Search

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.

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.

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.

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.

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.

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.

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.

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

The present experiments were designed to determine the ability of humans to estimate the roughness of raised dot surfaces using passive touch (subject immobile, surfaces scanned proximo-distally under the tip of the middle digit). In two initial experiments, the spatial period in the direction of the scan (longitudinal SP) was varied from 1.5 to 5 mm, while the transverse SP was kept constant at 2 mm. These surfaces were identical to those used in single unit recording experiments in primates which have shown that neurones in primary somatosensory cortex (SI) show graded changes in discharge over the same range of SPs. In the third and fourth experiments, roughness perception was studied over an extended range of longitudinal SPs (1.5-8.5 mm); transverse SP was either 1 or 2 mm. We also examined the effects on roughness perception of presenting the surfaces at two different scanning speeds (approximately 50 and approximately 95 mm/s), similar to those employed in the unit recordings. Magnitude estimates of perceived roughness increased in a nearly linear fashion as longitudinal SP was increased up to either 5 mm (experiments 1 and 2) or 8.5 mm (experiments 3 and 4). A modest increase in the transverse SP elicited a small increase in roughness estimates, with preservation of the linear relation between roughness and longitudinal SP. The roughness sensation evoked by each surface was not affected by the change of the temporal pattern of scanning (Low and High speeds). These results showed that there is perceptual constancy for roughness across a twofold change in passive scanning speed, and that roughness is a function of the spatial characteristics of these raised dot surfaces. While a spatial code may underlie this observation, it is suggested that a simpler intensive code may be sufficient to explain the psychophysical observations. Based on the results of neurophysiological recordings in SI cortex, it is suggested that an invariant central representation of surface roughness could be extracted from the ambiguous peripheral signals that covary with roughness and the stimulating conditions (e.g. speed) by means of a simple subtraction process.

Previous studies have shown that voluntary motor activity decreases the ability to detect near-threshold electrical stimuli applied to the skin, but has no effect on the perception of either suprathreshold electrical stimuli or natural thermal stimuli (warmth, heat pain). The present study was undertaken to determine if the perception of natural tactile stimuli (vibrotactile) is diminished by motor activity (rhythmical isometric flexions and extensions about the elbow). The stimuli were applied at three different sites on the operant arm--ventral forearm, thenar eminence and distal digit--to examine also the influence of the proximity of the stimulated site to the active muscles on perception. The ability to detect near-threshold stimuli at the two more proximal stimulation sites was significantly reduced during the motor task, and these effects were more pronounced and widespread with higher levels of target force (20 N vs 50 N). Discrimination of small differences in the intensity of suprathreshold stimuli, at all three sites, was unchanged during the motor task. Finally, the subjective intensity of suprathreshold vibrotactile stimuli was reduced, in a nonlinear fashion, during the motor task; proximity again influenced the degree of modulation. In contrast a previous study showed no change in magnitude estimates of suprathreshold electrical stimuli during isotonic flexion and extension. Some possible reasons for the discrepancy are discussed. In addition, our previous suggestion that movement produces a simple reduction in the signal-to-noise ratio (i.e. the gating signal modelled as a masking stimulus) cannot explain the present results, so more complex models are required.

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 investigate the neuronal mechanisms, at the level of the primary somatosensory cortex, which underlie the observation that somatosensory cortical potentials evoked by air puff stimuli directed at the forearm are decreased, in a nonspecific and widespread manner, during voluntary movements about the elbow. Unitary discharge was recorded from 131 cells receiving cutaneous input from the hairy skin of the forearm or hand (areas 3b and 1) of two monkeys trained to perform rapid movements of the contralateral arm (elbow flexion or extension). Evoked unitary responses to air puff stimuli applied to the centre of the cell's receptive field, at various delays before and after the onset of movement, were recorded. Movement produced a significant decrease in the short latency excitatory response to the air puff in 89% of the cells (117/131); the remaining 11% were not modulated by movement. This movement-related "gating" of cutaneous inputs occurred regardless of the response pattern of the cells to movement alone, being observed in 91% of the cells with no movement-related discharge, and 89% of those with movement-related discharge. The air puff responses of cells with inputs from the forearm and the dorsum of the hand were all similarly modulated by movement and the modulation was clearly present prior to the onset of movement (mean onset, -66 ms). Variation in the depth of modulation as a function of the direction of the movement, flexion or extension, was observed in only a very small proportion of the modulated units (16/117); most showed no relationship to direction. It is suggested that, in this experimental situation, much of the modulation appears to occur at a pre-cortical level since there was no relationship between the pattern of discharge of cells in relation to movement alone and the pattern of movement-related gating of their responses to the air puff. Effects which might be consistent with a cortical origin for the modulation were only infrequently observed. The present results are strikingly similar to those obtained using the evoked potential method, and thus support the hypothesis that in this task of rapid elbow movements, movement modulates the transmission of cutaneous signals from the hairy skin of the distal forelimb to primary somatosensory cortex in a nonspecific and widespread fashion.

