Differential effects of the mode of touch, active and passive, on experience-driven plasticity in the S1 cutaneous digit representation of adult macaque monkeys.

This study compared the receptive field (RF) properties and firing rates of neurons in the cutaneous hand representation of primary somatosensory cortex (areas 3b, 1, and 2) of 9 awake, adult macaques that were intensively trained in a texture discrimination task using active touch (fingertips scanned over the surfaces using a single voluntary movement), passive touch (surfaces displaced under the immobile fingertips), or both active and passive touch. Two control monkeys received passive exposure to the same textures in the context of a visual discrimination task. Training and recording extended over 1-2 yr per animal. All neurons had a cutaneous receptive field (RF) that included the tips of the stimulated digits (D3 and/or D4). In area 3b, RFs were largest in monkeys trained with active touch, smallest in those trained with passive touch, and intermediate in those trained with both; i.e., the mode of touch differentially modified the cortical representation of the stimulated fingers. The same trends were seen in areas 1 and 2, but the changes were not significant, possibly because a second experience-driven influence was seen in areas 1 and 2, but not in area 3b: smaller RFs with passive exposure to irrelevant tactile inputs compared with recordings from one naive hemisphere. We suggest that added feedback during active touch and higher cortical firing rates were responsible for the larger RFs with behavioral training; this influence was tempered by periods of more restricted sensory feedback during passive touch training in the active + passive monkeys.NEW & NOTEWORTHY We studied experience-dependent sensory cortical plasticity in relation to tactile discrimination of texture using active and/or passive touch. We showed that neuronal receptive fields in primary somatosensory cortex, especially area 3b, are largest in monkeys trained with active touch, smallest in those trained with passive touch, and intermediate in those trained using both modes of touch. Prolonged, irrelevant tactile input had the opposite influence in areas 1 and 2, favoring smaller receptive fields.

Context-dependent tactile texture-sensitivity in monkey M1 and S1 cortex.

Caudal primary motor cortex (M1, area 4) is sensitive to cutaneous inputs, but the extent to which the physical details of complex stimuli are encoded is not known. We investigated the sensitivity of M1 neurons (4 Macaca mulatta monkeys) to textured stimuli (smooth/rough or rough/rougher) during the performance of a texture discrimination task and, for some cells, during a no-task condition (same surfaces; no response). The recordings were made from the hemisphere contralateral to the stimulated digits; the motor response (sensory decision) was made with the nonstimulated arm. Most M1 cells were modulated during surface scanning in the task (88%), but few of these were texture-related (24%). In contrast, 44% of M1 neurons were texture related in the no-task condition. Recordings from the neighboring primary somatosensory cortex (S1), the potential source of texture-related signals to M1, showed that S1 neurons were significantly more likely to be texture related during the task (57 vs 24%) than M1. No difference was observed in the no-task condition (52 vs. 44%). In these recordings, the details about surface texture were relevant for S1 but not for M1. We suggest that tactile inputs to M1 were selectively suppressed when the animals were engaged in the task. S1 was spared these controls, as the same inputs were task-relevant. Taken together, we suggest that the suppressive effects are most likely exerted directly at the level of M1, possibly through the activation of a top-down gating mechanism specific to motor set/intention. NEW & NOTEWORTHY Sensory feedback is important for motor control, but we have little knowledge of the contribution of sensory inputs to M1 discharge during behavior. We showed that M1 neurons signal changes in tactile texture, but mainly outside the context of a texture discrimination task. Tactile inputs to M1 were selectively suppressed during the task because this input was not relevant for the recorded hemisphere, which played no role in generating the discrimination response.

Effects of transcranial direct current stimulation of primary somatosensory cortex on vibrotactile detection and discrimination.

