Agraphesthesia. A disorder of directional cutaneous kinesthesia or a disorientation in cutaneous space

Integrating Tactile Feedback Technologies Into Home-Based Telerehabilitation: Opportunities and Challenges in Light of COVID-19 Pandemic

The COVID-19 pandemic has highlighted the need for advancing the development and implementation of novel means for home-based telerehabilitation in order to enable remote assessment and training for individuals with disabling conditions in need of therapy. While somatosensory input is essential for motor function, to date, most telerehabilitation therapies and technologies focus on assessing and training motor impairments, while the somatosensorial aspect is largely neglected. The integration of tactile devices into home-based rehabilitation practice has the potential to enhance the recovery of sensorimotor impairments and to promote functional gains through practice in an enriched environment with augmented tactile feedback and haptic interactions. In the current review, we outline the clinical approaches for stimulating somatosensation in home-based telerehabilitation and review the existing technologies for conveying mechanical tactile feedback (i.e., vibration, stretch, pressure, and mid-air stimulations). We focus on tactile feedback technologies that can be integrated into home-based practice due to their relatively low cost, compact size, and lightweight. The advantages and opportunities, as well as the long-term challenges and gaps with regards to implementing these technologies into home-based telerehabilitation, are discussed.

Visual feedback of finger writing in a patient with sensory aphasia: a case report and theoretical considerations

Through cognitive task performance, we examined the functional role of finger writing (kūsho) in a Japanese patient with moderate sensory aphasia and reading difficulties. We hypothesized that the visual feedback of kūsho would improve visual language processing, which we tested with a "kanji construction task" using character subparts. Results showed a higher number of correct responses 1) when the patient used kūsho and 2) when visual feedback of finger movements was available. The results suggest that kūsho may not improve the retrieval of phonological information but does aid the visual processing necessary to assemble character subparts.

Tactile direction discrimination in humans after stroke

Sensing movements across the skin surface is a complex task for the tactile sensory system, relying on sophisticated cortical processing. Functional MRI has shown that judgements of the direction of tactile stimuli moving across the skin are processed in distributed cortical areas in healthy humans. To further study which brain areas are important for tactile direction discrimination, we performed a lesion study, examining a group of patients with first-time stroke. We measured tactile direction discrimination in 44 patients, bilaterally on the dorsum of the hands and feet, within 2 weeks (acute), and again in 28 patients 3 months after stroke. The 3-month follow-up also included a structural MRI scan for lesion delineation. Fifty-nine healthy participants were examined for normative direction discrimination values. We found abnormal tactile direction discrimination in 29/44 patients in the acute phase, and in 21/28 3 months after stroke. Lesions that included the opercular parietal area 1 of the secondary somatosensory cortex, the dorsolateral prefrontal cortex or the insular cortex were always associated with abnormal tactile direction discrimination, consistent with previous functional MRI results. Abnormal tactile direction discrimination was also present with lesions including white matter and subcortical regions. We have thus delineated cortical, subcortical and white matter areas important for tactile direction discrimination function. The findings also suggest that tactile dysfunction is common following stroke.

Spinal signalling of C-fiber mediated pleasant touch in humans

C-tactile afferents form a distinct channel that encodes pleasant tactile stimulation. Prevailing views indicate they project, as with other unmyelinated afferents, in lamina I-spinothalamic pathways. However, we found that spinothalamic ablation in humans, whilst profoundly impairing pain, temperature and itch, had no effect on pleasant touch perception. Only discriminative touch deficits were seen. These findings preclude privileged C-tactile-lamina I-spinothalamic projections and imply integrated hedonic and discriminative spinal processing from the body.

Writing in the air: A visualization tool for written languages

The present study investigated interactions between cognitive processes and finger actions called “kusho,” meaning “air-writing” in Japanese. Kanji-culture individuals often employ kusho behavior in which they move their fingers as a substitute for a pen to write mostly done when they are trying to recall the shape of a Kanji character or the spelling of an English word. To further examine the visualization role of kusho behavior on cognitive processing, we conducted a Kanji construction task in which a stimulus (i.e., sub-parts to be constructed) was simultaneously presented. In addition, we conducted a Kanji vocabulary test to reveal the relation between the kusho benefit and vocabulary size. The experiment provided two sets of novel findings. First, executing kusho behavior improved task performance (correct responses) as long as the participants watched their finger movements while solving the task. This result supports the idea that visual feedback of kusho behavior helps cognitive processing for the task. Second, task performance was positively correlated with the vocabulary score when stimuli were presented for a relatively long time, whereas the kusho benefits and vocabulary score were not correlated regardless of stimulus-presentation time. These results imply that a longer stimulus-presentation could allow participants to utilize their lexical resources for solving the task. The current findings together support the visualization role of kusho behavior, adding experimental evidence supporting the view that there are interactions between cognition and motor behavior.

Visual area V5/hMT+ contributes to perception of tactile motion direction: a TMS study

Human imaging studies have reported activations associated with tactile motion perception in visual motion area V5/hMT+, primary somatosensory cortex (SI) and posterior parietal cortex (PPC; Brodmann areas 7/40). However, such studies cannot establish whether these areas are causally involved in tactile motion perception. We delivered double-pulse transcranial magnetic stimulation (TMS) while moving a single tactile point across the fingertip, and used signal detection theory to quantify perceptual sensitivity to motion direction. TMS over both SI and V5/hMT+, but not the PPC site, significantly reduced tactile direction discrimination. Our results show that V5/hMT+ plays a causal role in tactile direction processing, and strengthen the case for V5/hMT+ serving multimodal motion perception. Further, our findings are consistent with a serial model of cortical tactile processing, in which higher-order perceptual processing depends upon information received from SI. By contrast, our results do not provide clear evidence that the PPC site we targeted (Brodmann areas 7/40) contributes to tactile direction perception.

