Modulation of somatosensory evoked responses in the primary somatosensory cortex produced by intracortical microstimulation of the motor cortex in the monkey

Microstimulation of human somatosensory cortex evokes task-dependent, spatially patterned responses in motor cortex

The primary motor (M1) and somatosensory (S1) cortices play critical roles in motor control but the signaling between these structures is poorly understood. To fill this gap, we recorded - in three participants in an ongoing human clinical trial (NCT01894802) for people with paralyzed hands - the responses evoked in the hand and arm representations of M1 during intracortical microstimulation (ICMS) in the hand representation of S1. We found that ICMS of S1 activated some M1 neurons at short, fixed latencies consistent with monosynaptic activation. Additionally, most of the ICMS-evoked responses in M1 were more variable in time, suggesting indirect effects of stimulation. The spatial pattern of M1 activation varied systematically: S1 electrodes that elicited percepts in a finger preferentially activated M1 neurons excited during that finger's movement. Moreover, the indirect effects of S1 ICMS on M1 were context dependent, such that the magnitude and even sign relative to baseline varied across tasks. We tested the implications of these effects for brain-control of a virtual hand, in which ICMS conveyed tactile feedback. While ICMS-evoked activation of M1 disrupted decoder performance, this disruption was minimized using biomimetic stimulation, which emphasizes contact transients at the onset and offset of grasp, and reduces sustained stimulation.

Decoding hand kinetics and kinematics using somatosensory cortex activity in active and passive movement

Area 2 of the primary somatosensory cortex (S1), encodes proprioceptive information of limbs. Several studies investigated the encoding of movement parameters in this area. However, the single-trial decoding of these parameters, which can provide additional knowledge about the amount of information available in sub-regions of this area about instantaneous limb movement, has not been well investigated. We decoded kinematic and kinetic parameters of active and passive hand movement during center-out task using conventional and state-based decoders. Our results show that this area can be used to accurately decode position, velocity, force, moment, and joint angles of hand. Kinematics had higher accuracies compared to kinetics and active trials were decoded more accurately than passive trials. Although the state-based decoder outperformed the conventional decoder in the active task, it was the opposite in the passive task. These results can be used in intracortical micro-stimulation procedures to provide proprioceptive feedback to BCI subjects.

Structural brain connectivity predicts early acute pain after mild traumatic brain injury

ABSTRACT

Mild traumatic brain injury (mTBI), is a leading cause of disability worldwide, with acute pain manifesting as one of its most debilitating symptoms. Understanding acute postinjury pain is important because it is a strong predictor of long-term outcomes. In this study, we imaged the brains of 157 patients with mTBI, following a motorized vehicle collision. We extracted white matter structural connectivity networks and used a machine learning approach to predict acute pain. Stronger white matter tracts within the sensorimotor, thalamiccortical, and default-mode systems predicted 20% of the variance in pain severity within 72 hours of the injury. This result generalized in 2 independent groups: 39 mTBI patients and 13 mTBI patients without whiplash symptoms. White matter measures collected at 6 months after the collision still predicted mTBI pain at that timepoint (n = 36). These white matter connections were associated with 2 nociceptive psychophysical outcomes tested at a remote body site-namely, conditioned pain modulation and magnitude of suprathreshold pain-and with pain sensitivity questionnaire scores. Our findings demonstrate a stable white matter network, the properties of which determine an important amount of pain experienced after acute injury, pinpointing a circuitry engaged in the transformation and amplification of nociceptive inputs to pain perception.

Deep Residual Convolutional Neural Networks for Brain-Computer Interface to Visualize Neural Processing of Hand Movements in the Human Brain

Concomitant with the development of deep learning, brain-computer interface (BCI) decoding technology has been rapidly evolving. Convolutional neural networks (CNNs), which are generally used as electroencephalography (EEG) classification models, are often deployed in BCI prototypes to improve the estimation accuracy of a participant's brain activity. However, because most BCI models are trained, validated, and tested via within-subject cross-validation and there is no corresponding generalization model, their applicability to unknown participants is not guaranteed. In this study, to facilitate the generalization of BCI model performance to unknown participants, we trained a model comprising multiple layers of residual CNNs and visualized the reasons for BCI classification to reveal the location and timing of neural activities that contribute to classification. Specifically, to develop a BCI that can distinguish between rest, left-hand movement, and right-hand movement tasks with high accuracy, we created multilayers of CNNs, inserted residual networks into the multilayers, and used a larger dataset than in previous studies. The constructed model was analyzed with gradient-class activation mapping (Grad-CAM). We evaluated the developed model via subject cross-validation and found that it achieved significantly improved accuracy (85.69 ± 1.10%) compared with conventional models or without residual networks. Grad-CAM analysis of the classification of cases in which our model produced correct answers showed localized activity near the premotor cortex. These results confirm the effectiveness of inserting residual networks into CNNs for tuning BCI. Further, they suggest that recording EEG signals over the premotor cortex and some other areas contributes to high classification accuracy.

Human Somatosensory Cortex Is Modulated during Motor Planning

Recent data and motor control theory argues that movement planning involves preparing the neural state of primary motor cortex (M1) for forthcoming action execution. Theories related to internal models, feedback control, and predictive coding also emphasize the importance of sensory prediction (and processing) before (and during) the movement itself, explaining why motor-related deficits can arise from damage to primary somatosensory cortex (S1). Motivated by this work, here we examined whether motor planning, in addition to changing the neural state of M1, changes the neural state of S1, preparing it for the sensory feedback that arises during action. We tested this idea in two human functional MRI studies (N = 31, 16 females) involving delayed object manipulation tasks, focusing our analysis on premovement activity patterns in M1 and S1. We found that the motor effector to be used in the upcoming action could be decoded, well before movement, from neural activity in M1 in both studies. Critically, we found that this effector information was also present, well before movement, in S1. In particular, we found that the encoding of effector information in area 3b (S1 proper) was linked to the contralateral hand, similarly to that found in M1, whereas in areas 1 and 2 this encoding was present in both the contralateral and ipsilateral hemispheres. Together, these findings suggest that motor planning not only prepares the motor system for movement but also changes the neural state of the somatosensory system, presumably allowing it to anticipate the sensory information received during movement.SIGNIFICANCE STATEMENT Whereas recent work on motor cortex has emphasized the critical role of movement planning in preparing neural activity for movement generation, it has not investigated the extent to which planning also modulates the activity in the adjacent primary somatosensory cortex. This reflects a key gap in knowledge, given that recent motor control theories emphasize the importance of sensory feedback processing in effective movement generation. Here, we find through a convergence of experiments and analyses, that the planning of object manipulation tasks, in addition to modulating the activity in the motor cortex, changes the state of neural activity in different subfields of the human S1. We suggest that this modulation prepares the S1 for the sensory information it will receive during action execution.

