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

Aging and the perception of tactile speed

Eighteen younger and older adults (mean ages were 20.4 and 72.8 years, respectively) participated in a tactile speed matching task. On any given trial, the participants felt the surfaces of rotating standard and test wheels with their index fingertip and were required to adjust the test wheel until its speed appeared to match that of the standard wheel. Three different standard speeds were utilized (30, 50, and 70 cm/s). The results indicated that while the accuracy of the participants' judgments was similar for younger and older adults, the precision (i.e., reliability across repeated trials) of the older participants' judgments deteriorated significantly relative to that exhibited by the younger adults. While adverse effects of age were obtained with regards to both the precision of tactile speed judgments and the participants' tactile acuity, there was nevertheless no significant correlation between the older adults' tactile acuities and the precision of their tactile speed judgments.

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.

Feeling fooled: Texture contaminates the neural code for tactile speed

Motion is an essential component of everyday tactile experience: most manual interactions involve relative movement between the skin and objects. Much of the research on the neural basis of tactile motion perception has focused on how direction is encoded, but less is known about how speed is. Perceived speed has been shown to be dependent on surface texture, but previous studies used only coarse textures, which span a restricted range of tangible spatial scales and provide a limited window into tactile coding. To fill this gap, we measured the ability of human observers to report the speed of natural textures—which span the range of tactile experience and engage all the known mechanisms of texture coding—scanned across the skin. In parallel experiments, we recorded the responses of single units in the nerve and in the somatosensory cortex of primates to the same textures scanned at different speeds. We found that the perception of speed is heavily influenced by texture: some textures are systematically perceived as moving faster than are others, and some textures provide a more informative signal about speed than do others. Similarly, the responses of neurons in the nerve and in cortex are strongly dependent on texture. In the nerve, although all fibers exhibit speed-dependent responses, the responses of Pacinian corpuscle–associated (PC) fibers are most strongly modulated by speed and can best account for human judgments. In cortex, approximately half of the neurons exhibit speed-dependent responses, and this subpopulation receives strong input from PC fibers. However, speed judgments seem to reflect an integration of speed-dependent and speed-independent responses such that the latter help to partially compensate for the strong texture dependence of the former.

Our ability to sense the speed at which a surface moves across our skin is highly unreliable and depends on the texture of the surface. This study shows that speed illusions can be predicted from the responses of a specific population of nerve fibers and of their downstream targets; because the skin is too sparsely innervated to compute tactile speed accurately, the nervous system relies on a heuristic to estimate it.

Illusory Motion Reversal in Touch

Psychophysical visual experiments have shown illusory motion reversal (IMR), in which the perceived direction of motion is the opposite of its actual direction. The tactile form of this illusion has also been reported. However, it remains unclear which stimulus characteristics affect the magnitude of IMR. We closely examined the effect of stimulus characteristics on IMR by presenting moving sinusoid gratings and random-dot patterns to 10 participants’ fingerpads at different spatial periods, speeds, and indentation depths. All participants perceived a motion direction opposite to the veridical direction some of the time. The illusion was more prevalent at spatial periods of 1 and 2 mm and at extreme speeds of 20 and 320 mm/s. We observed stronger IMR for gratings and much weaker IMR for a random-dot pattern, indicating that edge orientation might be a major contributor to this illusion. These results show that the optimal parameters for IMR are consistent with the characteristics of motion-selective neurons in the somatosensory cortex, as most of these neurons are also orientation-selective. We speculate that these neurons could be the neural substrate that accounts for tactile IMR.

