Tactile sensitivity in the rat: a correlation between receptor structure and function.

Single cutaneous fibers were recorded in the median nerve of the deeply anesthetized rat and the receptor morphology in the forelimb glabrous skin was analyzed to establish a probable correlation between receptor anatomy and physiology. Receptor complexes in the glabrous skin of the rat forelimb were stained immunologically with antibodies NF-200 and PGP-9.5, confirming the presence of Meissner corpuscles and Merkel complexes within the dermal papilla similar to other mammals including primates. Both the Meissner corpuscles and Merkel cell complexes were sparse and located in the pyramidal-shaped palmer pads and the apex of the digit extremities. They were almost totally absent elsewhere in the glabrous skin. No Ruffini receptors or Pacinian corpuscles were found in our samples. A total of 92 cutaneous fibers were retained long enough for analysis. Thirty-five (38%) were characterized as rapidly adapting fibers (RA) and 57 (62%) were slowly adapting afferents (SA). Despite the very limited number of receptors at the tip of the digit, RA receptors outnumbered SA fibers 3.2/1.0. In contrast, SA fibers on the thenar pad outnumbered RA receptors by a ratio of 3-1. Despite the very limited number of low threshold mechanoreceptors in the glabrous skin of the rat forelimb, the prevalence of SA afferents in the palm and more frequent occurrence of RA afferents in the digit extremity suggest differences in functionality both for locomotion and object manipulation.

Correlation of fingertip shear force direction with somatosensory cortical activity in monkey.

To examine the activity of somatosensory cortex (S1) neurons to self-generated shear forces on the index and thumb, two monkeys were trained to grasp a stationary metal tab with a key grip and exert forces without the fingers slipping in one of four orthogonal directions for 1 s. A majority (∼85%) of slowly adapting and rapidly adapting (RA) S1 neurons had activity modulated with shear force direction. The cells were recorded mainly in areas 1 and 2 of the S1, although some area 3b neurons also responded to shear direction or magnitude. The preferred shear vectors were distributed in every direction, with tuning arcs varying from 50° to 170°. Some RA neurons sensitive to dynamic shear force direction also responded to static shear force but within a narrower range, suggesting that the direction of the shear force may influence the adaptation rate. Other neurons were modulated with shear forces in diametrically opposite directions. The directional sensitivity of S1 cortical neurons is consistent with recordings from cutaneous afferents showing that shear direction, even without slip, is a powerful stimulus to S1 neurons.

Neuronal activity in somatosensory cortex related to tactile exploration.

The very light contact forces (∼0.60 N) applied by the fingertips during tactile exploration reveal a clearly optimized sensorimotor strategy. To investigate the cortical mechanisms involved with this behavior, we recorded 230 neurons in the somatosensory cortex (S1), as two monkeys scanned different surfaces with the fingertips in search of a tactile target without visual feedback. During the exploration, the monkeys, like humans, carefully controlled the finger forces. High-friction surfaces offering greater tangential shear force resistance to the skin were associated with decreased normal contact forces. The activity of one group of neurons was modulated with either the normal or tangential force, with little or no influence from the orthogonal force component. A second group responded to kinetic friction or the ratio of tangential to normal forces rather than responding to a specific parameter, such as force magnitude or direction. A third group of S1 neurons appeared to respond to particular vectors of normal and tangential force on the skin. Although 45 neurons correlated with scanning speed, 32 were also modulated by finger forces, suggesting that forces on the finger should be considered as the primary parameter encoding the skin compliance and that finger speed is a secondary parameter that co-varies with finger forces. Neurons (102) were also tested with different textures, and the activity of 62 of these increased or decreased in relation to the surface friction.

Tactile sensory system: encoding from the periphery to the cortex.

Specialized mechanoreceptors in the skin respond to mechanical deformation and provide the primary input to the tactile sensory system. Although the morphology of these receptors has been documented, there is still considerable uncertainty as to the relation between cutaneous receptor morphology and the associated physiological responses to stimulation. Labelled-line models of somatosensory processes in which specific mechanoreceptors are associated with particular sensory qualities fail to account for the evidence showing that all types of tactile afferent units respond to a varying extent to most types of natural stimuli. Neurophysiological and psychophysical experiments have provided the framework for determining the relation between peripheral afferent or cortical activity and tactile perception. Neural codes derived from these afferent signals are evaluated in terms of their capacity to predict human perceptual performance. One particular challenge in developing models of the tactile sensory system is the dual use of sensory signals from the skin. In addition to their perceptual function they serve as inputs to the sensorimotor control system involved in manipulation. Perceptions generated through active touch differ from those resulting from passive stimulation of the skin because they are the product of self-generated exploratory processes. Recent research in this area has highlighted the importance of shear forces in these exploratory movements and has shown that fingertip skin is particularly sensitive to shear generated during both object manipulation and tactile exploration.

