Variability of BOLD response evoked by foot vibrotactile stimulation: Influence of vibration amplitude and stimulus waveform

Human foot cutaneous receptors function: clinical findings and prospects of using medical devices to stimulate mechanoreceptors in neurorehabilitation

The effectiveness of the support stimulation of the mechanoreceptors of the feet has been first shown in space medicine. In space flight during support withdrawal with non-use of postural muscle, this method is a countermeasure against sensorimotor disorders. Later, it was applied in clinical practice as treatment of motor disorders after stroke, in Parkinson's disease, infantile cerebral palsy, neuropathies, and many others. The impact of such stimulation on motor control is due to spinal and supraspinal mechanisms, which are activated by creating an additional support afferent input through the plantar surface. Many studies confirmed the positive effect of support stimulation on motor control, but the protocols of such stimulation remain the subject of active discussion. This review includes (1) the features of sensitivity of the foot sole cutaneous afferents to the support mechanical stimuli, (2) data on spinal and supraspinal responses of the nervous system to support stimulation, and (3) the results of applying this approach in neurological practice via various techniques. Summarizing this information, the authors highlight the most promising ways and types of medical devices for foot support stimulation in neurology.

Modulation of the human sensorimotor system by afferent somatosensory input: evidence from experimental pressure stimulation and physiotherapy

Peripheral afferent input is critical for human motor control and motor learning. Both skin and deep muscle mechanoreceptors can affect motor behaviour when stimulated. Whereas some modalities such as vibration have been employed for decades to alter cutaneous and proprioceptive input, both experimentally and therapeutically, the central effects of mechanical pressure stimulation have been studied less frequently. This discrepancy is especially striking when considering the limited knowledge of the neurobiological principles of frequently used physiotherapeutic techniques that utilise peripheral stimulation, such as reflex locomotion therapy. Our review of the available literature pertaining to pressure stimulation focused on transcranial magnetic stimulation (TMS) and neuroimaging studies, including both experimental studies in healthy subjects and clinical trials. Our search revealed a limited number of neuroimaging papers related to peripheral pressure stimulation and no evidence of effects on cortical excitability. In general, the majority of imaging studies agreed on the significant involvement of cortical motor areas during the processing of pressure stimulation. Recent data also point to the specific role of subcortical structures, such as putamen or brainstem reticular formation. A thorough comparison of the published results often demonstrated, however, major inconsistencies which are thought to be due to variable stimulation protocols and statistical power. In conclusion, localised peripheral sustained pressure is a potent stimulus inducing changes in cortical activation within sensory and motor areas. Despite historical evidence for modulation of motor behaviour, no direct link can be established based on available fMRI and electrophysiological data. We highlight the limited amount of research devoted to this stimulus modality, emphasise current knowledge gaps, present recent developments in the field and accentuate evidence awaiting replication or confirmation in future neuroimaging and electrophysiological studies.

Disinhibition of Human Primary Somatosensory Cortex After Median Nerve Transection and Reinnervation

Despite state-of-the-art surgical and postoperative treatment, median nerve transection causes lasting impaired hand function due to limitations in the nerve’s reinnervation ability. The defective innervation and thus controllability of the affected hand can shape the brain’s control of manual behaviors. Earlier studies of changes in the processing of tactile stimuli have focused mainly on stimulation of the reinnervated hand and lack sufficient control over the brain’s use of the tactile input in perceptual terms. Here we used fMRI to measure brain activity (BOLD-signal) in 11 people with median nerve injury and healthy controls (N = 11) when performing demanding tactile tasks using the tip of either the index or little finger of either hand. For the nerve-injured group, the left median nerve had been traumatically transected in the distal forearm and surgically repaired on average 8 years before the study. The hand representation of their contralesional (right) primary somatosensory cortex (S1) showed greater activity compared to controls when the left reinnervated index finger was used, but also when the left-hand little finger and the fingers of the right hand innervated by uninjured nerves were used. We argue that the overall increase in activity reflects a general disinhibition of contralesional S1 consistent with an augmented functional reorganizational plasticity being an ongoing feature of chronic recovery from nerve injury. Also, the nerve-injured showed increased activity within three prefrontal cortical areas implicated in higher-level behavioral processing (dorsal anterior cingulate cortex, left ventrolateral prefrontal and right dorsolateral prefrontal cortex), suggesting that processes supporting decision-making and response-selection were computationally more demanding due to the compromised tactile sensibility.