Osseoperception in Dental Implants: A Systematic Review

PURPOSE

Replacement of lost teeth has significant functional and psychosocial effects. The capability of osseointegrated dental implants to transmit a certain amount of sensibility is still unclear. The phenomenon of developing a certain amount of tactile sensibility through osseointegrated dental implants is called osseoperception. The aim of this article is to evaluate the available literature to find osseoperception associated with dental implants.

MATERIALS AND METHODS

To identify suitable literature, an electronic search was performed using Medline and PubMed database. Articles published in English and articles whose abstract is available in English were included. The articles included in the review were based on osseoperception, tactile sensation, and neurophysiological mechanoreceptors in relation to dental implants. Articles on peri-implantitis and infection-related sensitivity were not included. Review articles without the original data were excluded, although references to potentially pertinent articles were noted for further follow-up. The phenomenon of osseoperception remains a matter of debate, so the search strategy mainly focused on articles on osseoperception and tactile sensibility of dental implants. This review presents the histological, neurophysiological, and psychophysical evidence of osseoperception and also the role of mechanoreceptors in osseoperception.

RESULTS

The literature on osseoperception in dental implants is very scarce. The initial literature search resulted in 90 articles, of which 81 articles that fulfilled the inclusion criteria were included in this systematic review.

CONCLUSION

Patients restored with implant-supported prostheses reported improved tactile and motor function when compared with patients wearing complete dentures.

Tactile asymbolia

Agraphesthesia has been attributed to impairment of the ability to detect more rudimentary directionality of lines written on the skin (directional cutaneous kinesthesia). We examined a patient who had a dissociation between preserved perception of line directionality and the loss of graphesthesia for letters and numbers. A man with a metastatic right parietal lesion was tested for the ability to determine the directionality of lines drawn on the palms and forehead and then evaluated for recognition of letters and numbers in these regions. Our patient could identify the directions of lines, letters and numbers drawn on paper. The ability to detect the direction and shape of lines drawn on the skin of the palms and on the forehead was preserved but he had agraphesthesia for numbers and letters in these same locations. The finding of isolated agraphesthesia for letters and numbers may be assigned to damage in the right parietal lobe. It represents a deficit of somatosensory processing that is of a higher order than detection of line directionality. The term "tactile asymbolia" may capture the dissociation. These clinical findings suggest that tactile cortex in humans, like visual cortex, may be hierarchically organized, as has been demonstrated in primates.

Writing in the Air: Contributions of Finger Movement to Cognitive Processing

The present study investigated the interactions between motor action and cognitive processing with particular reference to kanji-culture individuals. Kanji-culture individuals often move their finger as if they are writing when they are solving cognitive tasks, for example, when they try to recall the spelling of English words. This behavior is called kusho, meaning air-writing in Japanese. However, its functional role is still unknown. To reveal the role of kusho behavior in cognitive processing, we conducted a series of experiments, employing two different cognitive tasks, a construction task and a stroke count task. To distinguish the effects of the kinetic aspects of kusho behavior, we set three hand conditions in the tasks; participants were instructed to use either kusho, unrelated finger movements or do nothing during the response time. To isolate possible visual effects, two visual conditions in which participants saw their hand and the other in which they did not, were introduced. We used the number of correct responses and response time as measures of the task performance. The results showed that kusho behavior has different functional roles in the two types of cognitive tasks. In the construction task, the visual feedback from finger movement facilitated identifying a character, whereas the kinetic feedback or motor commands for the behavior did not help to solve the task. In the stroke count task, by contrast, the kinetic aspects of the finger movements influenced counting performance depending on the type of the finger movement. Regardless of the visual condition, kusho behavior improved task performance and unrelated finger movements degraded it. These results indicated that motor behavior contributes to cognitive processes. We discussed possible mechanisms of the modality dependent contribution. These findings might lead to better understanding of the complex interaction between action and cognition in daily life.

The cellular and molecular basis of direction selectivity of Aδ-LTMRs

The perception of touch, including the direction of stimulus movement across the skin, begins with activation of low-threshold mechanosensory neurons (LTMRs) that innervate the skin. Here, we show that murine Aδ-LTMRs are preferentially tuned to deflection of body hairs in the caudal-to-rostral direction. This tuning property is explained by the finding that Aδ-LTMR lanceolate endings around hair follicles are polarized; they are concentrated on the caudal (downward) side of each hair follicle. The neurotrophic factor BDNF is synthesized in epithelial cells on the caudal, but not rostral, side of hair follicles, in close proximity to Aδ-LTMR lanceolate endings, which express TrkB. Moreover, ablation of BDNF in hair follicle epithelial cells disrupts polarization of Aδ-LTMR lanceolate endings and results in randomization of Aδ-LTMR responses to hair deflection. Thus, BDNF-TrkB signaling directs polarization of Aδ-LTMR lanceolate endings, which underlies direction-selective responsiveness of Aδ-LTMRs to hair deflection.