Neural Coding of Contact Events in Somatosensory Cortex

Manual interactions with objects require precise and rapid feedback about contact events. These tactile signals are integrated with motor plans throughout the neuraxis to achieve dexterous object manipulation. To better understand the role of somatosensory cortex in interactions with objects, we measured, using chronically implanted arrays of electrodes, the responses of populations of somatosensory neurons to skin indentations designed to simulate the initiation, maintenance, and termination of contact with an object. First, we find that the responses of somatosensory neurons to contact onset and offset dwarf their responses to maintenance of contact. Second, we show that these responses rapidly and reliably encode features of the simulated contact events-their timing, location, and strength-and can account for the animals' performance in an amplitude discrimination task. Third, we demonstrate that the spatiotemporal dynamics of the population response in cortex mirror those of the population response in the nerves. We conclude that the responses of populations of somatosensory neurons are well suited to encode contact transients and are consistent with a role of somatosensory cortex in signaling transitions between task subgoals.

The somatosensory cortex receives information about motor output

During voluntary movement, the somatosensory system not only passively receives signals from the external world but also actively processes them via interactions with the motor system. However, it is still unclear how and what information the somatosensory system receives during movement. Using simultaneous recordings of activities of the primary somatosensory cortex (S1), the motor cortex (MCx), and an ensemble of afferent neurons in behaving monkeys combined with a decoding algorithm, we reveal the temporal profiles of signal integration in S1. While S1 activity before movement initiation is accounted for by MCx activity alone, activity during movement is accounted for by both MCx and afferent activities. Furthermore, premovement S1 activity encodes information about imminent activity of forelimb muscles slightly after MCx does. Thus, S1 receives information about motor output before the arrival of sensory feedback signals, suggesting that S1 executes online processing of somatosensory signals via interactions with the anticipatory information.

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

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

Facilitation of information processing in the primary somatosensory area in the ball rotation task

Somatosensory input to the brain is known to be modulated during voluntary movement. It has been demonstrated that the response in the primary somatosensory cortex (SI) is generally gated during simple movement of the corresponding body part. This study investigated sensorimotor integration in the SI during manual movement using a motor task combining movement complexity and object manipulation. While the amplitude of M20 and M30 generated in the SI showed a significant reduction during manual movement, the subsequent component (M38) was significantly higher in the motor task than in the stationary condition. Especially, that in the ball rotation task showed a significant enhancement compared with those in the ball grasping and stone and paper tasks. Although sensorimotor integration in the SI generally has an inhibitory effect on information processing, here we found facilitation. Since the ball rotation task seems to be increasing the demand for somatosensory information to control the complex movements and operate two balls in the palm, it may have resulted in an enhancement of M38 generated in the SI.

Cortical contributions to sensory gating in the ipsilateral somatosensory cortex during voluntary activity

KEY POINTS

It has long been known that the somatosensory cortex gates sensory inputs from the contralateral side of the body. Here, we examined the contribution of the ipsilateral somatosensory cortex (iS1) to sensory gating during index finger voluntary activity. The amplitude of the P25/N33, but not other somatosensory evoked potential (SSEP) components, was reduced during voluntary activity compared with rest. Interhemispheric inhibition between S1s and intracortical inhibition in the S1 modulated the amplitude of the P25/N33. Note that changes in interhemispheric inhibition between S1s correlated with changes in cortical circuits in the ipsilateral motor cortex. Our findings suggest that cortical circuits, probably from somatosensory and motor cortex, contribute to sensory gating in the iS1 during voluntary activity in humans.

ABSTRACT

An important principle in the organization of the somatosensory cortex is that it processes afferent information from the contralateral side of the body. The role of the ipsilateral somatosensory cortex (iS1) in sensory gating in humans remains largely unknown. Using electroencephalographic (EEG) recordings over the iS1 and electrical stimulation of the ulnar nerve at the wrist, we examined somatosensory evoked potentials (SSEPs; P14/N20, N20/P25 and P25/N33 components) and paired-pulse SSEPs between S1s (interhemispheric inhibition) and within (intracortical inhibition) the iS1 at rest and during tonic index finger voluntary activity. We found that the amplitude of the P25/N33, but not other SSEP components, was reduced during voluntary activity compared with rest. Interhemispheric inhibition increased the amplitude of the P25/N33 and intracortical inhibition reduced the amplitude of the P25/N33, suggesting a cortical origin for this effect. The P25/N33 receives inputs from the motor cortex, so we also examined the contribution of distinct sets of cortical interneurons by testing the effect of ulnar nerve stimulation on motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation over the ipsilateral motor cortex with the coil in the posterior-anterior (PA) and anterior-posterior (AP) orientation. Afferent input attenuated PA, but not AP, MEPs during voluntary activity compared with rest. Notably, changes in interhemispheric inhibition correlated with changes in PA MEPs. Our novel findings suggest that interhemispheric projections between S1s and intracortical circuits, probably from somatosensory and motor cortex, contribute to sensory gating in the iS1 during voluntary activity in humans.

Physiological and Perceptual Sensory Attenuation Have Different Underlying Neurophysiological Correlates

Sensory attenuation, the top-down filtering or gating of afferent information, has been extensively studied in two fields: physiological and perceptual. Physiological sensory attenuation is represented as a decrease in the amplitude of the primary and secondary components of the somatosensory evoked potential (SEP) before and during movement. Perceptual sensory attenuation, described using the analogy of a persons' inability to tickle oneself, is a reduction in the perception of the afferent input of a self-produced tactile sensation due to the central cancellation of the reafferent signal by the efference copy of the motor command to produce the action. The fields investigating these two areas have remained isolated, so the relationship between them is unclear. The current study delivered median nerve stimulation to produce SEPs during a force-matching paradigm (used to quantify perceptual sensory attenuation) in healthy human subjects to determine whether SEP gating correlated with the behavior. Our results revealed that these two forms of attenuation have dissociable neurophysiological correlates and are likely functionally distinct, which has important implications for understanding neurological disorders in which one form of sensory attenuation but not the other is impaired. Time-frequency analyses revealed a negative correlation over sensorimotor cortex between gamma-oscillatory activity and the magnitude of perceptual sensory attenuation. This finding is consistent with the hypothesis that gamma-band power is related to prediction error and that this might underlie perceptual sensory attenuation.

SIGNIFICANCE STATEMENT

We demonstrate that there are two functionally and mechanistically distinct forms of sensory gating. The literature regarding somatosensory evoked potential (SEP) gating is commonly cited as a potential mechanism underlying perceptual sensory attenuation; however, the formal relationship between physiological and perceptual sensory attenuation has never been tested. Here, we measured SEP gating and perceptual sensory attenuation in a single paradigm and identified their distinct neurophysiological correlates. Perceptual and physiological sensory attenuation has been shown to be impaired in various patient groups, so understanding the differential roles of these phenomena and how they are modulated in a diseased state is very important for aiding our understanding of neurological disorders such as schizophrenia, functional movement disorders, and Parkinson's disease.

The somatosensory-evoked potential in reaction time is gated and elicited earlier when the motor response to a somatosensory cue is faster

The aim of this study was to investigate whether the somatosensory-evoked potential (SEP) is gated and elicited earlier when a motor response to a somatosensory stimulus is faster. In healthy humans, the left index finger and thumb were pinched in response to a somatosensory stimulus administered over the left median nerve. The SEP elicited by this stimulus when producing a fast motor response was compared with that when producing a slow motor response. The amplitudes of N18 and frontal P40 when producing a fast motor response were smaller than those when producing a slow motor response. The latencies of frontal P22 and P40 when producing a fast motor response were shorter than those when producing a slow motor response. These findings indicated that the responses of the primary sensory and frontal areas elicited by a somatosensory stimulus are gated and that the responses of the frontal areas elicited by this stimulus occur earlier when a motor response to this stimulus is faster. The enhanced efficiency or increased speed of the stimulus identification process may be related to the gating and early onset of the SEP components when producing a fast motor response.

