The role of somatosensory input for the perception of verticality

Environment and body-brain interplay affect inhibition and decision-making

The fine-tuned interplay of brain and body underlies human ability to cope with changes in the internal and external milieus. Previous research showed that cardiac interoceptive changes (e.g., cardiac phase) affect cognitive functions, notably inhibition that is a key element for adaptive behaviour. Here we investigated the influence on cognition of vestibular signal, which provides the brain with sensory information about body position and movement. We used a centrifuge-based design to disrupt vestibular signal in healthy human volunteers while their inhibition and decision-making functions were assessed with the stop-signal paradigm. Participants performed the standard and a novel, sensorial version of the stop-signal task to determine whether disrupted vestibular signal influences cognition as a function of its relevance to the context. First, we showed that disrupted vestibular signal was associated with a larger variability of longest inhibition latencies, meaning that participants were even slower to inhibit in the trials where they had the most difficulty inhibiting. Second, we revealed that processing of bodily information, as required in the sensorial stop-signal task, also led to a larger variability of longest inhibition latencies, which was all the more important when vestibular signal was disrupted. Lastly, we found that such a degraded response inhibition performance was due in part to the acceleration of decision-making process, meaning that participants made a decision more quickly even when strength of sensory evidence was reduced. Taken together, these novel findings provide direct evidence that vestibular signal affects the cognitive functions of inhibition and decision-making.

Long-duration head down bed rest as an analog of microgravity: Effects on the static perception of upright

BACKGROUND:

Humans demonstrate many physiological changes in microgravity for which long-duration head down bed rest (HDBR) is a reliable analog. However, information on how HDBR affects sensory processing is lacking.

OBJECTIVE:

We previously showed [25] that microgravity alters the weighting applied to visual cues in determining the perceptual upright (PU), an effect that lasts long after return. Does long-duration HDBR have comparable effects?

METHODS:

We assessed static spatial orientation using the luminous line test (subjective visual vertical, SVV) and the oriented character recognition test (PU) before, during and after 21 days of 6° HDBR in 10 participants. Methods were essentially identical as previously used in orbit [25].

RESULTS:

Overall, HDBR had no effect on the reliance on visual relative to body cues in determining the PU. However, when considering the three critical time points (pre-bed rest, end of bed rest, and 14 days post-bed rest) there was a significant decrease in reliance on visual relative to body cues, as found in microgravity. The ratio had an average time constant of 7.28 days and returned to pre-bed-rest levels within 14 days. The SVV was unaffected.

CONCLUSIONS:

We conclude that bed rest can be a useful analog for the study of the perception of static self-orientation during long-term exposure to microgravity. More detailed work on the precise time course of our effects is needed in both bed rest and microgravity conditions.

Does somatosensory feedback from the plantar foot sole contribute to verticality perception?

AIM OF THE STUDY

In upright standing, the human foot sole is the only point of contact with the ground conveying information about the pressure distribution under the feet. We examined how the altered somatosensory input from the plantar foot receptors, when standing on a soft surface, affects the subjective estimation of the earth vertical in different sensory contexts.

MATERIALS AND METHODS

Twelve (12) healthy young females (mean age: 21.8 ± 2.4 years) adjusted the orientation of a visual line (35 × 1.5 cm) representing the roll orientation of a hand-held (attached on a 24.9 × 4 cm cylinder) or head-attached electromagnetic tracking sensor (Nest of Birds, Ascension Technologies Inc., VT. USA, 60 Hz) under two visual conditions (eyes open, eyes closed) while standing on a soft or firm surface. The mean absolute (accuracy) and variable (precision) error in the verticality estimate was depicted in the sensor's roll deviation from the gravitational vertical.

RESULTS

The accuracy and the precision of the estimate decreased in the absence of vision, while standing on the soft surface and when the estimate was provided by an active hand rather than head rotation. The surface effect was significant only in the absence of vision and when the estimate was provided by the hand.

CONCLUSIONS

The contribution of the plantar foot mechanoreceptors to gravity perception is sensory context dependent. Perception of the earth vertical is more accurate when estimated by active head rotation due to the integration of the vestibular and neck proprioceptive afferents.

Measuring Vestibular Contributions to Age-Related Balance Impairment: A Review

Aging is associated with progressive declines in both the vestibular and human balance systems. While vestibular lesions certainly contribute to imbalance, the specific contributions of age-related vestibular declines to age-related balance impairment is poorly understood. This gap in knowledge results from the absence of a standardized method for measuring age-related changes to the vestibular balance pathways. The purpose of this manuscript is to provide an overview of the existing body of literature as it pertains to the methods currently used to infer vestibular contributions to age-related imbalance.

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.

Comparison of two methods based on one psychophysical paradigm to measure the subjective postural vertical in standing

The perception of verticality can be altered with age or due to neurological diseases. Different procedures have been described to measure the subjective postural vertical (SPV). A deviation from the earth vertical was either described as a single position or as a sector defined by two positions representing the edges of the perceived verticality. In this study, for the first time, we investigated if these two methods produce equal values, and consequently can be merged to set normative values. SPV in standing was tested in 24 healthy young adults (28.4 (5.2) years of age, 12 women). Each participant performed both methods in the sagittal and the frontal plane. Absolute and constant error values were found to be similar for both methods in both planes with a mean difference of less than 0.3° (p > 0.148). The mean width of the SPV sector was 3.9° (0.9°) in the sagittal and 3.7° (1.4°) in the frontal plane, ranging in the mean from -5.5° to 8.1° in the sagittal and -5.3° to 4.3° in the frontal plane. SPV values significantly differed in range between both methods in both planes with a mean difference of more than 3.1° (p<0.002). Results show that both methods, SPVposition and SPVsector, produce equal error values when applied with otherwise similar methodological settings and can therefore be used alternatively or within the same meta-analysis. The SPVsector, however, led to wider range values and was less frequently rated as the preferred method to represent the participants' subjective verticality.