Roughness of simulated surfaces examined with a haptic tool: effects of spatial period, friction, and resistance amplitude.

A specifically designed force-feedback device accurately simulated textures consisting of lateral forces opposing motion, simulating friction. The textures were either periodic trapezoidal forces, or sinusoidal forces spaced at various intervals from 1.5 mm to 8.5 mm. In each of two experiments, 10 subjects interacted with the virtual surfaces using the index finger placed on a mobile plate that produced the forces. The subjects selected their own speed and contact force for exploring the test surface. The apparatus returned force fields as a function of both the finger position and the force normal to the skin allowing full control over the tangential interaction force. In Experiment #1, subjects used an integer, numerical scale of their own choosing to rate the roughness of eight identical, varyingly spaced force ramps superimposed on a background resistance. The results indicated that subjective roughness was significantly, but negatively, correlated (mean r = -0.84) with the spatial period of the resistances for all subjects. In a second experiment, subjects evaluated the roughness of 80 different sinusoidal modulated force fields, which included 4 levels of resistance amplitude, 4 levels of baseline friction, and 5 spatial periods. Multiple regression was used to determine the relationship between friction, tangential force amplitude, and spatial period to roughness. Together, friction and tangential force amplitude produced a combined correlation of 0.70 with subjective roughness. The addition of spatial period only increased the multiple regression correlation to 0.71. The correlation between roughness estimates and the rate of change in tangential force was 0.72 in Experiment #1 and 0.57 in Experiment #2. The results suggest that the sensation of roughness is strongly influenced by friction and tangential force amplitude, whereas the spatial period of simulated texture alone makes a negligible contribution to the sensation of roughness.

Perception of simulated local shapes using active and passive touch.

This study reexamined the perceptual equivalence of active and passive touch using a computer-controlled force-feedback device. Nine subjects explored a 6 x 10-cm workspace, with the index finger resting on a mobile flat plate, and experienced simulated Gaussian ridges and troughs (width, 15 mm; amplitude, 0.5 to 4.5 mm). The device simulated shapes by modulating either lateral resistance with no vertical movement or by vertical movement with no lateral forces, as a function of the digit position in the horizontal workspace. The force profiles and displacements recorded during active touch were played back to the stationary finger in the passive condition, ensuring that stimulation conditions were identical. For the passive condition, shapes simulated by vertical displacements of the finger had lower categorization thresholds and higher magnitude estimates compared with those of active touch. In contrast, the results with the lateral force fields showed that with passive touch, subjects recognized that a stimulus was present but were unable to correctly categorize its shape as convex or concave. This result suggests that feedback from the motor command can play an important role in processing sensory inputs during tactile exploration. Finally, subjects were administered a ring-block anesthesia of the digital nerves of the index finger and subsequently retested. Removing skin sensation significantly increased the categorization threshold for the perception of shapes generated by lateral force fields, but not for those generated by displacement fields.

Duchenne, Charcot and Babinski, three neurologists of La Salpetrière Hospital, and their contribution to concepts of the central organization of motor synergy.

Many currently accepted notions of motor control originate from a few seminal concepts developed in the latter half of the 19th century (see Bennett and Hacker, 2002). The goal of this review is to retrace some current ideas about motor control back to the thought of three French neurologists of Hospital of the Salpetrière hospital in Paris during the latter half of the 19th century and early 20th century (Fig. 1): Guillaume Duchenne de Boulogne (1806-1875), Jean-Martin Charcot (1825-1893), and Joseph Babinski (1857-1932). A common theoretical and methodological thread unites these three men as Charcot was taught neurology by Duchenne, and Babinski was trained by Charcot. The influential concepts developed by these pioneering French neurologists have been neglected for nearly a century and only rediscovered recently. We intend to highlight how these astute clinicians used their meticulous clinical observations of patients to reveal novel and original perspectives of motor co-ordination. Between 1850 and 1930, all three men played a major role in developing and shaping the entire field of normal and pathological motor control in addition to making important contributions to three major scientific issues; the centralist view of muscle sense, the emerging concept of muscle synergy in voluntary movements and in locomotion and finally the specific role of the cerebellum in muscle synergy. The important contributions of these men will be considered in the context of other significant schools of neurology from other countries. Finally, the concept of cerebellar asynergy as proposed by Babinski anticipated the development of the internal models which much later were able to provide a theoretical basis for understanding the mechanism of learned motor co-ordination involving the cerebellum.