Changes in Somatosensory Evoked Potentials and Hoffmann Reflexes during Fast Isometric Contraction of Foot Plantarflexor in Humans

In the present study, the extent to which the early component of somatosensory evoked potentials (SEPs) and the Hoffmann (H-) reflex induced by stimulation of the posterior tibial nerve are altered during the ascending and descending phases of fast plantarflexion was investigated. SEPSs and H-reflex of the soleus following tibial nerve stimulation were examined during fast plantarflexion when performed by nine normal subjects. The analyses focused on differences in amplitude modulation of the P30-P40 component of SEP and the H-reflex between the ascending and descending phases of full-wave rectified and averaged soleus electromyographic (EMG) activity. The H-reflex amplitude was significantly increased and decreased during the ascending and descending phases more than under resting control conditions, respectively. The reduction of SEP amplitude was 49% for the ascending phase and 83% for the descending phases with respect to the resting situation. Modulation of SEP during the ascending and descending phases was robustly retained even during ischemic nerve blockade of large diameter afferent fibers. These findings suggest that the transmission of afferent inputs from muscle spindles to motoneurons and to the somatosensory cortex during fast isometric contraction of the plantar flexor is regulated in a time-dependent fashion by descending commands.

Risonanza magnetica funzionale dell'encefalo in pazienti candidati ad intervento chirurgico con guida stereotassica

Negli ultimi anni la risonanza magnetica funzionale dell'encefalo (fMRI) si è proposta con successo nello studio dell'attività encefalica durante lo svolgimento di determinate funzioni. Si è inoltre delineato per essa un ruolo paricolarmente interessante nell'ambito della pianificazione preoperatoria di pazienti destinati ad un intervento chirurgico in sedi encefaliche critiche, ovvero ad alto rischio di invalidtà postoperatoria.

Il presente lavoro mira alla realizzazione di mappe di attivazione motoria in pazienti con lesioni occupanti spazio nell'encefalo mediante un tomografo a 1,5 T di non recentissima tecnologia; successivamente si è voluto valutare l'informatività di tali mappe, sia dal punto di vista neurochirurgico sia dal punto di vista neurofisiologico.

Sono stati presi in esame 5 pazienti, tutti portatori di una lesione occupante spazio in prossimità della corteccia perirolandica e tutti destinati ad un intervento chirurgico in stereotassi per l'asportazione di tale lesione. I pazienti venivano sottoposti in un'unica seduta allo studio funzionale e morfologico dell'encefalo con il casco stereotassico in sede.

Lo studio di fMRI si è svolto su uno o massimo due piani assiali orientati ortogonalmente al decorso della scissura di Rolando. Esso prevedeva come prova di attivazione il movimento della mano controlaterale all'emisfero con la lesione da asportare.

In un paziente la funzione motoria è stata studiata anche intraoperatoriamente mediante la stimolazione elettrica della corteccia cerebrale.

In 4 pazienti si è avuta una netta rappresentazione della corteccia sensitivo-motoria di tale emisfero. In 3 di essi si è vista inoltre un'attivazione dell'emisfero opposto, ipsilaterale alla mano in movimento. Altre zone di attivazione visualizzate nei nostri studi sono state messe in relazione con l'area motoria supplementare (3 pazienti) e l'area premotoria (2 pazienti).

Nel paziente in cui si è proceduto intraoperatoriamente con la stimolazione elettrica della corteccia cerebrale è stata riscontrata una buona concordanza tra lo studio elettrofisiologico e l'indagine di fMRI.

Analizzando i risultati da noi conseguiti possiamo concludere che anche un tomografo a 1,5 T di tipo convenzionale si presta ad un'indagine di fMRI in pazienti con lesioni encefaliche. Le mappe di attivazione realizzate su uno o due piani assiali sono in grado di fornire delle informazioni sui rapporti tra la lesione da asportare e la corteccia sensitivo-motoria, in particolare l'area della mano. Ciò ha notevoli implicazioni dal punto di vista neurochirurgico, considerando quanto sia importante la salvaguardia dell'integrità funzionale del soggetto che si sottopone ad un intervento neurochirurgico in una sede critica come la corteccia perirolandica.

Discrepancies between cortical and behavioural long-term readouts of hyperalgesia in awake freely moving rats

BACKGROUND

It is still unclear to what extent the most common animal models of pain and analgesia, based on indirect measures such as nocifensive behaviours, provide valid measures of pain perception.

METHODS

To address this issue, we developed a novel animal model comprising a more direct readout via chronically (>1 month) implanted multichannel electrodes (MCE) in rat primary somatosensory cortex (S1; known to be involved in pain perception in humans) and compared this readout to commonly used behavioural pain-related measures during development of hyperalgesia. A translational method to induce hyperalgesia, UVB irradiation of the skin, was used. Localized CO2 laser stimulation was made of twenty skin sites (20 stimulations/site/observation day) on the plantar hind paw, before and during the time period when enhanced pain perception is reported in humans after UVB irradiation.

RESULTS

We demonstrate a 2-10 fold significant enhancement of cortical activity evoked from both irradiated and adjacent skin and a time course that corresponds to previously reported enhancement of pain magnitude during development of primary and secondary hyperalgesia in humans. In contrast, withdrawal reflexes were only significantly potentiated from the irradiated skin area and this potentiation was significantly delayed as compared to activity in S1.

CONCLUSIONS

The present findings provide direct evidence that chronic recordings in S1 in awake animals can offer a powerful, and much sought for, translational model of the perception of pain magnitude during hyperalgesia. WHAT DOES THIS STUDY ADD?: In a novel animal model, chronic recordings of nociceptive activity in primary somatosensory cortex (S1) in awake freely moving rats are compared to behavioural readouts during UVB-induced hyperalgesia. Evoked activity in rat S1 replicates altered pain perception in humans during development of hyperalgesia, but withdrawal reflexes do not.

Effect of muscle contraction strength on gating of somatosensory magnetic fields

Afferent somatosensory information is modulated before the afferent input arrives at the primary somatosensory cortex during voluntary movement. The aim of the present study was to clarify the effect of muscular contraction strength on somatosensory evoked fields (SEFs) during voluntary movement. In addition, we examined the differences in gating between innervated and non-innervated muscle during contraction. We investigated the changes in gating effect by muscular contraction strength and innervated and non-innervated muscles in human using 306-channel magnetoencephalography. SEFs were recorded following the right median nerve stimulation in a resting condition and during isometric muscular contractions from 10 % electromyographic activity (EMG), 20 and 30 % EMG of the right extensor indicis muscle and abductor pollicis brevis muscle. Our results showed that the equivalent current dipole (ECD) strength for P35m decreased with increasing strength of muscular contraction of the right abductor pollicis brevis muscle. However, changes were observed only at 30 % EMG contraction level of the right extensor indicis muscle, which was not innervated by the median nerve. There were no significant changes in the peak latencies and ECD locations of each component in all conditions. The ECD strength did not differ significantly for N20m and P60m regardless of the strength of muscular contraction and innervation. Therefore, we suggest that the gating of SEF waveforms following peripheral nerve stimulation was affected by the strength of muscular contraction and innervation of the contracting muscle.