Anodal transcranial direct current stimulation (a-tDCS) of primary somatosensory cortex (S1) has been shown to enhance tactile spatial acuity, but there is little information as to the underlying neuronal mechanisms. We examined vibrotactile perception on the distal phalanx of the middle finger before, during, and after contralateral S1 tDCS [a-, cathodal (c)-, and sham (s)-tDCS]. The experiments tested our shift-gain hypothesis, which predicted that a-tDCS would decrease vibrotactile detection and discrimination thresholds (leftward shift of the stimulus-response function with increased gain/slope) relative to s-tDCS, whereas c-tDCS would have the opposite effects (relative to s-tDCS). The results showed that weak a-tDCS (1 mA, 20 min) led to a reduction in both vibrotactile detection and discrimination thresholds to 73-76% of baseline during the application of the stimulation in subjects categorized as responders. These effects persisted after the end of a-tDCS but were absent 30 min later. Most, but not all, subjects showed a decrease in threshold (8/12 for detection; 9/12 for discrimination). Intersubject variability was explained by a ceiling effect in the discrimination task. c-tDCS had no significant effect on either detection or discrimination threshold. Taken together, our results supported our shift-gain hypothesis for a-tDCS but not c-tDCS.

Tactile texture signals in primate primary somatosensory cortex and their relation to subjective roughness intensity.

This study investigated the hypothesis that a simple intensive code, based on mean firing rate, could explain the cortical representation of subjective roughness intensity and its invariance with scanning speed. We examined the sensitivity of neurons in the cutaneous, finger representation of primary somatosensory cortex (S1) to a wide range of textures [1 mm high, raised-dot surfaces; spatial periods (SPs), 1.5-8.5 mm], scanned under the digit tips at different speeds (40-115 mm/s). Since subjective roughness estimates show a monotonic increase over this range and are independent of speed, we predicted that the mean firing rate of a subgroup of S1 neurons would share these properties. Single-unit recordings were made in four alert macaques (areas 3b, 1 and 2). Cells whose discharge rate showed a monotonic increase with SP, independent of speed, were particularly concentrated in area 3b. Area 2 was characterized by a high proportion of cells sensitive to speed, with or without texture sensitivity. Area 1 had intermediate properties. We suggest that area 3b and most likely area 1 play a key role in signaling roughness intensity, and that a mean rate code, signaled by both slowly and rapidly adapting neurons, is present at the level of area 3b. Finally, the substantial proportion of neurons that showed a monotonic change in discharge limited to a small range of SPs (often independent of response saturation) could play a role in discriminating smaller changes in SP.

Haptic two-dimensional angle categorization and discrimination.

This study examined the extent to which haptic perception of two-dimensional (2-D) shape is modified by the design of the perceptual task (single-interval categorization vs. two-interval discrimination), the orientation of the angles in space (oblique vs. horizontal), and the exploration strategy (one or two passes over the angle). Subjects (n = 12) explored 2-D angles using the index finger of the outstretched arm. In the categorization task, subjects scanned individual angles, categorizing each as "large" or "small" (2 angles presented in each block of trials; range 80° vs. 100° to 89° vs. 91°; implicit standard 90°). In the discrimination task, a pair of angles was scanned (standard 90°; comparison 91-103°) and subjects identified the larger angle. The threshold for 2-D angle categorization was significantly lower than for 2-D angle discrimination, 4° versus 7.2°. Performance in the categorization task did not vary with either the orientation of the angles (horizontal vs. oblique, 3.9° vs. 4°) or the number of passes over the angle (1 vs. 2 passes, 3.9° vs. 4°). We suggest that the lower threshold with angle categorization likely reflects the reduced cognitive demands of this task. We found no evidence for a haptic oblique effect (higher threshold with oblique angles), likely reflecting the presence of an explicit external frame of reference formed by the intersection of the two bars forming the 2-D angles. Although one-interval haptic categorization is a more sensitive method for assessing 2-D haptic angle perception, perceptual invariances for exploratory strategy and angle orientation were, nevertheless, task-independent.

Neuronal correlates of tactile speed in primary somatosensory cortex.