Young and Older Adults Differ in Integration of Sensory Cues for Vertical Perception

Introduction

The subjective visual vertical (SVV) measures the perception of a person's spatial orientation relative to gravity. Weighted central integration of vestibular, visual, and proprioceptive inputs is essential for SVV perception. Without any visual references and minimal proprioceptive contribution, the static SVV reflects balance of the otolith organs. Normal aging is associated with bilateral and progressive decline in otolith organ function, but age-dependent effects on SVV are inconclusive. Studies on sensory reweighting for visual vertical and multisensory integration strategies reveal age-dependent differences, but most studies have included elderly participants in comparison to younger adults. The aim of this study was to compare young adults with older adults, an age group younger than the elderly.

Methods

Thirty-three young and 28 older adults (50–65 years old) adjusted a tilted line accurately to their perceived vertical. The rod's final position from true vertical was recorded as tilt error in degrees. For otolithic balance, visual vertical was recorded in the dark without any visual references. The rod and frame task (RFT) with tilted disorienting visual frames was used for creating visuovestibular conflict. We adopted Nyborg's analysis method to derive the rod and frame effect (RFE) and trial-to-trial variability measures. Rod alignment times were also analyzed.

Results

There was no age difference in signed tilts of SVV without visual reference. There was an age effect on RFE and on overall trial-to-trial variability of rod tilt, with older adults displaying larger frame effects and greater variability in rod tilts. Alignment times were longer in the tilted-frame conditions for both groups and in the older adults compared to their younger counterparts. The association between tilt accuracy and tilt precision was significant for older adults only during visuovestibular conflict, revealing an increase in RFE with an increase in tilt variability. Correlation of σ_SVV, which represents vestibular input precision, with RFE yielded exactly the same contribution of σ_SVV to the variance in RFE for both age groups.

Conclusions

Older adults have balanced otolithic input in an upright position. Increased reliance on visual cues may begin at ages younger than what is considered elderly. Increased alignment times for older adults may create a broader time window for integration of relevant and irrelevant sensory information, thus enhancing their multisensory integration. In parallel with the elderly, older adults may differ from young adults in their integration of sensory cues for visual vertical perception.

Misperception of the subjective visual vertical in neurological patients with or without stroke: A meta-analysis

BACKGROUND

The interpretation of the verticality of the environment is crucial for a proper body balance. The subjective visual vertical test (SVV) is a widely used method to determine the visual perception of the verticality, whose alteration has been related with poor functional status.

OBJECTIVE

To analyze the visual perception of the verticality in neurological patients in comparison with healthy controls.

METHODS

We searched PubMed, Scopus, and Scielo from the start of the databases until October 2017 and manually searched the reference lists of studies comparing SVV values between neurological patients and controls. Standardized mean difference (SMD) and subgroup analysis were used to analyze differences between neurological patients and healthy subjects and between stroke and non-stroke patients, respectively.

RESULTS

A total of 1,916 subjects from 31 studies were included. Neurological patients misestimate the true vertical in comparison with controls (SMD = 1.05; 95% CI: 0.81, 1.28). The misperception of the verticality was higher in stroke patients (SMD = 1.35; 95% CI: 1.02, 1.68) than in patients with other neurological conditions (SMD = 0.48; 95% CI: 0.29, 0.68).

CONCLUSIONS

Neurological patients showed a misperception of the verticality, estimated using the SVV. The neurological pathology that most alters the SVV is stroke.

Biases in the Visual and Haptic Subjective Vertical Reveal the Role of Proprioceptive/Vestibular Priors in Child Development

Investigation of the perception of verticality permits to disclose the perceptual mechanisms that underlie balance control and spatial navigation. Estimation of verticality in unusual body orientation with respect to gravity (e.g., laterally tilted in the roll plane) leads to biases that change depending on the encoding sensory modality and the amount of tilt. A well-known phenomenon is the A-effect, that is a bias toward the body tilt often interpreted in a Bayesian framework to be the byproduct of a prior peaked at the most common head and body orientation, i.e., upright. In this study, we took advantage of this phenomenon to study the interaction of visual, haptic sensory information with vestibular/proprioceptive priors across development. We tested children (5-13 y.o) and adults (>22 y.o.) in an orientation discrimination task laterally tilted 90° to their left-ear side. Experimental conditions differed for the tested sensory modality: visual-only, haptic-only, both modalities. Resulting accuracy depended on the developmental stage and the encoding sensory modality, showing A-effects in vision across all ages and in the haptic modality only for the youngest children whereas bimodal judgments show lack of multisensory integration in children. A Bayesian prior model nicely predicts the behavioral data when the peak of the prior distribution shifts across age groups. Our results suggest that vision is pivotal to acquire an idiotropic vector useful for improving precision when upright. The acquisition of such a prior might be related to the development of head and trunk coordination, a process that is fundamental for gaining successful spatial navigation.

Plantar cutaneous afferents influence the perception of Subjective Visual Vertical in quiet stance

The estimation of Subjective Visual Vertical (SVV) involves the allocentric, gravitational and egocentric references, which are built by visual, vestibular and somatosensory afferents. Our goals were to assess the influence of plantar cutaneous afferents on the perception of SVV, and to see if there is a difference according to the efficiency of plantar cutaneous afferents. We recruited 48 young and healthy subjects and assessed their SVV and postural performances in quiet stance with a force platform, at 40 or 200 cm, in four ground conditions: on firm ground, on foam, with a bilateral, or with a unilateral 3 mm arch support. We also assessed the efficiency of our subjects' plantar afferents with the plantar quotient method and divided them in two groups: subjects with a normal use of plantar afferents and subjects with Plantar Exteroceptive Inefficiency (PEI). The results showed significant decreases in the counter clockwise SVV deviation only with the unilateral arch support, at near distance, and among the typically behaving subjects. We conclude that asymmetric foot cutaneous afferents are able to bias the egocentric vertical reference and hence influence the perception of SVV. This influence disappears among subjects with PEI, probably because of a distortion of the plantar signal.

