Modulation of conscious experience by peripheral sensory stimuli

An Overview of the Bodily Awareness Representation and Interoception: Insights and Progress in the Field of Neurorehabilitation Research

In the last two decades, the scientific literature on so-called body representations has been increasing, and the notion of body awareness (BA) is particularly interesting for neurorehabilitation. In this article, we present results derived from recent studies on this representation, considering the different definitions and explicative models proposed as well as the empirical settings used to test it, providing an extensive overview of these issues. This article discusses the challenge of understanding how we integrate the sensory experiences of proprioception (knowing where our body is in space) and interoception (sensing internal bodily sensations, like hunger of thirst) with our perception of self. This is a difficult problem to analyze because our awareness of our body is inherently linked to our perspective, since the body is the means through which we interact with the world. Presenting the different viewpoints offered by recent theories on this concern, we highlighted that the neurorehabilitation and psychiatric settings offer two important fields useful for the study of BA because in them it is possible to analyze bodily representations by inducing/observing a controlled discrepancy between dysfunctional content and sensory inputs.

Spatial neglect encompasses impaired verticality representation after right hemisphere stroke

Spatial neglect after right hemisphere stroke (RHS) was recently found to encompass lateropulsion, a deficit in body orientation with respect to gravity caused by altered brain processing of graviception. By analogy, we hypothesized that spatial neglect after RHS might encompass an altered representation of verticality. We also assumed a strong relation between body neglect and impaired postural vertical, both referring to the body. To tackle these issues, we performed contingency and correlation analyses between two domains of spatial neglect (body, extra-body) and two modalities of verticality perception (postural, visual) in 77 individuals (median age = 67) with a first-ever subacute RHS (1-3 months). All individuals with a transmodal (postural and visual) tilt in verticality perception (n = 26) had spatial neglect, but the reverse was not found. Correlation and multivariate analyses revealed that spatial neglect (and notably body neglect) was associated more with postural than visual vertical tilts. These findings indicate that after RHS, an impaired verticality representation results from a kind of graviceptive neglect, bearing first on somaesthetic graviception and second on vestibular graviception. They also suggest that the human brain uses not only a mosaic of 2D representations but also 3D maps involving a transmodal representation of verticality.

The Effect of Somatosensorial System on Vestibular System

The aim of this study was to investigate the effects of the somatosensory system on the vestibular system and the interconnected ways they work together to maintain balance. The study was conducted on 54 individuals (27 females and 27 males), aged between 18-25 years. vHIT as well as cVEMP tests were used to evaluate the participants. Tests were carried out while sitting, standing on firm surface and standing on foam respectively. According to the posterior vHIT results, there was a significant difference between VOR gains obtained while sitting and standing on firm surface in right side as well as on the left side (p < 0,01). Moreover, when VOR gains in standing on firm and standing on foam results were compared to each other, statistical significance was found right and left posterior canals (p < 0,05). Concerning the results obtained from VEMP, a statistically significant difference was seen in the comparison of P1-N1 amplitudes of the right side on firm surface and standing on foam (p < 0,01). When the inputs from somatosensorial system are disturbed, the parts of the vestibular system that are primarily affected are the posterior SSC, saccule and inferior vestibular nerve. This can be interpreted as the inferior vestibular nerve being more affected than the superior vestibular nerve when posture is disturbed due to somatosensory cues being unavailable or unstable.

Attention to body parts prompts thermoregulatory reactions in Body Integrity Dysphoria

In healthy subjects, the transient perturbation of body part ownership is accompanied by regional skin temperature decrease. This observation leaves an open question about a possible body part-specific thermoregulatory response in pathological conditions, in which the sense of ownership over that body part is altered. For instance, Body Integrity Dysphoria (BID), a poorly understood neuropsychiatric disorder, is characterised by the non-acceptance of one or more of one's extremities. This unsettling feeling pervasively captures the individuals' attention towards the unwanted limb. Previous studies characterised BID in terms of absent ownership feeling with preserved ownership judgment. We explored for the first time whether this altered feeling is also associated with a specific thermoregulatory response. We recorded thermal image sequences of circumscribed regions of the limbs' skin in seven individuals with BID desiring to remove one leg while they were invited to focus their attention toward one particular limb (arm or leg). Their event-related thermoregulatory pattern was compared to a group of healthy matched controls. In individuals with BID but not in control persons, we found a bilateral decrease in leg temperature when focusing their attention on either the unwanted or accepted leg. The event-related thermoregulatory response for both upper limbs was similar between individuals with BID and healthy controls. Our results suggest that the alteration of the sense of body ownership in neuropsychiatric conditions such as BID may critically rest on specific event-related thermoregulatory patterns in response to modulation of attention to body parts.

