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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.

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.)

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.

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.

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.

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.

This study investigates how a change in the physical relation between objects (two-dimensional, 2-D, angles) and a subject, as well as scanning conditions, modify the ability to discriminate small changes in 2-D shape. Subjects scanned pairs of angles (90 masculine standard; 91 masculine-103 masculine comparison angles) with the right index finger of the out-stretched arm, identifying the larger of each pair. When joint rotation was restricted to the shoulder, the discrimination threshold significantly increased when the angles were explored with the shoulder in a more eccentric position rather than closer to the midline (60 masculine versus 30 masculine to the right). This result was attributed to changes in proprioceptive sensitivity, since explorations restricted to distal joints (wrist/second metacarpophalangeal joint) showed no change with shoulder position. The results showed, moreover, that discrimination threshold was similar for distal and proximal joints when the delay between scanning the pairs of angles was long (15 s). This observation suggests that regional variations in proprioceptive acuity (proximal>distal) may reflect an adaptation to generate an invariant central representation of haptic shape. Using a shorter interscan delay (5 s), a position-dependent increase in discrimination threshold was revealed for distal explorations, an effect that disappeared when the head was turned in the direction of the unseen angle (vision occluded). We suggest that these results can be explained by the existence of two competing egocentric frames of reference with different time courses, one of short duration that is centred on the arm/hand, and a second of longer duration centred on the head. At the short delay, the reference frames interacted to distort the haptic representation when they were misaligned. This distortion was resolved at the long delay, possibly through suppression of the arm/hand-centred reference frame.

The human ability to recognize objects on the basis of their shape, as defined by active exploratory movements, is dependent on sensory feedback from mechanoreceptors located both in the skin and in deep structures ( haptic feedback). Surprisingly, we have little information about the mechanisms for integrating these different signals into a single sensory percept. With the eventual aim of studying the underlying central neural mechanisms, we developed a shape discrimination test that required active exploration of objects, but was restricted to one component of shape, two-dimensional (2D) angles. The angles were machined from 1-cm-thick Plexiglas, and consisted of two 8-cm-long arms that met to form an angle of 90 degrees (standard) or 91 degrees to 103 degrees (comparison angles). Subjects scanned pairs of angles with the index finger of the outstretched arm and identified the larger angle of each pair explored. Discrimination threshold (75% correct) was 4.7 degrees (range 0.7 degrees to 12.1 degrees), giving a precision of 5.2% (0.8-13.4%: difference/standard). Repeated blocks of trials, either in the same session or on different days, had no effect on discrimination threshold. In contrast, the motor strategy was partly modified: scanning speed increased but dwell-time at the intersection did not change. Finally, 2D angle discrimination was not significantly modified by rotating the orientation of one of the angles in the pair (0 degrees, 4 degrees or 8 degrees rotation towards the midline, in the vertical plane), providing evidence that subjects evaluated each angle independently in each trial. Subject reports indicated that they relied on cutaneous feedback from the exploring digit (amount of compression of the finger at the angle) and mental images of the angles, most likely arising from proprioceptive information (from the shoulder) generated during the to-and-fro scans over the angle. In terms of shoulder angles, the mean discrimination threshold here was 0.54 degrees (range 0.08 degrees to 1.36 degrees). These values are lower than previous estimates of position sense at the shoulder. In light of the subjects' strategies, it therefore seems likely that both cutaneous and proprioceptive (including both dynamic and static position-related signals) feedback contributed to the haptic discrimination of 2D angles.

Using two-dimensional (2D) angles composed of two straight, 8-cm-long arms that formed an angle, we investigated the importance of cutaneous feedback from the exploring index finger, and proprioceptive feedback from the shoulder (scanning movements made with the outstretched arm), to the human ability to discriminate small differences in the angles. Using a two-alternative forced-choice paradigm, subjects identified the larger angle in each pair explored (standard angle, 90 degrees; comparison angles, 91 degrees to 103 degrees). Subjects were tested under four experimental conditions: (1) active touch (reference condition); (2) active touch with digital anaesthesia; (3) passive touch (a computer-controlled device displaced the angle under the subject's immobile digit); and (4) passive touch with digital anaesthesia. When only proprioceptive feedback from the shoulder was available (condition 2), there was a significant increase in discrimination threshold, from 4.0 degrees in the reference condition (condition 1) to 7.2 degrees, indicating that cutaneous feedback from the exploring digit contributed to task performance. When only cutaneous feedback from the finger was available (condition 3), there was also a significant increase in threshold from 4.2 degrees in the active condition to 8.7 degrees. This suggested that proprioceptive feedback from the shoulder, potentially from a variety of deep (muscle and joint) but also cutaneous receptors, contributed to the ability to discriminate small changes in 2D angles. When both sources of feedback were eliminated (condition 4), subjects were unable to discriminate even the largest difference presented (13 degrees). The results suggest that this sensory task is truly an integrative task drawing on sensory information from two different submodalities and so, following the definition of Gibson, is haptic in nature. The results are discussed in relation to the potential neural mechanisms that might underlie a task that requires integration across two anatomically separate body parts and two distinct modalities.