Normative data for human postural vertical: A systematic review and meta-analysis

Perception of verticality is required for normal daily function, yet the typical human detection error range has not been well characterized. Vertical misperception has been correlated with poor postural control and functionality in patients after stroke and after vestibular disorders. Until now, all the published studies that assessed Subjective Postural Vertical (SPV) in the seated position used small groups to establish a reference value. However, this sample size does not represent the healthy population for comparison with conditions resulting in pathological vertical. Therefore, the primary objective was to conduct a systematic review with meta-analyses of Subjective Postural Vertical (SPV) data in seated position in healthy adults to establish the reference value with a representative sample. The secondary objective was to investigate the methodological characteristics of different assessment protocols of SPV described in the literature. A systematic literature search was conducted using Medline, EMBASE, and Cochrane libraries. Mean and standard deviation of SPV in frontal and sagittal planes were considered as effect size measures. Sixteen of 129 identified studies met eligibility criteria for our systematic review (n = 337 subjects in the frontal plane; n = 187 subjects in sagittal plane). The meta-analyses measure was estimated using the pooled mean as the estimator and its respective error. Mean reference values were 0.12°±1.49° for the frontal plane and 0.02°±1.82° for the sagittal plane. There was a small variability of the results and this systematic review resulted in representative values for SPV. The critical analysis of the studies and observed homogeneity in the sample suggests that the methodological differences used in the studies did not influence SPV assessment of directional bias in healthy subjects. These data can serve as a reference for clinical studies in disorders of verticality.

Sensory information and the perception of verticality in post-stroke patients. Another point of view in sensory reweighting strategies

INTRODUCTION

Perception of verticality is highly related to balance control in human. Head-on-body tilt <60° results in the E-effect, meaning that a tilt of the perceived vertical is observed contralateral to the head tilt in the frontal plane. Furthermore, somatosensory loss also impacts the accuracy of verticality perception. However, when several input sources are absent or biased, less options for sensory weighting and balance control occur. Therefore, this study aims to identify the E-effect and assess the effect of somatosensory loss on the extent of the E-effect.

METHODS

All patients with a first stroke admitted to a Belgian rehabilitation hospital were eligible for inclusion. Patients aged above 80 with other neurological and orthopaedic impairments as well as brainstem, cerebellar or multiple lesions were excluded. In addition, patients with visuospatial neglect and pusher behaviour were also excluded as this can affect verticality perception. The Rivermead Assessment of Somatosensory Performance (RASP), the Subjective Visual (SVV) and Subjective Postural (SPV) Vertical Test were administered.

RESULTS

In total, 37 patients were included in the analysis of which 24 patients completed both SVV and SPV assessment. Results show that the E-effect occurred in our sample of stroke survivors for both SVV and SPV. In addition, the presence of somatosensory loss will increase the E-effect in both SVV as SPV assessment. A significant difference in verticality perception was noted for both SVV and SPV between the group with no (SVV: 5.13°(6.92); SPV: 0.30°(1.85)) and highly severe (SVV: 10.54°(13.19); SPV: 5.96°(9.27)) sensory loss.

CONCLUSIONS

The E-effect occurs in stroke subjects and increases when patients experience somatosensory loss. This suggests that the lack of available afferent information impede estimation of verticality. Therefore, stroke survivors have fewer alternative input sources as a result of impairments, leading to fewer options about sensory reweighting strategies and balance recovery after perturbations.

Gravity estimation and verticality perception

Gravity is a defining force that governs the evolution of mechanical forms, shapes and anchors our perception of the environment, and imposes fundamental constraints on our interactions with the world. Within the animal kingdom, humans are relatively unique in having evolved a vertical, bipedal posture. Although a vertical posture confers numerous benefits, it also renders us less stable than quadrupeds, increasing susceptibility to falls. The ability to accurately and precisely estimate our orientation relative to gravity is therefore of utmost importance. Here we review sensory information and computational processes underlying gravity estimation and verticality perception. Central to gravity estimation and verticality perception is multisensory cue combination, which serves to improve the precision of perception and resolve ambiguities in sensory representations by combining information from across the visual, vestibular, and somatosensory systems. We additionally review experimental paradigms for evaluating verticality perception, and discuss how particular disorders affect the perception of upright. Together, the work reviewed here highlights the critical role of multisensory cue combination in gravity estimation, verticality perception, and creating stable gravity-centered representations of our environment.

Change in the natural head-neck orientation momentarily altered sensorimotor control during sensory transition

Achilles tendon vibration generates proprioceptive information that is incongruent with the actual body position; it alters the perception of body orientation leading to a vibration-induced postural response. When a person is standing freely, vibration of the Achilles tendon shifts the internal representation of the verticality backward thus the vibration-induced postural response realigned the whole body orientation with the shifted subjective vertical. Because utricular otoliths information participates in the creation of the internal representation of the verticality, changing the natural orientation of the head-neck system during Achilles tendon vibration could alter the internal representation of the earth vertical to a greater extent. Consequently, it was hypothesized that compared to neutral head-neck orientation, alteration in the head-neck orientation should impair balance control immediately after Achilles tendon vibration onset or offset (i.e., sensory transition) as accurate perception of the earth vertical is required. Results revealed that balance control impairment was observed only immediately following Achilles tendon vibration offset; both groups with the head-neck either extended or flexed showed larger body sway (i.e., larger root mean square scalar distance between the center of pressure and center of gravity) compared to the group with the neutral head-neck orientation. The fact that balance control was uninfluenced by head-neck orientation immediately following vibration onset suggests the error signal needs to accumulate to a certain threshold before the internal representation of the earth vertical becomes incorrect.

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.

Evolution of Subjective Visual Vertical Perturbation After Stroke

Objective. The perception of visual verticality is often perturbed after stroke and might be an underlying component of imbalance. The aim of this study was to describe the evolution of visual vertical (VV) perturbation and to investigate the factors affecting it.