Modulation of somatosensory evoked potentials during force generation and relaxation

This study investigated the modulation of somatosensory evoked potentials (SEPs) during precisely controlled force generation and force relaxation in a visuomotor tracking task. Subjects were instructed to track a target line with a line that represented their own force generated by grip movement with the right hand as accurately as possible during concurrent electrical stimulation. The target force line moved up continuously from 0 to 20 % of maximal voluntary contraction (MVC) (the force generation phase: FG phase) and moved down from 20 to 0 % of MVC (the force relaxation phase: FR phase) in 7 s at a constant velocity. We separately obtained SEPs following electrical stimulation of the median nerve at the wrist in each phase. During the visuomotor tracking task, compared with the stationary condition, the N30 at Fz and P27 at C3' showed a significant reduction in amplitude in the FG and FR phases. In addition, the N30 and P27 were significantly smaller in amplitude in the FG than FR phase. Although the average amount of force exertion was the same in the FG and FR phases, the modulation of SEP amplitude was larger in the FG phase. These results indicated that sensorimotor integration in the somatosensory area was dependent on the context of movement exertion.

A critical speed for gating of tactile detection during voluntary movement

This study addressed the paradoxical observation that movement is essential for tactile exploration, and yet is accompanied by movement-related gating or suppression of tactile detection. Knowing that tactile gating covaries with the speed of movement (faster movements, more gating), we hypothesized that there would be no tactile gating at slower speeds of movement, corresponding to speeds commonly used during tactile exploration (<200 mm/s). Subjects (n = 21) detected the presence or absence of a weak electrical stimulus applied to the skin of the right middle finger during two conditions: rest and active elbow extension. Movement speed was systematically varied from 50 to ~1,000 mm/s. No subject showed evidence of tactile gating at the slowest speed tested, 50 mm/s (rest versus movement), but all subjects showed decreased detection at one or more higher speeds. For each subject, we calculated the critical speed, corresponding to the speed at which detection fell to 0.5 (chance). The mean critical speed was 472 mm/s and >200 mm/s in almost all subjects (19/21). This result is consistent with our hypothesis that subjects optimize the speed of movement during tactile exploration to avoid speeds associated with tactile gating. This strategy thus maximizes the quality of the tactile feedback generated during tactile search and improves perception.

Overestimation of force during matching of externally generated forces

If a weight is applied to a finger and the subject asked to produce the same force, the subject generates a force larger than the weight. That is, subjects overestimate the force applied by an external target when matching it. Details of this force overestimation are not well understood. We show that subjects overestimate small target weights, but not larger ones. Furthermore we show for the first time that the force overestimation consists of two components. The first component is a constant. The second component depends on the precise magnitude of the weight and is only present when subjects hold the target weight against gravity. We suggest that the two components are generated in different phases of the force-matching task, are due to different processes, and must have an influence on all proprioceptive judgements of force.

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

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

Hypohydration and muscular fatigue of the thumb alter median nerve somatosensory evoked potentials

The mechanisms by which dehydration impairs endurance performance remain unresolved but may involve alterations in afferent neural processing. The purpose of this study was to determine the effect of hypohydration on somatosensory evoked potentials (SEPs) at rest and during recovery from fatiguing exercise. Fourteen volunteers (12 men, 2 women) performed repetitive isometric thumb contractions (50% maximal voluntary contractions (MVC) and 100% MVC in a 5:1 ratio, each contraction separated by 5 s of rest) until exhaustion when euhydrated (EU) and when hypohydrated by 4% body mass (HY). SEPs were obtained from the median nerve. The results indicated that HY did not produce statistical differences in time to exhaustion (EU = 754 (SD 255); HY = 714 (SD 318) s; p = 0.66) or rate of muscle fatigue. However, HY was associated with greater subjective feelings of fatigue and loss of vigor after exhaustive exercise (p < 0.01). HY affected N20 latency with an interaction effect of hydration by fatigue state (EU-Rest: 18.5 (SD 1.6) ms; EU-Fatigue: 19.0 (SD 1.6) ms; HY-Rest: 18.3 (SD 1.3) ms; HY-Fatigue: 18.4 (SD 1.5) ms; p = 0.034), but N20 and N20-P22 amplitude responses were similar between HY and EU trials. We concluded that moderate water deficits appear to alter afferent signal processing within the cerebral cortex.

Perception of simulated local shapes using active and passive touch

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

Passive range of motion functional magnetic resonance imaging localizing sensorimotor cortex in sedated children

OBJECT

In this study, the authors examined whether passive range of motion (ROM) under conscious sedation could be used to localize sensorimotor cortex using functional MR (fMR) imaging in children as part of their presurgical evaluation.

METHODS

After obtaining institutional review board approval (for retrospective analysis of imaging data acquired for clinical purposes) and informed consent, 16 children underwent fMR imaging. All 16 had lesions; masses were found in 9 patients and cortical dysplasia was found in 4; the lesions in 3 patients were not diagnosed. Passive ROM was performed during blood oxygen level-dependent MR imaging sequences. Three of the patients also performed active motor tasks during the fMR imaging study. All patients were evaluated using passive ROM of the hand and/or foot; 3 patients were evaluated for passive touch of the face. In 9 cases, intraoperative electrocorticography (ECoG) was used. Five of the patients underwent intraoperative ECoG to evaluate for seizure activity. Four patients had intraoperative ECoG for motor mapping. Five of the patients had subdural grids placed for extraoperative monitoring.

RESULTS

In 3 cases, the active and passive ROMs colocalized. In 4 patients ECoG was used to identify motor cortex, and in all 4 motor ECoG yielded results consistent with the passive ROM localization. Thirteen of 16 children have undergone resection based on passive ROM fMR imaging findings with no unanticipated deficits.

CONCLUSIONS

These preliminary data suggest that passive ROM fMR imaging can accurately detect functional hand, leg, and face regions of the sensorimotor cortex in the sedated child. This extends current extraoperative mapping capabilities to patients unable or unwilling to cooperate for active motor tasks.

Enhanced spatial discrimination in paretic hands

OBJECTIVE

Proper interaction between the sensory and motor system is essential for adequate control of voluntary movement. The influence of sensory alteration on motor activity is well known, while the effect of motor disturbance on sensory activity has yet to be clarified. We performed this study to investigate the influence of motor deactivation on sensory discrimination.

METHODS

Using Johnson-Van Boven-Phillips domes, we evaluated tactile spatial discrimination (grating orientation threshold; GOT) in 62 patients with acute pure motor stroke and 75 age-matched healthy controls.

