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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.

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.

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., (Copyright © 2016 the American Physiological Society.)

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., (Copyright © 2016 the American Physiological Society.)

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.

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.

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.

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.

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.

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.

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.

The role of attention in modulating tactile sensitivity in primary (SI) and secondary somatosensory cortex (SII) was addressed using a cross-modal manipulation of attention, somatosensory versus visual. Two adult monkeys (Macaca mulatta) were trained to perform two tasks: tactile discrimination of a change in the texture of a surface presented to digits 3 and 4 and visual discrimination of a change in the intensity of a light. In each trial, standard texture (2 mm spatial period, SP) and visual stimuli were presented. These were followed by an increase in SP and/or luminance. Each trial was preceded by an instruction cue (colored light) that directed the animal to attend and respond to the change in one modality while ignoring any change in the other modality. The two tasks were interleaved during the recording, on a trial-by-trial basis. Extracellular recordings were made from 178 neurons (SI, 102; SII, 76), all with a cutaneous receptive field on the stimulated digit tips. Discharge was quantified in both tasks during the instruction, the standard-stimuli, and the texture-change periods. The results showed that selective attention to tactile stimuli had qualitatively and quantitatively greater and earlier effects in SII than SI. Twenty-four of 102 SI cells showed a significant change in discharge with the direction of attention. For almost all cells (20/24), discharge was enhanced when attention was directed toward the tactile stimuli; the effects were most frequent in the analysis interval that encompassed the change in SP (16/24). A significantly higher proportion of SII cells were attention-sensitive (47/76). The effects were concentrated in the texture-change period (39/47) but also included earlier periods in the trial (instruction period, n = 15; standard-stimuli period, n = 32). Attention-related modulation that spanned all three intervals (n = 11) likely reflected baseline changes in discharge. For the texture-sensitive cells (43 in SI, 37 in SII), the mean change in discharge frequency (post texture change - pre-texture change) in each task was significantly increased in SII but not SI with selective attention. The results are consistent with a two-stage modulation of parietal cortical discharge, an initial stage (SI) in which there is some enhancement of sensory responses to the salient feature, the texture change, and a second stage (SII) in which baseline changes occur, along with further feature selection. These controls may be independently exerted on SI and SII, or they may reflect top-down controls from SII to SI.

This study investigated the relative importance of central and peripheral signals for movement-related gating by comparing the time course and magnitude of movement-related decreases in tactile detection during a reference motor task, active isotonic digit 2 (D2) abduction, with that seen during three test tasks: a comparison with active isometric D2 abduction (movement vs. no movement) evaluated the contribution of peripheral reafference generated by the movement to gating; a comparison with passive D2 abduction (motor command vs. no motor command; movement generated by an external agent) allowed us to evaluate the contribution of the central motor command to tactile gating; and finally, the inclusion of an active "no apparatus," or freehand, D2 abduction task allowed us to evaluate the potential contribution of incidental peripheral reafference generated by the position detecting apparatus to the results (apparatus vs. no apparatus). Weak electrical stimuli (2-ms pulse; intensity, 90% detected at rest) were applied to D2 at different delays before and after movement onset or electromyographic (EMG) activity onset. Significant time-dependent movement-related decreases in detection were obtained with all tasks. When the results obtained during the active isotonic movement task were compared with those obtained in the three test tasks, no significant differences in the functions describing detection performance over time were seen. The results obtained with the isometric D2 abduction task show that actual movement of a body part is not necessary to diminish detection of tactile stimuli in a manner similar to the decrease produced by isotonic, active movement. In the passive test task, the peak decrease in detection clearly preceded the onset of passive movement (by 38 ms) despite the lack of a motor command and, presumably, no movement-related peripheral reafference. A slightly but not significantly earlier decrease was obtained with active movement (49 ms before movement onset). Expectation of movement likely did not contribute to the results because stimulus detection during sham passive movement trials (subjects expected but did not receive a passive movement) was not different from performance at rest (no movement). The results obtained with passive movement are best explained by invoking backward masking of the test stimuli by movement-related reafference and demonstrate that movement-related reafference is sufficient to produce decreases in detection with a time course and amplitude not significantly different from that produced by active movement.

