Interaction effects of galvanic vestibular stimulation and head position on the soleus H reflex in humans

Effects of cerebellar transcranial direct current stimulation on the excitability of spinal motor neurons and vestibulospinal tract in healthy individuals

Cerebellar transcranial direct current stimulation (ctDCS) modulates cerebellar cortical excitability in a polarity-dependent manner and affects inhibitory pathways from the cerebellum. The cerebellum modulates spinal reflex excitability via the vestibulospinal tract and other pathways projecting to the spinal motor neurons; however, the effects of ctDCS on the excitability of spinal motor neurons and vestibulospinal tract remain unclear. The experiment involved 13 healthy individuals. ctDCS (sham-ctDCS, anodal-ctDCS, and cathodal-ctDCS) was applied to the cerebellar vermis at 2 mA with an interval of at least 3 days between each condition. We measured the maximal M-wave (Mmax) and maximal H-reflex (Hmax) in the right soleus muscle to assess the excitability of spinal motor neurons. We applied galvanic vestibular stimulation (GVS) for 200 ms at 100 ms before tibial nerve stimulation to measure Hmax conditioned by GVS (GVS-Hmax) and calculated the change rate of Hmax by GVS as the excitability of vestibulospinal tract. We measured the Mmax, Hmax, and GVS-Hmax before, during, and after ctDCS in the sitting posture. No main effects of tDCS condition, main effects of time, or interaction effects were observed in Hmax/Mmax or the change rate of Hmax by GVS. It has been suggested that ctDCS does not affect the excitability of spinal motor neurons and vestibulospinal tract, as measured by neurophysiological methods, such as the H-reflex, in healthy individuals in a sitting posture. Effect of ctDCS on other descending pathways to spinal motor neurons, the neurological mechanism of tDCS and the cerebellar activity during the experiment may have contributed to these results. Therefore, we need to investigate the involvement of the cerebellum in Hmax/Mmax and the change rate of Hmax by GVS under different neuromodulation techniques and postural conditions.

Post-activation depression of the Hoffman reflex is not altered by galvanic vestibular stimulation in healthy subjects

The comprehension of the neural elements interacting in the spinal cord affected by vestibular input will contribute to the understanding of movement execution in normal and pathological conditions. In this context, Hoffman's reflex (H-reflex) has been used to evaluate transient excitability changes on the spinal cord descending pathways. The post-activation depression (P-AD) of the H-reflex consists of evoking consecutive responses (>1 Hz) provoking an amplitude depression, which has been shown to diminish in pathological conditions (i.e., spasticity, diabetic neuropathy). Galvanic Vestibular Stimulation (GVS) is a non-invasive method that activates the vestibular afferents and has been used to study the excitability of the H-reflex applied as a conditioning pulse. To our knowledge, there are no reports evaluating the P-AD during and after GVS. Our primary aim was to determine if GVS alters the P-AD evoked by stimulating the tibial nerve at 0.1, 1, 5, and 10 Hz, recording in the gastrocnemius and soleus muscles. Direct current stimulation of 2.0 ± 0.6 mA with the cathode ipsilateral (Ipsi) or contralateral (Contra) to the H-reflex electrode montage was applied bilaterally over the mastoid process in 19 healthy subjects. The P-AD's immediate post-GVS response (P Ipsi, P Contra) was also analyzed. Secondarily, we analyzed the excitability of the H-reflex during GVS. Responses evoked at 0.1 Hz with GVS, post-GVS, and a Control (no GVS) condition were used for comparisons. Our results show that P-AD persisted in all subjects despite increased excitability induced by GVS: statistical significance was found when comparing P-AD at 1, 5, and 10 Hz with the corresponding condition (Control, Ipsi, P Ipsi, Contra, P Contra) at 0.1 Hz (p < 0.001). Additionally, the increase in excitability produced by GVS was quantified for the first H-reflex of each P-AD stimulation frequency. The percentage change for all GVS conditions surpassed the Control by at least 20%, being statistically significant for Contra compared to Control (p < 0.01). In summary, although GVS increases the excitability of the vestibulospinal pathway at a premotor level, the neural inhibitory mechanism present in P-AD remains unaltered in healthy subjects.

Relationships between changes in lateral vestibulospinal tract excitability and postural control by dynamic balance intervention in healthy individuals: A preliminary study

Introduction

We conducted dynamic balance or static intervention on healthy young adults to examine the changes in lateral vestibulospinal tract (LVST) excitability and postural control that ensued following dynamic balance intervention and to investigate the correlation between these changes.

Methods

Twenty-eight healthy young adults were randomly assigned to either the dynamic balance group or the control group. They performed either a dynamic balance or static intervention for 10 trials of 30 s each and were assessed for head jerks during the intervention to confirm adaptation to the intervention. The dynamic balance intervention consisted of maintaining balance on a horizontally unstable surface, whereas the control intervention involved standing in the same foot position as the dynamic balance intervention on a stable surface while completing a maze task. LVST excitability and postural stability were assessed before and after the interventions. LVST excitability was assessed as the change rate in the soleus H-reflex amplitude with galvanic vestibular stimulation (GVSH). The velocity and area of the center of pressure (COP) were examined in the eyes closed/foam rubber condition.

Results

No significant main and interaction effects (task, time) were observed for GVSH and COP variables. In the dynamic balance intervention, head jerk significantly decreased, and GVSH-change and changes in head jerk and COP area were significantly negatively correlated.