RESULTS

In controls, the GOT was significantly lower in the dominant than in the non-dominant hands. The GOT was significantly reduced in patients' paretic hand, as compared to their unaffected hand and the hands of healthy controls in both the dominant and non-dominant side. The GOT of patients' paretic hand was significantly and inversely correlated to severity of their initial motor deficit in the non-dominant side (r=0.40, p<0.05), but not in the dominant side.

CONCLUSIONS

These results suggest that motor deprivation enhances ascending sensory inputs.

SIGNIFICANCE

This study provides additional information about the relationship between the motor and sensory system in humans.

Characteristics of sensori-motor interaction in the primary and secondary somatosensory cortices in humans: a magnetoencephalography study

We studied sensori-motor interaction in the primary (SI) and secondary somatosensory cortex (SII) using magnetoencephalography. Since SII in both hemispheres was activated following unilateral stimulation, we analyzed SIIc (contralateral to stimulation) as well as SIIi (ipsilateral to stimulation). Four tasks were performed in human subjects in which a voluntary thumb movement of the left or right hand was combined with electrical stimulation applied to the index finger of the left or right hand: L(M)-L(S) (movement of the left thumb triggered stimulation to the left finger), L(M)-R(S) (movement of the left thumb triggered electrical stimulation to the right finger), R(M)-R(S) (movement of the right thumb triggered electrical stimulation to the right finger), and R(M)-L(S) (movement of the right thumb triggered electrical stimulation to the left finger). Stimulation to the index finger only (S condition) was also recorded. In SI, the amplitude of N20m and P35m was significantly attenuated in the R(M)-R(S) and L(M)-L(S) tasks compared with the S condition, but that for other tasks showed no change, corresponding to a conventional gating phenomenon. In SII, the R(M)-L(S) task significantly enhanced the amplitude of SIIc but reduced that of SIIi compared with the S condition. The L(M)-L(S) and R(M)-R(S) tasks caused a significant enhancement only in SIIi. The L(M)-R(S) task enhanced the amplitude only in SIIc. The laterality index showed that SII modulation with voluntary movement was more dominant in the hemisphere ipsilateral to movement but was not affected by the side of stimulation. These results provided the characteristics of activities in somatosensory cortices, a simple inhibition in SI but complicated changes in SII depending on the side of movement and stimulation, which may indicate the higher cognitive processing in SII.

Spinal and Supraspinal Effects of Long-Term Stimulation of Sensorimotor Cortex in Rats

Sensorimotor cortex (SMC) modifies spinal cord reflex function throughout life and is essential for operant conditioning of the H-reflex. To further explore this long-term SMC influence over spinal cord function and its possible clinical uses, we assessed the effect of long-term SMC stimulation on the soleus H-reflex. In freely moving rats, the soleus H-reflex was measured 24 h/day for 12 wk. The soleus background EMG and M response associated with H-reflex elicitation were kept stable throughout. SMC stimulation was delivered in a 20-day-on/20-day-off/20-day-on protocol in which a train of biphasic 1-ms pulses at 25 Hz for 1 s was delivered every 10 s for the on-days. The SMC stimulus was automatically adjusted to maintain a constant descending volley. H-reflex size gradually increased during the 20 on-days, stayed high during the 20 off-days, and rose further during the next 20 on-days. In addition, the SMC stimulus needed to maintain a stable descending volley rose steadily over days. It fell during the 20 off-days and rose again when stimulation resumed. These results suggest that SMC stimulation, like H-reflex operant conditioning, induces activity-dependent plasticity in both the brain and the spinal cord and that the plasticity responsible for the H-reflex increase persists longer after the end of SMC stimulation than that underlying the change in the SMC response to stimulation.

Median nerve somatosensory evoked potentials and their high-frequency oscillations in amyotrophic lateral sclerosis

OBJECTIVE

To investigate sensory cortical changes in amyotrophic lateral sclerosis (ALS), we studied somatosensory evoked potentials (SEPs) and their high-frequency oscillation potentials.

METHODS

Subjects were 15 healthy volunteers and 26 ALS patients. Median nerve SEPs were recorded and several peaks of oscillations were obtained by digitally filtering raw SEPs. The patients were sorted into three groups according to the level of weakness of abductor pollicis brevis muscle (APB): mild, moderate and severe. The latencies and amplitudes of main and oscillation components of SEP were compared among normal subjects and the three patient groups.

RESULTS

The early cortical response was enlarged in the moderate weakness group, while it was attenuated in the severe weakness group. No differences were noted in the size ratios of oscillations to the main SEP component between the patients and normal subjects. The central sensory conduction time (CCT) and N20 duration were prolonged in spite of normal other latencies.

CONCLUSIONS

The median nerve SEP amplitude changes are associated with motor disturbances in ALS. The cortical potential enhancement of SEPs with moderate weakness in ALS may reflect some compensatory function of the sensory cortex for motor disturbances.

SIGNIFICANCE

The sensory cortical compensation for motor disturbances is shown in ALS, which must be important information for rehabilitation.

Centrifugal regulation of human cortical responses to a task-relevant somatosensory signal triggering voluntary movement

Many studies have reported a movement-related modulation of response in the primary and secondary somatosensory cortices (SI and SII) to a task-irrelevant stimulation in primates. In the present study, magnetoencephalography (MEG) was used to examine the top-down centrifugal regulation of neural responses in the human SI and SII to a task-relevant somatosensory signal triggering a voluntary movement. Nine healthy adults participated in the study. A visual warning signal was followed 2 s later by a somatosensory imperative signal delivered to the right median nerve at the wrist. Three kinds of warning signal informed the participants of the reaction which should be executed on presentation of the imperative signal (rest or extension of the right index finger, extension of the left index finger). The somatosensory stimulation was used to both generate neural responses and trigger voluntary movement and therefore was regarded as a task-relevant signal. The responses were recorded using a whole-head MEG system. The P35m response around the SI was reduced in magnitude without alteration of the primary SI response, N20m, when the signal triggered a voluntary movement compared to the control condition, whereas bilateral SII responses peaking at 70-100 ms were enhanced and the peak latency was shortened. The peak latency of the responses in the SI and SII preceded the onset of the earliest voluntary muscle activation in each subject. Later bilateral perisylvian responses were also enhanced with movement. In conclusion, neural activities in the SI and SII evoked by task-relevant somatosensory signals are regulated differently by motor-related neural activities before the afferent inputs. The present findings indicate a difference in function between the SI and SII in somatosensory-motor regulation.

Pre-movement modulation of tibial nerve SEPs caused by a self-initiated dorsiflexion

OBJECTIVE

To investigate the centrifugal effect on somatosensory evoked potentials (SEPs), we recorded the pre-movement modulation of SEPs following stimulation of the tibial nerve caused by a self-initiated dorsiflexion.

METHODS

SEPs following stimulation of the right tibial nerve at the popliteal fossa were recorded during self-initiated dorsiflexion of the right ankle every 5-7s. Based on the onset of Bereitschaftspotential and negative slope, the preparatory period before dorsiflexion was divided into four sub-periods (pre-BP, BP1a, BP1b and BP2 sub-period), and SEPs in each sub-period were averaged. SEPs were also recorded in a stationary condition.