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.

Neural Evidence of Mirror Self-Recognition in the Secondary Somatosensory Cortex of Macaque: Observations from a Single-Cell Recording Experiment and Implications for Consciousness

Despite mirror self-recognition being regarded as a classical indication of self-awareness, little is known about its neural underpinnings. An increasing body of evidence pointing to a role of multimodal somatosensory neurons in self-recognition guided our investigation toward the secondary somatosensory cortex (SII), as we observed single-neuron activity from a macaque monkey sitting in front of a mirror. The monkey was previously habituated to the mirror, successfully acquiring the ability of mirror self-recognition. While the monkey underwent visual and somatosensory stimulation, multimodal visual and somatosensory activity was detected in the SII, with neurons found to respond to stimuli seen through the mirror. Responses were also modulated by self-related or non-self-related stimuli. These observations corroborate that vision is an important aspect of SII activity, with electrophysiological evidence of mirror self-recognition at the neuronal level, even when such an ability is not innate. We also show that the SII may be involved in distinguishing self and non-self. Together, these results point to the involvement of the SII in the establishment of bodily self-consciousness.

Trigeminal input, pupil size and cognitive performance: From oral to brain matter

It has been observed that, in patients affected by temporomandibular disorders (TMDs) and edentulism, a left-right asymmetry in electromyographic (EMG) activity of masseter muscles during clenching and in pupil size at rest (anisocoria) is present. Both are greatly reduced by an orthotic-prosthetic correction. In parallel, the correction significantly improves cognitive performance. These effects are possibly due to the recovery of a cortical balance, via Locus Coeruleus (LC) modulation, whose activity is powerfully affected by the sensorimotor trigeminal input. The role of this functional axis was further investigated in subjects without overt occlusal or dental problems. In these individuals, the EMG asymmetry was significantly correlated to anisocoria at rest, with the dental arches open or in contact. Also in normal subjects, both the EMG and the pupil asymmetry during clenching could be significantly reduced by an orthotic (bite) correction. Closing the arches without bite increased anisocoria and reduced performance in the Spinnler-Tognoni matrices test, as well as the mydriasis induced by a haptic task. When the bite was interposed, anisocoria was reduced, while both performance and task-related mydriasis were enhanced. Since pupil size is considered a proxy of the LC activity, these results suggest that asymmetric occlusion biases the LC discharge and the hemispheric excitability, possibly via a sensorimotor trigeminal imbalance. Removing the anisocoria through bite correction re-establishes a symmetric LC discharge, improving performance and enhancing task-related mydriasis. Therefore, occlusal balancing may represent a tool for improving subjective performance and may be exploited for training and rehabilitative purposes.

Vestibular cognition: State-of-the-art and future directions

Vestibular information has been traditionally considered as a specialized input for basic orienting behaviours, such as oculo-motor adjustments, postural control and gaze orientation. However, in the past two decades a widespread vestibular network in the human brain has been identified, that goes far beyond the low-level reflex circuits emphasized by earlier work. Because this vestibular cortical network is so widely distributed, it could, in principle, impact multiple neurocognitive functions in health and disease. This paper focuses on the relations between vestibular input, vestibular networks, and vestibular interventions by providing the authors' personal viewpoint on the state-of-the-art of vestibular cognitive neuropsychology, and its potential relevance for neurorehabilitation.

Building the bodily self-awareness: Evidence for the convergence between interoceptive and exteroceptive information in a multilevel kernel density analysis study

Exteroceptive and interoceptive signals shape and sustain the bodily self-awareness. The existence of a set of brain areas, supporting the integration of information coming from the inside and the outside of the body in building the sense of bodily self-awareness has been postulated, yet the evidence remains limited, a matter of discussion never assessed quantitatively. With the aim of unrevealing where in the brain interoceptive and exteroceptive signals may converge, we performed a meta-analysis on imaging studies of the sense of body ownership, modulated by external visuotactile stimulation, and studies on interoception, which involves the self-awareness for internal bodily sensations. Using a multilevel kernel density analysis, we found that processing of stimuli of the two domains converges primarily in the supramarginal gyrus bilaterally. Furthermore, we found a right-lateralized set of areas, including the precentral and postcentral, and superior temporal gyri. We discuss these results and propose this set of areas as ideal candidates to match multiple body-related signals contributing to the creation of a multidimensional representation of the bodily self.