Discussion

The LVST excitability change for the dynamic balance intervention varied among the participants, although increased LVST excitability may have been related to increased postural stability.

Vestibular motor control

The vestibular system is an essential sensory system that generates motor reflexes that are crucial for our daily activities, including stabilizing the visual axis of gaze and maintaining head and body posture. In addition, the vestibular system provides us with our sense of movement and orientation relative to space and serves a vital role in ensuring accurate voluntary behaviors. Neurophysiological studies have provided fundamental insights into the functional circuitry of vestibular motor pathways. A unique feature of the vestibular system compared to other sensory systems is that the same central neurons that receive direct input from the afferents of the vestibular component of the 8th nerve can also directly project to motor centers that control vital vestibular motor reflexes. In turn, these reflexes ensure stabilize gaze and the maintenance of posture during everyday activities. For instance, a direct three-neuron pathway mediates the vestibulo-ocular reflex (VOR) pathway to provide stable gaze. Furthermore, recent studies have advanced our understanding of the computations performed by the cerebellum and cortex required for motor learning, compensation, and voluntary movement and navigation. Together, these findings have provided new insights into how the brain ensures accurate self-movement during our everyday activities and have also advanced our knowledge of the neurobiological mechanisms underlying disorders of vestibular processing.

Distinct representations of body and head motion are dynamically encoded by Purkinje cell populations in the macaque cerebellum

The ability to accurately control our posture and perceive our spatial orientation during self-motion requires knowledge of the motion of both the head and body. However, while the vestibular sensors and nuclei directly encode head motion, no sensors directly encode body motion. Instead, the integration of vestibular and neck proprioceptive inputs is necessary to transform vestibular information into the body-centric reference frame required for postural control. The anterior vermis of the cerebellum is thought to play a key role in this transformation, yet how its Purkinje cells transform multiple streams of sensory information into an estimate of body motion remains unknown. Here, we recorded the activity of individual anterior vermis Purkinje cells in alert monkeys during passively applied whole-body, body-under-head, and head-on-body rotations. Most Purkinje cells dynamically encoded an intermediate representation of self-motion between head and body motion. Notably, Purkinje cells responded to both vestibular and neck proprioceptive stimulation with considerable heterogeneity in their response dynamics. Furthermore, their vestibular responses were tuned to head-on-body position. In contrast, targeted neurons in the deep cerebellar nuclei are known to unambiguously encode either head or body motion across conditions. Using a simple population model, we established that combining responses of~40-50 Purkinje cells could explain the responses of these deep cerebellar nuclei neurons across all self-motion conditions. We propose that the observed heterogeneity in Purkinje cell response dynamics underlies the cerebellum’s capacity to compute the dynamic representation of body motion required to ensure accurate postural control and perceptual stability in our daily lives.

Reliability and laterality of the soleus H-reflex following galvanic vestibular stimulation in healthy individuals

The vestibulospinal tract (VST) plays an important role in the control of the ipsilateral antigravity muscles, and the balance of left and right VST excitability is important in human postural control. A method for measuring VST excitability is the application of galvanic vestibular stimulation (GVS) before tibial nerve stimulation that evokes the soleus H-reflex; the change rate of the H-reflex amplitude is then evaluated. Assessments of VST excitability and the left and right balance could be useful when determining the pathology for interventions in postural control impairments. However, the reliability and laterality of this assessment have not been clarified, nor has its relationship to postural control. We investigated the reliability, laterality and standing postural control in relation to the degree of facilitation of the H-reflex following GVS in 15 healthy adults. The assessments were performed in two sessions, one each for the left- and right-sides, in random order. The inter-session reliability of the short-interval assessments of an increase in the H-reflex following GVS on both sides were sufficient. The degree of H-reflex facilitation by GVS showed no significant difference between the left- and right-sides in any session. There was a moderate positive correlation between the mediolateral position of the center of pressure in the eyes-closed standing on foam condition and the left/right ratio of the degree of increased H-reflex in the first-session. We concluded that this method for evaluating the increase in the soleus H-reflex following GVS has high inter-session reliability in the short-interval that did not differ between sides.

Integration of vestibular and hindlimb inputs by vestibular nucleus neurons: multisensory influences on postural control

We recently demonstrated in decerebrate and conscious cat preparations that hindlimb somatosensory inputs converge with vestibular afferent input onto neurons in multiple central nervous system (CNS) locations that participate in balance control. Although it is known that head position and limb state modulate postural reflexes, presumably through vestibulospinal and reticulospinal pathways, the combined influence of the two inputs on the activity of neurons in these brainstem regions is unknown. In the present study, we evaluated the responses of vestibular nucleus (VN) neurons to vestibular and hindlimb stimuli delivered separately and together in conscious cats. We hypothesized that VN neuronal firing during activation of vestibular and limb proprioceptive inputs would be well fit by an additive model. Extracellular single-unit recordings were obtained from VN neurons. Sinusoidal whole body rotation in the roll plane was used as the search stimulus. Units responding to the search stimulus were tested for their responses to 10° ramp-and-hold roll body rotation, 60° extension hindlimb movement, and both movements delivered simultaneously. Composite response histograms were fit by a model of low- and high-pass filtered limb and body position signals using least squares nonlinear regression. We found that VN neuronal activity during combined vestibular and hindlimb proprioceptive stimulation in the conscious cat is well fit by a simple additive model for signals with similar temporal dynamics. The mean R² value for goodness of fit across all units was 0.74 ± 0.17. It is likely that VN neurons that exhibit these integrative properties participate in adjusting vestibulospinal outflow in response to limb state.NEW & NOTEWORTHY Vestibular nucleus neurons receive convergent information from hindlimb somatosensory inputs and vestibular inputs. In this study, extracellular single-unit recordings of vestibular nucleus neurons during conditions of passively applied limb movement, passive whole body rotations, and combined stimulation were well fit by an additive model. The integration of hindlimb somatosensory inputs with vestibular inputs at the first stage of vestibular processing suggests that vestibular nucleus neurons account for limb position in determining vestibulospinal responses to postural perturbations.