Methods. Thirty patients with hemiplegia after a single hemispheric stroke (17 left lesioned [LL] and 13 right lesioned [RL]) were studied. Visual verticality was tested within 45 days of stroke, and then at 3 and 6 months. Subjects sat in a dark room and adjusted a luminous rod to the vertical position. The differences between patients’ adjustments and vertical were calculated. The effects on VV evolution of the side, size, type, and location of the lesion were tested.

Results. Sixty percent of the recent stroke patients had an initial inaccurate perception of verticality, and 39% of these patients recovered during the 1st 3 months after stroke. The evolution of VV tilt depended on the side of the lesion ( P = 0.01), with better recovery in LL patients. None of the other factors studied affected VV normalization.

Conclusions. The poorer recovery of vertical perception after right-side stroke might be due to the predominant role of the right hemisphere in spatial cognition, and might be involved in the poorer recovery of balance after stroke in RL patients.

Do Visual and Vestibular Inputs Compensate for Somatosensory Loss in the Perception of Spatial Orientation? Insights from a Deafferented Patient

The present study aimed at investigating the consequences of a massive loss of somatosensory inputs on the perception of spatial orientation. The occurrence of possible compensatory processes for external (i.e., object) orientation perception and self-orientation perception was examined by manipulating visual and/or vestibular cues. To that aim, we compared perceptual responses of a deafferented patient (GL) with respect to age-matched Controls in two tasks involving gravity-related judgments. In the first task, subjects had to align a visual rod with the gravitational vertical (i.e., Subjective Visual Vertical: SVV) when facing a tilted visual frame in a classic Rod-and-Frame Test. In the second task, subjects had to report whether they felt tilted when facing different visuo-postural conditions which consisted in very slow pitch tilts of the body and/or visual surroundings away from vertical. Results showed that, much more than Controls, the deafferented patient was fully dependent on spatial cues issued from the visual frame when judging the SVV. On the other hand, the deafferented patient did not rely at all on visual cues for self-tilt detection. Moreover, the patient never reported any sensation of tilt up to 18° contrary to Controls, hence showing that she did not rely on vestibular (i.e., otoliths) signals for the detection of very slow body tilts either. Overall, this study demonstrates that a massive somatosensory deficit substantially impairs the perception of spatial orientation, and that the use of the remaining sensory inputs available to a deafferented patient differs regarding whether the judgment concerns external vs. self-orientation.

Dissociating vestibular and somatosensory contributions to spatial orientation

Inferring object orientation in the surroundings heavily depends on our internal sense of direction of gravity. Previous research showed that this sense is based on the integration of multiple information sources, including visual, vestibular (otolithic), and somatosensory signals. The individual noise characteristics and contributions of these sensors can be studied using spatial orientation tasks, such as the subjective visual vertical (SVV) task. A recent study reported that patients with complete bilateral vestibular loss perform similar as healthy controls on these tasks, from which it was conjectured that the noise levels of both otoliths and body somatosensors are roll-tilt dependent. Here, we tested this hypothesis in 10 healthy human subjects by roll tilting the head relative to the body to dissociate tilt-angle dependencies of otolith and somatosensory noise. Using a psychometric approach, we measured the perceived orientation, and its variability, of a briefly flashed line relative to the gravitational vertical (SVV). Measurements were taken at multiple body-in-space orientations (-90 to 90°, steps of 30°) and head-on-body roll tilts (30° left ear down, aligned, 30° right ear down). Results showed that verticality perception is processed in a head-in-space reference frame, with a systematic SVV error that increased with larger head-in-space orientations. Variability patterns indicated a larger contribution of the otolith organs around upright and a more substantial contribution of the body somatosensors at larger body-in-space roll tilts. Simulations show that these findings are consistent with a statistical model that involves tilt-dependent noise levels of both otolith and somatosensory signals, confirming dynamic shifts in the weights of sensory inputs with tilt angle.

Sensorimotor and cognitive factors associated with the age-related increase of visual field dependence: a cross-sectional study

Reliance on the visual frame of reference for spatial orientation (or visual field dependence) has been reported to increase with age. This has implications on old adults' daily living tasks as it affects stability, attention, and adaptation capacities. However, the nature and underlying mechanisms of this increase are not well defined. We investigated sensorimotor and cognitive factors possibly associated with increased visual field dependence in old age, by considering functions that are both known to degrade with age and important for spatial orientation and sensorimotor control: reliance on the (somatosensory-based) egocentric frame of reference, visual fixation stability, and attentional processing of complex visual scenes (useful field of view, UFOV). Twenty young, 18 middle-aged, and 20 old adults completed a visual examination, three tests of visual field dependence (RFT, RDT, and GEFT), a test of egocentric dependence (subjective vertical estimation with the body erect and tilted at 70°), a visual fixation task, and a test of visual attentional processing (UFOV®). Increased visual field dependence with age was associated with reduced egocentric dependence, visual fixation stability, and visual attentional processing. In addition, visual fixation instability and reduced UFOV were correlated. Results of middle-aged adults fell between those of the young and old, revealing the progressive nature of the age effects we evaluated. We discuss results in terms of reference frame selection with respect to ageing as well as visual and non-visual information processing. Inter-individual differences amongst old adults are highlighted and discussed with respect to the functionality of increased visual field dependence.

Sensory substitution in bilateral vestibular a-reflexic patients

Patients with bilateral vestibular loss have balance problems in darkness, but maintain spatial orientation rather effectively in the light. It has been suggested that these patients compensate for vestibular cues by relying on extravestibular signals, including visual and somatosensory cues, and integrating them with internal beliefs. How this integration comes about is unknown, but recent literature suggests the healthy brain remaps the various signals into a task-dependent reference frame, thereby weighting them according to their reliability. In this paper, we examined this account in six patients with bilateral vestibular a-reflexia, and compared them to six age-matched healthy controls. Subjects had to report the orientation of their body relative to a reference orientation or the orientation of a flashed luminous line relative to the gravitational vertical, by means of a two-alternative-forced-choice response. We tested both groups psychometrically in upright position (0°) and 90° sideways roll tilt. Perception of body tilt was unbiased in both patients and controls. Response variability, which was larger for 90° tilt, did not differ between groups, indicating that body somatosensory cues have tilt-dependent uncertainty. Perception of the visual vertical was unbiased when upright, but showed systematic undercompensation at 90° tilt. Variability, which was larger for 90° tilt than upright, did not differ between patients and controls. Our results suggest that extravestibular signals substitute for vestibular input in patients’ perception of spatial orientation. This is in line with the current status of rehabilitation programs in acute vestibular patients, targeting at recognizing body somatosensory signals as a reliable replacement for vestibular loss.