Moving stimuli activate all of the mechanoreceptive afferents involved in discriminative touch, but their signals covary with several parameters, including texture. Despite this, the brain extracts precise information about tactile speed, and humans can scale the tangential speed of moving surfaces as long as they have some surface texture. Speed estimates, however, vary with texture: lower estimates for rougher surfaces (increased spatial period, SP). We hypothesized that the discharge of cortical neurons playing a role in scaling tactile speed should covary with speed and SP in the same manner. Single-cell recordings (n = 119) were made in the hand region of primary somatosensory cortex (S1) of awake monkeys while raised-dot surfaces (longitudinal SPs, 2-8 mm; periodic or nonperiodic) were displaced under their fingertips at speeds of 40-105 mm/s. Speed sensitivity was widely distributed (area 3b, 13/25; area 1, 32/51; area 2, 31/43) and almost invariably combined with texture sensitivity (82% of cells). A subset of cells (27/64 fully tested speed-sensitive cells) showed a graded increase in discharge with increasing speed for testing with both sets of surfaces (periodic, nonperiodic), consistent with a role in tactile speed scaling. These cells were almost entirely confined to caudal S1 (areas 1 and 2). None of the speed-sensitive cells, however, showed a pattern of decreased discharge with increased SP, as found for subjective speed estimates in humans. Thus further processing of tactile motion signals, presumably in higher-order areas, is required to explain human tactile speed scaling.

Physical determinants of the shape of the psychophysical curve relating tactile roughness to raised-dot spacing: implications for neuronal coding of roughness.

There are conflicting reports as to whether the shape of the psychometric relation between perceived roughness and tactile element spacing [spatial period (SP)] follows an inverted U-shape or a monotonic linear increase. This is a critical issue because the former result has been used to assess neuronal codes for roughness. We tested the hypothesis that the relation's shape is critically dependent on tactile element height (raised dots). Subjects rated the roughness of low (0.36 mm)- and high (1.8 mm)-raised-dot surfaces displaced under their fingertip. Inverted U-shaped curves were obtained as the SP of low-dot surfaces was increased (1.3-6.2 mm, tetragonal arrays); a monotonic increase was observed for high-dot surfaces. We hypothesized that roughness is not a single sensory continuum across the tested SPs of low-dot surfaces, predicting that roughness discrimination would show deviations from the invariant relation between threshold (ΔS) and the value of the standard (S) surface (Weber fraction, ΔS/S) expected for a single continuum. The results showed that Weber fractions were increased for SPs on the descending limb of the inverted U-shaped curve. There was also an increase in the Weber fraction for high-dot surfaces but only at the peak (3 mm), corresponding to the SP at which the slope of the psychometric function showed a modest decline. Together the results indicate that tactile roughness is not a continuum across low-dot SPs of 1.3-6.2 mm. These findings suggest that correlating the inverted U-shaped function with neuronal codes is of questionable validity. A simple intensive code may well contribute to tactile roughness.

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.

Modulation of the response to a somatosensory stimulation of the hand during the observation of manual actions.

Observation of hand movements has been repeatedly demonstrated to increase the excitability of the motor cortical representation of the hand. Little attention, however, has been devoted to its effect on somatosensory processing. Movement execution is well known to decrease somatosensory cortical excitability, a phenomenon termed 'gating'. As executed and observed actions share common cortical representations, we hypothesized that action observation (hand movements) should also modulate the cortical response to sensory stimulation of the hand. Seventeen healthy subjects participated in these experiments in which electroencephalographic (EEG) recordings of the somatosensory steady-state response (SSSR) were obtained. The SSSR provides a continuous measure of somatosensory processing. Recordings were made during a baseline condition and five observation conditions in which videos showed either a: (1) hand action; (2) passive stimulation of a hand; (3) static hand; (4) foot action; or (5) static object. The method employed consisted of applying a continuous 25 Hz vibratory stimulation to the index finger during the six conditions and measuring potential gating effects in the SSSR within the 25 Hz band (corresponding to the stimulation frequency). A significant effect of condition was found over the contralateral parietal cortex. Observation of hand actions resulted in a significant gating effect when compared to baseline (average gating of 22%). Observation of passive touch of the hand also gated the response (17% decrease). In conclusion, the results show that viewing a hand performing an action or being touched interferes with the processing of somatosensory information arising from the hand.