Posture influences on vestibulospinal tract excitability

The human vestibulospinal tract has important roles in postural control, but it has been unknown whether vestibulospinal tract excitability is influenced by the body's postures. We investigated whether postures influence the vestibulospinal tract excitability by a neurophysiological method, i.e., applying galvanic vestibular stimulation (GVS) 100 ms before tibial nerve stimulation evoking the soleus H-reflex. GVS is a percutaneous stimulation, and it has not been clarified how the cutaneous input from GVS influences the facilitation effect of cathodal GVS on the soleus H-reflex amplitude. In Experiment 1, we evaluated the effects of GVS on the soleus H-reflex amplitude of subjects in the prone, supine, and sitting positions in random order to clarify the differences in the GVS effects among these postures. In Experiment 2, to determine whether the effects of GVS in the supine and sitting positions are due solely to cutaneous input from GVS, we provided GVS and cutaneous stimulations as conditioning stimuli and compared the effects in both postures. Interaction effects between postures and stimulus conditions were observed in both experiments. The facilitation rate of the maximum H-reflex amplitude by GVS in the sitting position was significantly higher than those in the prone and supine positions (Experiment 1). The facilitation rate of GVS was significantly larger than the cutaneous stimulation only in the sitting position (Experiment 2). These results indicate that vestibulospinal tract excitability may be higher in the sitting position than in either lying position (prone and supine), due mainly to the increased need for postural control.

Assessment of vestibulocortical interactions during standing in healthy subjects

The vestibular system is essential to produce adequate postural responses enabling voluntary movement. However, how the vestibular system influences corticospinal output during postural tasks is still unknown. Here, we examined the modulation exerted by the vestibular system on corticospinal output during standing. Healthy subjects (n = 25) maintained quiet standing, head facing forward with eyes closed. Galvanic vestibular stimulation (GVS) was applied bipolarly and binaurally at different delays prior to transcranial magnetic stimulation (TMS) which triggered motor evoked potentials (MEPs). With the cathode right/anode left configuration, MEPs in right Soleus (SOL) muscle were significantly suppressed when GVS was applied at ISI = 40 and 130ms before TMS. With the anode right/cathode left configuration, no significant changes were observed. Changes in the MEP amplitude were then compared to changes in the ongoing EMG when GVS was applied alone. Only the decrease in MEP amplitude at ISI = 40ms occurred without change in the ongoing EMG, suggesting that modulation occurred at a premotoneuronal level. We further investigated whether vestibular modulation could occur at the motor cortex level by assessing changes in the direct corticospinal pathways using the short-latency facilitation of the SOL Hoffmann reflex (H-reflex) by TMS. None of the observed modulation occurred at the level of motor cortex. Finally, using the long-latency facilitation of the SOL H-reflex, we were able to confirm that the suppression of MEP at ISI = 40ms occurred at a premotoneuronal level. The data indicate that vestibular signals modulate corticospinal output to SOL at both premotoneuronal and motoneuronal levels during standing.

Cerebellar Repetitive Transcranial Magnetic Stimulation and Noisy Galvanic Vestibular Stimulation Change Vestibulospinal Function

Background

The cerebellum strongly contributes to vestibulospinal function, and the modulation of vestibulospinal function is important for rehabilitation. As transcranial magnetic stimulation (TMS) and electrical stimulation may induce functional changes in neural systems, we investigated whether cerebellar repetitive TMS (crTMS) and noisy galvanic vestibular stimulation (nGVS) could modulate vestibulospinal response excitability. We also sought to determine whether crTMS could influence the effect of nGVS.

Methods

Fifty-nine healthy adults were recruited; 28 were randomly allocated to a real-crTMS group and 31 to a sham-crTMS group. The crTMS was conducted using 900 pulses at 1 Hz, while the participants were in a static position. After the crTMS, each participant was allocated to either a real-nGVS group or sham-nGVS group, and nGVS was delivered (15 min., 1 mA; 0.1–640 Hz) while patients were in a static position. The H-reflex ratio (with/without bilateral bipolar square wave pulse GVS), which reflects vestibulospinal excitability, was measured at pre-crTMS, post-crTMS, and post-nGVS.

Results

We found that crTMS alone and nGVS alone have no effect on H-reflex ratio but that the effect of nGVS was obtained after crTMS.

Conclusion

crTMS and nGVS appear to act as neuromodulators of vestibulospinal function.