RESULTS

P30, N40, P50 and N70 were identified at Cz in all subjects. The amplitude of P30 was significantly smaller in the BP2 sub-period than in the pre-BP sub-period. The N40 amplitude was significantly attenuated in the BP2 sub-period compared with the stationary condition, the pre-BP sub-period, the BP1a sub-period and the BP1b sub-period.

CONCLUSIONS

These results suggested that the motor-related areas involved in generating negative slope modulated the tibial nerve SEPs preceding a self-initiated contraction of the agonist muscle.

SIGNIFICANCE

The centrifugal gating effect on SEPs extends to the somatosensory information from the antagonistic body part.

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

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

Centrifugal regulation of a task-relevant somatosensory signal triggering voluntary movement without a preceding warning signal

A warning signal followed by an imperative signal generates anticipatory and preparatory activities, which regulate sensory evoked neuronal activities through a top-down centrifugal mechanism. The present study investigated the centrifugal regulation of neuronal responses evoked by a task-relevant somatosensory signal, which triggers a voluntary movement without a warning signal. Eleven healthy adults participated in this study. Electrical stimulation was delivered to the right median nerve at a random interstimulus interval (1.75-2.25 s). The participants were instructed to extend the second digit of the right hand as fast as possible when the electrical stimulus was presented (ipsilateral reaction condition), or extend that of the left hand (contralateral reaction condition). They also executed repetitively extension of the right second digit at a rate of about 0.5 Hz, irrespective of electrical stimulation (movement condition), to count silently the number of stimuli (counting condition). In the control condition, they had no task to perform. The amplitude of short-latency somatosensory evoked potentials, the central P25, frontal N30, and parietal P30, was significantly reduced in both movement and ipsilateral reaction conditions compared to the control condition. The amplitude of long-latency P80 was significantly enhanced only in the ipsilateral reaction condition compared to the control, movement, contralateral reaction, and counting conditions. The long-latency N140 was significantly enhanced in both movement and ipsilateral reaction conditions compared to the control condition. In conclusion, short- and long-latency neuronal activities evoked by task-relevant somatosensory signals were regulated differently through a centrifugal mechanism even when the signal triggered a voluntary movement without a warning signal. The facilitation of activities at a latency of around 80 ms is associated with gain enhancement of the task-relevant signals from the body part involved in the action, whereas that at a latency of around 140 ms is associated with unspecific gain regulation generally induced by voluntary movement. These may be dissociated from the simple effect of directing attention to the stimulation.

Centrifugal regulation of task-relevant somatosensory signals to trigger a voluntary movement

Many previous papers have reported the modulation of somatosensory evoked potentials (SEPs) during voluntary movement, but the locus and mechanism underlying the movement-induced centrifugal modulation of the SEPs elicited by a task-relevant somatosensory stimulus still remain unclear. We investigated the centrifugal modulation of the SEPs elicited by a task-relevant somatosensory stimulus which triggers a voluntary movement in a forewarned reaction time task. A pair of warning (S1: auditory) and imperative stimuli (S2: somatosensory) was presented with a 1 s interstimulus interval. Subjects were instructed to respond by moving the hand ipsilateral or contralateral to the somatosensory stimulation which elicits the SEPs. In four experiments, the locus and selectivity of the SEPs' modulation, the contribution of cutaneous afferents and the effect of contraction magnitude were examined, respectively. A control condition where subjects had no task to perform was compared to several task conditions. The amplitude of the frontal N30, parietal P30, and central P25 was decreased and that of the long latency P80 and N140 was increased when the somatosensory stimuli triggered a voluntary movement of the stimulated finger compared to the control condition. The N60 decreased with the movement of any finger. These results were considered to be caused by the centrifugal influence of neuronal activity which occurs before a somatosensory imperative stimulus. The present findings did not support the hypothesis that the inhibition of afferent inputs by descending motor commands can occur at subcortical levels. A higher contraction magnitude produced a further attenuation of the amplitude of the frontal N30, while it decreased the enhancement of the P80. Moreover, the modulation of neuronal responses seems to result mainly from the modulation of cutaneous afferents, especially from the moved body parts. In conclusion, the short- and long-latency somatosensory neuronal activities evoked by task-relevant ascending afferents from the moved body parts are regulated differently by motor-related neuronal activities before those afferent inputs. The latter activities may be associated with sensory gain regulation related to directing attention to body parts involved in the action.

Somatotopic blocking of sensation with navigated transcranial magnetic stimulation of the primary somatosensory cortex

We demonstrate that spatially accurate and selective stimulation is crucial when cortical functions are studied by the creation of temporary lesions with transcranial magnetic stimulation (TMS). Previously, the interpretation of the TMS results has been hampered by inaccurate knowledge of the site and strength of the induced electric current in the brain. With a Navigated Brain Stimulation (NBS) system, which provides real-time magnetic resonance image (MRI)-guided targeting of the TMS-induced electric field, we found that TMS of a spatially restricted cortical S1 thenar area is sufficient to abolish sensation from a weak electric stimulation of the corresponding skin area. We demonstrate that with real-time navigation, TMS can be repeatably directed at millimeter-level precision to a target area defined on the MRI. The stimulation effect was temporally and spatially specific: the greatest inhibition of sensation occurred when TMS was applied 20 ms after the cutaneous test stimulus and the TMS effect was sensitive to 8-13 mm displacements of the induced electric field pattern. The results also indicate that TMS selectively to S1 is sufficient to abolish perception of cutaneous stimulation of the corresponding skin area.

Differential modulation in human primary and secondary somatosensory cortices during the preparatory period of self-initiated finger movement

To elucidate the mechanisms underlying sensorimotor integration, we investigated modulation in the primary (SI) and secondary (SII) somatosensory cortices during the preparatory period of a self-initiated finger extension. Electrical stimulation of the right median nerve was applied continuously, while the subjects performed a self-initiated finger extension and were instructed not to pay attention to the stimulation. The preparatory period was divided into five sub-periods from the onset of the electromyogram to 3000 ms before movement and the magnetoencephalogram signals following stimulation in each sub-period were averaged. Multiple source analysis indicated that the equivalent current dipoles (ECDs) were located in SI and bilateral SII. Although the ECD moment for N 20 m (the upward deflection peaking at around 20 ms) was not significantly changed, that for P 30 m (the downward deflection peaking at around 30 m) was significantly smaller in the 0- to -500-ms sub-period than the -2000- to -3000-ms sub-period. As for SII, the ECD moment for the SII ipsilateral to movement showed no significant change, while that for the contralateral SII was significantly larger in the 0- to -500-ms sub-period than the -1500- to -2000-ms or -2000- to -3000-ms sub-period. The opposite effects of movement on SI and SII cortices indicated that these cortical areas play a different role in the function of the sensorimotor integration and are affected differently by the centrifugal process.