This study examined the effect of systematically varying stimulus intensity on the time course and magnitude of movement-related gating of tactile detection and scaling in 17 human subjects trained to perform a rapid abduction of the right index finger (D2) in response to a visual cue. Electrical stimulation was delivered to D2 at five different intensities. At the lowest intensity, approximately 90% of stimuli were detected at rest (1 x P(90)); four multiples of this intensity were also tested (1.25, 1.5, 1.75, and 2. 0 x P(90)). At all intensities of stimulation, detection of stimuli applied to the moving digit was diminished significantly and in a time-dependent manner, with peak decreases occurring within +/-12 ms of the onset of electromyographic activity in the first dorsal interosseous (25-45 ms before movement onset). Reductions in the proportion of stimuli detected were greatest at the lowest stimulus intensity and progressively smaller at higher intensities. No shift in the timing of the decreases in performance was seen with increasing intensity. Once the weakest intensity at which most stimuli were perceived during movement had been established (2 x P(90)), magnitude estimation experiments were performed using two stimulus intensities, 2 x P(90) (5 subjects) and 3 x P(90) (3 subjects). Significant movement-related decreases in estimated stimulus magnitude were observed at both intensities, the time course of which was similar to the time course of reductions in detection performance. As stimulus intensity increased, the magnitude of the movement-related decrease in scaling diminished. A model of detection performance that accurately described the effect of stimulus intensity and timing on movement-related reductions in detection was created. This model was then combined with a previous model that described the effects of stimulus localization and timing to predict detection performance at a given stimulation site, intensity, and time during movement. Movement-related gating of tactile perception represents the end result of movement-related effects on the transmission and subsequent processing of the stimulus. The combined model clearly defines many of the requirements that proposed physiological mechanisms of movement-related gating will have to fulfill.

The time course and spatial extent of movement-related suppression of the detection of weak electrical stimuli (intensity, 90% detected at rest) was determined in 118 experiments carried out in 47 human subjects. Subjects were trained to perform a rapid abduction of the right index finger (D2) in response to a visual cue. Stimulus timing was calculated relative to the onset of movement and the onset of electromyographic (EMG) activity. Electrical stimulation was delivered to 10 different sites on the body, including sites on the limb performing the movement (D2, D5, hand, forearm and arm) as well as several distant sites (contralateral arm, ipsilateral leg). Detection of stimuli applied to the moving digit diminished significantly and in a time-dependent manner, with the first significant decrease occurring 120 ms before movement onset and 70 ms before the onset of EMG activity. Movement-related and time-dependent effects were obtained at all stimulation sites on the homolateral arm as well as the adjacent trunk. A pronounced spatiotemporal gradient was observed: the magnitude of the movement-related decrease in detectability was greatest and earliest at sites closest to the moving finger and progressively weaker and later at more proximal sites. When stimuli were applied to the distant sites, only a small (approximately 10%), non-time-dependent decrease was observed during movement trials. A simple model of perceptual performance adequately described the results, providing insight into the distribution of movement-related inhibitory controls within the CNS.

Two rhesus monkeys were trained to discriminate, with the use of passive touch, a standard surface [rectangular arrays of raised dots with a spatial period (SP) of 2 mm across the rows and columns] from three modified surfaces in which the SP between rows was increased to 3, 4, or 5 mm over the second half of the surface. After the surface presentation (to digit tips 3 and 4 of one hand) the monkeys indicated the presence or absence of a change in texture by pulling or pushing a lever, respectively, with the opposite hand. Of 193 neurons recorded from primary somatosensory cortex (SI, 3 hemispheres) and 94 neurons from secondary somatosensory cortex (SII, 1 hemisphere), all contralateral to the stimulated hand, the discharge of 51 SI and 19 SII neurons was classified as texture related. Two types of texture-related responses were obtained. Graded neurons showed a linear relationship between mean discharge frequency and SP; nongraded neurons showed a significant change in discharge over the modified half of the surfaces but the discharge did not distinguish between the three modified surfaces. The distribution of these texture responses was significantly different in SI and SII: whereas most of the texture-related neurons in SI (44 of 51, 86%) were graded, the majority of those in SII (12 of 19, 63%) were nongraded. The results were interpreted as suggesting that the nongraded responses reflect feature extraction in SII, signaling the presence of a change in texture but not its magnitude, and so support the notion that texture signals are processed sequentially, first in SI and then in SII.