The Two-Wrongs model explains perception-action dissociations for illusions driven by distortions of the egocentric reference frame

Several studies have demonstrated a dissociation of the effects of illusion on perception and action, with perception generally reported to be susceptible to illusions, while actions are seemingly immune. These findings have been interpreted to support Milner and Goodale's Two Visual Systems model, which proposes the existence of separate visual processing streams for perception and action. However, an alternative interpretation suggests that this type of behavioral dissociation will occur for any illusion that is caused by a distortion of the observer's egocentric reference frame, without requiring the existence of separate perception and action systems that are differently affected by the illusion. In this scenario, movements aimed at illusory targets will be accurate if they are guided within the same distorted reference frame used for target encoding, since the error of motor guidance will cancel with the error of encoding (hence, for actions, two wrongs do make a right). We further test this Two-Wrongs model by examining two illusions for which the hypothesis makes very different predictions: the rod-and-frame illusion (which affects perception but not actions) and the simultaneous-tilt illusion (which affects perception and actions equally). We demonstrate that the rod-and-frame illusion is caused by a distortion of the observer's egocentric reference frame suitable for the cancellation of errors predicted by the Two-Wrongs model. In contrast, the simultaneous-tilt illusion is caused by local interactions between stimulus elements within an undistorted reference frame, precluding the cancellation of errors associated with the Two-Wrongs model such that the illusion is reflected in both perception and actions. These results provide evidence for a class of illusions that lead to dissociations of perception and action through distortions of the observer's spatial reference frame, rather than through the actions of functionally separate visual processing streams.

The role of the right superior parietal lobule in processing visual context for the establishment of the egocentric reference frame

Visual cues contribute to the creation of an observer's egocentric reference frame, within which the locations and orientations of objects can be judged. However, these cues can also be misleading. In the rod-and-frame illusion, for example, a large tilted frame distorts the observer's sense of vertical, causing an enclosed rod to appear tilted in the opposite direction. To determine the brain region responsible for processing these spatial cues, we used TMS to suppress neural activity in the superior parietal lobule of healthy observers. Stimulation of the right hemisphere, but not the left, caused a significant reduction in rod-and-frame susceptibility. In contrast, a tilt illusion caused by a mechanism that does not involve a distortion of the observer's egocentric reference frame was unaffected. These results demonstrate that the right superior parietal lobule is actively involved in processing the contextual cues that contribute to our perception of egocentric space.

A biological walker is faster and better recognized when aligned with body axis observer

The representation of the vertical direction is a compromise between the directions given by the egocentric and allocentric references. Dissociations between these two referentials in the discrimination of a biological walker which typically refers to a model of verticality questions the coordinate system (allocentric and/or egocentric) used to perceive it. With a point-light display paradigm, the characteristics of an artificial walking pattern were manipulated in order to offer to 10 healthy participants (5 men/5 women; 24.6±3.4 years) a female or male locomotion which had to be identified as such. The body position of the viewer (sitting/lying) and the walking pattern viewed (aligned/rotated in relation to the egocentric referential) were crossed. Three indices were analyzed and 200 trials recorded: percentage of correct identification, reaction time and confidence score. This paper confirms the validity of the walking pattern model since the more pronounced the gradient of the walking pattern (as female or male) the better the recognition. Furthermore, whatever the body position, artificial walking patterns were more easily identified when they were aligned with the egocentric referential rather than tilted. The participant gender had no influence on the walking pattern recognition. We conclude that the perception of a biological walker referenced to the vertical is exclusively improved by a representation of the spatial information in an egocentric coordinate system.

[Is the sense of verticality vestibular?]

The vestibular system constitutes an inertial sensor, which detects linear (otoliths) and angular (semicircular canals) accelerations of the head in the three dimensions. The otoliths are specialized in the detection of linear accelerations and can be used by the brain as a "plumb line" coding earth gravity acceleration (direction). This property of otolithic system suggested that the sense of verticality is supported by the vestibular system. The preeminence of vestibular involvement in the sense of verticality stated in the 1900s was progressively supplanted by the notion of internal models of verticality. The internal models of verticality involve rules and properties of integration of vestibular graviception, somaesthesic graviception, and vision. The construction of a mental representation of verticality was mainly modeled as a bottom-up organization integrating visual, somatosensory and vestibular information without any cognitive modulations. Recent studies reported that the construction of internal models of verticality is not an automatic multi-sensory integration process but corresponds to more complex mechanisms including top-down influences such as awareness of body orientation or spatial representations.

Disruption of spatial task performance in anorexia nervosa

In anorexia nervosa (AN), body distortions have been associated with parietal cortex (PC) dysfunction. The PC is the anatomical substrate for a supramodal reference framework involved in spatial orientation constancy. Here, we sought to evaluate spatial orientation constancy and the perception of body orientation in AN patients. In the present study, we investigated the effect of passive lateral body inclination on the visual and tactile subjective vertical (SV) and body Z-axis in 25 AN patients and 25 healthy controls. Subjects performed visual- and tactile-spatial judgments of axis orientations in an upright position and tilted 90° clockwise or counterclockwise. We observed a significant deviation of the tactile and visual SV towards the body (an A-effect) under tilted conditions, suggesting a multisensory impairment in spatial orientation. Deviation of the Z-axis in the direction of the tilt was also observed in the AN group. The greater A-effect in AN patients may reflect reduced interoceptive awareness and thus inadequate consideration of gravitational inflow. Furthermore, marked body weight loss could decrease the somatosensory inputs required for spatial orientation. Our study results suggest that spatial references are impaired in AN. This may be due to particular integration of visual, tactile and gravitational information (e.g. vestibular and proprioceptive cues) in the PC.