A System for Electrotactile Feedback Using Electronic Skin and Flexible Matrix Electrodes: Experimental Evaluation

Myoelectric prostheses are successfully controlled using muscle electrical activity, thereby restoring lost motor functions. However, the somatosensory feedback from the prosthesis to the user is still missing. The sensory substitution methods described in the literature comprise mostly simple position and force sensors combined with discrete stimulation units. The present study describes a novel system for sophisticated electrotactile feedback integrating advanced distributed sensing (electronic skin) and stimulation (matrix electrodes). The system was tested in eight healthy subjects who were asked to recognize the shape, trajectory, and direction of a set of dynamic movement patterns (single lines, geometrical objects, letters) presented on the electronic skin. The experiments demonstrated that the system successfully translated the mechanical interaction into the moving electrotactile profiles, which the subjects could recognize with a good performance (shape recognition: 86±8% lines, 73±13% geometries, 72±12% letters). In particular, the subjects could identify the movement direction with a high confidence. These results are in accordance with previous studies investigating the recognition of moving stimuli in human subjects. This is an important development towards closed-loop prostheses providing comprehensive and sophisticated tactile feedback to the user, facilitating the control and the embodiment of the artificial device into the user body scheme.

The tactile motion aftereffect suggests an intensive code for speed in neurons sensitive to both speed and direction of motion

Neurophysiological studies in primates have found that direction-sensitive neurons in the primary somatosensory cortex (SI) generally increase their response rate with increasing speed of object motion across the skin and show little evidence of speed tuning. We employed psychophysics to determine whether human perception of motion direction could be explained by features of such neurons and whether evidence can be found for a speed-tuned process. After adaptation to motion across the skin, a subsequently presented dynamic test stimulus yields an impression of motion in the opposite direction. We measured the strength of this tactile motion aftereffect (tMAE) induced with different combinations of adapting and test speeds. Distal-to-proximal or proximal-to-distal adapting motion was applied to participants' index fingers using a tactile array, after which participants reported the perceived direction of a bidirectional test stimulus. An intensive code for speed, like that observed in SI neurons, predicts greater adaptation (and a stronger tMAE) the faster the adapting speed, regardless of the test speed. In contrast, speed tuning of direction-sensitive neurons predicts the greatest tMAE when the adapting and test stimuli have matching speeds. We found that the strength of the tMAE increased monotonically with adapting speed, regardless of the test speed, showing no evidence of speed tuning. Our data are consistent with neurophysiological findings that suggest an intensive code for speed along the motion processing pathways comprising neurons sensitive both to speed and direction of motion.

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.

Path integration in tactile perception of shapes

Whenever we move the hand across a surface, tactile signals provide information about the relative velocity between the skin and the surface. If the system were able to integrate the tactile velocity information over time, cutaneous touch may provide an estimate of the relative displacement between the hand and the surface. Here, we asked whether humans are able to form a reliable representation of the motion path from tactile cues only, integrating motion information over time. In order to address this issue, we conducted three experiments using tactile motion and asked participants (1) to estimate the length of a simulated triangle, (2) to reproduce the shape of a simulated triangular path, and (3) to estimate the angle between two-line segments. Participants were able to accurately indicate the length of the path, whereas the perceived direction was affected by a direction bias (inward bias). The response pattern was thus qualitatively similar to the ones reported in classical path integration studies involving locomotion. However, we explain the directional biases as the result of a tactile motion aftereffect.

A multi-digit tactile motion stimulator

BACKGROUND

One of the hallmarks of haptic exploration is that it typically involves movement between skin and object. Explored objects may contact multiple digits simultaneously so information about motion must be integrated across digits, a process about which little is known.

NEW METHOD

To fill this gap, we have developed a stimulator that allows for the simultaneous and independent delivery of motion stimuli to multiple digits. The stimulator consists of individual units that deliver motion with three degrees of freedom: rotation (to produce motion), vertical excursion (to control depth of indentation into the skin) and arm orientation (to control the direction of motion). Each degree of freedom is controlled by a single motor. The compact design of the simulator allows for the side-by-side arrangement of the stimulator units such that they impinge upon adjacent fingers.

RESULTS

To demonstrate the functionality of the stimulator, we performed a series of psychophysical experiments that investigate the perception of motion on multiple fingers. We find that, while the sensitivity to changes in motion direction is equivalent whether stimuli are presented to the same or to different fingers, the perceived direction of motion depends on the relative configuration of the digits.

COMPARISON WITH EXISTING METHODS

We replicated the results of previous experiments investigating motion discrimination with a single digit and were able to extend these findings by investigating motion perception across multiple digits.