Tactile suppression of displacement.

In vision, the discovery of the phenomenon of saccadic suppression of displacement has made important contributions to the understanding of the stable world problem. Here, we report a similar phenomenon in the tactile modality. When scanning a single Braille dot with two fingers of the same hand, participants were asked to decide whether the dot was stationary or whether it was displaced from one location to another. The stimulus was produced by refreshable Braille devices that have dots that can be swiftly raised and recessed. In some conditions, the dot was stationary. In others, a displacement was created by monitoring the participant's finger position and by switching the dot activation when it was not touched by either finger. The dot displacement was of either 2.5 mm or 5 mm. We found that in certain cases, displaced dots were felt to be stationary. If the displacement was orthogonal to the finger movements, tactile suppression occurred effectively when it was of 2.5 mm, but when the displacement was of 5 mm, the participants easily detected it. If the displacement was medial-lateral, the suppression effect occurred as well, but less often when the apparent movement of the dot opposed the movement of the finger. In such cases, the stimulus appeared sooner than when the brain could predict it from finger movement, supporting a predictive rather than a postdictive differential processing hypothesis.

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.

Tactile perception of roughness: raised-dot spacing, density and disposition.

Recently, we showed that tactile speed estimates are modified by the spatial parameters of moving raised-dot surfaces, specifically dot spacing but not dot disposition (regular, irregular) or density. The purpose of this study was to determine the extent to which tactile roughness perception resembles tactile speed with respect to its dependence and/or independence of the spatial properties of raised-dot surfaces. Subjects scaled the roughness of surfaces displaced under the finger. Dot spacing (centre-to-centre) ranged from 1.5 to 8.5 mm in the direction of the scan (longitudinal). Mean dot density varied from 2.2 to 46.2 dots/cm2. Dot disposition was varied: repeating rows (periodic) or quasi-random (non-periodic). In the first experiment (n = 8), the periodic and non-periodic surfaces were matched for mean dot density. Roughness showed a monotonic increase with 1/dot density, but non-periodic surfaces were judged to be smoother than the periodic surfaces. Subjective equality was obtained when the data were re-expressed relative to longitudinal SP. In the second experiment (n = 7), the periodic and non-periodic surfaces were matched for longitudinal dot spacing. Perceptual equivalence was observed when the results were plotted relative to dot spacing, but not 1/dot density. Dot spacing in the orthogonal direction (transverse) was excluded as a contributing factor. Thus, as found for tactile speed scaling, roughness is critically dependent on longitudinal dot spacing, but independent of dot disposition and dot density (over much of the tested range). These results provide a set of predictions to identify cortical neurones that play critical roles in roughness appreciation.

Instructed delay discharge in primary and secondary somatosensory cortex within the context of a selective attention task.

The neuronal mechanisms that contribute to tactile perception were studied using single-unit recordings from the cutaneous hand representation of primate primary (S1) and secondary (S2) somatosensory cortex. This study followed up on our recent observation that S1 and S2 neurons developed a sustained change in discharge during the instruction period of a directed-attention task. We determined the extent to which the symbolic light cues, which signaled the modality (tactile, visual) to attend and discriminate, elicited changes in discharge rate during the instructed delay (ID) period of the attention task and the functional importance of this discharge. ID responses, consisting of a sustained increase or decrease in discharge during the 2-s instruction period, were present in about 40% of the neurons in S1 and S2. ID responses in both cortical regions were very similar in most respects (frequency, sign, latency, amplitude), suggesting a common source. A major difference, however, was related to attentional modulation during the ID period: attentional influences were almost entirely restricted to S2 and these effects were always superimposed on the ID response (additive effect). These findings suggest that the underlying mechanisms for ID discharge and attention are independent. ID discharge significantly modified the initial response to the standard stimuli (competing texture and visual stimuli), usually enhancing responsiveness. We also showed that tactile detection in humans is enhanced during the ID period. Together, the results suggest that ID discharge represents a priming mechanism that prepares cortical areas to receive and process sensory inputs.