Role of primary somatosensory cortex in the coding of pain

The intensity and submodality of pain are widely attributed to stimulus encoding by peripheral and subcortical spinal/trigeminal portions of the somatosensory nervous system. Consistent with this interpretation are studies of surgically anesthetized animals, demonstrating that relationships between nociceptive stimulation and activation of neurons are similar at subcortical levels of somatosensory projection and within the primary somatosensory cortex (in cytoarchitectural areas 3b and 1 of somatosensory cortex, SI). Such findings have led to characterizations of SI as a network that preserves, rather than transforms, the excitatory drive it receives from subcortical levels. Inconsistent with this perspective are images and neurophysiological recordings of SI neurons in lightly anesthetized primates. These studies demonstrate that an extreme anterior position within SI (area 3a) receives input originating predominantly from unmyelinated nociceptors, distinguishing it from posterior SI (areas 3b and 1), long recognized as receiving input predominantly from myelinated afferents, including nociceptors. Of particular importance, interactions between these subregions during maintained nociceptive stimulation are accompanied by an altered SI response to myelinated and unmyelinated nociceptors. A revised view of pain coding within SI cortex is discussed, and potentially significant clinical implications are emphasized.

Somatotopic mismatch following stroke: a pathophysiological condition escaping detection

Clinical evaluation of somatosensory deficits in stroke patients is very limited and usually does not include testing of somatotopic organisation, which is a prerequisite for meaningful interpretation of sensory input and sensorimotor control. Detailed tactile testing of the left hand of a 54-year-old patient suffering from sensory deficit and central pain after a right-sided stroke revealed severe distortion of somatotopic sensory maps as evidenced by incorrect localisation of the point stimuli. Unlike previously reported gross somatotopic remapping taking place within reduced representational space after lesion, this is the first case report revealing chaotic scrambled somatosensory maps. While the incidence of such scrambled somatotopic representation of tactile input is not yet known in stroke patients, current observations indicate that in-depth investigations of somatotopic organisation of affected area may reveal the underlying cause for various functional deficits including central pain. Thus, new rehabilitation strategies may need to be developed specifically for such patients.

Cortical processing of tactile direction discrimination based on spatiotemporal cues in man

Tactile direction discrimination (TDD), the ability to determine the direction of an object's movement across the skin, is used clinically to detect and quantify tactile dysfunction. We have previously identified a cortical network for TDD based on skin stretch information that includes the second somatosensory, anterior insular and dorsolateral prefrontal cortices. In the present study we investigated cortical processing of TDD based on spatiotemporal cues. Sixteen healthy subjects (8 females; mean age, 25.5 years; range, 23-32 years) were stimulated with a low-friction, spatiotemporal rolling wheel on the right thigh during functional magnetic resonance imaging (fMRI). The subjects were instructed to indicate the distal or proximal rolling direction of the stimulus. The fMRI contrast between rolling wheel stimulation and rest showed activations in several areas which included the left (contralateral) primary somatosensory, bilateral second somatosensory, bilateral anterior insular, and bilateral dorsolateral prefrontal cortices. We conclude that, spatiotemporal TDD is processed in a largely similar distributed cortical network as skin stretch TDD. Further, spatiotemporal TDD activated primary somatosensory cortex whereas a role for this area in processing of skin stretch TDD has not been demonstrated.

Exteroceptive aspects of nociception: insights from graphesthesia and two-point discrimination

The exteroceptive capabilities of the nociceptive system have long been thought to be considerably more limited than those of the tactile system. However, most investigations of spatio-temporal aspects of the nociceptive system have largely focused on intensity coding as consequence of spatial or temporal summation. Graphesthesia, the identification of numbers "written" on the skin, and assessment of the two-point discrimination thresholds were used to compare the exteroceptive capabilities of the tactile and nociceptive systems. Numbers were "written" on the forearm and the abdomen by tactile stimulation and by painful non-contact infrared laser heat stimulation. Subjects performed both graphesthesia tasks better than chance. The tactile graphesthesia tasks were performed with 89% (82-97%) correct responses on the forearm and 86% (79-94%) correct responses on the abdomen. Tactile graphesthesia tasks were significantly better than painful heat graphesthesia tasks that were performed with 31% (23-40%) and 44% (37-51%) correct responses on the forearm and abdomen, respectively. These findings demonstrate that the central nervous system is capable of assembling complex spatio-temporal patterns of nociceptive information from the body surface into unified mental objects with sufficient accuracy to enable behavioral discrimination.

Tactile direction discrimination and vibration detection in diabetic neuropathy

OBJECTIVE

To evaluate the clinical usefulness of quantitative testing of tactile direction discrimination (TDD) in patients with diabetic neuropathy.

MATERIALS AND METHODS

TDD and vibration detection were examined on the dorsum of the feet in 43 patients with type 1 diabetes mellitus and clinical signs and symptoms indicating mild neuropathy, and abnormal results for neurography, temperature detection, or heart rate variability. Test-retest examination of TDD was performed in nine of the patients.

RESULTS

Twenty-six of the patients had abnormal TDD (sensitivity 0.60) and 20 had abnormal vibration detection (sensitivity 0.46). Ten of the patients had abnormal TDD and normal vibration detection. Four of the patients had abnormal vibration detection and normal TDD. Test-retest examination of TDD showed a high degree of reproducibility (r = 0.87).

CONCLUSION

TDD seems more useful than vibration detection in examination of diabetic neuropathy.

Perceptual changes in the peri-implant soft tissues assessed by directional cutaneous kinaesthesia and graphaesthesia: a prospective study

BACKGROUND

The innervation of skin and oral mucosa plays a major physiological role in exteroception. This innervation is also clinically relevant as sensory changes occur after neurosurgical procedures.