Changes in the centrifugal gating effect on somatosensory evoked potentials depending on the level of contractile force

In this study, we investigated the somatosensory evoked potentials (SEPs) during the preparatory period of self-initiated plantar flexion at different force levels of muscle contraction and elucidated the mechanism behind the centrifugal gating effect on somatosensory information processing. We recorded SEPs following stimulation of the tibial nerve at the popliteal fossa during the preparatory period of a 20% maximal voluntary contraction (MVC) and 50% MVC. The preparatory period was divided into two sub-periods based on the components of movement-related cortical potentials, the negative slope (NS sub-period) and the Bereitschaftspotential (BP sub-period). The subjects were instructed to concentrate on the movement and not to pay attention to the continuous electrical stimulation. Pre-movement SEPs were averaged separately during the two sub-periods under each MVC condition. The mean amplitudes of BP and NS were larger during the 50% MVC than the 20% MVC. As for the components of SEPs, during the NS sub-period the amplitude of P30 under the 50% MVC and N40 under both conditions were significantly smaller than that in the stationary sequence, and N40 amplitude was significantly smaller during the 50% MVC than the 20% MVC. During the BP sub-period, the amplitude of P30 and N40 during the 50% MVC was significantly smaller than during the stationary sequence, while it was not significantly different between the 20% and 50% MVCs. In conclusion, the extent of the centrifugal gating effect on SEPs was dependent on the activities of motor-related areas, which generated the NS and BP.

Load- and cadence-dependent modulation of somatosensory evoked potentials and Soleus H-reflexes during active leg pedaling in humans

Modulation of transmission in group I muscle afferent pathways to the somatosensory cortex and those to the alpha-motoneuron were investigated during active leg pedaling. Cerebral somatosensory evoked potentials (SEPs) and Soleus (Sol) H-reflexes following posterior tibial nerve stimulation were recorded at four different pedaling phases. The subjects were asked to perform pedaling at three different cadences (30, 45 and 60 rpm with 0.5 kp, cadence task; C-task) and with three different workloads (at 45 rpm with 0.0, 0.5 and 1.0 kp, load task; L-task). In both C- and L-tasks, Sol H-reflexes were modulated in a phase-dependent manner, showing an increase in the power phase and a decrease in the recovery phase. In contrast, the early SEP (P30-N40) components were modulated in a phase-dependent manner when the cadence and load were low. When focusing on the power phases, significant cadence- and load-dependent modulations of the P30-N40 were found, and inversely graded with the cadence and load. The H-reflex was found to be significantly decreased at the highest cadence, i.e., cadence-dependent modulation. In contrast, the H-reflex during the L-task was found to be proportional to the load. The correlation analysis between the size of H-reflex and the amount of background (BG) electromyographic (EMG) activity demonstrated that the H-reflex in the power phase did not depend on the BG EMG in either C- or L-task. These findings suggested that transmission of muscle afferents along the ascending pathways to the cerebral cortex and the spinal cord is independently controlled in accordance with the biomechanical constraints of active pedaling.

Differential modulation of the short- and long-latency somatosensory evoked potentials in a forewarned reaction time task

OBJECTIVE

We investigated modulation of the short- and long-latency somatosensory evoked potentials (SEPs) in a forewarned reaction time task.

METHODS

A pair of warning (auditory) and imperative stimuli (somatosensory) was presented with a 2 s interstimulus interval. In movement condition, subjects responded by grip movement with the ipsilateral hand to the somatosensory stimulation when the imperative stimulus was presented. In counting condition, they silently counted the number of imperative stimuli. The SEPs in response to the imperative stimuli were recorded.

RESULTS

Frontal N30 and central N60 amplitudes were significantly smaller in the movement than in the counting or rest conditions. None of the short-latency components differed between the counting and rest conditions. In contrast to the short-latency components, P80 was significantly larger in the counting than in the rest condition, and showed a further increase from the counting to the movement condition. The N140 amplitude was significantly larger in the movement than the rest condition, but was not changed between the counting and the rest conditions.

CONCLUSIONS

The attenuation of the frontal N30 and central N60, and the enhancement of the P80 and possibly the N140 resulted from the centrifugal mechanism. The present findings may show the different effects of voluntary movement on the early and subsequent cortical processing of the relevant somatosensory information requiring a behavioral response.

SIGNIFICANCE

The present study demonstrated the differential modulation of short- and long-latency components of SEPs in a forewarned reaction time task.

Time course and magnitude of movement-related gating of tactile detection in humans. III. Effect of motor tasks

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.

Changes in tooth pulpal detection and pain thresholds in relation to jaw movement in man

The effect of jaw movements on pulpal sensory thresholds to electrical stimulation was studied in healthy humans. The movements consisted of repeated jaw opening and closing at two different frequencies (1 and 3 s(-1)). The detection/perception and pain thresholds of an upper or lower central incisor were determined by stimulation with monopolar constant current pulses at two different durations (0.5 and 5.0 ms). In the absence of jaw movement, the control (baseline) pain threshold was significantly higher than the detection threshold, and both thresholds were significantly decreased with an increase of the stimulus pulse duration. During jaw movement, pulpal detection and pain thresholds were significantly elevated, independent of the duration of the stimulus pulse. The jaw movement-related increase in detection thresholds was significantly dependent on the rate of cyclical jaw movements and on the site of stimulation. An increase in pulpal sensory thresholds was observed with stimulation of the lower incisor only; there was no change in thresholds for the upper incisor. Pulpal detection thresholds were significantly more elevated during jaw movement than pulpal pain thresholds. The results indicate that the reduction in pulpal sensitivity is related to the jaw movements. The effect of jaw movement on pulpal detection thresholds was segmentally restricted. In contrast, modulation of the pulpal pain thresholds was considerably weaker. The jaw movement-related suppression of pulpal sensitivity may be explained by activation of segmental afferent-induced inhibition, corollary efferent barrage from motor to sensory areas, or a combination of both.

Characterizing somatosensory evoked potential sources with dipole models: Advantages and limitations

Several methods have been developed to investigate the cerebral generators of scalp somatosensory evoked potentials (SEPs), because simple visual inspection of the electroencephalographic signal does not allow for immediate identification of the active brain regions. When the neurons fired by the afferent inputs are closely grouped, as usually occurs in SEP generation, they can be represented as a dipole, that is, as a linear source with two opposite poles. Several techniques for dipolar source modeling, which use different algorithms, have been employed to build source models of early, middle-latency, and late cognitive SEPs. Modifications of SEP dipolar activities after experimental maneuvers or in pathological conditions have also been observed. Although the effectiveness of dipolar source analysis should not be overestimated due to the intrinsic limitations of the approach, dipole modeling provides a means to assess SEPs in terms of cerebral sources and voltage fields that they produce over the head.

Habituation of cutaneomuscular reflexes recorded from the first dorsal interosseous and triceps muscle in man

Cutaneomuscular reflexes have been recorded from the first dorsal interosseous muscle during a sustained abduction of the index finger of 20 subjects (25 recordings) following stimulation of the digital nerves at the following frequencies: 2 Hz, 3 Hz, 5 Hz, 7 Hz and 9 Hz, presented in random order. Five hundred stimuli were given at each frequency. EMG was rectified and consecutive batches of 100 sweeps of each set of 500 responses were averaged time locked to the stimulus. All reflex components, E1, I1 and E2, exhibit habituation with the E1 component habituating the most and the I1 component the least. There was considerable variation in the rate of habituation between subjects. The rate of habituation was independent of the frequency of stimulation. Reflex responses were recorded from the triceps brachii muscle in eight subjects; this reflex response habituated at a faster rate than the E2 component recorded from the first dorsal interosseous muscle. These results are discussed in relation to the choice of stimulus parameters for the clinical testing of cutaneous reflexes. We conclude that it is important to consistently average the same number of responses.

