Human Somatosensory System Research Articles

A brush stimulator for functional brain imaging

Objective To describe a novel non-magnetic hand-held device to stimulate various parts of the skin and to evaluate its performance in magnetoencephalographic (MEG) recordings. Methods The hand-held part of the device consists of an optic fiber bundle that forms a small brush. Half of the fibers emit modulated red light and the other half detect the reflected light from the skin so that the brush-to-skin contact is detected by means of reflectance. Results Light tapping of the back of the hand at the innervation area of the radial nerve elicited clear responses in all 10 subjects studied, with the main deflections peaking 40–70 ms after the stimulus. The earliest responses, obtained with a higher number of averaged trials, peaked 27–28 ms after the tap to the left hand dorsum. Source analysis of the MEG signals indicated neuronal sources at the primary somatosensory (SI) cortex, with a clear somatotopical order for face vs. hand. Conclusions The device seems feasible for both MEG and functional magnetic resonance imaging experiments to address functional anatomy of the human somatosensory system with a real-life like stimulation. Significance Non-magnetic and artefact-free tactile stimulator with a selective stimulus offers new possibilities for experimental designs to study the human mechanoreceptor system.

Dopaminergic influences on changes in human tactile acuity induced by tactile coactivation

As shown in animal experiments, dopaminergic mechanisms participate in N-methyl-D-aspartate (NMDA) receptor-dependent neuroplasticity. Dopamine is thought to play a similar role in humans, where it influences learning and memory. Here, we tested the dopaminergic action on learning in the tactile domain. To induce tactile non-associative learning, we applied a tactile coactivation protocol, which is known to improve tactile two-point discrimination of the stimulated finger. We studied the influence of a single oral dose of levodopa (25, 50, 100, 250 or 350 mg) administered preceding the coactivation protocol on changes in tactile performance in different groups of subjects. In addition, 3 x 100 mg levodopa was administered over a time period of 3 h in another group. Under placebo conditions, tactile two-point discrimination was improved on the coactivated index finger. Similar improvement was found when 25, 50 and 250 mg levodopa was applied. On the contrary, tactile improvement was completely eliminated by 1 x 100 and 3 x 100 mg levodopa. No drug effects were found on the left index finger indicating that the drug had no effect on performance per se. In contrast to previous findings in the motor and speech domain, we found that the administration of levodopa exerts either no or even negative effects on non-associative learning in the human somatosensory system. Whenever levodopa is used in neurorehabilitative context, it has to be kept in mind that beneficial effects in the motor or speech domain cannot be easily generalized to other systems.

Oscillatory activity reflects the excitability of the human somatosensory system

The neuronal activity of the resting human brain is dominated by spontaneous oscillations in primary sensory and motor areas. These oscillations are thought to reflect the excitability of sensory and motor systems that can be modulated according to the actual behavioral demands. However, so far, evidence for an association between oscillatory activity and excitability has been inconsistent. Here, we used magnetoencephalography to reinvestigate the relationship between oscillatory activity and excitability in the somatosensory system on a single trial basis. Brief painful stimuli were applied to relate pain-induced suppressions of oscillatory activity to pain-induced increases in excitability. The analysis reveals a significant negative correlation between sensorimotor oscillatory activity, particularly in the α-band, and excitability of somatosensory cortices. Oscillatory activity outside the somatosensory system did not correlate with somatosensory excitability. These findings demonstrate that modulations of sensorimotor oscillatory activity specifically reflect modulations in excitability of the somatosensory system and thus provide direct evidence for the basic tenet of an association between oscillatory activity and cortical excitability.

Static Standing Trunk Sway Assessment in Amputees – Effects of Sub-Threshold Stimulation

Sub-threshold electrical stimulation can enhance the sensitivity of the human somatosensory system to improve the balance control capability of elderly was shown in recent rehabilitation articles. The purpose of this study was to evaluate the postural sway of trans-tibial amputees when performing single leg quiet standing on firm surface. Four unilateral trans-tibial amputees who consecutively wore prosthetics over 2 years were recruited in this study. Subjects performed single leg quiet standing trails with sub-threshold electrical stimulation applied at the quadriceps muscle during the trails. Spatial co-ordinates for the determination kinematic data (sway distance) of the center of mass (COM) on second sacral (S2) were collected using an ultrasound-based Zebris CMS-HS system. The single leg quiet standing test is measure considered to assess postural steadiness in a static position by a spatial measurement. The common notion is that a better postural steadiness, i.e. less postural sway, allows for longer time single leg quiet standing. However, there is lack of evidence how postural steadiness during single leg quiet standing changes over time. In this article, we hypothesized that the static balance of single leg quiet standing could be improved for providing proprioceptive neuromuscular facilitation using sub-sensory stimulation in amputees. To test this hypothesis, a computerized sub-threshold low-level electrical stimulation device was developed and proposed for clinical study. Experimental results show that reduction in all of the postural sway indices (constant time sway length, max sway distance and average sway distance) and increase in single leg support time index during single leg quiet standing by applying sub-sensory stimulation. The single leg quiet standing test findings suggest that sub-threshold electrical stimulation rehabilitation strategies may be effective in improving static balance performance for amputees.