Vestibular processing during natural self-motion: implications for perception and action

How the brain computes accurate estimates of our self-motion relative to the world and our orientation relative to gravity in order to ensure accurate perception and motor control is a fundamental neuroscientific question. Recent experiments have revealed that the vestibular system encodes this information during everyday activities using pathway-specific neural representations. Furthermore, new findings have established that vestibular signals are selectively combined with extravestibular information at the earliest stages of central vestibular processing in a manner that depends on the current behavioural goal. These findings have important implications for our understanding of the brain mechanisms that ensure accurate perception and behaviour during everyday activities and for our understanding of disorders of vestibular processing.

Vestibulospinal and Corticospinal Modulation of Lumbosacral Network Excitability in Human Subjects

As part of a project aimed to develop a novel, non-invasive techniques for comprehensive assessment of supraspinal-spinal connectivity in humans, the present study sought to explore the convergence of descending vestibulospinal and corticospinal pathways onto lumbosacral motor pools. Transcutaneous electrical spinal stimulation-evoked motor potentials were recorded from knee and ankle flexors and extensors in resting neurologically intact participants. Descending influences on lumbosacral motor neurons were studied using galvanic vestibular (GVS) or transcranial magnetic stimulation (TMS) to elicit descending vestibulospinal or corticospinal volleys, respectively. Facilitatory conditioning effects of descending corticospinal volleys were manifested by a significant increase of spinally evoked motor potentials in recorded knee and ankle muscles bilaterally, and were observed at the 10–30 ms conditioning-test intervals (CTIs); whereas, facilitatory conditioning effects of vestibulospinal volleys manifested at longer latencies (CTIs of 90 and 110 ms), and lasted up to 250 ms. TMS mediated volleys revealed the conditioning effects at both short and long latencies, suggestive of both direct and indirect influence. In contrast, vestibulospinally mediated conditioning effects occurred at longer latencies, consistent with this pathway’s known anatomical and functional interfaces with other descending systems including the reticulospinal pathway and, suggestively, propriospinal interneurons. Our work demonstrates the utility and sensitivity of transcutaneous spinal stimulation in human neurophysiological studies as a technique for quantitative characterization of excitatory conditioning effects in multiple lumbosacral motor pools, obtained through descending pathways. This characterization becomes critical in understanding the neuroplasticity in the central nervous system during motor learning and neurological recovery.

When one is Enough: Impaired Multisensory Integration in Cerebellar Agenesis

In the last two decades, an intriguing shift in the understanding of the cerebellum has led to consider the nonmotor functions of this structure. Although various aspects of perceptual and sensory processing have been linked to the cerebellar activity, whether the cerebellum is essential for binding information from different sensory modalities remains uninvestigated. Multisensory integration (MSI) appears very early in the ontogenesis and is critical in several perceptual, cognitive, and social domains. For the first time, we investigated MSI in a rare case of cerebellar agenesis without any other associated brain malformations. To this aim, we measured reaction times (RTs) after the presentation of visual, auditory, and audiovisual stimuli. A group of neurotypical age-matched individuals was used as controls. Although we observed the typical advantage of the auditory modality relative to the visual modality in our patient, a clear impairment in MSI was found. Beyond the obvious prudence necessary for inferring definitive conclusions from this single-case picture, this finding is of interest in the light of reduced MSI abilities reported in several neurodevelopmental and psychiatric disorders-such as autism, dyslexia, and schizophrenia-in which the cerebellum has been implicated.

Rapid limb-specific modulation of vestibular contributions to ankle muscle activity during locomotion

KEY POINTS

The vestibular influence on human walking is phase-dependent and modulated across both limbs with changes in locomotor velocity and cadence. Using a split-belt treadmill, we show that vestibular influence on locomotor activity is modulated independently in each limb. The independent vestibular modulation of muscle activity from each limb occurs rapidly at the onset of split-belt walking, over a shorter time course relative to the characteristic split-belt error-correction mechanisms (i.e. muscle activity and kinematics) associated with locomotor adaptation. Together, the present results indicate that the nervous system rapidly modulates the vestibular influence of each limb separately through processes involving ongoing sensory feedback loops. These findings help us understand how vestibular information is used to accommodate the variable and commonplace demands of locomotion, such as turning or navigating irregular terrain.

ABSTRACT

During walking, the vestibular influence on locomotor activity is phase-dependent and modulated in both limbs with changes in velocity. It is unclear, however, whether this bilateral modulation is due to a coordinated mechanism between both limbs or instead through limb-specific processes that remain masked by the symmetric nature of locomotion. Here, human subjects walked on a split-belt treadmill with one belt moving at 0.4 m s-1 and the other moving at 0.8 m s-1 while exposed to an electrical vestibular stimulus. Muscle activity was recorded bilaterally around the ankles of each limb and used to compare vestibulo-muscular coupling between velocity-matched and unmatched tied-belt walking. In general, response magnitudes decreased by ∼20-50% and occurred ∼13-20% earlier in the stride cycle at the higher belt velocity. This velocity-dependent modulation of vestibular-evoked muscle activity was retained during split-belt walking and was similar, within each limb, to velocity-matched tied-belt walking. These results demonstrate that the vestibular influence on ankle muscles during locomotion can be adapted independently to each limb. Furthermore, modulation of vestibular-evoked muscle responses occurred rapidly (∼13-34 strides) after onset of split-belt walking. This rapid adaptation contrasted with the prolonged adaptation in step length symmetry (∼128 strides) as well as EMG magnitude and timing (∼40-100 and ∼20-70 strides, respectively). These results suggest that vestibular influence on ankle muscle control is adjusted rapidly in sensorimotor control loops as opposed to longer-term error correction mechanisms commonly associated with split-belt adaptation. Rapid limb-specific sensorimotor feedback adaptation may be advantageous for asymmetric overground locomotion, such as navigating irregular terrain or turning.