Differential effects of visual feedback on subjective visual vertical accuracy and precision

The brain constructs an internal estimate of the gravitational vertical by integrating multiple sensory signals. In darkness, systematic head-roll dependent errors in verticality estimates, as measured by the subjective visual vertical (SVV), occur. We hypothesized that visual feedback after each trial results in increased accuracy, as physiological adjustment errors (A-/E-effect) are likely based on central computational mechanisms and investigated whether such improvements were related to adaptational shifts of perceived vertical or to a higher cognitive strategy. We asked 12 healthy human subjects to adjust a luminous arrow to vertical in various head-roll positions (0 to 120deg right-ear down, 15deg steps). After each adjustment visual feedback was provided (lights on, display of previous adjustment and of an earth-vertical cross). Control trials consisted of SVV adjustments without feedback. At head-roll angles with the largest A-effect (90, 105, and 120deg), errors were reduced significantly (p<0.001) by visual feedback, i.e. roll under-compensation decreased, while precision of SVV was not significantly (p>0.05) influenced. In seven subjects an additional session with two consecutive blocks (first with, then without visual feedback) was completed at 90, 105 and 120deg head-roll. In these positions the error-reduction by the previous visual feedback block remained significant over the consecutive 18-24 min (post-feedback block), i.e., was still significantly (p<0.002) different from the control trials. Eleven out of 12 subjects reported having consciously added a bias to their perceived vertical based on visual feedback in order to minimize errors. We conclude that improvements of SVV accuracy by visual feedback, which remained effective after removal of feedback for ≥18 min, rather resulted from a cognitive strategy than by adapting the internal estimate of the gravitational vertical. The mechanisms behind the SVV therefore, remained stable, which is also supported by the fact that SVV precision - depending mostly on otolith input - was not affected by visual feedback.

Influence of sensory loss on the perception of verticality in stroke patients

PURPOSE

The aim of this study was to investigate the relationship between somatosensory loss and perception of verticality in stroke patients suffering single-hemisphere lesions.

METHOD

Somatosensory loss was measured using the Rivermead Assessment for Somatosensory Performance (RASP). Perception of verticality was assessed with the Subjective Visual Vertical (SVV) and the Subjective Postural Vertical (SPV) tests. Absolute Values of SVV and SPV were used to analyze the amount of deviation in relation to somatosensory loss.

RESULTS

Thirty-two patients were included in the study (mean age = 45.91 SD = 31.88 years). Analysis showed that somatosensory loss was related to results of the SVV (r = -0.552, p = 0.001, Pearson Rank) and the SPV (r = -0.661, p < 0.001, Spearman Ï). Furthermore, results showed that both joint-related (SVV: r = -0.411, p = 0.019, Pearson Rank; SPV: r = -0.597, p = 0.001, Spearman Ï) and skin-related (SVV: r = -0.595, p < 0.001, Pearson Rank; SPV: r = -0.663, p < 0.001, Spearman Ï) somatosensory information is related to verticality perception.

CONCLUSIONS

This study provides evidence that perception of verticality is related to somatosensory loss, which means that somatosensory loss will lead to a larger amount of deviation of SVV and SPV in relation to the gravitational vector. Furthermore, it is interesting to note that both SVV and SPV are influenced by somatosensory loss.

IMPLICATIONS FOR REHABILITATION

• Somatosensory information is related to both visual and postural aspects of verticality perception. • Both joint- and cutaneous-related modalities of sensory information are related to perception of verticality. • Sensory training could be important in the recovery of verticality perception.

Does the integration of haptic and visual cues reduce the effect of a biased visual reference frame on the subjective head orientation?

Background

The selection of appropriate frames of reference (FOR) is a key factor in the elaboration of spatial perception and the production of robust interaction with our environment. The extent to which we perceive the head axis orientation (subjective head orientation, SHO) with both accuracy and precision likely contributes to the efficiency of these spatial interactions. A first goal of this study was to investigate the relative contribution of both the visual and egocentric FOR (centre-of-mass) in the SHO processing. A second goal was to investigate humans' ability to process SHO in various sensory response modalities (visual, haptic and visuo-haptic), and the way they modify the reliance to either the visual or egocentric FORs. A third goal was to question whether subjects combined visual and haptic cues optimally to increase SHO certainty and to decrease the FORs disruption effect.

Methodology/Principal Findings

Thirteen subjects were asked to indicate their SHO while the visual and/or egocentric FORs were deviated. Four results emerged from our study. First, visual rod settings to SHO were altered by the tilted visual frame but not by the egocentric FOR alteration, whereas no haptic settings alteration was observed whether due to the egocentric FOR alteration or the tilted visual frame. These results are modulated by individual analysis. Second, visual and egocentric FOR dependency appear to be negatively correlated. Third, the response modality enrichment appears to improve SHO. Fourth, several combination rules of the visuo-haptic cues such as the Maximum Likelihood Estimation (MLE), Winner-Take-All (WTA) or Unweighted Mean (UWM) rule seem to account for SHO improvements. However, the UWM rule seems to best account for the improvement of visuo-haptic estimates, especially in situations with high FOR incongruence. Finally, the data also indicated that FOR reliance resulted from the application of UWM rule. This was observed more particularly, in the visual dependent subject. Conclusions: Taken together, these findings emphasize the importance of identifying individual spatial FOR preferences to assess the efficiency of our interaction with the environment whilst performing spatial tasks.