Altered gravity highlights central pattern generator mechanisms.

In many nonprimate species, rhythmic patterns of activity such as locomotion or respiration are generated by neural networks at the spinal level. These neural networks are called central pattern generators (CPGs). Under normal gravitational conditions, the energy efficiency and the robustness of human rhythmic movements are due to the ability of CPGs to drive the system at a pace close to its resonant frequency. This property can be compared with oscillators running at resonant frequency, for which the energy is optimally exchanged with the environment. However, the ability of the CPG to adapt the frequency of rhythmic movements to new gravitational conditions has never been studied. We show here that the frequency of a rhythmic movement of the upper limb is systematically influenced by the different gravitational conditions created in parabolic flight. The period of the arm movement is shortened with increasing gravity levels. In weightlessness, however, the period is more dependent on instructions given to the participants, suggesting a decreased influence of resonant frequency. Our results are in agreement with a computational model of a CPG coupled to a simple pendulum under the control of gravity. We demonstrate that the innate modulation of rhythmic movements by CPGs is highly flexible across gravitational contexts. This further supports the involvement of CPG mechanisms in the achievement of efficient rhythmic arm movements. Our contribution is of major interest for the study of human rhythmic activities, both in a normal Earth environment and during microgravity conditions in space.

Paradoxical effect of digital anaesthesia on force and corticospinal excitability.

The role played by sensory information in maintaining motor cortical representations is still incompletely understood. We investigated the effect of digital anaesthesia of the index finger and thumb on the amplitude of motor evoked potentials to transcranial magnetic stimulation (TMS) recorded from the first dorsal interosseus, F-wave response probability and maximal key pinch force. Whereas digital anaesthesia led to a 29% decrease in maximal force, both motor evoked potential amplitudes and F-wave probability remained unchanged. This dramatic decrease in maximal voluntary contraction following digital anaesthesia may result from a lack of proper sensory feedback during the task.

The cutaneous contribution to adaptive precision grip.

Only after injury, or perhaps prolonged exposure to cold that is sufficient to numb the fingers, do we suddenly appreciate the complex neural mechanisms that underlie our effortless dexterity in manipulating objects. The nervous system is capable of adapting grip forces to a wide range of object shapes, weights and frictional properties, to provide optimal and secure handling in a variety of potentially perturbing environments. The dynamic interplay between sensory information and motor commands provides the basis for this flexibility, and recent studies supply somewhat unexpected evidence of the essential role played by cutaneous feedback in maintaining and acquiring predictive grip force control. These examples also offer new insights into the adaptive control of other voluntary movements.

Responses of cerebellar interpositus neurons to predictable perturbations applied to an object held in a precision grip.

Two monkeys were trained to lift and hold an instrumented object at a fixed height for 2.5 s using a precision grip. The device was equipped with load cells to measure both the grip and lifting or load forces. On selected blocks of 20-30 trials, a downward force-pulse perturbation was applied to the object after 1.5 s of stationary holding. The animals were required to resist the perturbation to obtain a fruit juice reward. The perturbations invariably elicited a reflex-like, time-locked increase in grip force at latencies between 50 and 100 ms. In this study, we searched for single cells in the interpositus and dentate nuclei with activity related to grasping and lifting, and we tested 127/150 task-related cells for their responses to the perturbation. Of the 127 cells, reflex-like increases or decreases in discharge frequency occurred in 75 cells (59%) at a mean latency of 36 ms. Preparatory increases in grip force preceding the perturbation appeared gradually and increased in strength with repetition in 39/127 (31%) cells. These preparatory increases did not immediately disappear when the perturbations were withdrawn but decreased progressively over repeated trials. Although a few cells showed anticipatory activity without a reflex-like response (15/127 or 12%), the majority of these cells (24/39) displayed both anticipatory and reflex-like responses. From an examination of the histological sections, cells with both anticipatory and reflex-like responses appeared to be confined to the dorsal anterior interpositus, adjacent to, but not within, the dentate nucleus. These results confirm and extend the suggestion by Dugas and Smith that the cerebellum plays a major role in organizing anticipatory responses to predictable perturbations in a manner that medial and lateral premotor areas of the cerebral cortex do not.