DESIGN OF A NEW BIOFEEDBACK PROPRIOCEPTIVE NEUROMUSCULAR FACILITATION SYSTEM FOR BELOW-KNEE AMPUTEES

Proprioceptive neuromuscular facilitation and foot sensory compensation are critical to balance control and ambulatory performance in below-knee amputees. Sub-sensory stimulation has been shown to be effective in enhancing the sensitivity of the human somatosensory system. In addition, visual-auditory biofeedback to improve foot sensory compensation for amputees was suggested in recent articles. The purpose of this study is to develop a new biofeedback proprioceptive neuromuscular facilitation system for improving balance control and foot sensory compensation in below-knee amputees. The proposed system functioned with sub-threshold electrical stimulation and visual-auditory biofeedback was developed for clinical study. Two unilateral trans-tibial amputees who consecutively wore prosthetics over 10 years were participated in this study. Subjects performed multiple single leg quite standing trails with sub-sensory electrical stimulation applied at the quadriceps muscle during half of the trails. Four static balance performance indices (i.e. Holding Time Index, HTI; Sway Length Index, SLI; Max Sway Distance Index, MSDI; Average Sway Distance Index, ASDI) were characterized using Zebris motion analysis system. The improvement ratio of these static balance performance indices across subjects for single leg quiet standing tests were resulted in a 209.7% in HTI, 39.1% in SLI, 24.3% in MSDI, and 65.4% in ASDI respectively. In addition, multiple treadmill ambulatory trails with or without visual-auditory biofeedback were evaluated. Four dynamic gait performance indices (i.e. Double Support Time Index, DSTI; Constant Time Cadence Index, CTCI; Single Support Time Index, SSTI; Stance/Swing Phase Index, SSPI) were characterized with Zebris instrumented insole and associated FMS analysis software. With visual-auditory biofeedback, the improvement of all four dynamic gait performance indices in below-knee amputees was verified. The improvement ratio of four gait performance indices across subjects resulted in a 14.81% in DSTI (sound side), 14.29% in DSTI (affected side), 14% in CTCI, 13.00% in SSTI (sound side), 6.02% in SSTI (affected side), 45.17% in SSPI (sound side), and 27.49% in SSPI (affected side) respectively. These findings suggest that sub-threshold electrical stimulation and visual-auditory biofeedback proprioceptive neuromuscular facilitation strategies may be effective in compensating foot sensory loss and improving balance control for below-knee amputees.

Oscillatory activity reflects the excitability of the human somatosensory system

The neuronal activity of the resting human brain is dominated by spontaneous oscillations in primary sensory and motor areas. These oscillations are thought to reflect the excitability of sensory and motor systems that can be modulated according to the actual behavioral demands. However, so far, evidence for an association between oscillatory activity and excitability has been inconsistent. Here, we used magnetoencephalography to reinvestigate the relationship between oscillatory activity and excitability in the somatosensory system on a single trial basis. Brief painful stimuli were applied to relate pain-induced suppressions of oscillatory activity to pain-induced increases in excitability. The analysis reveals a significant negative correlation between sensorimotor oscillatory activity, particularly in the α-band, and excitability of somatosensory cortices. Oscillatory activity outside the somatosensory system did not correlate with somatosensory excitability. These findings demonstrate that modulations of sensorimotor oscillatory activity specifically reflect modulations in excitability of the somatosensory system and thus provide direct evidence for the basic tenet of an association between oscillatory activity and cortical excitability.

