Caloric vestibular stimulation: interaction between somatosensory system and vestibular apparatus

Effects of unilateral neck muscle vibration on standing postural orientation and spatial perception in healthy subjects based on stimulus duration and simultaneous stimulation of trunk muscles

Neck muscle vibration (NMV) influences proprioceptive sensations and modulates standing postural orientation and spatial perception. However, the effects of NMV in healthy participants would vary based on the influence of stimulus duration and combination with trunk muscle vibration. Therefore, this study with a cross-over design clarified these effects. Twenty-four healthy participants (mean age, 25.7±3.7 years) were enrolled. To assess standing postural orientation, standing center-of-pressure (COP) measurements were recorded on a COP platform, starting with closed eyes and then with open eyes. The mean mediolateral (ML) and anteroposterior (AP) position [mm] of COP and other parameters were calculated. To assess spatial perception, subjective straight ahead (SSA) measurements were recorded, wherein participants were instructed to point and project the position of the manubrium of sternum on the touch panel using their right index finger with their eyes closed. Measurements were taken before and after four conditions: no vibration (control), left NMV for 30 s, left NMV for 10 min, and left NMV and left lumbar back vibration for 10 min. Vibratory stimulation was performed with the eyes closed at 80 Hz. The measurements under the four conditions were conducted with random cross-over and 5-min resting period between the conditions. COP and SSA values were subtracted before and after each condition for standardized variation and compared. NMV combined with trunk muscle vibration for 10 min resulted in significant deviations of the ML-COP toward the stimulation side and AP-COP toward the anterior side compared to the control condition with closed eyes. SSA showed no significant differences. These findings suggest that NMV-induced nervous system modulation would be amplified by proprioceptive sensory input to trunk muscles. Therefore, this method could provide a new option for clinical trials on postural orientation using NMV. SSA based on proprioceptive sensation may not be biased without visual illusions.

Asymmetrical influence of bi-thermal caloric vestibular stimulation on a temporal order judgment task

Recent evidences suggest that binaural vestibular stimulation affects tactile temporal processing. However, it remains difficult to determine the physiological mechanisms supporting the vestibular-somatosensory interactions observed during a TOJ task. Controlling the activation of the right or left vestibular system separately could allow to better understand the physiological bases of these findings and reconcile previous studies. The objective of the present study was to examine tactile temporal processing using a temporal order judgment task following selective stimulation of the right and left vestibular system with bi-thermal caloric vestibular stimulation (CVS). A total of 24 right-handed participants received bi-thermal CVS either in the right ear (n = 12) or the left ear (n = 12). Participants held vibrators in both hands which delivered a signal temporally separated by a variable asynchrony. Participants had to report the hand where the vibration was perceived first. The task was performed in three different CVS conditions: (1) baseline, (2) warm CVS, and (3) cold CVS. Analysis of the logistics curve parameters-just noticeable difference (JND) and point of subjective simultaneity (PSS)-for each participant in each CVS conditions revealed an increase in JND greater following warm CVS. A significant increase in JND following warm CVS was measured bilaterally. However, cold CVS increased JND only when CVS was applied in the left ear, but not in the right ear. Finally, no influence of CVS on PSS was observed.

Auditory perception is influenced by the orientation of the trunk relative to a sound source

The study investigated how hearing depends on the whole body, head and trunk orientation relative to a sound source. In normal hearing humans we examined auditory thresholds and their ability to recognize logatomes (bi-syllabic non-sense words) at different whole body, head and trunk rotation relative to a sound source. We found that auditory threshold was increased and logatome recognition was impaired when the body or the trunk were rotated 40° away from a sound source compared to when the body or the trunk was oriented towards the sound source. Conversely, no effects were seen when only the head was rotated. Further, an increase of thresholds and impairment of logatome recognition were also observed after unilateral vibration of dorsal neck muscles that induces, per se, long-lasting illusory trunk displacement relative to the head. Thus, our findings support the idea that processing of acoustic signals depends on where a sound is located within a reference system defined by the subject's trunk coordinates.