CONCLUSION

The novel motion stimulator will be an invaluable tool to investigate how motion information is integrated across multiple digits.

Context-dependent changes in tactile perception during movement execution

Tactile perception is inhibited during movement execution, a phenomenon known as tactile suppression. Here, we investigated whether the type of movement determines whether or not this form of sensory suppression occurs. Participants performed simple reaching or exploratory movements. Tactile discrimination thresholds were calculated for vibratory stimuli delivered to participants' wrists while executing the movement, and while at rest (a tactile discrimination task, TD). We also measured discrimination performance in a same vs. different task for the explored materials during the execution of the different movements (a surface discrimination task, SD). The TD and SD tasks could either be performed singly or together, both under active movement and passive conditions. Consistent with previous results, tactile thresholds measured at rest were significantly lower than those measured during both active movement and passive touch (that is, tactile suppression was observed). Moreover, SD performance was significantly better under conditions of single-tasking, active movements, as well as exploratory movements, as compared to conditions of dual-tasking, passive movements, and reaching movements, respectively. Therefore, the present results demonstrate that when active hand movements are made with the purpose of gaining information about the surface properties of different materials an enhanced perceptual performance is observed. As such, it would appear that tactile suppression occurs for irrelevant tactual features during both reaching and exploratory movements, but not for those task-relevant features that result from action execution during tactile exploration. Taken together, then, these results support a context-dependent modulation of tactile suppression during movement execution.

Effects of distance and direction on tangential tactile perception of the index finger pad

Tangential motion on a finger pad is a promising method of transmitting directional tactile information to human users. This study examined the identification and discrimination of tangential force motion on an index finger pad. An experimental device was built to automatically and randomly move a small probe in eight radial directions (45° apart) and two distances (0.5 and 1.5 mm). Index fingers of 62 subjects were tested. The results showed that moving the probe at 1.5 mm was detected with more accuracy than the 0.5 mm one. And, the absolute direction was not a statistically significant variable affecting accuracy for 1.5 mm distance, but was a significant effect for 0.5 mm distance. Implications of these results are discussed and future developments are offered within the context of a proposed Braille design with tangential actuators.

Cross-modal sensory integration of visual-tactile motion information: instrument design and human psychophysics

Information obtained from multiple sensory modalities, such as vision and touch, is integrated to yield a holistic percept. As a haptic approach usually involves cross-modal sensory experiences, it is necessary to develop an apparatus that can characterize how a biological system integrates visual-tactile sensory information as well as how a robotic device infers object information emanating from both vision and touch. In the present study, we develop a novel visual-tactile cross-modal integration stimulator that consists of an LED panel to present visual stimuli and a tactile stimulator with three degrees of freedom that can present tactile motion stimuli with arbitrary motion direction, speed, and indentation depth in the skin. The apparatus can present cross-modal stimuli in which the spatial locations of visual and tactile stimulations are perfectly aligned. We presented visual-tactile stimuli in which the visual and tactile directions were either congruent or incongruent, and human observers reported the perceived visual direction of motion. Results showed that perceived direction of visual motion can be biased by the direction of tactile motion when visual signals are weakened. The results also showed that the visual-tactile motion integration follows the rule of temporal congruency of multi-modal inputs, a fundamental property known for cross-modal integration.

A critical speed for gating of tactile detection during voluntary movement

This study addressed the paradoxical observation that movement is essential for tactile exploration, and yet is accompanied by movement-related gating or suppression of tactile detection. Knowing that tactile gating covaries with the speed of movement (faster movements, more gating), we hypothesized that there would be no tactile gating at slower speeds of movement, corresponding to speeds commonly used during tactile exploration (<200 mm/s). Subjects (n = 21) detected the presence or absence of a weak electrical stimulus applied to the skin of the right middle finger during two conditions: rest and active elbow extension. Movement speed was systematically varied from 50 to ~1,000 mm/s. No subject showed evidence of tactile gating at the slowest speed tested, 50 mm/s (rest versus movement), but all subjects showed decreased detection at one or more higher speeds. For each subject, we calculated the critical speed, corresponding to the speed at which detection fell to 0.5 (chance). The mean critical speed was 472 mm/s and >200 mm/s in almost all subjects (19/21). This result is consistent with our hypothesis that subjects optimize the speed of movement during tactile exploration to avoid speeds associated with tactile gating. This strategy thus maximizes the quality of the tactile feedback generated during tactile search and improves perception.