Perception of subjective visual vertical and horizontal in patients with chronic neck pain: a cross-sectional observational study

Previous studies have shown that chronic neck pain (CNP) patients have a larger spread of perceptual errors for subjective visual vertical (SVV) than those exhibited by asymptomatic controls. The current study investigated whether this was also the case for perception of subjective visual horizontal (SVH) and whether there was a correlation between the two measurements. Fifty patients with CNP were compared with a group of 50 age- and gender-matched controls. All subjects were required to complete a test to measure SVH as well as SVV using the computerised rod and frame (CRAF) test. These tests were conducted under various frame conditions. No difference was found between the errors of the CNP and control groups in the absence of a surrounding frame. When a tilted frame was added to the CRAF test, the range of errors observed in the CNP group increased for both SVV and SVH. In particular, significantly more CNP patients fell outside the reference range of errors and a subgroup of patients, characterised by higher neck pain disability indices, was identified who demonstrated higher than expected errors for both SVV and SVH. However no conclusion could be drawn with regards to the direction of error asymmetry and laterality of pain as those patients with unilateral pain exhibited errors both towards and away from the affected area.

Perception threshold for tilt

The aim of this study was to determine the thresholds for perception of tilt and translation using 3 motion/tilt profile paradigms. Healthy subjects were submitted to the following: 1) unilateral and bilateral eccentric rotations (centrifugation), 2) whole body translatory decelerations opposite to the movement direction while seated on a linear sled, and 3) discrete slow velocity platform tilts. Subjects were instructed to verbally indicate the perceived direction of tilt or translation. Fifteen healthy subjects (12 male and 3 female subjects, 18-31 yr) without any history or evidence of any ophthalmologic or neuro-otologic disorder participated in this study. Our results from unilateral centrifugation indicate a threshold for body tilt perception of approximately 2 degrees with a substantial interindividual range (1.9-5.6 degrees, 52% interindividual and 34% intraindividual variability), which, to our interpretation, mainly depends on otolithic function. Tilt perception during whole body decelerations and discrete platform tilts mainly depends on somatosensory information, showing the dominant role of the somatosensory system for the perception of body orientation. Thus, tilt sensations during eccentric rotations seems to be a promising tool for the evaluation of utricular dysfunction.

Kinaesthetic and visual perceptions of orientations

In the present study we compare the kinaesthetic and visual perception of the vertical and horizontal orientations (subjective vertical and subjective horizontal) to determine whether the perception of cardinal orientations is amodal or modality-specific. The influence of methodological factors on the accuracy of perception is also investigated by varying the stimulus position as a function of its initial tilt (clockwise or counterclockwise) and its angle (22 degrees, 45 degrees, 67 degrees, and 90 degrees) in respect to its physical orientation. Ten participants estimated the vertical and horizontal orientations by repositioning a rod in the kinaesthetic condition or two luminous points, forming a 'virtual line' in the visual condition. Results within the visual modality replicated previous findings by showing that estimation of the physical orientations is very accurate regardless of the initial position of the virtual line. In contrast, the perception of orientation with the kinaesthetic modality was less accurate and systematically influenced by the angle between the initial position of the rod and the required orientation. The findings question the assumption that the subjective vertical is derived from an internal representation of gravity and highlight the necessity of taking into account methodological factors in studies on subjective orientations.

Roll-dependent modulation of the subjective visual vertical: contributions of head- and trunk-based signals

Precision and accuracy of the subjective visual vertical (SVV) modulate in the roll plane. At large roll angles, systematic SVV errors are biased toward the subject's body-longitudinal axis and SVV precision is decreased. To explain this, SVV models typically implement a bias signal, or a prior, in a head-fixed reference frame and assume the sensory input to be optimally tuned along the head-longitudinal axis. We tested the pattern of SVV adjustments both in terms of accuracy and precision in experiments in which the head and the trunk reference frames were not aligned. Twelve subjects were placed on a turntable with the head rolled about 28 degrees counterclockwise relative to the trunk by lateral tilt of the neck to dissociate the orientation of head- and trunk-fixed sensors relative to gravity. Subjects were brought to various positions (roll of head- or trunk-longitudinal axis relative to gravity: 0 degrees , +/-75 degrees ) and aligned an arrow with perceived vertical. Both accuracy and precision of the SVV were significantly (P < 0.05) better when the head-longitudinal axis was aligned with gravity. Comparing absolute SVV errors for clockwise and counterclockwise roll tilts, statistical analysis yielded no significant differences (P > 0.05) when referenced relative to head upright, but differed significantly (P < 0.001) when referenced relative to trunk upright. These findings indicate that the bias signal, which drives the SVV toward the subject's body-longitudinal axis, operates in a head-fixed reference frame. Further analysis of SVV precision supports the hypothesis that head-based graviceptive signals provide the predominant input for internal estimates of visual vertical.

The peripheral nervous system and the perception of verticality

Orientation of the body with respect to gravity is based on integration of visual, vestibular and somatosensory signals. Here, we investigated the subjective postural vertical (SPV) and visual vertical (SVV) in three patients with bilateral somatosensory deafferentation and a group of age-matched normal subjects. Our hypothesis was that the patients with bilateral somatosensory deafferentation may show tilt induced bias in the construction of their SPV, with a normal SVV. Patient 1 had a severe Guillain Barré syndrome and almost complete absence of peripheral sensation, the two other patients had a thoracic spinal injury with a sensory loss from T6-7 down. On initial testing, compared with normal subjects and the patients with spinal injury, Patient 1 had a significant bias in SPV towards the side of a preceding tilt in both directions. Several months later, after significant improvement of sensation, this tilt-induced bias in SPV had resolved completely. In addition, Patient 1 had a significantly enlarged "cone of verticality", which did not change following improvement in peripheral sensation, reflecting persisting disturbance in the perception of body verticality. In the two patients with spinal injury, bias towards the side of a preceding tilt was not significant. These findings confirm the importance of somatosensory input from the trunk to the perception of SPV in the seated position.

Influence of subjective visual vertical misperception on balance recovery after stroke

BACKGROUND

Subjective visual vertical (SVV) perception can be perturbed after stroke, but its effect on balance recovery is not yet known.

AIM

To evaluate the influence of SVV perturbations on balance recovery after stroke.