1. The discharge patterns of 359 single neurons in the hand representation of primary somatosensory cortex (SI) of two monkeys (Macaca mulatta) were recorded during the performance of a passive texture discrimination task with the contralateral hand (104 in area 3b, 149 in area 1, and 106 in area 2). Three nyloprint surfaces were mounted on a drum that was rotated under the digit tips. One surface was entirely smooth, whereas the other two were smooth over the first half and rough over the second half (smooth/ rough) (raised dots, 1 mm high and 1 mm diam, in a rectangular array; spatial period of 3 mm across the rows and columns for most recordings; 9 mm between columns for selected recordings). The monkeys were trained to distinguish between the smooth and smooth/rough surfaces. After the surface presentation, the monkey indicated the texture of the second half of the surface by pushing or pulling, respectively, on a lever with the other arm. For most recordings an average tangential speed of 49 mm/s was tested. For selected recordings motor speed was incremented (63, 75, or 89 mm/s). 2. Two hundred eighty-three neurons had a cutaneous receptive field (RF) on the hand (96 in area 3b, 120 in area 1, and 67 in area 2). Thirty-five neurons had a deep RF (4 in area 3b, 15 in area 1, and 16 in area 2). Seven neurons had mixed cutaneous and deep RFs (4 in area 1, 3 in area 2). Thirty-four neurons had no identifiable RF (4 in area 3b, 10 in area 1, and 20 in area 2). 3. The discharge of 185 of 359 neurons was significantly modulated during the presentation of one or both surfaces compared with the discharge at rest. Cells with a cutaneous RF that included part or all of the distal phalangeal pads of the digits used in the task (usually digits III and IV) were more likely to be modulated during surface presentation (132 of 179, 74%) than those with a cutaneous RF not in contact with the surfaces (24 of 104, 23%). The remaining neurons (mixed, deep, or no RF) were also infrequently modulated (29 of 76, 38%). 4. Of the 185 modulated units, 118 cells were classified as texture related because there was a significant difference in the discharge rate evoked by the smooth/rough and smooth surfaces. Cells with a cutaneous RF that included the digital pads in contact with the surfaces were frequently texture related (100 of 132, 76%). Texture sensitivity was less frequently observed in the remaining modulated neurons (18 of 53, 34%: cutaneous RF not in contact with the surfaces, deep RF, mixed cutaneous and deep RF, no identifiable RF). 5. Texture-related neurons were found in areas 3b, 1, and 2. Two patterns of texture-related responses were observed in the 100 cutaneous units with an RF in contact with the surfaces. Thirty-one units were classified as showing a phasic response at the time the digits encountered the leading edge of the rough half of the surface. Fifty-eight cells were classified as phasic-tonic (or sometimes tonic at the slowest motor speeds) because the response lasted for the duration of the presentation of the rough portion of the surface. The remaining 11 neurons could not be readily classified into one or the other category and, indeed, generally showed clear texture-related responses only at higher motor speeds (> 49 mm/s, 9 of 11). 6. Speed sensitivity was systematically evaluated in 41 of 100 texture-related units with a cutaneous RF in contact with the surfaces. The discharge of 66% of the units (27 of 41) varied significantly with the speed of surface presentation, with discharge increasing at higher speeds. Speed sensitivity was found in all three cytoarchitectonic areas (6 of 6 cells in area 3b, 11 of 22 in area 1, and 10 of 13 in area 2). 7. Contact force was also systematically monitored in these experiments (69 of 100 texture-related cells with a cutaneous RF in contact with the surfaces). Linear regression analyses indicated than 22% (15 of 69) of the texture-related units were sensitive to contact force (13

Single-unit recordings were obtained from 404 neurons in the dentate and interposed nuclei in two monkeys trained to perform simple movements of the elbow in response to three different sensory cues: a light, a tone, and a small, brief perturbation of the trained forearm. Both flexion and extension movements were investigated. Those dentate neurons that showed a clear modulation before the onset of movement (149 of 318 cells recorded) were classified as stimulus related or movement related on the basis of an analysis of the timing of the initial change in discharge. Seventy-one percent of these dentate neurons (106/149) were classified as stimulus related, and 21% (31/149) were classified as movement related. Within the stimulus-related group 87% responded selectively to only one or two of the sensory cues (selective stimulus-related neurons), most often the teleceptive cues, whereas the remaining 13% responded nonselectively to all three cues (nonselective stimulus-related neurons). Interposed neurons, in contrast, showed principally movement-related discharge, and this represented the initial change in discharge in 89% of the neurons. Eleven percent of the interposed cells showed a selective response to the somesthetic cue. The discharge of 28 out of 91 dentate neurons tested with both flexion and extension movements varied with the direction of movement. Few dentate neurons (9%) were found to display any direction sensitivity when considering the discharge preceding the onset of movement, and none of these showed a reciprocal pattern. The discharge of a greater proportion of neurons (24%) was direction sensitive during movement and was occasionally reciprocal. In the same monkeys, however, 78% of the neurons in the neighboring interposed nucleus were direction sensitive, and one-quarter of these displayed reciprocal patterns of discharge. Thus, the discharge of dentate neurons, occurring well in advance of a conditioned movement, cannot specify direction in this simple reaction-time (RT) task. The sensory responses of selective stimulus-related dentate cells ended near the onset of movement but were time locked to the stimulus and not to the movement. When a neuron was responsive to two of the cues the response did not vary with the modality of the stimulus apart from changes in the latency. The initial sensory response was usually followed by later "secondary" changes in discharge that were temporally related to the movement.(ABSTRACT TRUNCATED AT 250 WORDS)