Secondary somatosensory cortex of primates: beyond body maps, toward conscious self-in-the-world maps

Recent human imaging studies have revealed the involvement of the secondary somatosensory cortex (SII) in processes that require high-level information integration, such as self-consciousness, social relations, whole body representation, and metaphorical extrapolations. These functions are far beyond its known role in the formation of body maps (even in their most complex forms), requiring the integration of different information modalities in addition to somatosensory information. However, no evidence of such complex processing seems to have been detected at the neuronal level in animal experiments, which would constitute a major discrepancy between human and non-human animals. This article scrutinizes this gap, introducing experimental evidence of human and non-human primates’ SII functions set in context with their evolutionary significance and mechanisms, functionally situating the human SII as a primate brain. Based on the presented data, a new concept of a somatocentric holistic self is proposed, represented as a more comprehensive body-in-the-world map in the primate SII, taking into account evolutionary aspects that characterize the human SII and its implication in the emergence of self-consciousness. Finally, the idea of projection is introduced from the viewpoint of cognitive science, providing a logical explanation to bridge this gap between observed behavior and neurophysiological data.

Modulation of Body Representation Impacts on Efferent Autonomic Activity

The afferent branch of the autonomic nervous system contributes with interoception to the multimodal sensory correlation continuously needed to update our representation of the body. To test whether the modulation of body representation would have an impact on the efferent branch of the autonomic nervous system, nonspecific skin conductance has been measured in three rubber hand illusion (RHI) experiments, controlled with asynchronous brush-stroking and incongruent fake hand position. Nonspecific skin conductance standard deviation (SCSD) computed along the whole 90 sec of stroking was found to be increased by the illusion and to correlate with all the typical measures of embodiment. Computing SCSD in shorter time windows strongly enhanced the difference between illusion and controls. The highest difference was found in the 10-55 sec window, being the 14-34 sec window as the most informative one. The higher correlations with the validated measures of embodiment (all but the proprioceptive drift) were found for time windows ranging between 35 and 65 sec. The SCSD was no longer significantly higher when the RHI was repeated twice (two trials each iteration), but it was still significantly higher in synchronous stroking even when considering only the second trial. However, after the first iteration of the RHI paradigm, the effect of the embodiment on nonspecific skin conductance response results to be attenuated, suggesting that novelty in presentation of the RHI can contribute to the effect on nonspecific skin conductance response. Results candidate SCSD as a noninvasive, cheap, easy, and objective measure of embodiment, especially sensible to onset and strength of the illusion. Alike the already known enhanced autonomic reaction to a threatening, SCSD does not interfere with the collection of other behavioral measures. Correlations and their dynamics, presence of the effect in the second presentation of the setup but relative low robustness against multiple repetition, suggest that the increased fluctuations of skin conductance caught by SCSD are not just the effect of different presented sensory stimuli but more likely a stronger arousal response to the novelty of the updated perceptual status.

Vestibular and Multi-Sensory Influences Upon Self-Motion Perception and the Consequences for Human Behavior

In this manuscript, we comprehensively review both the human and animal literature regarding vestibular and multi-sensory contributions to self-motion perception. This covers the anatomical basis and how and where the signals are processed at all levels from the peripheral vestibular system to the brainstem and cerebellum and finally to the cortex. Further, we consider how and where these vestibular signals are integrated with other sensory cues to facilitate self-motion perception. We conclude by demonstrating the wide-ranging influences of the vestibular system and self-motion perception upon behavior, namely eye movement, postural control, and spatial awareness as well as new discoveries that such perception can impact upon numerical cognition, human affect, and bodily self-consciousness.

Trigeminal, Visceral and Vestibular Inputs May Improve Cognitive Functions by Acting through the Locus Coeruleus and the Ascending Reticular Activating System: A New Hypothesis

It is known that sensory signals sustain the background discharge of the ascending reticular activating system (ARAS) which includes the noradrenergic locus coeruleus (LC) neurons and controls the level of attention and alertness. Moreover, LC neurons influence brain metabolic activity, gene expression and brain inflammatory processes. As a consequence of the sensory control of ARAS/LC, stimulation of a sensory channel may potential influence neuronal activity and trophic state all over the brain, supporting cognitive functions and exerting a neuroprotective action. On the other hand, an imbalance of the same input on the two sides may lead to an asymmetric hemispheric excitability, leading to an impairment in cognitive functions. Among the inputs that may drive LC neurons and ARAS, those arising from the trigeminal region, from visceral organs and, possibly, from the vestibular system seem to be particularly relevant in regulating their activity. The trigeminal, visceral and vestibular control of ARAS/LC activity may explain why these input signals: (1) affect sensorimotor and cognitive functions which are not directly related to their specific informational content; and (2) are effective in relieving the symptoms of some brain pathologies, thus prompting peripheral activation of these input systems as a complementary approach for the treatment of cognitive impairments and neurodegenerative disorders.