PURPOSE

The goal of this study was to compare the perception of mechanical stimuli applied to the buccal mucosa in the vicinity of osseointegrated oral implants with that in the controlateral dentate side. The role of the previously reported increased innervation in the peri-implant soft tissues in the oral sensorimotor function was thus examined.

MATERIALS AND METHODS

Seventeen subjects with 20 implants were tested. Directional cutaneous kinaesthesia (DCK) and graphesthesia (G) were performed on the buccal side of the alveolar mucosa before and at planned intervals after implant placement. The observation was pursued until 6 months after the prosthetic rehabilitation. In each subject, the contralateral mucosa served as a control to the implant sites. Average percentages of correct responses in a four-choice task for DCK and a three-choice task for G were calculated.

RESULTS

Despite an intersubject variation in both the DCK and G, high intraindividual correlations were found (p < .005). The implant sites showed a significant difference toward the control sites at the four interval test for both tests. For DCK and G, the average of correct responses decreased after abutment connection (i.e., after the implant uncovering surgery) to increase afterwards to reach a level close to, but still lower than, the control sites 3 to 6 months after the prosthetic rehabilitation.

CONCLUSION

The DCK and G are simple but reliable sensory tests that can be easily applied in the oral region. This prospective study indicates that tooth loss reduces tactile function compared with implant-supported prostheses. The peri-implant soft tissues could be partially involved in the osseoperception function.

Tactile functions after cerebral hemispherectomy

Patients that were hemispherectomized due to brain lesions early in life sometimes have remarkably well-preserved tactile functions on their paretic body half. This has been attributed to developmental neuroplasticity. However, the tactile examinations generally have been fairly crude, and subtle deficits may not have been revealed. We investigated monofilament detection and three types of tactile directional sensibility in four hemispherectomized patients and six healthy controls. Patients were examined bilaterally on the face, forearm and lower leg. Normal subjects were examined unilaterally. Following each test of directional sensibility, subjects were asked to rate the intensity of the stimulation. On the nonparetic side, results were almost always in the normal range. On the paretic side, the patients' capacity for monofilament detection was less impaired than their directional sensibility. Despite the disturbed directional sensibility on their paretic side the patients rated tactile sensations evoked by the stimuli, on both their paretic and nonparetic body halves, as more intense than normals. Thus, mechanisms of plasticity seem adequate for tactile detection and intensity coding but not for more complex tactile functions such as directional sensibility. The reason for the high vulnerability of tactile directional sensibility may be that it depends on spatially and temporally precise afferent information processed in a distributed cortical network.

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

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

Mechanosensory and thermosensory changes across the border of impaired sensitivity to pinprick after mandibular nerve injury

PURPOSE

The study goal was to determine how sensory function varies across the border of impaired sensitivity to pinprick in patients with mandibular nerve injuries.

PATIENTS AND METHODS

Borders of decreased sensitivity to pinprick were mapped in 15 patients who reported altered sensation. Four mechanoreceptive, 2 thermoreceptive, and 2 thermonociceptive functions were studied at 5 sites separated by 0.6 cm across the border. The tests were repeated to evaluate day-to-day consistency in the pattern of variation for each sensory measure.

RESULTS

The estimates of sensory function were not found to vary in a systematic manner from outside to inside the pinprick-impaired area for all patients for any of the 8 tests. However, for every test, some patients exhibited large variations. On average, the magnitudes of loss in contact detection, subjective intensity of light touch, and direction discrimination were greatest; the magnitudes of loss in 2-point perception and in heat and cold pain perception were least. Some patients provided no evidence of impairment on certain tests. For some patients, the estimates suggested increased sensitivity within the pinprick-impaired area (eg, to noxious cold stimuli).

CONCLUSIONS

Although certain patients exhibit impairment, there is no obligatory loss in light touch, 2-point perception, direction discrimination, or temperature perception across the border of decreased sensitivity to pinprick. The differences among patients suggest that the data from individual patients should be evaluated in clinical studies and in clinical practice. Researchers should not rely solely on average values and summary statistics.

Capturing the spatial percepts evoked by moving tactile stimuli: a novel approach

Forced-choice procedures are conventionally used to study the percepts evoked by stimuli that move across the skin and enable an unbiased estimation of subjects' sensory capacities. These procedures, however, require subjects to assign complicated percepts to one of a small number of experimenter-defined response categories, none of which may satisfactorily describe the perceptual experience. To address this limitation, we developed a psychophysical approach, which graphically captures spatial information about a moving stimulus in a holistic manner. Briefly summarized, the stimulus object controlled for location, velocity, direction and distance is moved across the skin of a blind-folded subject, after which the subject draws its path on a life-size, two-dimensional photograph of the body region stimulated. Using this approach, we demonstrated that the drawings contain perceptually relevant information, estimates of direction discrimination and subjective traverse length derived from the drawings closely parallel data obtained with forced-choice and magnitude estimation methods, respectively, and generate comparable psychophysical functions of stimulus velocity. In addition, information is represented in the complex shapes of the curves and in the locations at which they are drawn. Analyses of these latter features support the hypothesis that non-sensory factors (individual subject biases) also affect the drawings.

Spatial acuity after digit amputation

Digit amputation in human and non-human primates results in reorganization of somatosensory cortex in which the representations of adjacent, intact digits expand to fill the cortical region previously devoted to the amputated digits. Whether this expanded representation results in improved sensory performance has not been determined. Consequently, we measured the ability to recognize small objects (raised letters) with a digit adjacent to the amputation and the same digit on the normal, contralateral hand in 15 amputees. The same digits were also tested in 15 age-matched, amputation-free subjects. There was no significant difference in recognition scores between digits in the amputees or between amputees and control subjects. More detailed analyses of specific confusion patterns and of the improvement with practice showed no significant differences. As far as we could determine, the cortical expansion that is presumed to accompany digit amputation had no effect on tactile pattern recognition performance.