Tactile acuity in the blind: a psychophysical study using a two-dimensional angle discrimination task.

Growing evidence suggests that blind subjects outperform the sighted on certain tactile discrimination tasks depending on cutaneous inputs. The purpose of this study was to compare the performance of blind (n = 14) and sighted (n = 15) subjects in a haptic angle discrimination task, depending on both cutaneous and proprioceptive feedback. Subjects actively scanned their right index finger over pairs of two-dimensional (2-D) angles (standard 90 degrees ; comparison 91-103 degrees ), identifying the larger one. Two exploratory strategies were tested: arm straight or arm flexed at the elbow so that joint movement was, respectively, mainly proximal (shoulder) or distal (wrist, finger). The mean discrimination thresholds for the sighted subjects (vision occluded) were similar for both exploratory strategies (5.7 and 5.8 degrees , respectively). Exploratory strategy likewise did not modify threshold in the blind subjects (proximal 4.3 degrees ; distal 4.9 degrees ), but thresholds were on average lower than for the sighted subjects. A between-group comparison indicated that blind subjects had significantly lower thresholds than did the sighted subjects, but only for the proximal condition. The superior performance of the blind subjects likely represents heightened sensitivity to haptic inputs in response to visual deprivation, which, in these subjects, occurred prior to 14 years of age.

Tactile speed scaling: contributions of time and space.

A major challenge for the brain is to extract precise information about the attributes of tactile stimuli from signals that co-vary with multiple parameters, e.g., speed and texture in the case of scanning movements. We determined the ability of humans to estimate the tangential speed of surfaces moved under the stationary fingertip and the extent to which the physical characteristics of the surfaces modify speed perception. Scanning speed ranged from 33 to 110 mm/s (duration of motion constant). Subjects could scale tactile scanning speed, but surface structure was essential because the subjects were poor at scaling the speed of a moving smooth surface. For textured surfaces, subjective magnitude estimates increased linearly across the range of speeds tested. The spatial characteristics of the surfaces influenced speed perception, with the roughest surface (8 mm spatial period, SP) being perceived as moving 15% slower than the smoother, textured surfaces (2-3 mm SP). Neither dot disposition (periodic, non periodic) nor dot density contributed to the results, suggesting that the critical factor was dot spacing in the direction of the scan. A single monotonic relation between subjective speed and temporal frequency (speed/SP) was obtained when the ratings were normalized for SP. This provides clear predictions for identifying those cortical neurons that play a critical role in tactile motion perception and the underlying neuronal code. Finally, the results were consistent with observations in the visual system (decreased subjective speed with a decrease in spatial frequency, 1/SP), suggesting that stimulus motion is processed similarly in both sensory systems.

Haptic discrimination of two-dimensional angles: influence of exploratory strategy.