Oscillatory activity reflects the excitability of the human somatosensory system

The neuronal activity of the resting human brain is dominated by spontaneous oscillations in primary sensory and motor areas. These oscillations are thought to reflect the excitability of sensory and motor systems that can be modulated according to the actual behavioral demands. However, so far, evidence for an association between oscillatory activity and excitability has been inconsistent. Here, we used magnetoencephalography to reinvestigate the relationship between oscillatory activity and excitability in the somatosensory system on a single trial basis. Brief painful stimuli were applied to relate pain-induced suppressions of oscillatory activity to pain-induced increases in excitability. The analysis reveals a significant negative correlation between sensorimotor oscillatory activity, particularly in the α-band, and excitability of somatosensory cortices. Oscillatory activity outside the somatosensory system did not correlate with somatosensory excitability. These findings demonstrate that modulations of sensorimotor oscillatory activity specifically reflect modulations in excitability of the somatosensory system and thus provide direct evidence for the basic tenet of an association between oscillatory activity and cortical excitability.

Localization of Tactile Stimuli Depends on Conscious Detection

Neurological reports of "tactile blindsight" suggest that the human somatosensory system can extract behaviorally useful information about the location of a tactile stimulus in the absence of conscious awareness that the stimulus occurred (Paillard et al., 1983; Rossetti et al., 1995). However, in a series of psychophysical experiments with neurologically intact subjects, we found no evidence for such a dissociation. Our subjects' ability to name the finger on which a tactile stimulus had been presented was dependent on their ability to consciously detect that stimulus (Harris et al., 2004). The present experiments followed up on this study and specifically sought evidence for a dissociation when subjects were required to indicate the location of the stimulus either by pointing at or moving the stimulated finger, the same response made by the neurological patients. Once again, localization accuracy was correlated with detection, and, crucially, when both detection and localization were measured using equivalent forced-choice tasks, the subjects were completely unable to identify the location of stimuli that they had not detected. These findings are inconsistent with the dissociation implied by the cases of tactile blindsight, but are consistent with other neurological evidence that detection of a tactile stimulus does not depend on localization (Head and Holmes, 1911; Halligan et al., 1995; Rapp et al., 2002).

High-Frequency Oscillations in Somatosensory System

The recent revival of interest in high-frequency oscillation (HFO) is triggered by getting an opportunity to noninvasively monitor the timing of highly synchronized and rapidly repeating population spikes generated in the human somatosensory system. HFOs could be recorded from brainstem, cuneothalamic relay neurons, thalamus, thalamocortical radiation, thalamocortical terminals and cortex with deep brain or surface electrodes, or with magnetoencephalography. Here we briefly review the HFOs at each level of somatosensory pathways. HFOs recorded at brainstem might be produced by volume conduction from oscillations of the medial lemniscus. Thalamic HFOs at around 1000 Hz frequency would be generated within the somatosensory thalamus. Cortical HFOs would be generated by at least a few different mechanisms, thalamo-cortical projection terminals, interneurons and pyramidal cells of the primary sensory cortex. HFOs have been studied in several ways: their modulation by arousal changes, movements or drugs, their recovery function, effects of transcranial magnetic stimulation on them and also their changes in patients with various neurological diseases.

Percept-related activity in the human somatosensory system: functional magnetic resonance imaging studies

In this paper, we review blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) studies addressing the neural correlates of touch, thermosensation, pain and the mechanisms of their cognitive modulation in healthy human subjects. There is evidence that fMRI signal changes can be elicited in the parietal cortex by stimulation of single mechanoceptive afferent fibers at suprathreshold intensities for conscious perception. Positive linear relationships between the amplitude or the spatial extents of BOLD fMRI signal changes, stimulus intensity and the perceived touch or pain intensity have been described in different brain areas. Some recent fMRI studies addressed the role of cortical areas in somatosensory perception by comparing the time course of cortical activity evoked by different kinds of stimuli with the temporal features of touch, heat or pain perception. Moreover, parametric single-trial functional MRI designs have been adopted in order to disentangle subprocesses within the nociceptive system. Available evidence suggest that studies that combine fMRI with psychophysical methods may provide a valuable approach for understanding complex perceptual mechanisms and top-down modulation of the somatosensory system by cognitive factors specifically related to selective attention and to anticipation. The brain networks underlying somatosensory perception are complex and highly distributed. A deeper understanding of perceptual-related brain mechanisms therefore requires new approaches suited to investigate the spatial and temporal dynamics of activation in different brain regions and their functional interaction.

Spike times make sense

Many behavioral responses are completed too quickly for the underlying sensory processes to rely on estimation of neural firing rates over extended time windows. Theoretically, first-spike times could underlie such rapid responses, but direct evidence has been lacking. Such evidence has now been uncovered in the human somatosensory system. We discuss these findings and their potential generalization to other sensory modalities, and we consider some future challenges for the neuroscientific community.

Noise-enhanced balance control in older adults.