Dissociation between preserved body structural description and impaired body image following a pediatric spinal trauma

In adult patients, Spinal Cord Injury (SCI) may influence the mental Body Representation (BR). Currently, there is no evidence on the modulation of SCI on BR during early stages of cognitive development. Here, we investigated BR in a 3-year-old child with complete SCI. The patient was administered with a specific battery assessing different BR components. We found evidence for putative classical neuropsychological dissociation between a preserved topological map with impaired semantic knowledge of the body. This finding sheds new light on the impact of SCI on BR in childhood, as well as on the level of interdependence between BR's components..

The vestibular body: Vestibular contributions to bodily representations

Vestibular signals are integrated with signals from other sensory modalities. This convergence could reflect an important mechanism for maintaining the perception of the body. Here we review the current literature in order to develop a framework for understanding how the vestibular system contributes to body representation. According to recent models, we distinguish between three processes for body representation, and we look at whether vestibular signals might influence each process. These are (i) somatosensation, the primary sensory processing of somatic stimuli, (ii) somatoperception, the processes of constructing percepts and experiences of somatic objects and events and (iii) somatorepresentation, the knowledge about the body as a physical object in the world. Vestibular signals appear to contribute to all three levels in this model of body processing. Thus, the traditional view of the vestibular system as a low-level, dedicated orienting module tends to underestimate the pervasive role of vestibular input in bodily self-awareness.

Multisensory effects on somatosensation: a trimodal visuo-vestibular-tactile interaction

Vestibular information about self-motion is combined with other sensory signals. Previous research described both visuo-vestibular and vestibular-tactile bilateral interactions, but the simultaneous interaction between all three sensory modalities has not been explored. Here we exploit a previously reported visuo-vestibular integration to investigate multisensory effects on tactile sensitivity in humans. Tactile sensitivity was measured during passive whole body rotations alone or in conjunction with optic flow, creating either purely vestibular or visuo-vestibular sensations of self-motion. Our results demonstrate that tactile sensitivity is modulated by perceived self-motion, as provided by a combined visuo-vestibular percept, and not by the visual and vestibular cues independently. We propose a hierarchical multisensory interaction that underpins somatosensory modulation: visual and vestibular cues are first combined to produce a multisensory self-motion percept. Somatosensory processing is then enhanced according to the degree of perceived self-motion.

Hypnotically induced somatosensory alterations: Toward a neurophysiological understanding of hypnotic anaesthesia

Whereas numerous studies have investigated hypnotic analgesia, few have investigated hypnotic anaesthesia. Using magnetoencephalography (MEG) we investigated and localized brain responses (event-related fields and oscillatory activity) during sensory processing under hypnotic anaesthesia. Nineteen right handed neurotypical individuals with moderate-to-high hypnotizability received 100 vibrotactile stimuli to right and left index fingers in a random sequence. Thereafter a hypnotic state was induced, in which anaesthetic suggestion was applied to the left hand only. Once anaesthetic suggestion was achieved, a second, identical, session of vibrotactile stimuli was commenced. We found greater brain activity in response to the stimuli delivered to the left (attenuated) hand before hypnotic anaesthesia, than under hypnotic anaesthesia, in both the beta and alpha bands. In the beta band, the reduction of activity under hypnotic anaesthesia was found around 214-413ms post-stimuli and was located mainly in the right insula. In the alpha band, it was found around 253-500ms post-stimuli and was located mainly in the left inferior frontal gyrus. In a second experiment, attention modulation per se was ruled out as the underlying cause of the effects found. These findings may suggest that the brain mechanism underlying hypnotic anaesthesia involves top-down somatosensory inhibition and, therefore, a reduction of somatosensory awareness. The result of this mechanism is a mental state in which individuals lose bodily sensation.

Left caloric vestibular stimulation as a tool to reveal implicit and explicit parameters of body representation

Homeostatic parameters, such as temperature, are related to body representation. In this study, we measured whether caloric vestibular stimulation (CVS) alters body temperature and tactile processing, and if in the direction predicted by a holistic body matrix representation. Skin temperature and tactile two-point discrimination (TPD) acuity were measured for both arms before, immediately after and with a delay from CVS. Participants were also administered a personality questionnaire and an anxiety inventory to rule out confounding factors. Two control experiments were planned to exclude casual variations. Our results show that temperature drops significantly in both arms after CVS. CVS also induces a bilateral improvement in tactile acuity (even though not immediately after but in the delayed condition). Finally, these effects are not due to learning, as demonstrated by the control experiment. In summary, our results suggest that vestibular stimulation updates body representation, supporting the evidence in favor of a body matrix.