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

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

Tactile motion aftereffects produced by appropriate presentation for mechanoreceptors

Tactile motion perception is one of the most important functions for realizing a delicate appreciation of the tactile world. To explore the neural dynamics of motion processing in the brain, the motion adaptation phenomenon can be a useful probe. Tactile motion aftereffects (MAE), however, have not been reported in a reproducible fashion, and the indistinctive outcomes of the previous studies can be ascribed to the non-optimal choice of adapting and testing stimuli. Considering the features of the stimuli used in the studies, the stimuli activated the different mechanoreceptors in the adapting and testing phase. Consequently, we tested tactile MAE using appropriate combinations of adapting and testing stimuli. We used three pins to generate sensation of apparent motion on the finger cushion. They were sequentially vibrated with the frequency of 30 Hz both in adapting and testing phases. It is expected that this procedure ensured stimulation for the same mechanoreceptor (Rapid-Adapting mechanoreceptor) in both the adaptation and test phases. Using this procedure, we found robust tactile MAEs in the various tactile motions such as the short-distance motion within the fingertip, the long-distance motion from the finger base to the fingertip, and the circular motion on the fingertip. Our development of a protocol that reliably produces tactile MAEs will provide a useful psychophysical probe into the neural mechanisms of tactile motion processing.

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.

SI neuron response variability is stimulus tuned and NMDA receptor dependent

Skin brushing stimuli were used to evoke spike discharge activity in single skin mechanoreceptive afferents (sMRAs) and anterior parietal cortical (SI) neurons of anesthetized monkeys (Macaca fascicularis). In the initial experiments 10-50 presentations of each of 8 different stimulus velocities were delivered to the linear skin path from which maximal spike discharge activity could be evoked. Mean rate of spike firing evoked by each velocity (MFR) was computed for the time period during which spike discharge activity exceeded background, and an across-presentations estimate of mean firing rate (MFR) was generated for each velocity. The magnitude of the trial-by-trial variation in the response (estimated as CV; where CV = standard deviation in MFR/MFR) was determined for each unit at each velocity. MFR for both sMRAs and SI neurons (MFRsMRA and MFRSI, respectively) increased monotonically with velocity over the range 1-100 cm/s. At all velocities the average estimate of intertrial response variation for SI neurons (CVSI) was substantially larger than the corresponding average for sMRAs (CVsMRA). Whereas CVsMRA increased monotonically over the range 1-100 cm/s, CVSI decreased progressively with velocity over the range 1-10 cm/s, and then increased with velocity over the range 10-100 cm/s. The position of the skin brushing stimulus in the receptive field (RF) was varied in the second series of experiments. It was found that the magnitude of CVSI varied systematically with stimulus position in the RF: that is, CVSI was lowest for a particular velocity and direction of stimulus motion when the skin brushing stimulus traversed the RF center, and CVSI increased progressively as the distance between the stimulus path and the RF center increased. In the third series of experiments, either phencylidine (PCP; 100-500 microg/kg) or ketamine (KET; 0.5-7.5 mg/kg) was administered intravenously (iv) to assess the effect of block of N-methyl-D-aspartate (NMDA) receptors on SI neuron intertrial response variation. The effects of PCP on both CVSI and MFRSI were transient, typically with full recovery occurring in 1-2 h after drug injection. The effects of KET on CVSI and MFRSI were similar to those of PCP, but were shorter in duration (15-30 min). PCP and KET administration consistently was accompanied by a reduction of CVSI. The magnitude of the reduction of CVSI by PCP or KET was associated with the magnitude of CVSI before drug administration: that is, the larger the predrug CVSI, the larger the reduction in CVSI caused by PCP or KET. PCP and KET exerted variable effects on SI neuron mean firing rate that could differ greatly from one neuron to the next. The results are interpreted to indicate that SI neuron intertrial response variation is 1) stimulus tuned (intertrial response variation is lowest when the skin stimulus moves at 10 cm/s and traverses the neuron's RF center) and 2) NMDA receptor dependent (intertrial response variation is least when NMDA receptor activity contributes minimally to the response, and increases as the contribution of NMDA receptors to the response increases).