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)

Previous studies have shown that the amplitude of somatosensory evoked potentials is diminished prior to, and during, voluntary limb movement. The present study investigated the role of the motor cortex in mediating this movement-related modulation in three chronically prepared, awake monkeys by applying low intensity intracortical microstimulation (ICMS) to different sites within the area 4 representation of the arm. Air puff stimuli were applied to the contralateral arm or adjacent trunk at various delays following the ICMS. Somatosensory evoked potentials were recorded from the primary somatosensory cortex, areas 1 and 3b, with an intracortical microelectrode. The principal finding of this study was that very weak ICMS, itself producing at most a slight, localized, muscle twitch, produced a profound decrease in the magnitude of the short latency component of the somatosensory evoked potentials in the awake money. Higher intensities of ICMS (suprathreshold for eliciting electromyographic (EMG) activity in the "target" muscle, i.e. that muscle activated by area 4 stimulation) were more likely to decrease the evoked response and produced an even greater decrease. The modulation appeared to be, in part, central in origin since (i) it preceded the onset of EMG activity in 23% of experiments, (ii) direct stimulation of the muscle activated by ICMS, which mimicked the feedback associated with the small ICMS-induced twitch, was often ineffective and (iii) the modulation was observed in the absence of EMG activity. Peripheral feedback, however, may also make a contribution. The results also indicate that the efferent signals from the motor cortex can diminish responses in the somatosensory cortex evoked by cutaneous stimuli, in a manner related to the somatotopic order. The effects are organized so that the modulation is directed towards those neurones serving skin areas overlying, or distal to, the motor output.

Modulation of sensory transmission in the lemniscal system was investigated in 2 monkeys trained to perform a simple elbow flexion in response to an auditory cue. Evoked responses to peripheral stimulation were recorded in the medial lemniscus, sensory thalamus (ventral posterior lateral nucleus, caudal division, VPLc) and somatosensory cortex. Simultaneous recordings were made from the cortex and either the medial lemniscus or VPLc. At all recording sites, evoked responses to natural (air puff) or electrical, percutaneous stimulation were depressed prior to and during active movement. The time course of the depression was similar at all three levels; the magnitude of the decrease during movement was most pronounced at the cortical level. Cortical evoked responses to central stimulation of effective sites in either the medial lemniscus or VPLc were decreased during, but not before, the onset of movement. The decrease was less than that seen for peripheral evoked potentials. Passive movement of the forearm significantly decreased all but the lemniscal evoked potential. The results indicate that there is a centrally mediated suppression of somatosensory transmission prior to, and during movement, occurring at the level of the first relay, the dorsal column nuclei. During movement, reafferent signals from the moving arm decrease transmission at the thalamocortical level.

The ability of subjects to perceive innocuous stimuli in the presence and absence of movement was evaluated using electrical stimulation of the skin. The subjective intensity of suprathreshold stimuli was unchanged during movement. Discrimination of small differences in the intensity of suprathreshold stimuli (difference thresholds) was also not altered by movement while, in the same subjects, detection thresholds were increased during movement of the stimulated arm. These results suggest that the elevation of detection thresholds during movement can be explained by masking. Both active and passive movement of the stimulated limb increased detection thresholds, with active movement having a slightly greater and more consistent effect than passive movement. Thus, both central and peripheral feedback factors appear to play a role in diminishing one's ability to detect weak stimuli during movement. Attention was also shown to influence performance of the detection task.