Neurons in the pontomedullary reticular formation receive converging inputs from the hindlimb and labyrinth

The integration of inputs from vestibular and proprioceptive sensors within the central nervous system is critical to postural regulation. We recently demonstrated in both decerebrate and conscious cats that labyrinthine and hindlimb inputs converge onto vestibular nucleus neurons. The pontomedullary reticular formation (pmRF) also plays a key role in postural control, and additionally participates in regulating locomotion. Thus, we hypothesized that like vestibular nucleus neurons, pmRF neurons integrate inputs from the limb and labyrinth. To test this hypothesis, we recorded the responses of pmRF neurons to passive ramp-and-hold movements of the hindlimb and to whole-body tilts, in both decerebrate and conscious felines. We found that pmRF neuronal activity was modulated by hindlimb movement in the rostral-caudal plane. Most neurons in both decerebrate (83% of units) and conscious (61% of units) animals encoded both flexion and extension movements of the hindlimb. In addition, hindlimb somatosensory inputs converged with vestibular inputs onto pmRF neurons in both preparations. Pontomedullary reticular formation neurons receiving convergent vestibular and limb inputs likely participate in balance control by governing reticulospinal outflow.

Abnormal postural reflexes in a patient with pontine ischaemia

The control of body posture is a complex activity that needs a very close relationship between different structures, such as the vestibular system, and the muscle and joint receptors of the neck. Damage of even one of these structures can lead to abnormal postural reflexes. We describe a case of a woman with a left pontine ischaemia who developed a 'dystonic' extensor posture of the left limbs while turned on the right side. This clinical picture differs from previous reports on the subject, and may relate to ischaemic damage of a pontine structure involved in posture control, or of adjacent neural connections to be yet identified. To the best of our knowledge, this is the first case reported in the literature. Clinical examples of an altered interplay between vestibular and neck receptors are rare.

A paradigm of galvanic vestibular stimulation diminishes the soleus muscle H-reflex in healthy volunteers

STUDY DESIGN

In this study, we explored how galvanic vestibular stimulation can modify the soleus H-reflex (Hoffman reflex), that is, the excitability of the spinal cord circuits, in healthy humans.

OBJECTIVES

Our aim was to demonstrate H-reflex amplitude modulation caused by changing the duration and the intensity of the anodal galvanic vestibular stimulation. Therefore, we measured H-reflex before and after applied vestibular stimulation.

SETTINGS

This study was conducted in Rehabilitation Clinic, Belgrade, Serbia.

METHODS

The measurements were performed on 5 male volunteers aged 22-30 years. Anodal galvanic stimulation was applied on the right mastoid in prone position. H-reflex was elicited by nervus tibialis stimulation and measured from the right soleus muscle. In three subjects, trains of weak and strong galvanic stimuli (1, 5 and 9) were applied. In two subjects, only a train of 9 strong stimuli was applied.

RESULTS

A statistically significant decrease of the H-reflex amplitude after anodal galvanic stimulation was demonstrated in all subjects. The percentage of H-reflex amplitude diminution was between 6 and 18 in subjects with weak and strong stimuli and 5 and 6 in subjects with only 9 strong stimuli.

CONCLUSION

We intend to use this paradigm of stimulation to explore whether the vestibulospinal function exists after spinal cord injury (SCI). If it exists, it can be used to influence the preserved spinal cord circuits after SCI.

SPONSORSHIP

One of the authors (Nadica Miljković) was partly supported by the Ministry of education, science and technological development, Republic of Serbia, grant OS175016.

Recording and assessment of evoked potentials with electrode arrays

In order to optimize procedure for the assessment of evoked potentials and to provide visualization of the flow of action potentials along the motor systems, we introduced array electrodes for stimulation and recording and developed software for the analysis of the recordings. The system uses a stimulator connected to an electrode array for the generation of evoked potentials, an electrode array connected to the amplifier, A/D converter and computer for the recording of evoked potentials, and a dedicated software application. The method has been tested for the assessment of the H-reflex on the triceps surae muscle in six healthy humans. The electrode array with 16 pads was positioned over the posterior aspect of the thigh, while the recording electrode array with 16 pads was positioned over the triceps surae muscle. The stimulator activated all the pads of the stimulation electrode array asynchronously, while the signals were recorded continuously at all the recording sites. The results are topography maps (spatial distribution of evoked potentials) and matrices (spatial visualization of nerve excitability). The software allows the automatic selection of the lowest stimulation intensity to achieve maximal H-reflex amplitude and selection of the recording/stimulation pads according to predefined criteria. The analysis of results shows that the method provides rich information compared with the conventional recording of the H-reflex with regard the spatial distribution.

Galvanic vestibular stimulation: a novel modulatory countermeasure for vestibular-associated movement disorders

Motion sickness or kinetosis is the result of the abnormal neural output originated by visual, proprioceptive and vestibular mismatch, which reverses once the dysfunctional sensory information becomes coherent. The space adaptation syndrome or space sickness relates to motion sickness; it is considered to be due to yaw, pith, and roll coordinates mismatch. Several behavioural and pharmacological measures have been proposed to control these vestibular-associated movement disorders with no success. Galvanic vestibular stimulation has the potential of up-regulating disturbed sensory-motor mismatch originated by kinetosis and space sickness by modulating the GABA-related ion channels neural transmission in the inner ear. It improves the signal-to-noise ratio of the afferent proprioceptive volleys, which would ultimately modulate the motor output restoring the disordered gait, balance and human locomotion due to kinetosis, as well as the spatial disorientation generated by gravity transition.