Velocity invariance of receptive field structure in somatosensory cortical area 3b of the alert monkey

This is the second in a series of studies of the neural representation of tactile spatial form in cortical area 3b of the alert monkey. We previously studied the spatial structure of 330 area 3b neuronal receptive fields (RFs) on the fingerpad with random dot patterns scanned at one velocity (40 mm/sec; ). Here, we analyze the temporal structure of 84 neuronal RFs by studying their spatial structure at three scanning velocities (20, 40, and 80 mm/sec). As in the previous study, most RFs contained a single, central, excitatory region and one or more surrounding or flanking inhibitory regions. The mean time delay between skin stimulation and its excitatory effect was 15.5 msec. Except for differences in mean rate, each neuron's response and the spatial structure of its RF were essentially unaffected by scanning velocity. This is the expected outcome when excitatory and inhibitory effects are brief and synchronous. However, that interpretation is consistent neither with the reported timing of excitation and inhibition in somatosensory cortex nor with the third study in this series, which investigates the effect of scanning direction and shows that one component of inhibition lags behind excitation. We reconcile these observations by showing that overlapping (in-field) inhibition delayed relative to excitation can produce RF spatial structure that is unaffected by changes in scanning velocity. Regardless of the mechanisms, the velocity invariance of area 3b RF structure is consistent with the velocity invariance of tactile spatial perception (e.g., roughness estimation and form recognition).

Task- and subject-related differences in sensorimotor behavior during active touch

Rats explore objects by rhythmically whisking them with their mystacial vibrissae. On two types of tactile discrimination tasks, macrogeometric and microgeometric, better performers palpated the discrimnanda for longer periods of time and used movement patterns that appeared to optimize whisking frequency bandwidth and the extent to which the vibrissae would be bent by object contact. On a task involving finely textured surfaces, good and poor performers differed in the temporal components of their whisking patterns, whereas the spatial domain was more important for animals palpating surfaces with widely separated features. These findings are consistent with increasing neurophysiological evidence that the central representation of the tactile periphery, in rodents and other mammals, is both integrative and dynamic.

The response of SI directionally selective neurons to stimulus motion occurring at two sites within the receptive field

Data from two classes of primary somatosensory (SI) neurons (termed "direction-invariant" and "direction-variant") were analyzed to evaluate their capacity to process the directional information provided by two moving (i.e., brushing) stimuli delivered to nonoverlapping skin sites within the receptive field (RF). The stimulus sites were arranged either end to end or side by side on the skin. The two stimuli were delivered at the same time (i.e., simultaneously) or asynchronously in precisely defined orders. For both classes of neurons, and with both the end-to-end and side-by-side dual-stimulus arrangements, the response elicited by dual-site stimulation was usually much less than a linear summation of the responses elicited by independent stimulation of each site. For the direction-invariant neurons, when the two sites were arranged end to end and direction of motion at both sites was the same, directional sensitivity with dual-site stimulation most often matched or exceeded a vectorial sum of the sensitivities observed at each site when stimulated alone. In contrast, with the side-by-side arrangement, the level of directional sensitivity achieved with dual-site stimulation often failed to attain that predicted by vectorial summation of the sensitivities observed at each site. Instead, directional sensitivity under this dual-stimulus condition only approximated that attained with single-site stimulation at the more sensitive site. When noncorresponding directions of motion were presented at two sites within the RF (using either the end-to-end or side-by-side arrangement), direction-invariant neurons failed to respond differentially to opposing patterns of dual-site stimulation. For the direction-variant SI neurons, a particular end-to-end arrangement of the two sites within the RF was studied: Sites were identified on opposite sides of the within-RF boundary that in these neurons separates regions with opposite directional preferences. With this arrangement, the differential response was greater when opposite directions of motion were applied to the two sites than it was when the same direction of motion was delivered at both sites. The observations suggest that for both groups of SI neurons, the magnitude of directional sensitivity is dependent on the same attributes of dual-site stimulation that influence cutaneous directional sensitivity--that is, on the spatial arrangement of and temporal delay between the two stimuli, and on the correspondence of their directions. The effects of dual-site stimulation on the behavior of these two neuron populations appear to be in good agreement with the hypothesis that they subserve a function in tactile motion perception.