Segregation of somatosensory activation in the human rolandic cortex using fMRI

The segregation of sensory information into distinct cortical areas is an important organizational feature of mammalian sensory systems. Here, we provide functional magnetic resonance imaging (fMRI) evidence for the functional delineation of somatosensory representations in the human central sulcus region. Data were collected with a 3-Tesla scanner during two stimulation protocols, a punctate tactile condition without a kinesthetic/motor component, and a kinesthetic/motor condition without a punctate tactile component. With three-dimensional (3-D) anatomical reconstruction techniques, we analyzed data in individual subjects, using the pattern of activation and the anatomical position of specific cortical areas to guide the analysis. As a complimentary analysis, we used a brain averaging technique that emphasized the similarity of cortical features in the morphing of individual subjects and thereby minimized the distortion of the location of cortical activation sites across individuals. A primary finding of this study was differential activation of the cortex on the fundus of the central sulcus, the position of area 3a, during the two tasks. Punctate tactile stimulation of the palm, administered at 3 Hz with a 5.88(log10.mg) von Frey filament, activated discrete regions within the precentral (PreCG) and postcentral (PoCG) gyri, corresponding to areas 6, 3b, 1, and 2, but did not activate area 3a. Conversely, kinesthetic/motor stimulation, 3-Hz flexion and extension of the digits, activated area 3a, the PreCG (areas 6 and 4), and the PoCG (areas 3b, 1, and 2). These activation patterns were observed in individual subjects and in the averaged data, providing strong evidence for the existence of a distinct representation within area 3a in humans. The percentage signal changes in the PreCG and PoCG regions activated by tactile stimulation, and in the intervening gap region, support this functional dissociation. In addition to this distinction within the fundus of the central sulcus, the combination of high-resolution imaging and 3-D analysis techniques permitted localization of activation within areas 6, 4, 3a, 3b, 1, and 2 in the human. With the exception of area 4, which showed inconsistent activation during punctate tactile stimulation, activation in these areas in the human consistently paralleled the pattern of activity observed in previous studies of monkey cortex.

Effect of movement on dipolar source activities of somatosensory evoked potentials

The early scalp somatosensory evoked potentials (SEPs) to median and tibial nerve stimulation were recorded at rest and during voluntary movement of the stimulated hand and foot, respectively. Both tibial and median nerve SEP distributions at rest could be explained by four-dipole models, in which one dipole was activated at the same latency as the subcortical far field and the three remaining dipolar sources were located in the perirolandic region contralateral to the stimulated side. Voluntary movement reduced all cortical dipoles in strength, while the subcortical one remained unchanged, suggesting that the effect of movement occurs above the cervicomedullary junction. In animals, cutaneous inputs are suppressed during movement and we therefore interpreted the depression of activity in the primary somatosensory cortex induced by movement as due to selective "gating" of cutaneous afferents. Because the reduction in strength of the cortical dipoles was generally lower during passive than active movement, both centrifugal and centripetal mechanisms probably contribute to the phenomenon of "gating."

Muscular sense is attenuated when humans move

1. Muscle receptors play an important role in our conscious perception of movement, but there are no published accounts of our ability to detect their signals during different motor tasks. The present experiments introduce a method to test muscular sense when humans move. 2. Muscle receptors were excited by an electrically induced twitch of the right extensor carpi ulnaris muscle. The muscle was stimulated via percutaneously inserted intramuscular electrodes or using surface stimulation through anaesthetized skin. Muscular sense was represented by the ability to detect the twitch and was compared between various tasks and stationary control trials. 3. Three hertz voluntary wrist movements significantly attenuated muscular sense to 37 % of control. This velocity-dependent attenuation was present over a range of twitch amplitudes suggesting it does not simply reflect a masking of low intensity stimuli. Perceptual ratings of twitch amplitude during fast imposed passive movements were reduced by 40 %, though this did not quite reach statistical significance. However, perceptual ratings of twitches evoked up to 2 s after the termination of the passive movements were significantly different from control. 4. Reaching with the stimulated, but not the contralateral, arm also significantly reduced muscular sense (to 40 %). 5. Attenuation to 58 % of control during cyclic stretching of the skin on the dorsum of the hand showed that signals from peripheral receptors may play a role. Attenuation prior to a single wrist flexion movement indicated that central sources can also contribute. 6. The results are consistent with current findings of a general attenuation of sensory feedback during movement and raise questions regarding the role of muscular sense in movement control.

Time course and magnitude of movement-related gating of tactile detection in humans. I. Importance of stimulus location

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.

Selective modification of somatosensory evoked potential during voluntary finger movement in humans

We examined changes in somatosensory evoked potentials (SEPs) during voluntary movement of fingers innervated by the stimulated nerve and those not innervated by the stimulated nerve and the relationship to the kind of movement modality. Analysis showed that the amplitude of most components at F3, C3', and P3, except for P45 at C3, N35 and P45 at P3, decreased during voluntary finger movement tasks. Further, we found that the components of P40 at F3, P45 at C3', and N35 at P3 were increased during the voluntary pulling movement of the second and the third digits compared to those during the voluntary pushing movement of the fourth and the fifth digits, whereas all other components were decreased at F3, C3', and P3. We also found that not all components of SEPs were decreased while some SEPs in middle latency were increased. In conclusion, we confirmed the selectivity in attenuation of the SEPs. Moreover, we noted an interesting finding that the selectivity of attenuation of the SEPs was most frequently observed in the N20, P30 (P25 at F3), N35 (N30 at F3), and P45 (P40 at F3) components at F3, C3', and P3.

Selective gating of lower limb cortical somatosensory evoked potentials (SEPs) during passive and active foot movements

We evaluated subcortical and cortical somatosensory evoked potentials (SEPs) in response to posterior tibial nerve stimulation in 4 experimental conditions of foot movement and compared them with the baseline condition of full relaxation. The experimental conditions were: (a) active flexion-extension of the stimulated foot; (b) active flexion-extension of the non-stimulated foot; (c) passive flexion-extension of the stimulated foot in complete relaxation; (d) tonic active flexion of the stimulated foot. We analyzed latencies and amplitudes of the subcortical P30 potential, of the contralateral pre-rolandic N37 and P50 responses and of the P37, N50 and P60 potentials recorded over the vertex. Latencies did not vary in any of the paradigms. The amplitude of subcortical P30 potential did not change during any of the paradigms. Among the cortical waves, P37, N50 and P60 amplitudes were significantly attenuated in all conditions except active movement of the non-stimulated foot (b). This attenuation was less during passive (c) than during active movements of the stimulated foot (a and d). The contralateral pre-rolandic waves N37 and P50 showed no significant decrease during any of the paradigms. These results suggest that gating occurs rostrally to the cervico-medullary junction, probably at cortical level. The different behavior of N37, P50 and P37, N50 cortical responses during movement of the stimulated foot provides evidence suggestive of a highly localized gating process occurring at cortical level. These potentials could reflect activation of separate, functionally distinct generators.