Effects of muscimol inactivation of the cerebellar nuclei on precision grip.

A single monkey was trained to perform a grasp, lift, and hold task in which a stationary hand- held object was sometimes subjected to brief, predictable force-pulse perturbations. The displacement, grip, and lifting forces were measured as well the three-dimensional forces and torques to quantify specific motor deficits after reversible inactivation of the cerebellar nuclei. A prior single-cell recording study in the same monkey provided the stereotaxic coordinates used to guide intranuclear injections of muscimol. In total, 34 penetrations were performed at 28 different loci throughout the cerebellar nuclei. On each penetration, two 1.0-microl injections of 5 microg/microl muscimol, were made 1.0 mm apart either within the nuclei or in the white matter just lateral or posterior to the dentate nucleus. Injections in the region corresponding to the anterior interpositus nucleus produced pronounced dynamic tremor and dysmetric movements of the ipsilateral arm when the animal performed unrestrained reaching and grasping movements. In contrast, no relatively short-latency (15-20 min.) deficits were observed after injection in the dentate nucleus, although some effects were observed after several hours. When tested in a primate chair with the forearm supported and restrained at the wrist and elbow, the monkey performed the lift and hold task without tremor or dysmetria. However, with the restraint removed, the forces and torques applied to the manipulandum were poorly controlled and erratic. The monkey's arm was ataxic and a 5-Hz intention tremor was clearly visible. In addition, the animal was generally unable to compensate for the predictable perturbations and the anticipatory grip force increases were absent. However, overall the results suggest that reversible cerebellar nuclear inactivation with muscimol has little effect on isolated distal movements of the wrist and fingers.

Importance of cutaneous feedback in maintaining a secure grip during manipulation of hand-held objects.

Previous research has shown that grip and load forces are modulated simultaneously during manipulation of a hand-held object. This close temporal coupling suggested that both forces are controlled by an internal model within the CNS that predicts the changes in tangential force on the fingers. The objective of the present study was to examine how the internal model would compensate for the loss of cutaneous sensation through local anesthesia of the index and thumb. Ten healthy adult subjects (5 men and 5 women aged 20-57 yr) were asked to grasp, lift, and hold stationary, a 250 g object for 20 s. Next, the subjects were asked to perform vertical oscillatory movements over a distance of 20 cm at a rate of 1.0 Hz for 30 s. Eleven trials were performed with intact sensation, and 11 trials after a local ring-block anesthesia of the index and thumb with bupivacain (5 mg/ml). During static holding, loss of cutaneous sensation produced a significant increase in the safety margin. However, the grip force declined significantly over the 20-s static hold period. During oscillatory arm movements, grip and load forces were continuously modulated together in a predictive manner as suggested by Flanagan and Wing. Again, the grip force declined over the 30-s movement, and 7/10 subjects dropped the object at least once. With intact sensation, the object was never dropped; but with the fingers anesthetized, it was dropped on 36% of the trials, and a significant slip occurred on a further 12%. The mean correlation between the grip and load forces for all subjects deteriorated from 0.71 with intact sensation to 0.48 after digital anesthesia. However, a cross-correlation calculated between the grip and load forces indicated that the phase lag was approximately zero both with and without digital anesthesia. Taken together, the data from the present study suggest that cutaneous afferents are required for setting and maintaining the background level of the grip force in addition to their phasic slip-detection function and their role in adapting the grip force/load force ratio to the friction on initial contact with an object. Finally, at a more theoretical level, they correct and maintain an internal model of the physical properties of hand-held objects.

The effects of digital anesthesia on force control using a precision grip.