Tactile directional sensibility and diabetic neuropathy

Five different procedures used to diagnose neuropathy were compared in a "blind" study with diabetic patients. The aim was to evaluate tests of tactile directional sensibility. Three matched groups were examined, two groups with type I diabetes, either with or without suspected neuropathy, and one of healthy controls. Testing consisted of: (1) examination by a specialist in neurology, (2) electrophysiologic measurement of nerve conduction velocity and determination of cool sensitivity, and (3) determination of directional sensibility in two stages, with categorical and quantitative techniques. Abnormal test results were obtained for both groups of diabetic patients. Quantitatively measured directional sensibility had the highest sensitivity (89%) and specificity (85%) when calculated for patients who had received a diagnosis of neuropathy from the neurologist, despite one case of abnormal directional sensibility among the healthy controls. Conduction velocity testing was almost comparably sensitive (80%) and cool sensitivity, comparably specific (85%) when calculated in the same manner.

Somesthetic disconnection syndromes in patients with callosal lesions

Somesthetic disconnection syndromes were investigated in relation to the sites of lesions in the corpus callosum in 3 patients with callosal lesions, in order to identify the callosal regions responsible for the interhemispheric transfer of somesthetic information. Cases 1 and 2 with lesions in the posterior truncus exhibited transfer deficits of discriminative sensations between the left and right hands, left-sided tactile anomia and left-sided somesthetic alexia. Case 3 with lesions in the posteroventral part of the posterior truncus showed no signs of somesthetic disconnection syndromes. The results suggest the importance of the anterior and/or dorsal part of the posterior truncus of the corpus callosum for interhemispheric transfer of the discriminative sensations and integrated somesthetic information necessary for tactile naming and somesthetic reading.

Tactile directional sensibility: peripheral neural mechanisms in man

Tactile directional sensibility, i.e. the ability to tell the direction of an object's motion across the skin, is an easily observed sensory function that is highly sensitive to disturbances of the somatosensory system. Based on previous psychophysical experiments on healthy subjects it was concluded that directional sensibility depends on two kinds of information from cutaneous mechanoreceptors; spatio-temporal information and information about friction-induced changes in skin stretch. In the present study responses to similar probe movements as in the psychophysical experiments were recorded from human single mechanoreceptors in the forearm skin. All slowly adapting type 2 (SA2) units were spontaneously active, and with increasing force of friction their discharge rates were modified by probe movements at increasing distances from the Ruffini end-organ, reflecting the high stretch-sensitivity of these units. Slowly adapting type 1 (SA1) and field units responded to the moving probe within well-defined skin areas directly overlying the individual receptor terminals, and compared to the SA2 units their response properties were less dependent on the force of friction. The results suggest that SA1 and field units have the capacity to signal spatio-temporal information, whereas a population of SA2 units have the capacity to signal direction-specific information about changes in lateral skin stretch.

Remarkable capacity for perception of the direction of skin pull in man

We determined the ability to appreciate the direction of a skin pull caused by a moving pin that was glued to the forearm skin. A majority of the subjects were able to tell the direction of pin movements with an excursion of 0.13 mm (>/=66% correct responses, p<0.05). Local skin anaesthesia showed that stretch sensitive receptors located over 15 mm in front and behind the pin correctly signalled the direction of these minute skin pulls. It was concluded that information about patterns of skin stretch is an important component of the somatosensory system that may contribute not only to kinaesthetic, but also to cutaneous sensations.

Directional sensibility for quantification of tactile dysfunction

Examination of tactile directional sensibility, i.e., the ability to tell the direction of an object's motion across the skin, has been recommended by several authors for examination of patients with somatosensory disorders. Recent findings about the physiological mechanisms underlying directional sensibility suggested possibilities to further improve the test. In the present investigation a test was constructed that allowed a semiquantification of the directional sensibility of six body areas within 20 min. Normal values were obtained by testing healthy subjects (n = 40), and the normal values were compared to those obtained in a group of patients with tactile symptoms (n = 20). Ten of the patients had abnormal sensory conduction in one or several nerves, and they also had abnormal directional sensibility. Hence, examination of directional sensibility, according to the present protocol, provides a semiquantitative test that appears to be as sensitive as electrophysiological measurement of conduction in detecting dysfunction in tactile nerves.

Midcervical central cord syndrome: numb and clumsy hands due to midline cervical disc protrusion at the C3-4 intervertebral level

Eight patients with midline cervical disc protrusion at the C3-4 intervertebral level showed unusual clinical signs: numbness in the finger tips and palms, clumsiness of the hands, and a tightening sensation at the midthoracic level. The proprioceptive and cutaneous sensory afferents essential for motor control of the upper limbs were preferentially involved, tactile discrimination of passively given stimuli being spared. Somatosensory evoked potentials subsequent to median nerve stimulation showed conduction failure through the fasciculus cuneatus, as evidenced by absent or delayed and attenuated medullary and scalp potentials. The potential originating in the lower cervical cord (N13a) had a low amplitude, indicative of the caudal extension of the lesion. On the basis of the functional anatomy of the intraspinal pathways, especially of the dorsal columns, it is concluded that involvement of the central cord at the C3-4 intervertebral level and its caudal extension is responsible for the syndrome.