Diverse spatial reference frames of vestibular signals in parietal cortex

Reference frames are important for understanding how sensory cues from different modalities are coordinated to guide behavior, and the parietal cortex is critical to these functions. We compare reference frames of vestibular self-motion signals in the ventral intraparietal area (VIP), parietoinsular vestibular cortex (PIVC), and dorsal medial superior temporal area (MSTd). Vestibular heading tuning in VIP is invariant to changes in both eye and head positions, indicating a body (or world)-centered reference frame. Vestibular signals in PIVC have reference frames that are intermediate between head and body centered. In contrast, MSTd neurons show reference frames between head and eye centered but not body centered. Eye and head position gain fields were strongest in MSTd and weakest in PIVC. Our findings reveal distinct spatial reference frames for representing vestibular signals and pose new challenges for understanding the respective roles of these areas in potentially diverse vestibular functions.

Frequency response of vestibular reflexes in neck, back, and lower limb muscles

Vestibular pathways form short-latency disynaptic connections with neck motoneurons, whereas they form longer-latency disynaptic and polysynaptic connections with lower limb motoneurons. We quantified frequency responses of vestibular reflexes in neck, back, and lower limb muscles to explain between-muscle differences. Two hypotheses were evaluated: 1) that muscle-specific motor-unit properties influence the bandwidth of vestibular reflexes; and 2) that frequency responses of vestibular reflexes differ between neck, back, and lower limb muscles because of neural filtering. Subjects were exposed to electrical vestibular stimuli over bandwidths of 0–25 and 0–75 Hz while recording activity in sternocleidomastoid, splenius capitis, erector spinae, soleus, and medial gastrocnemius muscles. Coherence between stimulus and muscle activity revealed markedly larger vestibular reflex bandwidths in neck muscles (0–70 Hz) than back (0–15 Hz) or lower limb muscles (0–20 Hz). In addition, vestibular reflexes in back and lower limb muscles undergo low-pass filtering compared with neck-muscle responses, which span a broader dynamic range. These results suggest that the wider bandwidth of head-neck biomechanics requires a vestibular influence on neck-muscle activation across a larger dynamic range than lower limb muscles. A computational model of vestibular afferents and a motoneuron pool indicates that motor-unit properties are not primary contributors to the bandwidth filtering of vestibular reflexes in different muscles. Instead, our experimental findings suggest that pathway-dependent neural filtering, not captured in our model, contributes to these muscle-specific responses. Furthermore, gain-phase discontinuities in the neck-muscle vestibular reflexes provide evidence of destructive interaction between different reflex components, likely via indirect vestibular-motor pathways.

The neural encoding of self-motion

As we move through the world, information can be combined from multiple sources in order to allow us to perceive our self-motion. The vestibular system detects and encodes the motion of the head in space. In addition, extra-vestibular cues such as retinal-image motion (optic flow), proprioception, and motor efference signals, provide valuable motion cues. Here I focus on the coding strategies that are used by the brain to create neural representations of self-motion. I review recent studies comparing the thresholds of single versus populations of vestibular afferent and central neurons. I then consider recent advances in understanding the brain's strategy for combining information from the vestibular sensors with extra-vestibular cues to estimate self-motion. These studies emphasize the need to consider not only the rules by which multiple inputs are combined, but also how differences in the behavioral context govern the nature of what defines the optimal computation.

The vestibular system: multimodal integration and encoding of self-motion for motor control

Understanding how sensory pathways transmit information under natural conditions remains a major goal in neuroscience. The vestibular system plays a vital role in everyday life, contributing to a wide range of functions from reflexes to the highest levels of voluntary behavior. Recent experiments establishing that vestibular (self-motion) processing is inherently multimodal also provide insight into a set of interrelated questions. What neural code is used to represent sensory information in vestibular pathways? How do the interactions between the organism and the environment shape encoding? How is self-motion information processing adjusted to meet the needs of specific tasks? This review highlights progress that has recently been made towards understanding how the brain encodes and processes self-motion to ensure accurate motor control.

Multimodal integration in rostral fastigial nucleus provides an estimate of body movement

The ability to accurately control posture and perceive self-motion and spatial orientation requires knowledge of the motion of both the head and body. However, whereas the vestibular sensors and nuclei directly encode head motion, no sensors directly encode body motion. Instead, the convergence of vestibular and neck proprioceptive inputs during self-motion is generally believed to underlie the ability to compute body motion. Here, we provide evidence that the brain explicitly computes an internal estimate of body motion at the level of single cerebellar neurons. Neuronal responses were recorded from the rostral fastigial nucleus, the most medial of the deep cerebellar nuclei, during whole-body, body-under-head, and head-on-body rotations. We found that approximately half of the neurons encoded the motion of the body in space, whereas the other half encoded the motion of the head in space in a manner similar to neurons in the vestibular nuclei. Notably, neurons encoding body motion responded to both vestibular and proprioceptive stimulation (accordingly termed bimodal neurons). In contrast, neurons encoding head motion were sensitive only to vestibular inputs (accordingly termed unimodal neurons). Comparison of the proprioceptive and vestibular responses of bimodal neurons further revealed similar tuning in response to changes in head-on-body position. We propose that the similarity in nonlinear processing of vestibular and proprioceptive signals underlies the accurate computation of body motion. Furthermore, the same neurons that encode body motion (i.e., bimodal neurons) most likely encode vestibular signals in a body-referenced coordinate frame, since the integration of proprioceptive and vestibular information is required for both computations.

