next queried the ideal observer to discover the RF structure and number of cortical neurons that best matched each participant's performance. Human perception was matched with a median of 24 model neurons firing throughout a 1-s period. The 10 lowest-performing participants were fit with RFs lacking inhibitory sidebands, whereas 12 of the 14 higher-performing participants were fit with RFs containing inhibitory sidebands. Participants whose discrimination improved as bar length increased to 10 mm were fit with longer RFs; those who performed well on the 2-mm bar, with narrower RFs. These results suggest plausible RF features and computational strategies underlying tactile spatial perception and may have implications for perceptual learning.

Enhanced tactile acuity through mental states

Bodily training typically evokes behavioral and perceptual gains, enforcing neuroplastic processes and affecting neural representations. We investigated the effect on somatosensory perception of a three-day Zen meditation exercise, a purely mental intervention. Tactile spatial discrimination of the right index finger was persistently improved by only 6 hours of mental–sensory focusing on this finger, suggesting that intrinsic brain activity created by mental states can alter perception and behavior similarly to external stimulation.

Differences in two-point discrimination and sensory threshold in the blind between braille and text reading: a pilot study

[Purpose] This study investigated two-point discrimination (TPD) and the electrical sensory threshold of the blind to define the effect of using Braille on the tactile and electrical senses. [Subjects and Methods] Twenty-eight blind participants were divided equally into a text-reading and a Braille-reading group. We measured tactile sensory and electrical thresholds using the TPD method and a transcutaneous electrical nerve stimulator. [Results] The left palm TPD values were significantly different between the groups. The values of the electrical sensory threshold in the left hand, the electrical pain threshold in the left hand, and the electrical pain threshold in the right hand were significantly lower in the Braille group than in the text group. [Conclusion] These findings make it difficult to explain the difference in tactility between groups, excluding both palms. However, our data show that using Braille can enhance development of the sensory median nerve in the blind, particularly in terms of the electrical sensory and pain thresholds.

Feeling better: separate pathways for targeted enhancement of spatial and temporal touch

People perceive spatial form and temporal frequency through touch. Although distinct somatosensory neurons represent spatial and temporal information, these neural populations are intermixed throughout the somatosensory system. Here, we show that spatial and temporal touch can be dissociated and separately enhanced via cortical pathways that are normally associated with vision and audition. In Experiments 1 and 2, we found that anodal transcranial direct current stimulation (tDCS) applied over visual cortex, but not auditory cortex, enhances tactile perception of spatial orientation. In Experiments 3 and 4, we found that anodal tDCS over auditory cortex, but not visual cortex, enhances tactile perception of temporal frequency. This double dissociation reveals separate cortical pathways that selectively support spatial and temporal channels. These results bolster the emerging view that sensory areas process multiple modalities and suggest that supramodal domains may be more fundamental to cortical organization.

Tactile spatial acuity in childhood: effects of age and fingertip size

Tactile acuity is known to decline with age in adults, possibly as the result of receptor loss, but less is understood about how tactile acuity changes during childhood. Previous research from our laboratory has shown that fingertip size influences tactile spatial acuity in young adults: those with larger fingers tend to have poorer acuity, possibly because mechanoreceptors are more sparsely distributed in larger fingers. We hypothesized that a similar relationship would hold among children. If so, children’s tactile spatial acuity might be expected to worsen as their fingertips grow. However, concomitant CNS maturation might result in more efficient perceptual processing, counteracting the effect of fingertip growth on tactile acuity. To investigate, we conducted a cross-sectional study, testing 116 participants ranging in age from 6 to 16 years on a precision-controlled tactile grating orientation task. We measured each participant's grating orientation threshold on the dominant index finger, along with physical properties of the fingertip: surface area, volume, sweat pore spacing, and temperature. We found that, as in adults, children with larger fingertips (at a given age) had significantly poorer acuity, yet paradoxically acuity did not worsen significantly with age. We propose that finger growth during development results in a gradual decline in innervation density as receptive fields reposition to cover an expanding skin surface. At the same time, central maturation presumably enhances perceptual processing.

Two-point orientation discrimination versus the traditional two-point test for tactile spatial acuity assessment

Two-point discrimination is widely used to measure tactile spatial acuity. The validity of the two-point threshold as a spatial acuity measure rests on the assumption that two points can be distinguished from one only when the two points are sufficiently separated to evoke spatially distinguishable foci of neural activity. However, some previous research has challenged this view, suggesting instead that two-point task performance benefits from an unintended non-spatial cue, allowing spuriously good performance at small tip separations. We compared the traditional two-point task to an equally convenient alternative task in which participants attempt to discern the orientation (vertical or horizontal) of two points of contact. We used precision digital readout calipers to administer two-interval forced-choice versions of both tasks to 24 neurologically healthy adults, on the fingertip, finger base, palm, and forearm. We used Bayesian adaptive testing to estimate the participants’ psychometric functions on the two tasks. Traditional two-point performance remained significantly above chance levels even at zero point separation. In contrast, two-point orientation discrimination approached chance as point separation approached zero, as expected for a valid measure of tactile spatial acuity. Traditional two-point performance was so inflated at small point separations that 75%-correct thresholds could be determined on all tested sites for fewer than half of participants. The 95%-correct thresholds on the two tasks were similar, and correlated with receptive field spacing. In keeping with previous critiques, we conclude that the traditional two-point task provides an unintended non-spatial cue, resulting in spuriously good performance at small spatial separations. Unlike two-point discrimination, two-point orientation discrimination rigorously measures tactile spatial acuity. We recommend the use of two-point orientation discrimination for neurological assessment.