A total of 20 right-handed subjects were asked to perform a grasp-lift-and-hold task using a precision grip. The grasped object was a one-degree-of-freedom manipuladum consisting of a vertically mounted linear motor capable of generating resistive forces to simulate a range of object weights. In the initial study, seven subjects (6 women, 1 man; ages 24-56 yr) were first asked to lift and hold the object stationary for 4 s. The object presented a metal tab with two different surface textures and offered one of four resistive forces (0.5, 1.0, 1.5, and 2.0 N). The lifts were performed both with and without visual feedback. Next, the subjects were asked to perform the same grasping sequence again after ring block anesthesia of the thumb and index finger with mepivacaine. The objective was to determine the degree to which an internal model obtained through prior familiarity might compensate for the loss of cutaneous sensation. In agreement with previous studies, it was found that all subjects applied significantly greater grip force after digital anesthesia, and the coordination between grip and load forces was disrupted. It appears from these data, that the internal model alone is insufficient to completely compensate for the loss of cutaneous sensation. Moreover, the results suggest that the internal model must have either continuous tonic excitation from cutaneous receptors or at least frequent intermittent reiteration to function optimally. A subsequent study performed with 10 additional subjects (9 women, 1 man; ages 24-49 yr) indicated that with unimpaired cutaneous feedback, the grasping and lifting forces were applied together with negligible forces and torques in other directions. In contrast, after digital anesthesia, significant additional linear and torsional forces appeared, particularly in the horizontal and frontal planes. These torques were thought to arise partially from the application of excessive grip force and partially from a misalignment of the two grasping fingers. These torques were further increased by an imbalance in the pressure exerted by the two opposing fingers. Vision of the grasping hand did not significantly correct the finger misalignment after digital anesthesia. Taken together, these results suggest that mechanoreceptors in the fingertips signal the source and direction of pressure applied to the skin. The nervous system uses this information to adjust the fingers and direct the pinch forces optimally for grasping and object manipulation.

Deployment of fingertip forces in tactile exploration.

The purpose of this study was to examine how contact forces normal to the skin surface and shear forces tangential to the skin surface are deployed during tactile exploration of a smooth surface in search of a tactile target. Six naive subjects participated in two experiments. In the first experiment, the subjects were asked to explore a series of unseen smooth plastic surfaces by using the index finger to search for either a raised or recessed target. The raised targets were squares with a height of 280 micro m above the background surface and that varied in side lengths from 0.2 mm to 8.0 mm. A second series of smooth plastic surfaces consisted of small recessed squares (side lengths: 2.0, 3.0, 4.0 and 8.0 mm) that were etched to a depth of 620 micro m. Although made of an identical material, the plastic substrate had a lower coefficient of friction against the skin because only the recessed square had been subjected to the electrolytic etching process. The surfaces were mounted on a six-axes force and torque sensor connected to a laboratory computer. From the three axes of linear force, the computer was able to calculate the instantaneous position of the index finger and the instantaneous tangential force throughout the exploratory period. When exploring for the raised squares, the subjects maintained a relatively constant, average normal force of about 0.49 N with an average exploration speed of 8.6 cm/s. In contrast, all subjects used a significantly higher average normal force (0.64 N) and slightly slower mean exploration speed (7.67 cm/s) when searching for the small recessed squares. This appeared to be an attempt to maximize the amount of skin penetrating the recessed squares to improve the probability of target detection. In a second experiment, subjects were requested to search for an identical set of raised squares but with the fingertip having been coated with sucrose to impede the scanning movement by increasing the friction. Overall, the subjects maintained the same constant normal force that they used on the uncoated surface. However, they increased the tangential force significantly. The similarity of the search strategy employed by all subjects supports the hypothesis that shear forces on the skin provide a significant stimulus to mechanoreceptors in the skin during tactile exploration. Taken together, these data suggest that, in active tactile exploration with the fingertip, the tangential finger speed, the normal contact force, and the tangential shear force are adjusted optimally depending on the surface friction and whether the target is a raised asperity or a recessed indentation.

Role of friction and tangential force variation in the subjective scaling of tactile roughness.