Multimodal Contributions to Body Representation

Our body is a unique entity by which we interact with the external world. Consequently, the way we represent our body has profound implications in the way we process and locate sensations and in turn perform appropriate actions. The body can be the subject, but also the object of our experience, providing information from sensations on the body surface and viscera, but also knowledge of the body as a physical object. However, the extent to which different senses contribute to constructing the rich and unified body representations we all experience remains unclear. In this review, we aim to bring together recent research showing important roles for several different sensory modalities in constructing body representations. At the same time, we hope to generate new ideas of how and at which level the senses contribute to generate the different levels of body representations and how they interact. We will present an overview of some of the most recent neuropsychological evidence about multisensory control of pain, and the way that visual, auditory, vestibular and tactile systems contribute to the creation of coherent representations of the body. We will focus particularly on some of the topics discussed in the symposium on Multimodal Contributions to Body Representation held on the 15th International Multisensory Research Forum (2015, Pisa, Italy).

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.

Can Postural Instability Respond to Galvanic Vestibular Stimulation in Patients with Parkinson's Disease?

Objective

Galvanic vestibular stimulation (GVS) activates the vestibular afferents, and these changes in vestibular input exert a strong influence on the subject’s posture or standing balance. In patients with Parkinson’s disease (PD), vestibular dysfunction might contribute to postural instability and gait disorders.

Methods

Current intensity was increased to 0.7 mA, and the current was applied to the patients for 20 minutes. To perform a sham stimulation, the current intensity was increased as described and then decreased to 0 mA over the course of 10 seconds. The patient’s status was recorded continuously for 20 minutes with the patient in the supine position.

Results

Three out of 5 patients diagnosed with PD with postural instability and/or abnormal axial posture showed a reduction in postural instability after GVS. The score for item 12 of the revised Unified Parkinson’s Disease Rating Scale part 3 was decreased in these patients.

Conclusions

The mechanism of postural instability is complex and not completely understood. In 2 out of the 5 patients, postural instability was not changed in response to GVS. Nonetheless, the GVS-induced change in postural instability for 3 patients in our study suggests that GVS might be a therapeutic option for postural instability.

Distinct vestibular effects on early and late somatosensory cortical processing in humans

In non-human primates several brain areas contain neurons that respond to both vestibular and somatosensory stimulation. In humans, vestibular stimulation activates several somatosensory brain regions and improves tactile perception. However, less is known about the spatio-temporal dynamics of such vestibular-somatosensory interactions in the human brain. To address this issue, we recorded high-density electroencephalography during left median nerve electrical stimulation to obtain Somatosensory Evoked Potentials (SEPs). We analyzed SEPs during vestibular activation following sudden decelerations from constant-velocity (90°/s and 60°/s) earth-vertical axis yaw rotations and SEPs during a non-vestibular control period. SEP analysis revealed two distinct temporal effects of vestibular activation: An early effect (28-32ms post-stimulus) characterized by vestibular suppression of SEP response strength that depended on rotation velocity and a later effect (97-112ms post-stimulus) characterized by vestibular modulation of SEP topographical pattern that was rotation velocity-independent. Source estimation localized these vestibular effects, during both time periods, to activation differences in a distributed cortical network including the right postcentral gyrus, right insula, left precuneus, and bilateral secondary somatosensory cortex. These results suggest that vestibular-somatosensory interactions in humans depend on processing in specific time periods in somatosensory and vestibular cortical regions.

Caloric vestibular stimulation modulates nociceptive evoked potentials

Vestibular stimulation has been reported to alleviate central pain. Clinical and physiological studies confirm pervasive interactions between vestibular signals and somatosensory circuits, including nociception. However, the neural mechanisms underlying vestibular-induced analgesia remain unclear, and previous clinical studies cannot rule out explanations based on alternative, non-specific effects such as distraction or placebo. To investigate how vestibular inputs influence nociception, we combined caloric vestibular stimulation (CVS) with psychophysical and electrocortical responses elicited by nociceptive-specific laser stimulation in humans (laser-evoked potentials, LEPs). Cold water CVS applied to the left ear resulted in significantly lower subjective pain intensity for experimental laser pain to the left hand immediately after CVS, relative both to before CVS and to 1 h after CVS. This transient reduction in pain perception was associated with reduced amplitude of all LEP components, including the early N1 wave reflecting the first arrival of nociceptive input to primary somatosensory cortex. We conclude that cold left ear CVS elicits a modulation of both nociceptive processing and pain perception. The analgesic effect induced by CVS could be mediated either by subcortical gating of the ascending nociceptive input, or by direct modulation of the primary somatosensory cortex.