Receptive field characteristics of tactile units with myelinated afferents in hairy skin of human subjects

1. Impulses in single nerve fibres from the lateral antebrachial cutaneous nerve were recorded using the microneurography technique in human subjects. 2. In a sample of fifty-five mechanoreceptive units with fast-conducting nerve fibres, five types were identified, i.e. SAI (slowly adapting type I, Merkel), SAII (slowly adapting type II, Ruffini), hair units, field units and Pacinian-type units. The latter three unit types were all rapidly adapting. 3. The detailed structure of thirty-five receptive fields of SAI, SAII, hair and field units was explored with a method which was objective and independent of the experimenter's skill and experience. A lightweight probe was used to scan the receptive field area in a series of tracks 0.23 mm apart while single-unit activity was recorded. 4. SAI fields were small and composed of two to four well-separated high-sensitivity spots and often, in addition, one minor spot of lower sensitivity. SAII units typically fired spontaneously at a low and regular rate. Most fields consisted of one single spot of high sensitivity with diffuse borders. The hair units innervated ten to thirty-three (or more) hairs, which were evenly distributed over a large area. The field units were characterized by a number of small and closely packed high-sensitivity spots with diffuse borders. A conservative estimate indicated eleven spots per unit. 5. The findings indicate that the sheet of mechanoreceptors on the skin of the forearm is distinctly different from that on the dorsum of the hand and in the face. It seems reasonable to assume that the former is more representative for the hairy skin covering the main parts of the body.

Spatial cues serving the tactile directional sensibility of the human forearm

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

Effect of stimulus force on perioral direction discrimination: clinical implications

Clinicians have used the same instrument (viz, Semmes-Weinstein pressure aesthesiometers [Research Design, Inc, Houston, TX] or "von Frey hairs") for tests of both contact detection and direction discrimination. However, patients' ability to discriminate direction may be underestimated by barely detectable moving stimuli. To determine whether the aesthesiometers underestimate direction discrimination, we evaluated the capacity of 13 normal subjects to distinguish opposing directions provided by 10 different hairs. The hairs were selected to deliver forces below and above the contact-detection threshold. Each was stroked over 1.0 cm of perioral skin at the velocity at which the subject was predicted to best discriminate direction of motion. It was found that valid estimates of perioral direction discrimination can be obtained with appropriately selected aesthesiometers. Specifically, the least stiff hair whose handle displays a manufacturer's marking two units greater than that of the contact-detection "threshold hair" should be used to deliver the moving stimuli. The resultant force applied by this hair will exceed 10 times the subject's contact-detection threshold force. If a less-stiff hair is used, the capacity to distinguish direction may be underestimated.

Observations on human tactile directional sensibility

1. The ability to tell the direction of a motion across the skin deserve attention for being an easily observed function which provides a sensitive test for disturbances of the peripheral and central nervous systems. The mode of operation, on the other hand, of this tactile directional sensibility is still uncertain. 2. The dependence of directional sensibility on the contact load and distance of movement of a blunt metal tip, has now been determined for the skin of the forearm of normal subjects with the two-alternative forced-choice method. The testing was done under two conditions: elbow bent or straight. Straightening of the arm always reduced the accuracy of the directional sensibility. It also caused measurable changes of cutaneous mechanical properties, which presumably decreased the reliability of afferent information about lateral distension. 3. The average accuracy of the directional sensibility was found to be correlated linearly to the logarithm of the contact load, and straightening of the arm decreased the accuracy for each load by corresponding amounts. Similar relationships were found between the accuracy and the distance of movement. 4. Straightening of the arm did not cause any significant average reduction of the contact threshold for point stimulation of the same receptive field. A consistently lowered contact sensitivity, however, was observed for some of the subjects, which may have contributed to the reduction of the directional sensibility in these cases. 5. Correct directional estimations of the movement of the metal tip were obtained for a distance which was a fifth of the shortest distance for a corresponding estimation of the movement of a frictionless stimulus. The findings thus indicated that the friction between a moving object and the underlying skin, which can be mediated via stretch-sensitive cutaneous receptors, is critical for the determination of its direction of motion. 6. The present observations and previous observations by various authors are suggested to indicate that typical tactile directional sensibility depends on parallel processing of direction-selective data, and spatial data expressed as a function of time.

Human, tactile, directional sensibility and its peripheral origins

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

Directional sensitivity along the upper limb in humans

The capacity of four neurologically healthy young adults to distinguish opposing directions of cutaneous motion was determined at five different sites along the proximal-distal axis of the upper limb. Constant-velocity brushing stimuli (ranging from 0.5 to 32.0 cm/sec) were delivered through an aperture in a Teflon plate that was securely positioned in light contact with the skin. In one series of experiments, directional sensitivity in d' units was assessed at each site, using an aperture length of 0.75 cm. In a second series of experiments, the aperture length required to obtain the same criterion level of directional sensitivity at each site was determined. To attain the sensitivity reached at distal sites, a proximal stimulus had to traverse a longer chord of skin. Specifically, chords 5.9 times longer on average (range = 5.4-6.2) were required on the proximal forearm than on the index finger pad. This finding suggests that relative directional sensitivity increases sixfold from the proximal forearm to the finger pad. Moreover, relative directional sensitivity on the shoulder was comparable to that observed on the proximal forearm for two of the subjects, and approximately one-half that observed on the proximal forearm for the other two subjects. In addition to such a prominent spatial gradient in relative directional sensitivity, the velocity of stimulus motion at which directional sensitivity was highest increased systematically as the test site was shifted from the finger pad to the proximal forearm. Specifically, the optimal velocity on the finger pad varied among subjects from 1.5 to 9.4 cm/sec (mean = 5.4 cm/sec), and on the proximal forearm from 11.5 to 31.2 cm/sec (mean = 18.6 cm/sec). The optimal velocity on the shoulder was not significantly different from that observed on the proximal forearm. The results suggest that effective and informed clinical testing of patients' capacity to distinguish opposing directions of motion on cutaneous regions that differ in peripheral innervation density requires appreciation of the sensitivities of different skin regions, as well as the unique velocity dependency of direction discrimination at each skin site.