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

The discharge patterns of 144 single cortical neurones, within the cutaneous representation of the hand in area 2 (primary somatosensory cortex, SI), were studied in two rhesus monkeys during the performance of an active tactile discrimination task. These were compared to those previously described for units within areas 3b and 1 recorded from the same animals. The task consisted of making a single scanning movement of the digit tips over a surface (first half smooth; second half either smooth or rough). The nature of the texture encountered over the second half of the surface was indicated by the monkey making a differential lever response (push or pull) with the opposite hand. During the task, area 2 units with cutaneous receptive fields (RFs) on the digit tips of interest (those scanned over the surfaces) generally showed an increase in their discharge (75%); patterns of decreased discharge or no modulation (respectively, 12 and 13%) were rarely observed. Units with digital cutaneous RFs not in contact with the stimuli were much more likely to show either a pattern of decreased discharge or no modulation whatsoever (47% in each case), suggesting that there is some selection of cutaneous inputs in this task in that non-active inputs are selectively gated. For units with a cutaneous RF, the sign of modulation changed significantly across SI, in a manner consistent with a pattern of increased convergence onto the more caudal regions of SI. Overall, the proportions of area 2 units with digital RFs on the tips of interest that were classified as either texture-related (25%) or movement-related (26%) were similar to those reported previously for areas 3b and 1, suggesting that their presumed roles in, respectively, the analysis of surface texture and the representation of the physical parameters of movement are shared and distributed across the three cytoarchitectonic subdivisions of SI under consideration. In addition, the discharge patterns of single texture-related cells in areas 3b, 1 and 2 did not reliably signal whether or not the animal successfully discriminated the surfaces, suggesting that information from a population of cells is required for the performance of the task. Texture-related responses in area 2 were, however, unique in two ways. Firstly, 35% of the texture-related units had additional discharges related to the performance of the scanning movement (texture- and movement-related cells); no such units were found in area 3b, and only one was encountered in area 1.(ABSTRACT TRUNCATED AT 400 WORDS)

The present experiments were designed to characterize the discharge patterns of single cortical neurons within the cutaneous representation of the hand in postcentral cortex (areas 3b and 1) in awake monkeys during the performance of an active tactile discrimination task. The task consisted of making a single scanning movement over a surface (first half smooth; second half smooth or rough); the texture encountered over the second half of the surface was indicated by the animal, respectively, pushing or pulling a lever. Unitary discharge was recorded from 118 cells receiving input from the hand or distal forearm of two monkeys. Units with cutaneous fields on the digit tips in contact with the surfaces to be discriminated showed an increase in discharge (58%), a decrease in discharge (11%) or no change (31%) during the task. Units with cutaneous fields not in contact with the discriminanda were much more likely to show decreased discharge during the task (25%), suggesting that there is some selection of cutaneous inputs in this task. Cutaneous units in areas 3b and 1 were equally likely to signal differences in texture (respectively, 18% and 26% of those with digital receptive fields (RFs] and most of the texture-related units (78%) had a large RF, spanning several digits. The discharge patterns of single texture-related cells did not reliably signal whether or not the animal successfully discriminated the surfaces: unitary responses were occasionally absent even though the animal correctly identified the surface or they were present when an incorrect response was made. This observation suggested that information derived from a population of cells is required for the performance of the task, since no single cell's discharge contained sufficient information upon which the animal could base its behavioural response. A group of cells with digital RFs (24% of area 3b cells and 15% of area 1 units) were classified as movement-related. Their discharge signalled precisely the onset and/or end of movement, and they were generally insensitive to the texture of the surfaces scanned. Such cells may serve as an independent source of information for primary somatosensory cortex related to the physical parameters of movement. Most cells with digital RFs were more responsive during active tactile discrimination than during passive movement of the digits over the surfaces (monkey no longer required to discriminate the surface texture). For area 3b units, peripheral factors (RF orientation, speed of movement) were likely responsible for this observation.(ABSTRACT TRUNCATED AT 400 WORDS)