Characterization of the percepts evoked by discontinuous motion over the perioral skin

The capacity of human subjects to process information about discontinuous and continuous movement was evaluated. Constant-velocity brushing stimuli were delivered through aperture plates that rested lightly upon the mandibular skin. Each plate consisted of either two spatially separated, slit-like openings or a single continuous, longer opening. It was discovered that percepts of smooth apparent motion were achieved with the split apertures (i.e., from discontinuous movement) for only limited ranges of stimulus velocity. Moreover, the optimal velocity supporting smooth apparent motion increased with the separation between the slit-like openings. In a second series of experiments, subjects' ability to discriminate opposing directions of discontinuous and continuous movement was evaluated. It was found that subjects could derive directional information from percepts elicited by discontinuous movement. However, the capacity to discriminate opposing directions of continuous movement cannot be explained solely in terms of the ability to process information about the change in position of a stimulus from its onset to its offset.

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.

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.

Visual responses of neurons in somatosensory cortex of hamsters with experimentally induced retinal projections to somatosensory thalamus

These experiments investigate the capacity of thalamic and cortical structures in a sensory system to process information of a modality normally associated with another system. Retinal ganglion cells in newborn Syrian hamsters were made to project permanently to the main thalamic somatosensory (ventrobasal) nucleus. When the animals were adults, single unit recordings were made in the somatosensory cortices, the principal targets of the ventrobasal nucleus. The somatosensory neurons responded to visual stimulation of distinct receptive fields, and their response properties resembled, in several characteristic features, those of normal visual cortical neurons. In the visual cortex of normal animals and the somatosensory cortex of operated animals, the same functional categories of neurons occurred in similar proportions, and the neurons' selectivity for the orientation or direction of movement of visual stimuli was comparable. These results suggest that thalamic nuclei or cortical areas at corresponding levels in the visual and somatosensory pathways perform similar transformations on their inputs.

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.

Neural mechanisms of absolute tactile localization in monkeys

Macaca nemestrina monkeys were trained to indicate the location of suprathreshold tactile stimuli delivered to the glabrous skin of either foot. The testing paradigm involved self-initiated trials (a bar press), followed by 10-Hz stimulation at one of six locations (e.g., on the distal phalanx of the second toe on the left foot), providing the opportunity for the animal to press one of six buttons located on a facing panel. The buttons were positioned on a picture of a monkey's feet at locations corresponding to the skin loci that were stimulated on different trials. If the animal first pressed the button corresponding to the position stimulated, liquid reward was delivered; responses to any other button terminated stimulation without reward, requiring initiation of another trial for the opportunity to receive reinforcement. The localization errors for normal monkeys were reliably greater along the mediolateral dimension of the foot than they were proximodistally. For example, stimulation of the tip of toe 4 elicited responses to the button at the tip of toe 2 on 25% of the trials, as compared with only 10% errors between the tip of toe 4 and the pad at the base of toe 4. Following unilateral interruption of the dorsal spinal columns at an upper thoracic level, the capacity for absolute tactile localization was unchanged over months of testing. The greater localization accuracy along the proximodistal axis of the foot remained after dorsal column transection. In order to evaluate neural substrates of localization by monkeys, single-neuron receptive field (RF) sizes and distributions within the first somatosensory (SI) cortex were examined to determine the overlap or separation of the representations of different points on glabrous skin. The sample of neurons that provided the RF data was obtained in previous investigations of unanesthetized, neuromuscularly blocked Macaca fascicularis monkeys. Analysis of RF overlap revealed that greater than 50% of cytoarchitectural area 1 units that responded to stimulation of one digit tip also responded to another digit or to the pad at the base of a digit. These large RFs seem poorly suited to subserve a high degree of spatial localization and are compatible with the frequent localization errors by the monkeys in the behavioral experiments. However, the area 1 RF data do not explain the tendency of these animals to exhibit better localization accuracy along the proximodistal axis than along the mediolateral axis of the volar foot.(ABSTRACT TRUNCATED AT 250 WORDS)