Multisensory interactions between vestibular, visual and somatosensory signals

Vestibular inputs are constantly processed and integrated with signals from other sensory modalities, such as vision and touch. The multiply-connected nature of vestibular cortical anatomy led us to investigate whether vestibular signals could participate in a multi-way interaction with visual and somatosensory perception. We used signal detection methods to identify whether vestibular stimulation might interact with both visual and somatosensory events in a detection task. Participants were instructed to detect near-threshold somatosensory stimuli that were delivered to the left index finger in one half of experimental trials. A visual signal occurred close to the finger in half of the trials, independent of somatosensory stimuli. A novel Near infrared caloric vestibular stimulus (NirCVS) was used to artificially activate the vestibular organs. Sham stimulations were used to control for non-specific effects of NirCVS. We found that both visual and vestibular events increased somatosensory sensitivity. Critically, we found no evidence for supra-additive multisensory enhancement when both visual and vestibular signals were administered together: in fact, we found a trend towards sub-additive interaction. The results are compatible with a vestibular role in somatosensory gain regulation.

Vestibular contributions to a right-hemisphere network for bodily awareness: combining galvanic vestibular stimulation and the "Rubber Hand Illusion"

An altered sense of one's own body is a common consequence of vestibular damage, and also of damage to vestibular networks in the right hemisphere. However, few experimental studies have investigated whether vestibular signals contribute to bodily awareness. We addressed this issue by combining an established experimental model of bodily awareness (Rubber Hand Illusion -RHI) with galvanic vestibular stimulation (GVS) in healthy participants. Brief left anodal and right cathodal GVS (which predominantly activates vestibular networks in the right hemisphere), or right anodal and left cathodal GVS, or sham stimulation were delivered at random, while participants experienced either synchronous or asynchronous visuo-tactile stimulation of a rubber hand and their own hand. The drift in the perceived position of the participant's hand towards the rubber hand was used as a proxy measure of the resulting multisensory illusion of body ownership. GVS induced strong polarity-dependent effects on this measure of RHI: left anodal and right cathodal GVS produced significantly lower proprioceptive drift than right anodal and left cathodal GVS. We suggest that vestibular inputs influence the multisensory weighting functions that underlie bodily awareness: the right hemisphere vestibular projections activated by the left anodal and right cathodal GVS increased the weight of intrinsic proprioceptive signals about hand position, and decreased the weight of visual information responsible for visual capture during the RHI.

Age-related decline in functional connectivity of the vestibular cortical network

In the elderly, major complaints include dizziness and an increasing number of falls, possibly related to an altered processing of vestibular sensory input. In this study, we therefore investigate age-related changes induced by processing of vestibular sensory stimulation. While previous functional imaging studies of healthy aging have investigated brain function during task performance or at rest, we used galvanic vestibular stimulation during functional MRI in a task-free sensory stimulation paradigm to study the effect of healthy aging on central vestibular processing, which might only become apparent during stimulation processing. Since aging may affect signatures of brain function beyond the BOLD-signal amplitude-such as functional connectivity or temporal signal variability--we employed independent component analysis and partial least squares analysis of temporal signal variability. We tested for age-associated changes unrelated to vestibular processing, using a motor paradigm, voxel-based morphometry and diffusion tensor imaging. This allows us to control for general age-related modifications, possibly originating from vascular, atrophic or structural connectivity changes. Age-correlated decreases of functional connectivity and increases of BOLD--signal variability were associated with multisensory vestibular networks. In contrast, no age-related functional connectivity changes were detected in somatosensory networks or during the motor paradigm. The functional connectivity decrease was not due to structural changes but to a decrease in response amplitude. In synopsis, our data suggest that both the age-dependent functional connectivity decrease and the variability increase may be due to deteriorating reciprocal cortico-cortical inhibition with age and related to multimodal vestibular integration of sensory inputs.

Vestibular–Somatosensory Interactions: A Mechanism in Search of a Function?

No unimodal vestibular cortex has been identified in the human brain. Rather, vestibular inputs are strongly integrated with signals from other sensory modalities, such as vision, touch and proprioception. This convergence could reflect an important mechanism for maintaining a perception of the body, including individual body parts, relative to the rest of the environment. Neuroimaging, electrophysiological and psychophysical studies showed evidence for multisensory interactions between vestibular and somatosensory signals. However, no convincing overall theoretical framework has been proposed for vestibular–somatosensory interactions, and it remains unclear whether such percepts are by-products of neural convergence, or a functional multimodal integration. Here we review the current literature on vestibular–multisensory interactions in order to develop a framework for understanding the functions of such multimodal interaction. We propose that the target of vestibular–somatosensory interactions is a form of self-representation.