Effects of Focal Vibration over Upper Limb Muscles on the Activation of Sensorimotor Cortex Network: An EEG Study

Studying the therapeutic effects of focal vibration (FV) in neurorehabilitation is the focus of current research. However, it is still not fully understood how FV on upper limb muscles affects the sensorimotor cortex in healthy subjects. To explore this problem, this experiment was designed and conducted, in which FV was applied to the muscle belly of biceps brachii in the left arm. During the experiment, electroencephalography (EEG) was recorded in the following three phases: before FV, during FV, and two minutes after FV. During FV, a significant lower relative power at C3 and C4 electrodes and a significant higher connection strength between five channel pairs (Cz-FC1, Cz-C3, Cz-CP6, C4-FC6, and FC6-CP2) in the alpha band were observed compared to those before FV. After FV, the relative power at C4 in the beta band showed a significant increase compared to its value before FV. The changes of the relative power at C4 in the alpha band had a negative correlation with the relative power of the beta band during FV and with that after FV. The results showed that FV on upper limb muscles could activate the bilateral primary somatosensory cortex and strengthen functional connectivity of the ipsilateral central area (FC1, C3, and Cz) and contralateral central area (CP2, Cz, C4, FC6, and CP6). These results contribute to understanding the effect of FV over upper limb muscles on the brain cortical network.

Transcranial direct current stimulation modulates the brain's response to foot stimuli under dual-task condition: A fMRI study in elderly adults

Previous behavioral studies have shown that high-intensity cognitive tasks weaken balance control in elder adults. Moreover, age-related loss of plantar sensation is considered to be an important contributing factor to the occurrence of falls. Recently, we have realized that transcranial direct current stimulation (tDCS) can effectively improve the balance of the elderly under the dual-task, but its underlying regulatory mechanism is not clear. In this study, task functional Magnetic Resonance Imaging (fMRI) was used to study the brain's response to foot stimuli under foot stimuli or dual-task (foot stimuli and cognitive task) conditions to explain the effect of the addition of cognitive tasks during balance in 16 healthy elderly adults. To study whether tDCS would counteract the effect of the added cognitive task, we further compared the differences in activity of the cerebral cortices of dual-task and tDCS-dual-task conditions. The results suggested added cognitive tasks significantly attenuated the response of the brain to foot stimuli in elderly adults. Moreover, the cortex excitability weakened by cognitive tasks was significantly promoted after 20 min of tDCS. In conclusion, a portion of the resources originally used for plantar sensory processing may be assigned to the processing of the cognitive task when the cognitive tasks are added, which results in insufficient resources for plantar sensory processing. tDCS improves the ability of the brain to respond to foot stimuli by modulating the excitability of the cognitive cortex and reverses the effects of cognitive tasks.

Visual conflict and cognitive load modify postural responses to vibrotactile noise

BACKGROUND

Underlying the increased incidence of falls during multitasking is a reduced ability to detect or attend to the sensory information signaling postural instability. Adding noise to a biological system has been shown to enhance the detection and transmission of weakened or sub-threshold cutaneous signals. If stochastic resonance is to become an effective adjunct to rehabilitation, we need to determine whether vibrotactile noise can be effective when added to an environment presenting with other sensory noise.

METHODS

Sub-threshold vibration noise was applied for 30 sec at the soles of the feet in 21 healthy adults (20-29 yrs) between two 30-sec periods of no vibration. During the trials, subjects stood quietly with eyes closed or while viewing a visual scene that rotated in continuous upward pitch at 30 deg/sec. Subjects were also tested with these two visual conditions while performing a mental calculation task. It was hypothesized that sub-threshold vibration would increase regularity of postural sway, thereby improving postural stabilization during an attention demanding task but exerting less effect with multiple sensory demands. An ellipse fit to the covariance matrix revealed excursion of center of pressure (COP) and center of mass (COM) responses in the anterior-posterior and lateral planes. RMS values and approximate entropy of the COP and COM were calculated and statistically compared.

RESULTS

The addition of vibrotactile noise to the plantar surface during quiet stance with eyes closed reduced the area of the COM and COP responses, which then returned to pre-vibration levels after vibration was removed. Postural sway was generally increased with both visual field rotations and mental calculation compared to the eyes closed condition. The effect of sub-threshold vibratory noise on postural behavior was modified when visual field rotations and mental calculation was combined. It was shown that the measure of approximate entropy reflected increased task complexity.

CONCLUSIONS

Our results suggest that the impact of destabilizing signals is modulated when combined with vibrotactile stimulation. The strong aftereffects of the vibration stimulus suggest that the system has adapted to the sensory array even in the short time period tested here. The results imply that application of vibrotactile stimulation has the potential for diminishing sway magnitudes while increasing the potential for response variability, thereby presenting a non-invasive method of reducing the potential for falls.

Effect of a local vibration stimulus training programme on postural sway and gait in chronic stroke patients: a randomized controlled trial

Objective:

To investigate the effect of a local vibration stimulus training programme on postural sway and gait in stroke patients.