The disappearing hand: vestibular stimulation does not improve hand localisation

Background

Bodily self-consciousness depends on the coherent integration of sensory information. In addition to visual and somatosensory information processing, vestibular contributions have been proposed and investigated. Vestibular information seems especially important for self-location, but remains difficult to study.

Methods

This randomised controlled experiment used the MIRAGE multisensory illusion box to induce a conflict between the visually- and proprioceptively-encoded position of one hand. Over time, the perceived location of the hand slowly shifts, due to the fact that proprioceptive input is progressively weighted more heavily than the visual input. We hypothesised that left cold caloric vestibular stimulation (CVS) augments this shift in hand localisation.

Results

The results from 24 healthy participants do not support our hypothesis: CVS had no effect on the estimations with which the perceived position of the hand shifted from the visually- to the proprioceptively-encoded position. Participants were more likely to report that their hand was 'no longer there' after CVS. Taken together, neither the physical nor the subjective data provide evidence for vestibular enhanced self-location.

Modulation of somatoparaphrenia following left-hemisphere damage

Somatoparaphrenic symptoms after left-hemisphere damage are rare. To verify the potential role of body-related sensory (proprioceptive, visual, and somatosensory) manipulation in patients experiencing sensations of hand disownership, the symptoms of a patient suffering from right-hand somatoparaphrenia were monitored and clinical and neuropsychological variables were controlled. Four types of manipulation were administered: changes in spatial position of the hand, multisensory stimulation, and self-observation using video or mirrors. Multisensory visuo-tactile stimulation was efficacious in terms of reducing somatoparaphrenia, and changes in the position of the hand produced some positive effects. Third-person perspective self-observation did not, however, result in any changes.

Influence of Caloric Vestibular Stimulation on Body Experience in Healthy Humans

The vestibular system has more connections with and influence on higher cortical centers than previously thought. These interactions with higher cortical centers and the phenomena that they elicit require a structural intact cerebral cortex. To date, little is known about the role and influence of the vestibular system on one's body experience. In this study we show that caloric vestibular stimulation (CVS) in healthy participants has an effect on the perceptive component of one's body experience. After CVS all participants showed a statistically significant difference of thigh width estimation. In contrast to previous studies, which demonstrated an influence of CVS on higher cortical centers with an intact cerebral cortex both the cognitive and affective component of body experience were not effected by the CVS. Our results demonstrate the influence of the vestibular system on body perception and emphasize its role in modulating different perceptive-qualities which contributes to our body experience. We found that CVS has a limited influence on one's conscious state, thought process and higher cortical functions.

Multisensory Integration in Self Motion Perception

Self motion perception involves the integration of visual, vestibular, somatosensory and motor signals. This article reviews the findings from single unit electrophysiology, functional and structural magnetic resonance imaging and psychophysics to present an update on how the human and non-human primate brain integrates multisensory information to estimate one’s position and motion in space. The results indicate that there is a network of regions in the non-human primate and human brain that processes self motion cues from the different sense modalities.

The spatial side of somatoparaphrenia: a case study

The perception of the bodily self in space is a composite cognitive function requiring a dynamic integrated brain mechanism. Somatoparaphrenia (SP), a delusional belief concerning the experienced disownership for the contralesional paralyzed arm, represents the disruption of such mechanism. In two experiments, we have investigated the alteration of limb disownership after spatial manipulations in a right-brain-damaged patient affected by chronic SP. In experiment 1 the patient's spatial attention was switched between the left and right sides of space. SP signs worsened when the patient was interviewed from the left compared to the right bedside. In the second experiment we showed the first systematic transient remission of SP using left caloric vestibular stimulation (CVS), a physiologic manipulation mainly acting on the spatial frame of reference. Taken together, these results shed further light on the spatial nuance of SP and on the importance of vestibular signals for the generation of a coherent body representation. Furthermore, our case study demonstrated the possibility of eliciting more severe SP signs if the patient is interviewed from the left bedside. Additionally, CVS applications may have an important impact on the rehabilitation of these symptoms.