The present study examined the contribution of normal (Fz) and tangential (Fx) forces, and their ratio, kinetic friction (Fx/Fz), to the subjective magnitude estimations of roughness. The results suggested that the rate of variation in tangential stroking force is a significant determinant of roughness perception. In the first experiment, six volunteer subjects scaled the roughness of eight surfaces explored with a single, active scan of the middle finger. The surfaces were 7.5x2.4-cm polymer strips embossed with truncated cones 1.8 mm high with a spatial period of 2.0 mm in the transverse direction and 1.5-8.5 mm in the longitudinal, scanning direction. The surfaces were mounted on a six-axis force and torque sensor that measured the perpendicular, contact force (normal to the skin surface) and the tangential force along the axis of stroking. The results confirmed the findings of an earlier study that magnitude estimates of perceived roughness increase approximately linearly up to a longitudinal spatial period of 8.5 mm. Across subjects, no consistent correlations were found between perceived roughness and either the mean normal or tangential force alone. Although significant positive correlations were found between roughness and mean kinetic friction for all subjects, they were not as consistently robust as one might have expected. Furthermore, instantaneous kinetic friction varied widely over the course of a single stroke because of within trial oscillations in the tangential force. The amplitude of these oscillations increased with the longitudinal spatial period and their frequency was determined by a combination of the spatial period and the stroking velocity. These oscillations were even more conspicuous in the first derivative or rate of change of the tangential force (dFx/d t), which was quantified as the root mean square (RMS) of the tangential force rate. The mean normalized RMS proved to be strongly correlated with subjective roughness, averaging 0.88 for all subjects. In order to dissociate the fluctuations in tangential force from both the surface structure and the mean kinetic friction, a second experiment was performed on six additional subjects who estimated the roughness of identical lubricated and unlubricated (dry) surfaces. Lubrication with liquid soap reduced the mean kinetic friction by approximately 40%, the RMS of the tangential force rate by slightly more than 21% and the subjective estimates of roughness by 16.4%. Taken together, the results suggest that in tactile exploration, the RMS of the tangential force rate may be an important determinant of subjective roughness.

Distribution and terminal arborizations of cutaneous mechanoreceptors in the glabrous finger pads of the monkey.

Recent electrophysiological studies demonstrated that neurons in the somatosensory cortex of monkeys respond to tangential forces applied to glabrous skin. To unravel the peripheral basis for this cortical response, we determined the distribution of presumptive low-threshold mechanoreceptors innervating the distal finger pads of monkeys. Endings were reconstructed in immunolabeled serial sections imaged by epifluorescence and confocal microscopy. Although classically implicated as cutaneous stretch receptors, no Ruffini corpuscles were found in the glabrous skin. Ruffini-like endings were only detected at the base of the finger nails. Pacinian corpuscles were sparsely distributed in the deep dermis. Meissner corpuscles (MCs) in dermal papillary ridges had a comparably high density in the thumb, index, and fifth fingers. Each MC was innervated by several large-caliber axons. Within the limits of our reconstructions, some of these axons terminated in only one MC, whereas others innervated several MCs. Merkel endings covered about 80% of the base of the intermediate epidermal ridges that form the pattern of fingerprints. In some cases, the distal tip of a Merkel-related axon gave rise to a several terminal branches that supplied endings to tightly circumscribed (30-70 microm) clusters of Merkel cells. In other cases, the nodes of axons gave rise to en passant branches that formed extended chains of endings among Merkel cells spread over territories up to 300 microm long. Based on their relatively diffuse distributions, the axons that innervate multiple MCs or the axons with en passant Merkel terminations seem most suited to transduce tangential forces.

Magnitude estimation of tangential force applied to the fingerpad.

Prior research on large-fibre skin mechanoreceptors in humans and monkeys has demonstrated their sensitivity to perpendicular skin indentation and to the rate of force application. Although some studies have examined skin afferent responses to stretch, relatively few investigations have examined the neural and perceptual correlates of shear forces applied tangentially to the skin. The present study assessed the ability of human subjects to scale different levels of tangential force applied to the distal pad of the index finger. Subjects were instructed to choose their own magnitude estimation scale. Seven force levels ranging from 0.15 to 0.70 N were delivered randomly at rates of 0.10 N/s, 0.15 N/s or 0.30 N/s. Tangential forces were produced with a smooth metal spatula coated with an adhesive to insure a shear force on the underlying skin without slip. The same procedures were also used to generate skin indentation with normal forces. The results showed that most human subjects were able to scale different magnitudes of both tangential and normal forces applied to the tip of the index finger. The rate of force change did not influence the perception of the applied forces. These results highlight the potentially important role of tangential forces in haptic perception.