Artificial vestibular feedback in conditions of a modified body scheme

Subjects standing in the dark on a rigid immobile support maintained a vertical posture, which was destabilized by vibrostimulation of both Achilles tendons. Feedback was created via the vestibular pathway using transmastoid galvanic stimulation. Changes in the current in the feedback envelope were made using a linear function based on the amplitude and rate of head displacement. Changes in the body scheme were produced using rotation of the head relative to the trunk, rotation of the trunk with the relative head position fixed, and simultaneous rotation of the head and trunk. The result of these manipulations was that the head could be rotated through essentially 90 degrees relative to the feet. In addition, rotation of one foot relative to the other through 90 degrees was used. Artificial feedback damped head oscillations induced by vibration, but only those in the vertical plane passing through the interaural axis of the head. It is suggested that changes in the vectorial characteristics of vestibular responses and the results of applying artificial feedback on the background of modified orientation of the head relative to the feet may be associated with substitution of the ensembles of vestibular hair sets providing the dominant signals in the responses of vestibulospinal neurons.

Vestibulospinal responses in motor incomplete spinal cord injury

OBJECTIVE

Postural instability limits ambulatory capacity in patients with spinal cord injury (SCI). Galvanic vestibular stimulation (GVS) was used to investigate the integrity of vestibulospinal pathways and related changes in postural responses in SCI.

METHODS

Binaural bipolar galvanic stimuli of 400 ms duration and 3 mA intensity were applied in 8 motor incomplete SCI and 8 control subjects who stood facing towards the left. EMG responses were recorded from the right soleus muscle and the trajectory of the centre of pressure (CoP) was measured with a force plate.

RESULTS

There was no difference in excitability and amplitude of the responses between the groups. However, the latency and duration of the medium latency EMG response and all CoP responses were significantly longer in the SCI group. Additionally, postural stability was reduced in the SCI group, as shown by a greater tendency to fall due to GVS.

CONCLUSIONS

Despite early EMG responses proving the basic connectivity of the direct vestibulospinal pathways, the delayed GVS responses suggest a vestibulospinal deficit in the SCI subjects.

SIGNIFICANCE

GVS can be applied in incomplete SCI to supplement the neurological examination by revealing changes in vestibulospinal responses and impairment of postural stability.

Modulation of multisegmental monosynaptic responses in a variety of leg muscles during walking and running in humans

Motor responses evoked by stimulating the spinal cord percutaneously between the T11 and T12 spinous processes were studied in eight human subjects during walking and running. Stimulation elicited responses bilaterally in the biceps femoris, vastus lateralis, rectus femoris, medial gastrocnemius, soleus, tibialis anterior, extensor digitorum brevis and flexor digitorum brevis. The evoked responses were consistent with activation of Ia afferent fibres through monosynaptic neural circuits since they were inhibited when a prior stimulus was given and during tendon vibration. Furthermore, the soleus motor responses were inhibited during the swing phase of walking as observed for the soleus H-reflex elicited by tibial nerve stimulation. Due to the anatomical site and the fibre composition of the peripheral nerves it is difficult to elicit H-reflex in leg muscles other than the soleus, especially during movement. In turn, the multisegmental monosynaptic responses (MMR) technique provides the opportunity to study modulation of monosynaptic reflexes for multiple muscles simultaneously. Phase-dependent modulation of the MMR amplitude throughout the duration of the gait cycle period was observed in all muscles studied. The MMR amplitude was large when the muscle was activated whereas it was generally reduced, or even suppressed, when the muscle was quiescent. However, during running, there was a systematic anticipatory increase in the amplitude of the MMR at the end of swing in all proximal and distal extensor muscles. The present findings therefore suggest that there is a general control scheme by which the transmission in the monosynaptic neural circuits is modulated in all leg muscles during stepping so as to meet the requirement of the motor task.

Postural control in patients with unilateral vestibular lesions is more impaired in the roll than in the pitch plane: a static and dynamic posturography study

The postural instability of patients with vestibular loss (11 with bilateral and 101 with unilateral vestibular loss) at different times following the lesion was investigated by means of posturography and compared to healthy subjects. In addition, subjects submitted to galvanic vestibular stimulation were also studied to compare their postural performances with those of patients with complete unilateral vestibular lesion. The platform consisted of a static computerized force platform, on which a seesaw platform could be placed to test the subjects in dynamic conditions. The displacement of the center of foot pressure was measured under different conditions: subjects standing on the fixed platform, eyes open and eyes closed and subjects standing on the seesaw platform, eyes open and eyes closed. In the last condition, balance was tested in the subject's pitch plane by allowing the platform to rotate forwards and backwards only and in the patient's roll plane by allowing the platform to rotate to the left and to the right. The results showed that in static conditions, only bilateral vestibular loss patients had abnormal values compared to controls. In contrast, in dynamic eyes-closed conditions, both bilateral and unilateral patients could be differentiated from controls. Bilateral patients were unable to stand up without falling in both pitch and roll planes. Unilateral patients fell in the first week following the lesion and exhibited increased postural oscillations in both planes from the 2-week up to the 1-year postlesion stage. In addition and more importantly, they fell more often or had higher sway in the roll than in the pitch plane. Therefore, this study suggests that dynamic posturography on a seesaw platform could be a valuable tool for clinical diagnosis and quantitative analysis of imbalance in patients suffering from a unilateral vestibular loss up to 1 year after the lesion.