Design:

A randomized controlled trial with two groups: a local vibration stimulus training programme group and a sham group.

Setting:

Inpatient rehabilitation centre.

Subjects:

Thirty-one chronic stroke patients.

Interventions:

Both groups underwent a standard rehabilitation programme. The local vibration stimulus training programme group ( n = 16) participated in the local vibration stimulus training programme for 30 minutes a day, five times a week, for six weeks. The sham group ( n = 15) participated in a sham local vibration stimulus training programme for 30 minutes a day, five times a week, for six weeks.

Main measures:

A forceplate was used to measure postural sway under two conditions: standing with eyes open and eyes closed. Gait ability was measured using the GAITRite system.

Results:

In postural sway, greater improvements in the postural sway distance with eyes-open (–11.91 vs. 0.80) and eyes-closed (–20.67 vs. –0.34) conditions and postural sway velocity with eyes-open (–0.40 vs. 0.03) and eyes-closed (–0.69 vs. –0.01) conditions were observed in the local vibration stimulus training programme group, compared with the sham group ( P < 0.05). In gait ability, greater improvement in gait speed (15.06 vs. 2.85), cadence (8.46 vs. 1.55), step length (7.90 vs. 3.64), and single limb support time (0.12 vs. 0.01) were observed in the local vibration stimulus training programme group, compared with the sham group ( P < 0.05).

Conclusions:

These findings suggest that local vibration stimulus training programme is an effective method for improvement of the postural sway and gait ability of chronic stroke patients.

BOLD responses to tactile stimuli in visual and auditory cortex depend on the frequency content of stimulation

Although some brain areas preferentially process information from a particular sensory modality, these areas can also respond to other modalities. Here we used fMRI to show that such responsiveness to tactile stimuli depends on the temporal frequency of stimulation. Participants performed a tactile threshold-tracking task where the tip of either their left or right middle finger was stimulated at 3, 20, or 100 Hz. Whole-brain analysis revealed an effect of stimulus frequency in two regions: the auditory cortex and the visual cortex. The BOLD response in the auditory cortex was stronger during stimulation at hearable frequencies (20 and 100 Hz) whereas the response in the visual cortex was suppressed at infrasonic frequencies (3 Hz). Regardless of which hand was stimulated, the frequency-dependent effects were lateralized to the left auditory cortex and the right visual cortex. Furthermore, the frequency-dependent effects in both areas were abolished when the participants performed a visual task while receiving identical tactile stimulation as in the tactile threshold-tracking task. We interpret these findings in the context of the metamodal theory of brain function, which posits that brain areas contribute to sensory processing by performing specific computations regardless of input modality.

Novel MRI-compatible tactile stimulator for cortical mapping of foot sole pressure stimuli with fMRI

Foot sole somatosensory feedback is critical to motor control and declines with aging and disease. To enable study of cortical networks underlying foot sole somatosensation, we developed a pneumatic tactile stimulator capable of producing one degree-of-freedom (DOF) oscillations with preset waveform, frequency (≤10 Hz), force magnitude (5-500 N), duty cycle (20-100%), and contacted surface area over which pressures are applied to the foot sole. Image tests (anatomical/functional/field map) of a phantom demonstrated that the device is compatible with 3 T MRI. Gradient-recalled echo-planar images of seven healthy young adults using a typical block-designed 1 Hz sinusoidal stimulation protocol revealed significant activation contralaterally within the primary somatosensory cortex and paracentral gyrus, and bilaterally within the secondary somatosensory cortex. The stimulation system may therefore serve as a research tool to study functional brain networks involved in the perception and modulation of foot sole somatosensation and its relationship to motor control.

Mechanical flutter stimulation induces a lasting response in the sensorimotor cortex as revealed with BOLD fMRI

It has been recently shown that 20 min of mechanical flutter stimulation induces lasting motor cortical excitability changes, as assessed by transcranial magnetic stimulation in relaxed hand muscles. The present functional magnetic resonance imaging (fMRI) study aims to examine if such neuromodulatory changes are reflected in the BOLD signal during a motor test. Therefore, two groups were recruited: one group receiving whole-hand flutter stimulation with a frequency of 25 Hz (FSTIM group, n = 22) and a second group receiving no stimulation (NOSTIM group, n = 22). As motor test finger-to-thumb tapping was performed to activate a wide sensorimotor network during the fMRI measurements. Three fMRI measurements were obtained with this test: before stimulation (PRE), after stimulation (POST1), and 1 h after stimulation (POST2). Three regions of interest (ROIs) were defined: primary motor area (M1), primary somatosensory area (S1), and supplementary motor area. In the absence of baseline differences between both groups, the FSTIM group showed increased movement-related brain activations compared with the NOSTIM group, both at POST1 and POST2. ROI analysis revealed increased blood-oxygenation-level-dependent (BOLD) responses within contralateral S1 (+20%) and M1 (+25%) at POST1, which lasted until POST2. These poststimulatory effects within S1 and M1 obviously reflect neuroplastic changes associated with augmented cortical excitability. These findings are of high clinical relevance, for example, to improve the treatment of stroke patients.