The aim of this study was to define the relative contribution of self-generated cutaneous and proprioceptive feedback to haptic shape discrimination by systematically constraining the exploratory strategy. Subjects (n = 23) explored pairs of two-dimensional (2-D) angles (standard angle, 90 degrees; comparison angles, 91 degrees -103 degrees) placed at arm's length from the subject, and identified the larger angle of each pair. The exploratory strategies included a reference condition, dynamic scan of the index finger over the entire object [combined cutaneous and proprioceptive (shoulder) feedback], and modified conditions, static touch of the intersection of the two bars that formed the angle using the index finger (cutaneous feedback) and dynamic scans of the object using a hand-held tool (proprioceptive feedback, shoulder). Discrimination thresholds (75% correct) were very similar for dynamic and static touch with the index finger. Thresholds varied as a function of the static contact duration (<1 s, 7.2 degrees +/- 0.6 degrees; approximately 3 s, 4.2 degrees +/- 0.5 degrees), but were not different from the reference condition (6.0 degrees +/- 0.9 degrees). The higher threshold with short static touch likely reflects movement-related gating of self-generated tactile inputs. Together, the results suggested that cutaneous feedback alone may be sufficient to explain 2-D angle discrimination, because the added proprioceptive feedback did not improve performance. Also, threshold did not vary with the number of dynamic scans (one or two), suggesting that the critical information was gathered on the first pass over the angle. In contrast, when the angles were explored with the tool, the threshold increased relative to the corresponding reference condition from the same session (tool, 9.6 degrees +/- 0.9 degrees; dynamic scan with the finger, 6.2 degrees +/- 1.0 degrees). Thus, performance was poorer with proprioceptive feedback alone, suggesting that cutaneous feedback was relatively more important for 2-D haptic angle discrimination in the present experiment.

Differential controls over tactile detection in humans by motor commands and peripheral reafference.

The purpose of this study was to determine the extent to which motor commands and peripheral reafference differentially control the detection of near-threshold, tactile stimuli. Detection of weak electrical stimuli applied to the index finger (D2) was evaluated with two bias-free measures of sensory detection, the index of detectability (d') and the proportion of stimuli detected. Stimuli were presented at different delays prior to and during two motor tasks, D2 abduction, and elbow extension; both tasks were tested in two modes, active and passive. For both active tasks, the peak decrease in tactile suppression occurred at the onset of electromyographic activity. The time course for the suppression of detection during active and passive D2 abduction was identical, and preceded the onset of movement (respectively, -35 and -47 ms). These results suggest that movement reafference alone, acting through a mechanism of backward masking, could explain the modulation seen with D2 movement. In contrast, tactile suppression was significantly earlier for active elbow movements (-59 ms) as compared with passive (-21 ms), an observation consistent with both the motor command and peripheral reafference contributing to the suppression of detection of stimuli applied to D2 during movements about a proximal joint. A role for the motor command in tactile gating during distal movements cannot be discounted, however, because differences in the strength and distribution of the peripheral reafference may also have contributed to the proximo-distal differences in the timing of the suppression.

Independent controls of attentional influences in primary and secondary somatosensory cortex.

The neuronal mechanisms underlying enhanced perception of tactile stimuli with directed attention were investigated using single-unit recordings from primary (S1, n = 53) and secondary (S2, n = 50) somatosensory cortex in macaque monkeys. Neuronal responses to textures scanned under the digit tips (spatial periods, SP, of 2, 3.7 or 4.7 mm) were recorded while attention was directed either to discriminating a change in texture or to the reward and also in a neutral no-task condition. Cell discharge was quantified in three periods of the trials: salient Delta texture (directed attention), postreward, and static (both cases, attention directed to the reward). S1 texture- and non-texture-sensitive cells, as well as S2 non-texture-sensitive cells, showed a modest enhancement of discharge during the salient Delta texture period (approximately 25%) but no change in response gain, consistent with an additive increase in neuronal responsiveness with directed attention. In contrast, S2 texture-related cells showed a larger enhancement with directed attention to salient inputs (82%) and increased response gain, suggesting that directed attention produces a multiplicative increase in S2 responsiveness. During the postreward period, and also in no-task testing, S1 texture-sensitive cells preserved their sensitivity to SP. In contrast, S2 texture-, but not non-texture-, sensitive cells showed a marked suppression of discharge and decreased gain after the discrimination response. Together, the results support the notion that S2 discharge reflects stimulus parameters in relation to ongoing behavioral demands. The results also support the existence of two independent attentional mechanisms in somatosensory cortex, one generalized (S1 and S2), and the other focused on S2 texture-related cells.