Somatosensory information is critical to balance control and fall prevention in older adults. Recently, it has been shown that low-level input noise (electrical or mechanical) can enhance the sensitivity of the human somatosensory system. In this study, we tested the effect of low-level electrical noise, applied at the knee, on balance control in 13 healthy elderly volunteers. Subjects performed multiple single-legged stance trials with imperceptible electrical noise applied at the knee during half of the trials. Balance performance was characterized using a force platform to measure the displacement of the center of pressure (COP) under the subject's stance foot. Seven sway parameters were extracted from the COP time series. Improved balance was defined as a reduction in postural sway as indicated by decreases in the COP measures. Six of the seven sway parameters decreased with electrical noise. Three of these parameters decreased significantly ( < 0.05), and a fourth parameter was borderline significant. Averaged across subjects, the application of electrical noise resulted in a 3.8% reduction in mediolateral COP standard deviation ( = 0.04), a 5.4% decrease in the maximum anteroposterior COP excursion ( = 0.03), a 3.1% reduction in the COP path length ( = 0.04), and a 7.8% decrease in swept area ( = 0.05). The results suggest that imperceptible electrical noise, when applied to the knee, can enhance the balance performance of healthy older adults. These findings suggest that electrical noise-based devices may be effective in improving balance control in elderly people.

Neuroplastic Changes Related to Pain Occur at Multiple Levels of the Human Somatosensory System: A Somatosensory-Evoked Potentials Study in Patients with Cervical Radicular Pain

Studies suggest that pain may play a major role in determining cortical rearrangements in the adult human somatosensory system. Most studies, however, have been performed under conditions whereby pain coexists with massive deafferentation (e.g., amputations). Moreover, no information is available on whether spinal and brainstem changes contribute to pain-related reorganizational processes in humans. Here we assess the relationships between pain and plasticity by recording somatosensory-evoked potentials (SEPs) in patients who complained of pain to the right thumb after a right cervical monoradiculopathy caused by compression of the sixth cervical root, but did not present with clinical or neurophysiological signs of deafferentation. Subcortical and cortical potentials evoked by stimulation of digital nerves of the right thumb and middle finger were compared with those obtained after stimulation of the left thumb and middle finger and with those obtained in a control group tested in comparable conditions. Amplitudes of spinal N13, brainstem P14, parietal N20 and P27, and frontal N30 potentials after stimulation of the painful right thumb were greater than those of the nonpainful left thumb and showed a positive correlation with magnitude of pain. This right-left asymmetry was absent after stimulation of the patients' middle fingers and in control subjects. Results suggest that chronic cervical radicular pain is associated with changes in neural activity at multiple levels of the somatosensory system. The absence of correlation between the amplitude of spinal, brainstem, and cortical components of SEPs suggests that enhancement of cortical activity is not a simple amplification of subcortical enhancement.

High-frequency (300–800 Hz) components in cat geniculate (dLGN) early visual responses

We analysed the early visual responses of relay cells of the dorsal part of cat lateral geniculate nucleus (dLGN) for the occurrence and characteristics of high-frequency (>300 Hz) spike patterns comparable to the high-frequency oscillations (HFO) found in the human somatosensory system. By using a special algorithm for correcting response latency, we can show that the vast majority of dLGN visual responses which were elicited by a sudden change in contrast show HFOs in the range of 300 to more than 800 Hz. After response time correction these HFOs are clearly visible in summed responses, indicating that these patterns are highly reproducible by identical stimuli. On this basis we analysed the HFOs in more detail. We found the oscillation frequency to increase with stimulus contrast and the area of the receptive field centre covered by an excitatory stimulus. Inhibition reduces the oscillation frequency as demonstrated with additional stimulation of the antagonistic surround of the receptive field and by blocking inhibition with micro-iontophoretical application of bicuculline methiodide. The HFO was almost independent of the state of the system as estimated from the EEG pattern. Based on these findings we discuss whether bursts of action potentials triggered by the low-threshold calcium spike (LTS) can contribute to this pattern of visual thalamic activity.