New insights into pathophysiology of vestibular migraine

Vestibular migraine (VM) is a common disorder in which genetic, epigenetic, and environmental factors probably contribute to its development. The pathophysiology of VM is unknown; nevertheless in the last few years, several studies are contributing to understand the neurophysiological pathways involved in VM. The current hypotheses are mostly based on the knowledge of migraine itself. The evidence of trigeminal innervation of the labyrinth vessels and the localization of vasoactive neuropeptides in the perivascular afferent terminals of these trigeminal fibers support the involvement of the trigemino-vascular system. The neurogenic inflammation triggered by activation of the trigeminal-vestibulocochlear reflex, with the subsequent inner ear plasma protein extravasation and the release of inflammatory mediators, can contribute to a sustained activation and sensitization of the trigeminal primary afferent neurons explaining VM symptoms. The reciprocal connections between brainstem vestibular nuclei and the structures that modulate trigeminal nociceptive inputs (rostral ventromedial medulla, ventrolateral periaqueductal gray, locus coeruleus, and nucleus raphe magnus) are critical to understand the pathophysiology of VM. Although cortical spreading depression can affect cortical areas involved in processing vestibular information, functional neuroimaging techniques suggest a dysmodulation in the multimodal sensory integration and processing of vestibular and nociceptive information, resulting from a vestibulo-thalamo-cortical dysfunction, as the pathogenic mechanism underlying VM. The elevated prevalence of VM suggests that multiple functional variants may confer a genetic susceptibility leading to a dysregulation of excitatory–inhibitory balance in brain structures involved in the processing of sensory information, vestibular inputs, and pain. The interactions among several functional and structural neural networks could explain the pathogenic mechanisms of VM.

Neck proprioception shapes body orientation and perception of motion

This review article deals with some effects of neck muscle proprioception on human balance, gait trajectory, subjective straight-ahead (SSA), and self-motion perception. These effects are easily observed during neck muscle vibration, a strong stimulus for the spindle primary afferent fibers. We first remind the early findings on human balance, gait trajectory, SSA, induced by limb, and neck muscle vibration. Then, more recent findings on self-motion perception of vestibular origin are described. The use of a vestibular asymmetric yaw-rotation stimulus for emphasizing the proprioceptive modulation of motion perception from the neck is mentioned. In addition, an attempt has been made to conjointly discuss the effects of unilateral neck proprioception on motion perception, SSA, and walking trajectory. Neck vibration also induces persistent aftereffects on the SSA and on self-motion perception of vestibular origin. These perceptive effects depend on intensity, duration, side of the conditioning vibratory stimulation, and on muscle status. These effects can be maintained for hours when prolonged high-frequency vibration is superimposed on muscle contraction. Overall, this brief outline emphasizes the contribution of neck muscle inflow to the construction and fine-tuning of perception of body orientation and motion. Furthermore, it indicates that tonic neck-proprioceptive input may induce persistent influences on the subject's mental representation of space. These plastic changes might adapt motion sensitiveness to lasting or permanent head positional or motor changes.

Interaction of brain areas of visual and vestibular simultaneous activity with fMRI

Static body equilibrium is an essential requisite for human daily life. It is known that visual and vestibular systems must work together to support equilibrium. However, the relationship between these two systems is not fully understood. In this work, we present the results of a study which identify the interaction of brain areas that are involved with concurrent visual and vestibular inputs. The visual and the vestibular systems were individually and simultaneously stimulated, using flickering checkerboard (without movement stimulus) and galvanic current, during experiments of functional magnetic resonance imaging. Twenty-four right-handed and non-symptomatic subjects participated in this study. Single visual stimulation shows positive blood-oxygen-level-dependent (BOLD) responses (PBR) in the primary and associative visual cortices. Single vestibular stimulation shows PBR in the parieto-insular vestibular cortex, inferior parietal lobe, superior temporal gyrus, precentral gyrus and lobules V and VI of the cerebellar hemisphere. Simultaneous stimulation shows PBR in the middle and inferior frontal gyri and in the precentral gyrus. Vestibular- and somatosensory-related areas show negative BOLD responses (NBR) during simultaneous stimulation. NBR areas were also observed in the calcarine gyrus, lingual gyrus, cuneus and precuneus during simultaneous and single visual stimulations. For static visual and galvanic vestibular simultaneous stimulation, the reciprocal inhibitory visual-vestibular interaction pattern is observed in our results. The experimental results revealed interactions in frontal areas during concurrent visual-vestibular stimuli, which are affected by intermodal association areas in occipital, parietal, and temporal lobes.