Perioral somesthetic sensibility: do the skin of the lower face and the midface exhibit comparable sensitivity?

Studies of the perioral somatosensory capacities of neurologically normal adults were reviewed to determine whether sensitivities within the mental and infraorbital nerve distributions are comparable. It was found that tactile detection sensitivity, spatial acuity, and sensitivity to warmth are greater on skin sites located on the midface than on the lower face. In contrast, sensitivity to direction of motion and to differences in surface texture may be greater on skin sites located on the lower face. The literature further suggests that sensitivity within the distribution of each nerve varies appreciably. For example, the vermilion of the lips exhibits considerably greater vibrotactile detection sensitivity, spatial acuity, and sensitivity to direction of motion than does the perioral hairy skin. In addition, spatial acuity is notably greater on midline structures. These findings suggest that knowledge of the patterns of spatial variations in perioral tactile sensibilities can be effectively used during neurosensory examination to select control skin sites for comparison with areas of suspected neurosensory impairment and to distinguish apparent pathological alterations in tactile sensitivity from normal regional differences that characterize the perioral complex.

Effects of trauma to the mandibular nerve on human perioral directional sensitivity

The capacity of 4 patients who had previously experienced trauma to their mandibular nerves to distinguish opposing directions of tactile motion over the distribution of the mental nerve was compared to that of 8 neurologically normal adults. Brushing stimuli were delivered to the perioral region and were precisely controlled for their velocity, the length of skin traversed, the width of skin contacted, and the orientation and direction of motion. A temporal, 2-alternative, forced choice method was used to obtain estimates of directional sensitivity, d'. It was discovered that impairment in cutaneous directional sensitivity could be readily detected within areas of hypaesthesia. Although directional sensitivity was found to increase linearly with the length of skin traversed for both the patients and the neurologically normal adults, the slope and the x-intercept of the linear relationship differed between the two groups. The difference in the slope suggests that direction discrimination within the hypaesthetic areas is relatively insensitive to changes in the length of skin traversed. The difference in the x-intercept suggests that a greater length of skin must be traversed before any information about direction is made available at the hypaesthetic sites. The dependency of the capacity of neurologically normal and impaired individuals to process information about direction of tactile motion on the length of skin traversed and the velocity of stimulation suggests that a high degree of stimulus control is required for the detection and quantification of subtle neurosensory deficits.

Effects of traverse length on human perioral directional sensitivity

The capacity of 8 neurologically healthy adults to distinguish direction of motion on the skin overlying the mental foramen was determined. The velocity, orientation, and the length and width of skin traversed by the moving tactile stimuli were precisely controlled. Directional sensitivity, d', was found to depend on both stimulus velocity and the length of skin traversed. Since the relationship between d' and velocity at each traverse length was well described by a generalized gamma function, it was possible to quantitatively characterize the effects of changes in traverse length on the relationship between d' and velocity. Specifically, peak (i.e., maximal) directional sensitivity increased as the length of skin traversed was increased, yet the velocity which resulted in peak directional sensitivity (i.e., the optimal or model velocity) remained invariant over the range of traverse lengths investigated (0.35-1.0 cm). The effect of stimulus velocity on directional sensitivity was least at the longest traverse lengths used. The generalized gamma function model fit the relationship between directional sensitivity and velocity equally well at all traverse lengths studied. The results lead us to anticipate that stimuli of the type used in this study should prove valuable for the detection and quantification of disturbances in orofacial tactile spatiotemporal integration in patients with peripheral nerve injury.

Human perioral directional sensitivity

The capacity of 41 neurologically healthy young adults to distinguish opposing directions of brush motion across the skin innervated by the mental nerve was determined. The velocity and orientation and the length and width of skin traversed by the moving tactile stimuli were carefully controlled. Directional sensitivity, d', was found to vary curvilinearly with velocity over the range 0.5 to 32 cm/s. Because the data from most subjects were well described by a generalized gamma function, it was possible to characterize this velocity dependency quantitatively. Specifically, indices derived from these functions were found to describe the subject's peak (i.e., maximal) sensitivity, the velocity which resulted in peak sensitivity (i.e., the optimal velocity), and the degree to which stimulus velocity influenced the ability to recognize direction of motion (i.e., the velocity-tuning of d'). Peak sensitivity, optimal velocity, and the degree of global velocity-tuning were found to differ between males and females. Confidence limits (the lower and upper 2.5% points) for the normative data were determined to enable detection and characterization of deficits in orofacial tactile motion sensitivity in individuals with damaged mandibular nerves.