METHODS

28 patients (14 with a right hemisphere lesion (RHL) and 14 with a left hemisphere lesion (LHL)) were included, 5 were lost to follow-up. SVV perception was initially tested within 3 months after stroke, then at 6 months, using a luminous line, which the patients adjusted to the vertical position in a dark room. Mean deviation (V) and uncertainty (U), defined as the standard deviation of the SVV, were calculated for eight trials. Balance was initially assessed by the Postural Assessment Scale for Stroke (PASS), and at 6 months by the PASS (PASS6), a force platform (lateral and sagittal stability limits (LSL6 and SSL6)), the Rivermead Mobility Index (RMI6) and gait velocity (v6). Functional outcome was also assessed by the Functional Independence Measure at 6 months (FIM6).

RESULTS

The scores for balance and for FIM6 were related to the initial V value: PASS6 (p = 0.01, tau = -0.38); RMI6 (p = 0.002, tau = -0.48), LSL6 (p = 0.06, tau = -0.29), SSL6 (p = 0.004, tau = -0.43), v6 (p = 0.01, tau = -0.36) and FIM6 (p = 0.001, tau = -0.49), as well as to the initial U value: PASS6 (p = 0.03, tau = -0.32), RMI6 (p = 0.02, tau = -0.35), SSL6 (p = 0.005, tau = -0.43) and FIM6 (p = 0.01, tau = -0.38).

CONCLUSIONS

Initial misperception of verticality was related to a poor score for balance after stroke. This relationship seems to be independent of motricity and neglect. Rehabilitation programmes should take into account verticality misperceptions, which could be an important factors influencing balance recovery after stroke.

Influence of pitch tilts on the perception of gravity-referenced eye level in labyrinthine defective subjects

We investigate the role of vestibular information in judging the gravity-referenced eye level (i.e., earth-referenced horizon or GREL) during sagittal body tilt whilst seated. Ten bilateral labyrinthine-defective subjects (LDS) and 10 age-matched controls set a luminous dot to their perception of GREL in darkness, with and without arm pointing. Although judgements were linearly influenced by the magnitude of whole-body tilt, results showed no significant difference between LDS and age-matched controls in the subjective GREL accuracy or in the intra-subject variability of judgement. However, LDS performance without arm pointing was related to the degree of vestibular compensation inferred from another postural study performed with the same patients. LDS did not utilize upper limb input during arm pointing movements as a source of graviceptive information to compensate for the vestibular loss. The data suggest that vestibular cues are not of prime importance in GREL estimates in static conditions. The absence of difference between controls and LDS GREL performance, and the correlation between the postural task and GREL accuracy, indicate that somatosensory input may convey as much graviceptive information required for GREL judgements as the vestibular system.

Subjective Visual Vertical Perception Relates to Balance in Acute Stroke

OBJECTIVE

To determine whether misperception of the subjective visual vertical (SVV) underlies balance difficulties in hemiplegic patients.

DESIGN

Descriptive study, using a convenience sample.

SETTING

Department of physical medicine of a university hospital.

PARTICIPANTS

Thirty inpatients with hemiplegia after a hemispheric stroke during the 3 previous months.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

The SVV was tested while subjects sat in a dark room and were asked to adjust a luminous line to the vertical position. Mean SVV deviation and uncertainty, defined as the standard deviation, were calculated for 8 trials. Balance was assessed by the Postural Assessment Scale for Stroke (PASS) and while patients sat on a laterally rocking platform placed on a Satel force platform. The mean body position and the instability score (Lx), calculated as the length of the course of the center of pressure, were recorded. Functional outcome was also evaluated by the FIM instrument.

RESULTS

An abnormal SVV was recorded for 20 of 30 patients. Balance (ie, PASS, Lx) and FIM correlated significantly with SVV tilt (P<.001, P=.01, and P<.001, respectively) and with uncertainty (PASS, P=.006; FIM, P=.003).

CONCLUSIONS

Verticality misperception was related to poor balance and might be an important element in the assessment of contributing factors to balance disorders after stroke. It should probably be taken into account when establishing balance rehabilitation programs for patients with hemiplegia.

Otolith Function Assessed with the Subjective Postural Horizontal and Standardised Stance and Gait Tasks

If otolith function is essential to maintain upright standing while moving along slanted or uneven surfaces, subjects with an otolith deficit should have difficulty judging whether the inclination of the surface on which they are standing is tilted or not. We tested this judgement and compared it with the ability to control trunk sway during standardised stance and gait tests. Thirteen patients with unilateral vestibular nerve neurectomy at least 6 months prior to testing and 39 age-matched controls were asked to move a dynamic posturography platform on which they were standing back to their subjective 'horizontal' position after the platform had been slowly tilted at 0.4 degrees/s to 5 degrees in 8 different directions. Normal subjects left the platform deviated in pitch (forwards-backwards) at about 0.7 degrees on describing the platform as levelled off for all directions of tilt. Patients showed larger deviations of about 1.3 degrees in pitch with significant differences for forward right tilt (1.58+/-0.73 degrees compared to 0.73+/-0.11 degrees for normals; mean and SEM) and for forward left. Roll (lateral) deviations were about 0.4 degrees for normals and 0.5 degrees larger for the patients (for example, for backward left, 1.13+/-0.24 degrees compared to 0.4+/-0.07 degrees in normals). Except for a tendency towards greater deviations to the lesion side of patients with eyes closed, no differences were noted between tests under eyes open and closed conditions. However, for backward and roll tilts patients needed to steady themselves first by grasping a handrail when tested with eyes closed. Stance tests on foam showed increases in roll and pitch trunk sway with respect to controls. Patients had significantly larger trunk roll sway deviations during 1-legged stance tests and during gait trials. For stance trials, the patients lost their balance control prior to the end of the standard 20-second recording time. We conclude that a unilateral loss of otolith inputs due to nerve resection permanently impairs the ability to judge whether the support surface is horizontal, and leads to excessive trunk sway when standing on a compliant surface as well as excessive trunk roll sway during gait.