The capacity of human subjects to process directional information provided at two skin sites

The ability of human subjects to discriminate direction of tactile stimulus motion on the dorsum of the hand was determined (1) in the absence and (2) in the presence of a moving stimulus delivered to a second skin site on the ipsilateral or contralateral forelimb. When the two skin sites were simultaneously contacted by stimuli moving in the same direction, directional sensitivity was typically below that predicted for a hypothetical subject who could independently process the information provided at each of the two skin sites. Even when the stimulus delivered to a second site was deliberately ignored, it could still alter a subject's perception of stimulus direction on the dorsal hand. Moreover, its influence was greatest whenever it moved in a direction opposite to that of the attended stimulus. Whenever the two moving stimuli were delivered nonsimultaneously to two skin sites, directional sensitivity rarely matched the levels predicted for a hypothetical subject who could independently process the information provided at each site. This, in part, resulted from the subjects' utilization of "long-range" cues provided by the temporal order of stimulation. Subjects frequently failed to distinguish these cues from the sensation of stimulus direction provided at each skin site.

Discrimination and scaling of velocity of stimulus motion across the skin

The capacity of human subjects to discriminate and to scale the velocity of tactile brushing stimuli was assessed. Signal detection and classical psychophysical techniques were employed to estimate the Weber fraction over a wide range of velocities (from 1.5 to 140 cm/sec). In addition, free magnitude estimates of (1) the velocity and (2) the duration of moving tactile stimuli were obtained. It was found that human capacity to discriminate stimuli delivered to a 4 to 6-cm chord of skin on the dorsal forearm and differing in velocity remains grossly constant over the range of velocities tested and is relatively poor (i.e., the Weber fraction = 0.2-0.25). A simple power function (exponent = 0.6) satisfactorily describes the psychophysical relation (1) between the perceived and actual velocity and (2) between the perceived and actual duration of these stimuli. Since a direct proportionality between the reciprocal of a subject's estimate of duration and his or her estimate of velocity was observed, it is suggested that these two sensory attributes may reflect the operation of a neural mechanism sensitive to the duration of stimulation. Moreover, the data are inconsistent with the hypothesis that the subjects computed estimates of mean velocity from the ratio of perceived distance to perceived duration.

Perception of the length of voluntary movements

Two experiments were performed to study the ability of blindfolded subjects to estimate distance on the basis of proprioceptive cues. In the first experiment, subjects judged the length of metal rods that they were allowed to explore freely. With this access to positional as well as other cues, subjects' estimates were a nearly linear function of actual length. These data closely paralleled control measurements obtained under conditions of visual, rather than haptic, inspection. In the second experiment, each subject slid his or her index finger laterally along a straight path delimited by the apparatus, and then gave a magnitude estimate of the distance through which the finger had moved. Velocity of movement was manipulated by asking subjects, on each trial, to move at one of five speeds ranging from "very slow" to "very fast"; these instructions elicited velocities spanning a 100-to-1 range. Magnitude estimates of distance in this second experiment increased as a function of actual distance, but decreased as a function of velocity. This latter phenomenon resembles the dependence of perceived distance on velocity that has been shown by other investigators to occur when a stimulus object is drawn across the skin. The data of the present study are consistent with the hypothesis that the perceived length of an active movement depends on a combination of movement and position signals from primary and secondary sensory fibers in muscle spindles.

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.