Probing the human vestibular system with galvanic stimulation

Galvanic vestibular stimulation (GVS) is a simple, safe, and specific way to elicit vestibular reflexes. Yet, despite a long history, it has only recently found popularity as a research tool and is rarely used clinically. The obstacle to advancing and exploiting GVS is that we cannot interpret the evoked responses with certainty because we do not understand how the stimulus acts as an input to the system. This paper examines the electrophysiology and anatomy of the vestibular organs and the effects of GVS on human balance control and develops a model that explains the observed balance responses. These responses are large and highly organized over all body segments and adapt to postural and balance requirements. To achieve this, neurons in the vestibular nuclei receive convergent signals from all vestibular receptors and somatosensory and cortical inputs. GVS sway responses are affected by other sources of information about balance but can appear as the sum of otolithic and semicircular canal responses. Electrophysiological studies showing similar activation of primary afferents from the otolith organs and canals and their convergence in the vestibular nuclei support this. On the basis of the morphology of the cristae and the alignment of the semicircular canals in the skull, rotational vectors calculated for every mode of GVS agree with the observed sway. However, vector summation of signals from all utricular afferents does not explain the observed sway. Thus we propose the hypothesis that the otolithic component of the balance response originates from only the pars medialis of the utricular macula.

Multiple reference frames for motion in the primate cerebellum

Knowledge of body motion through space is necessary for spatial orientation, self-motion perception, and postural control. Yet, sensory afferent signals may not directly provide such information to the brain. Because motion detected by the vestibular end organs is encoded in a head-fixed frame of reference, a coordinate transformation is thus required to encode body motion. In this study, we investigated whether cerebellar motion-sensitive neurons encode the translation of the body through space. We systematically changed both the direction of motion relative to the body and the static orientation of the head relative to the trunk. The activities of motion-sensitive neurons in the most medial of the deep cerebellar nuclei, the rostral fastigial nucleus, were compared with those in the brainstem vestibular nuclei. We found a distributed representation of reference frames for motion in the rostral fastigial nucleus, in contrast to cells in the vestibular nuclei, which primarily encoded motion in a head-fixed reference frame. This differential representation of motion-related information implies potential differences in the functional roles of these areas.

Vestibulospinal influences on lower limb motoneurons

Galvanic vestibular stimulation (GVS) is a research tool used to activate the vestibular system in human subjects. When a low-intensity stimulus (1–4 mA) is delivered percutaneously to the vestibular nerve, a transient electromyographic response is observed a short time later in lower limb muscles. Typically, galvanically evoked responses are present when the test muscle is actively engaged in controlling standing balance. However, there is evidence to suggest that GVS may be able to modulate the activity of lower limb muscles when subjects are not in a free-standing situation. The purpose of this review is to examine 2 studies from our laboratory that examined the effects of GVS on the lower limb motoneuron pool. For instance, a monopolar monaural galvanic stimulus modified the amplitude of the ipsilateral soleus H-reflex. Furthermore, bipolar binaural GVS significantly altered the onset of activation and the initial firing frequency of gastrocnemius motor units. The following paper examines the effects of GVS on muscles that are not being used to maintain balance. We propose that GVS is modulating motor output by influencing the activity of presynaptic inhibitory mechanisms that act on the motoneuron pool.Key words: galvanic vestibular stimulation, h-reflex, motor unit, vestibulospinal, human.

Trunk position influences vestibular responses of fastigial nucleus neurons in the alert monkey

Vestibulospinal reflexes play an important role for body stabilization during locomotion and for postural control. For an appropriate distribution of vestibular signals to spinal motoneurons, the orientation of the body relative to the head needs to be taken into account. For different trunk positions, identical vestibular stimuli must activate different sets of muscles to ensure body stabilization. Because the cerebellar vermis and the underlying fastigial nucleus (FN) might be involved in this task, vestibular neurons in the rostral FN of alert rhesus monkeys were recorded during sinusoidal vestibular stimulation (0.1-1.0 Hz) in the roll and pitch planes at different trunk-re-head positions (center and +/-45 degrees ). From the sensitivity and phase values measured in these planes, the response properties in the intermediate planes and the stimulus orientation eliciting the optimal response [response vector orientation (RVO)] were calculated. In most neurons, the RVOs rotated systematically with respect to the head, when trunk-re-head position was altered, so that they tended to maintain their orientation with respect to the trunk. Sensitivity and phase at the RVO were not affected. This pattern was the same for neurons in the right and left FN and independent of stimulus frequency. The average sensitivity of this partially compensatory RVO shift in response to trunk-re-head displacements, evaluated by linear regression analyses, was 0.59 degrees / degrees (n = 73 neurons). These data show that FN neurons may encode vestibular information in a coordinate system that is closer to a trunk-centered than to a head-centered reference frame. They indicate an important role of this nucleus in motor programs related to posture and gait control.