Within-limb somatotopic organization in human SI and parietal operculum for the leg: an fMRI study

Somatotopic organizations in human somatosensory cortex (SI and SII) for scattered portions of the leg have not been systematically observed with functional magnetic resonance imaging (fMRI). In this research we compared functional representations in the contralateral SI and bilateral parietal operculum (that contained subregions OP1, 3-4 of SII and OP2) of four acupoints in right leg in proximal-distal and medial-lateral arrangement. The results were: (1) somatotopy of SI demonstrated a lateral-to-medial and inferior-to-superior pattern when acupoints were shifting from proximal to distal or from medial to lateral; (2) the contralateral OP1 also showed a clear somatotopic organization for the four separated leg portions, and the ipsilateral OP1 showed a similar pattern to the contralateral OP1, thus arrangements of responses in the two areas were mirror-symmetric against y-axis; (3) the contralateral OP2 showed a somatotopic organization when acupoints shifting from proximal to distal, while the contralateral OP3 presented a trend of somatotopy opposite to that of the contralateral OP1. These results first show definite within-leg somatotopy of human SI for scattered leg portions in medial-lateral arrangement using fMRI. Within-limb somatotopic organization of OP1 for leg portions arranging from proximal to distal as well as from medial to lateral, and somatotopy of OP2 for leg portions arranging from proximal to distal, are also shown for the first time. Our results also reinforce the proposal of a somatotopic map existing in human OP3, and indicating a fourth somatotopic map in OP2 in human parietal operculum, which suggests that OP 2 is not just a vestibular area. In addition, separable activations in somatosensory cortex induced by adjacent acupoints should play a fundamental role in acupoint-specific effects in the brain.

Differential effects of cognitive demand on human cortical activation associated with vibrotactile stimulation

This event-related functional MRI study examines the neural correlates of vibrotactile sensation within the context of different psychophysical demands. Nine subjects received vibrotactile stimuli on the right volar forearm during detection, localization, and passive tasks. In the detection task, subjects indicated the offset (end) of each stimulus by pressing a response key with their left hand. In the localization task, subjects identified the location of the stimulus ("distal?" or "proximal?") by pressing the appropriate response key 4 s after the end of the stimulus. In the passive task, subjects received the same vibrotactile stimuli, but no response was required. Analysis of stimulus-evoked activity compared with the resting baseline period revealed significant bilateral secondary somatosensory cortex activation for all three tasks. However, only in the offset-detection and localization tasks was stimulus-evoked activation observed in other expected areas of tactile processing, such as contralateral primary somatosensory cortex neighboring the posterior parietal cortex (SI/PPC) and in bilateral anterior insular cortex (aIC). During the localization task, we identified vibrotactile-evoked activation in the right aIC, which was maintained after the termination of the stimulus. Results suggest that vibrotactile-related activation within SI/PPC and aIC is enhanced by the increased levels of attention and cognitive demands required by the detection and localization tasks. Activation of aIC not only during vibrotactile stimulation, but also during the poststimulus delay in the localization trials, is consistent with the growing literature linking this area with the perception and short-term memory of tactile information.

Automated classification of fMRI data employing trial-based imagery tasks

Automated interpretation and classification of functional MRI (fMRI) data is an emerging research field that enables the characterization of underlying cognitive processes with minimal human intervention. In this work, we present a method for the automated classification of human thoughts reflected on a trial-based paradigm using fMRI with a significantly shortened data acquisition time (less than one minute). Based on our preliminary experience with various cognitive imagery tasks, six characteristic thoughts were chosen as target tasks for the present work: right-hand motor imagery, left-hand motor imagery, right foot motor imagery, mental calculation, internal speech/word generation, and visual imagery. These six tasks were performed by five healthy volunteers and functional images were obtained using a T(*)(2)-weighted echo planar imaging (EPI) sequence. Feature vectors from activation maps, necessary for the classification of neural activity, were automatically extracted from the regions that were consistently and exclusively activated for a given task during the training process. Extracted feature vectors were classified using the support vector machine (SVM) algorithm. Parameter optimization, using a k-fold cross validation scheme, allowed the successful recognition of the six different categories of administered thought tasks with an accuracy of 74.5% (mean)+/-14.3% (standard deviation) across all five subjects. Our proposed study for the automated classification of fMRI data may be utilized in further investigations to monitor/identify human thought processes and their potential link to hardware/computer control.