Somatosensory cortex responses to median nerve stimulation: fMRI effects of current amplitude and selective attention

Objectives: The aim of this study was to localize and to investigate response properties of the primary (SI) and the secondary (SII) somatosensory cortex upon median nerve electrical stimulation.Methods: Functional magnetic resonance imaging (fMRI) was used to quantify brain activation under different paradigms using electrical median nerve stimulation in healthy right-handed volunteers. In total 11 subjects were studied using two different stimulus current values in the right hand: at motor threshold (Imax) and at Imin (1/2 Imax). In 7 of these 11 subjects a parametric study was then conducted using 4 stimulus intensities (6/6, 5/6, 4/6 and 3/6 Imax). Finally, in 10 subjects an attention paradigm in which they had to perform a counting task during stimulation with Imin was done.Results: SI activation increased with current amplitude. SI did not show significant activation during stimulation at Imin. SII activation did not depend on current amplitude. Also the posterior parietal cortex appeared to be activated at Imin. The Imin response in SII significantly increased by selective attention compared to Imin without attention. At Imax significant SI activity was observed only in the contralateral hemisphere, the ipsilateral cerebellum, while other areas possibly showed bilateral activation.Conclusions: Distributed activation in the human somatosensory cortical system due to median nerve stimulation was observed using fMRI. SI, in contrast to SII, appears to be exclusively activated on the contralateral side of the stimulated hand at Imax, in agreement with the concept of SI's important role in processing of proprioceptive input. Only SII remains significantly activated in case of lower current values, which are likely to exclusively stimulate the sensible fibres mediating cutaneous receptor input. Selective attention only enhances SII activity, indicating a higher-order role for SII in the processing of somatosensory input.

Static Standing Trunk Sway Assessment in Amputees – Effects of Sub‐Threshold Stimulation

Sub‐threshold electrical stimulation can enhance the sensitivity of the human somatosensory system to improve the balance control capability of elderly was shown in recent rehabilitation articles. The purpose of this study was to evaluate the postural sway of trans‐tibial amputees when performing single leg quiet standing on firm surface. Four unilateral trans‐tibial amputees who consecutively wore prosthetics over 2 years were recruited in this study. Subjects performed single leg quiet standing trails with sub‐threshold electrical stimulation applied at the quadriceps muscle during the trails. Spatial co‐ordinates for the determination kinematic data (sway distance) of the center of mass (COM) on second sacral (S2) were collected using an ultrasound‐based Zebris CMS‐HS system. The single leg quiet standing test is measure considered to assess postural steadiness in a static position by a spatial measurement. The common notion is that a better postural steadiness, i.e. less postural sway, allows for longer time single leg quiet standing. However, there is lack of evidence how postural steadiness during single leg quiet standing changes over time. In this article, we hypothesized that the static balance of single leg quiet standing could be improved for providing proprioceptive neuromuscular facilitation using sub‐sensory stimulation in amputees. To test this hypothesis, a computerized sub‐threshold low‐level electrical stimulation device was developed and proposed for clinical study. Experimental results show that reduction in all of the postural sway indices (constant time sway length, max sway distance and average sway distance) and increase in single leg support time index during single leg quiet standing by applying sub‐sensory stimulation. The single leg quiet standing test findings suggest that sub‐threshold electrical stimulation rehabilitation strategies may be effective in improving static balance performance for amputees.

High-frequency (600 Hz) SEP activities originating in the subcortical and cortical human somatosensory system

Digitally high-pass filtered median nerve SEP show an oscillatory burst of low-amplitude high-frequency (600 Hz) wavelets superimposed on the N20 component which itself is generated by excitatory postsynaptic potentials of area 3b pyramidal cells. Prior studies using magnetoencephalography (MEG) localized one wavelet generator close to the primary somatosensory hand cortex. Since MEG recordings are biased towards tangentially oriented and superficial generators, a dipole source analysis of 32-channel electric SEP recordings was employed here to test for the possibility of deep and/or radially oriented burst generators: in 10 normal subjects low noise (16 000 averages) median nerve SEP were evaluated using dipole source analysis before and after applying a digital 475 Hz high-pass filter. Two main oscillatory 600 Hz burst sources were modeled; (i) a deep burst source close to the thalamus, most active in a time window between the brain-stem P14 and the cortical N20 sources, detectable in 7 of 10 subjects; most probably, this activity originates from deep axon segments of thalamocortical fibers; and (ii) a subsequent burst source timed around the N20 and located in the vicinity of the primary somatosensory hand cortex in all subjects, which was already known from MEG data. This superficial oscillatory source may be dominated by repetitive activity conducted in the terminal segments of the thalamocortical projection fibers initiated by the thalamic burst generator.

High-frequency (600 Hz) SEP activities originating in the subcortical and cortical human somatosensory system

High-frequency (600 Hz) SEP activities originating in the subcortical and cortical human somatosensory system

Multimodality imaging of human somatosensory system: combined fMRI and MEG study

Multimodality imaging of human somatosensory system: combined fMRI and MEG study

SS-7-5 The generator of high-frequency magnetic oscillations in the human somatosensory system

SS-7-5 The generator of high-frequency magnetic oscillations in the human somatosensory system