Caloric vestibular stimulation as a treatment for conversion disorder: a case report and medical hypothesis

Conversion disorder is a medical condition in which a person has paralysis, blindness, or other neurological symptoms that cannot be clearly explained physiologically. To date, there is neither specific nor conclusive treatment. In this paper, we draw together a number of disparate pieces of knowledge to propose a novel intervention to provide transient alleviation for this condition. As caloric vestibular stimulation has been demonstrated to modulate a variety of cognitive functions associated with brain activations, especially in the temporal-parietal cortex, anterior cingulate cortex, and insular cortex, there is evidence to assume an effect in specific mental disorders. Therefore, we go on to hypothesize that lateralized cold vestibular caloric stimulation will be effective in treating conversion disorder and we present provisional evidence from one patient that supports this conclusion. If our hypothesis is correct, this will be the first time in psychiatry and neurology that a clinically well-known mental disorder, long considered difficult to understand and to treat, is relieved by a simple or common, non-invasive medical procedure.

Spatial cognition, body representation and affective processes: the role of vestibular information beyond ocular reflexes and control of posture

A growing number of studies in humans demonstrate the involvement of vestibular information in tasks that are seemingly remote from well-known functions such as space constancy or postural control. In this review article we point out three emerging streams of research highlighting the importance of vestibular input: (1) Spatial Cognition: Modulation of vestibular signals can induce specific changes in spatial cognitive tasks like mental imagery and the processing of numbers. This has been shown in studies manipulating body orientation (changing the input from the otoliths), body rotation (changing the input from the semicircular canals), in clinical findings with vestibular patients, and in studies carried out in microgravity. There is also an effect in the reverse direction; top-down processes can affect perception of vestibular stimuli. (2) Body Representation: Numerous studies demonstrate that vestibular stimulation changes the representation of body parts, and sensitivity to tactile input or pain. Thus, the vestibular system plays an integral role in multisensory coordination of body representation. (3) Affective Processes and Disorders: Studies in psychiatric patients and patients with a vestibular disorder report a high comorbidity of vestibular dysfunctions and psychiatric symptoms. Recent studies investigated the beneficial effect of vestibular stimulation on psychiatric disorders, and how vestibular input can change mood and affect. These three emerging streams of research in vestibular science are-at least in part-associated with different neuronal core mechanisms. Spatial transformations draw on parietal areas, body representation is associated with somatosensory areas, and affective processes involve insular and cingulate cortices, all of which receive vestibular input. Even though a wide range of different vestibular cortical projection areas has been ascertained, their functionality still is scarcely understood.

Apotemnophilia, body integrity identity disorder or xenomelia? Psychiatric and neurologic etiologies face each other

This review summarizes the available studies of a rare condition in which individuals seek the amputation of a healthy limb or desire to be paraplegic. Since 1977, case reports and group studies have been produced, trying to understand the cause of this unusual desire. The main etiological hypotheses are presented, from the psychological/psychiatric to the most recent neurologic explanation. The paradigms adopted and the clinical features are compared across studies and analyzed in detail. Finally, future directions and ethical implications are discussed. A proposal is made to adopt a multidisciplinary approach that comprises state-of-the-art technologies and a variety of theoretical models, including both body representation and psychological and sexual components.