Dependence of subjective traverse length on velocity of moving tactile stimuli

Two series of experiments were performed to assess the effects of stimulus velocity on human subjects' perception of the distance traversed by a moving tactile stimulus. In all experiments, constant-velocity stimuli were applied to the dorsal surface of the left forearm; velocities ranging between 1.0 and 256 cm/sec were used. In some experiments the stimuli moved from distal to proximal over the skin, and in others they moved from proximal to distal. The length of skin contacted by the moving stimulus was defined by a plate having an aperture of 4.0 X 0.5 cm. In the first series of experiments, subjects were required to compare the distance traversed by a test stimulus delivered 2 sec after a standard stimulus, and also to report the on-locus and the off-locus of the brushing stimulus. In the second series of experiments, the subjects rated the perceived distance on the skin using a free-magnitude-estimation procedure. The data from both series of experiments defined the same relationship between stimulus velocity and perceived stimulus distance. More specifically, although the length of skin contacted by the stimulus was the same at all velocities, subjects' estimates of stimulus distance decreased with increasing stimulus velocity. In addition, the function relating estimates of stimulus distance to velocity was flat for velocities between 5 and 20 cm/sec, but possessed an appreciable negative slope at lower and higher velocities. It is interesting that the plateau of the relationship between perceived stimulus distance and velocity occurred within the range of velocities that human subjects employ to scan textured surfaces; it also corresponded precisely with the range of stimulus velocities at which the directional sensitivity of somatosensory cortical neurons and human subjects is optimal.

Assessment of the capacity of human subjects and S-I neurons to distinguish opposing directions of stimulus motion across the skin

The ability of human subjects and the capacities of single S-I neurons of macaque monkeys to distinguish opposing directions of movement over the skin were investigated by employing experimental paradigms and data analyses based on sensory decision theory (SDT). It is shown that these techniques can be utilized to provide behavioral and neurophysiological indices of directional sensitivity which have the same metric, and are amenable to statistical tests for significance. The influences of 3 different paradigms and modes of relative operating characteristic (ROC) curve construction on SDT indices of human cutaneous directional sensitivity were investigated. Response latency (RL) was used as an objective indication of certainty in all 3 paradigms; in one of the 3 paradigms the subject also rated the certainty of each report. The SDT indices of cutaneous directional sensitivity and response bias were shown to be independent of the paradigm and mode of ROC curve construction investigated, and the SDT 'Gaussian-equal variance' hypothesis was concluded to be consistent with the data provided by all 3 paradigms. A considerable amount of inter-subject as well as intra-subject variability in human cutaneous directional sensitivity is demonstrated for all subjects tested. This variability appears to be an attribute of the processes underlying the sensing of stimulus direction since it is present even when stimulus conditions are maintained constant. Experimental designs were developed which account for this variability, thus allowing detection and quantitation of the influence of variations in stimulus conditions on human directional sensitivity. It is demonstrated that for S-I neurons, an ROC curve can be generated from the responses to multiple replications of opposing directions of movement across the receptive field. The large number of stimulus presentations required to estimate directional sensitivity from ROC curves involves a prolonged period of single neuron recording that is difficult to achieve even under ideal experimental conditions. It is shown that one can obtain a reliable estimate of single neuron directional sensitivity (i.e. delta'e) using relatively few stimulus replications when mean firing rate is assumed to represent that aspect of the neural response carrying information about stimulus direction. These indices allow assessment of the selectivity of single S-I neurons for direction as stimulus parameters are varied. Examples are provided which show (utilizing delta'e) that those stimulus conditions evoking maximal firing rates from S-I neurons are often not optimal for signalling direction of movement across the skin.

Factors influencing cutaneous directional sensitivity: a correlative psychophysical and neurophysiological investigation

The effects of 4 parameters of moving tactile stimuli (i.e., velocity, traverse length, position and orientation) on human cutaneous directional sensitivity and on the behavior of directionally sensitive neurons in S-I of unanesthetized macaque monkeys are studied. The experimental paradigms and approaches to data analysis are based on sensory decision theory (SDT), and provide indices of single neuron and of perceptual cutaneous direction sensitivity that can be compared. Human cutaneous directional sensitivity is shown to be maximal when the stimuli move at velocities between 5 and 30 cm/s, and to fall off either at lower or higher velocities. The neurophysiological studies of the effects of velocity reveal a heterogeneity in the population of directionally sensitive S-I neurons. Some neurons are shown to exhibit maximal directional sensitivity at velocities between 5 and 30 cm/s, whereas others possess maximal directional sensitivity at lower velocities (i.e., less than 5 cm/s). Human cutaneous directional sensitivity is determined at each of 5 different forelimb regions. The data reveal that a pronounced gradient in human cutaneous directional sensitivity exists along the proximodistal axis of the forelimb, with the greatest sensitivity existing at the most distal forelimb site studied. The companion neurophysiological studies reveal that a change in the position of the moving stimulus within the receptive field of an individual directionally sensitive S-I neuron is usually accompanied by a change in the magnitude of its directional sensitivity. Two major classes of directionally sensitive S-I neurons can be distinguished on the basis of the in-field variations in directional sensitivity they exhibit. For one neuron class, preferred direction remains the same at all regions within the receptive field; these are termed 'direction invariant neurons' and they appear to be capable of signalling direction of motion unambiguously under most of the experimental conditions used in this study. For the neurons of the second class, preferred direction varies with the position of the stimulus within the receptive field; these are termed 'direction variant' neurons. Direction variant S-I neurons signal movement toward or away from a given point within the receptive field. As a consequence, a reversal in cutaneous directional sensitivity within their receptive fields can typically be demonstrated. For every direction variant neuron studied the receptive field position at which cutaneous directional sensitivity reversed was located over a joint.(ABSTRACT TRUNCATED AT 400 WORDS)