Beyond the Non-Specific Attentional Effect of Caloric Vestibular Stimulation: Evidence from Healthy Subjects and Patients

Caloric vestibular stimulation (CVS) is a simple physiological manipulation that has been used for a long time in different clinical fields due to its rapid and relevant effects on behaviour. One of the most debated issues in this research field concerns the degree of specificity of such stimulation, namely whether the effects of CVS can be, and to what extent are, independent of the mere influence of non-specific factors such as general arousal, ocular movements or attentional shift towards the stimulated side. The hypothesis that CVS might cause a shift of attention towards the side of the stimulation has been largely supported; moreover, a large amount of evidence is available nowadays to corroborate the specific effect of CVS, providing behavioural and neurophysiological data in both patients and normal subjects. These data converge in indicating that the effects of CVS can be independent of eye deviation and general arousal, can modulate different symptoms in different directions, and do not merely depend on a general shift of attention. The present article is divided into three main sections. In the first section, we describe classical studies that investigate the effects of CVS on neglect and related symptoms. In the second and third parts, we provide an overview of the modulatory effects of CVS on somatosensory processes and body representation in both brain-damaged patients and healthy subjects. Finally, we conclude by discussing the relevance of these new findings for the understanding of the neural mechanisms underlying the modulatory effects of CVS.

Nobel Prize centenary: Robert Bárány and the vestibular system

The hundredth anniversary of Robert Bárány's Nobel Prize in Medicine offers the opportunity to highlight the importance of his discoveries on the physiology and pathophysiology of the vestibular organs. Bárány developed the method of caloric vestibular stimulation that revolutionized the investigation of the semicircular canals and that is still widely used today. Caloric vestibular stimulation launched experimental vestibular research that was relevant to comprehend the evolution of human locomotion, and Bárány's tests continue to be used in neuroscience to understand the influence of vestibular signals on bodily perceptions, cognition and emotions. Only during the last 20 years has caloric vestibular stimulation been merged with brain imaging to localize the human vestibular cortex.

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.

Vestibular-somatosensory interactions: effects of passive whole-body rotation on somatosensory detection

Vestibular signals are strongly integrated with information from several other sensory modalities. For example, vestibular stimulation was reported to improve tactile detection. However, this improvement could reflect either a multimodal interaction or an indirect interaction driven by vestibular effects on spatial attention and orienting. Here we investigate whether natural vestibular activation induced by passive whole-body rotation influences tactile detection. In particular, we assessed the ability to detect faint tactile stimuli to the fingertips of the left and right hand during spatially congruent or incongruent rotations. We found that passive whole-body rotations significantly enhanced sensitivity to faint shocks, without affecting response bias. Critically, this enhancement of somatosensory sensitivity did not depend on the spatial congruency between the direction of rotation and the hand stimulated. Thus, our results support a multimodal interaction, likely in brain areas receiving both vestibular and somatosensory signals.

A neuroscientific account of how vestibular disorders impair bodily self-consciousness

The consequences of vestibular disorders on balance, oculomotor control, and self-motion perception have been extensively described in humans and animals. More recently, vestibular disorders have been related to cognitive deficits in spatial navigation and memory tasks. Less frequently, abnormal bodily perceptions have been described in patients with vestibular disorders. Altered forms of bodily self-consciousness include distorted body image and body schema, disembodied self-location (out-of-body experience), altered sense of agency, as well as more complex experiences of dissociation and detachment from the self (depersonalization). In this article, I suggest that vestibular disorders create sensory conflict or mismatch in multisensory brain regions, producing perceptual incoherence and abnormal body and self perceptions. This hypothesis is based on recent functional mapping of the human vestibular cortex, showing vestibular projections to the primary and secondary somatosensory cortex and in several multisensory areas found to be crucial for bodily self-consciousness.

Caloric vestibular stimulation modulates affective control and mood

BACKGROUND

Clinical evidence suggests a link between vestibular dysfunctions and mood disorders. No study has yet investigated mood and affective control during vestibular stimulation in healthy participants.

OBJECTIVE

We predicted a modulating effect of caloric vestibular stimulation (CVS) on affective control measured in an affective Go/NoGo task (AGN).

METHODS

Thirty-two participants performed an AGN task while they were exposed to cold left or right ear CVS (20 °C) and sham stimulation (37 °C). In each block, either positive or negative pictures (taken from the International Affective Picture System) were defined as targets. Participants had to respond to targets (Go), and withhold responses to distractors (NoGo).

RESULTS

The sensitivity index d' (hits - false alarms) was used to measure affective control. Affective control improved during right ear CVS when viewing positive stimuli (P = .005), but decreased during left ear CVS when compared to sham stimulation (P = .009). CVS had a similar effect on positive mood ratings (Positive and Negative Affect Schedule). Positive mood ratings decreased during left ear CVS when compared to sham stimulation, but there was no effect after right ear CVS.

DISCUSSION

The results suggest that CVS, depending on side of stimulation, has a modulating effect on mood and affective control. The results complement previous findings in manic patients and provide new evidence for the clinical potential of CVS.

Vestibular insights into cognition and psychiatry

The vestibular system has traditionally been thought of as a balance apparatus; however, accumulating research suggests an association between vestibular function and psychiatric and cognitive symptoms, even when balance is measurably unaffected. There are several brain regions that are implicated in both vestibular pathways and psychiatric disorders. The present review examines the anatomical associations between the vestibular system and various psychiatric disorders. Despite the lack of direct evidence for vestibular pathology in the key psychiatric disorders selected for this review, there is a substantial body of literature implicating the vestibular system in each of the selected psychiatric disorders. The second part of this review provides complimentary evidence showing the link between vestibular dysfunction and vestibular stimulation upon cognitive and psychiatric symptoms. In summary, emerging research suggests the vestibular system can be considered a potential window for exploring brain function beyond that of maintenance of balance, and into areas of cognitive, affective and psychiatric symptomology. Given the paucity of biological and diagnostic markers in psychiatry, novel avenues to explore brain function in psychiatric disorders are of particular interest and warrant further exploration.

Transcutaneous electrical nerve stimulation effects on neglect: a visual-evoked potential study

We studied the effects of transcutaneous electrical nerve stimulation (TENS) in six right-brain-damaged patients with left unilateral spatial neglect (USN), using both standard clinical tests (reading, line, and letter cancelation, and line bisection), and electrophysiological measures (steady-state visual-evoked potentials, SSVEP). TENS was applied on left neck muscles for 15′, and measures were recorded before, immediately after, and 60′ after stimulation. Behavioral results showed that the stimulation temporarily improved the deficit in all patients. In cancelation tasks, omissions and performance asymmetries between the two hand-sides were reduced, as well as the rightward deviation in line bisection. Before TENS, SSVEP average latency to stimuli displayed in the left visual half-field [LVF (160 ms)] was remarkably longer than to stimuli shown in the right visual half-field [RVF (120 ms)]. Immediately after TENS, latency to LVF stimuli was 130 ms; 1 h after stimulation the effect of TENS faded, with latency returning to baseline. TENS similarly affected also the latency SSVEP of 12 healthy participants, and their line bisection performance, with effects smaller in size. The present study, first, replicates evidence concerning the positive behavioral effects of TENS on the manifestations of left USN in right-brain-damaged patients; second, it shows putatively related electrophysiological effects on the SSVEP latency. These behavioral and novel electrophysiological results are discussed in terms of specific directional effects of left somatosensory stimulation on egocentric coordinates, which in USN patients are displaced toward the side of the cerebral lesion. Showing that visual-evoked potentials latency is modulated by proprioceptive stimulation, we provide electrophysiological evidence to the effect that TENS may improve some manifestations of USN, with implications for its rehabilitation.

Full body illusion is associated with widespread skin temperature reduction

A central feature of our consciousness is the experience of the self as a unified entity residing in a physical body, termed bodily self-consciousness. This phenomenon includes aspects such as the sense of owning a body (also known as body ownership) and has been suggested to arise from the integration of sensory signals from the body. Several studies have shown that temporally synchronous tactile stimulation of the real body and visual stimulation of a fake or virtual body can induce changes in bodily self-consciousness, typically resulting in a sense of illusory ownership over the fake body. The present study assessed the effect of anatomical congruency of visuo-tactile stimulation on bodily self-consciousness. A virtual body was presented and temporally synchronous visuo-tactile stroking was applied simultaneously to the participants' body and to the virtual body. We manipulated the anatomical locations of the visuo-tactile stroking (i.e., on the back, on the leg), resulting in congruent stroking (stroking was felt and seen on the back or the leg) or incongruent stroking (i.e., stroking was felt on the leg and seen on the back). We measured self-identification with the virtual body and self-location as well as skin temperature. Illusory self-identification with the avatar as well as changes in self-location were experienced in the congruent stroking conditions. Participants showed a decrease in skin temperature across several body locations during congruent stimulation. These data establish that the full-body illusion (FBI) alters bodily self-consciousness and instigates widespread physiological changes in the participant's body.