Fusion of Visual and Vestibular Tilt Cues in the Perception of Visual Vertical

Influence of scene aspect ratio and depth cues on verticality perception bias

Perceiving verticality is crucial for accurate spatial orientation. Previous research has revealed that tilted scenes can bias verticality perception. Verticality perception bias can be represented as the sum of multiple periodic functions that play a role in the perception of visual orientation, where the specific factors affecting each periodicity remain uncertain. This study investigated the influence of the width and depth of an indoor scene on each periodic component of the bias. The participants were presented with an indoor scene showing a rectangular checkerboard room (Experiment 1), a rectangular aperture on the wall (Experiment 2), or a rectangular dotted room (Experiment 3), with various aspect ratios. The stimuli were presented with roll orientations ranging from 90° clockwise to 90° counterclockwise. The participants were asked to report their subjective visual vertical (SVV) perceptions. The contributions of 45°, 90°, and 180° periodicities to the SVV error were assessed by the weighted vector sum model. In Experiment 1, the periodic components of the SVV error increased with the aspect ratio. In Experiments 2 and 3, only the 90° component increased with the aspect ratio. These findings suggest that extended transverse surfaces may modulate the periodic components of verticality perception.

Perceptual lateralization on the Rod-and-Frame Test in young and older adults

INTRODUCTION

There is an overall left visual field/right hemisphere advantage in young adults for masked, tachistoscopically presented images on the Rod-and-Frame Test (RFT). This study explored potential age-related lateralization differences in processing of visual context on the RFT.

METHODS

The 35 young and 33 older adults aligned a rod surrounded either by no frame, a vertical, or leftward/rightward tilted frame to their perceived vertical. Algebraic errors of rod alignment were used to derive the rod-and-frame effect (RFE) and asymmetry index.

RESULTS

Young adults had frequent indirect effects, mostly to the right-tilted frame, while older adults hardly produced any. Compared with nontilted frames, young adults displayed larger alignment errors with left-tilted frames; however, older adults exhibited this same effect for both frame tilt conditions. Young adults had smaller RFE values than older adults for the right-tilted frame, with no age-related difference in RFE for the left-tilted frame or asymmetry index. The negative asymmetry index was statistically different from the true vertical only in young adults.

CONCLUSION

There is an age-related reduction in the right hemisphere processing of left-sided visual contexts on the RFT. Such findings can assist clinicians to improve interpretation of RFT findings in clinical patients.

Cognition in vestibular disorders: state of the field, challenges, and priorities for the future

Vestibular disorders are prevalent and debilitating conditions of the inner ear and brain which affect balance, coordination, and the integration of multisensory inputs. A growing body of research has linked vestibular disorders to cognitive problems, most notably attention, visuospatial perception, spatial memory, and executive function. However, the mechanistic bases of these cognitive sequelae remain poorly defined, and there is a gap between our theoretical understanding of vestibular cognitive dysfunction, and how best to identify and manage this within clinical practice. This article takes stock of these shortcomings and provides recommendations and priorities for healthcare professionals who assess and treat vestibular disorders, and for researchers developing cognitive models and rehabilitation interventions. We highlight the importance of multidisciplinary collaboration for developing and evaluating clinically relevant theoretical models of vestibular cognition, to advance research and treatment.

Humans gradually integrate sudden gain or loss of visual information into spatial orientation perception

Introduction

Vestibular and visual information is used in determining spatial orientation. Existing computational models of orientation perception focus on the integration of visual and vestibular orientation information when both are available. It is well-known, and computational models capture, differences in spatial orientation perception with visual information or without (i.e., in the dark). For example, during earth vertical yaw rotation at constant angular velocity without visual information, humans perceive their rate of rotation to decay. However, during the same sustained rotation with visual information, humans can continue to more accurately perceive self-rotation. Prior to this study, there was no existing literature on human motion perception where visual information suddenly become available or unavailable during self-motion.

Methods

Via a well verified psychophysical task, we obtained perceptual reports of self-rotation during various profiles of Earth-vertical yaw rotation. The task involved transitions in the availability of visual information (and control conditions with visual information available throughout the motion or unavailable throughout).

Results

We found that when visual orientation information suddenly became available, subjects gradually integrated the new visual information over ~10 seconds. In the opposite scenario (visual information suddenly removed), past visual information continued to impact subject perception of self-rotation for ~30 seconds. We present a novel computational model of orientation perception that is consistent with the experimental results presented in this study.

Discussion

The gradual integration of sudden loss or gain of visual information is achieved via low pass filtering in the visual angular velocity sensory conflict pathway. In conclusion, humans gradually integrate sudden gain or loss of visual information into their existing perception of self-motion.

Nonhuman primates exploit the prior assumption that the visual world is vertical

When human subjects tilt their heads in dark surroundings, the noisiness of vestibular information impedes precise reports on objects' orientation with respect to Earth's vertical axis. This difficulty is mitigated if a vertical visual background is available. Tilted visual backgrounds induce feelings of head tilt in subjects who are in fact upright. This is often explained as a result of the brain resorting to the prior assumption that natural visual backgrounds are vertical. Here, we tested whether monkeys show comparable perceptual mechanisms. To this end we trained two monkeys to align a visual arrow to a vertical reference line that had variable luminance across trials, while including a large, clearly visible background square whose orientation changed from trial to trial. On ∼20% of all trials, the vertical reference line was left out to measure the subjective visual vertical (SVV). When the frame was upright, the monkeys' SVV was aligned with the gravitational vertical. In accordance with the perceptual reports of humans, however, when the frame was tilted it induced an illusion of head tilt as indicated by a bias in SVV toward the frame orientation. Thus all primates exploit the prior assumption that the visual world is vertical.NEW & NOTEWORTHY Here we show that the principles that characterize the human perception of the vertical are shared by another old world primate species, the rhesus monkey, suggesting phylogenetic continuity. In both species the integration of visual and vestibular information on the orientation of the head relative to the world is similarly constrained by the prior assumption that the visual world is vertical in the sense of having an orientation that is congruent with the gravity vector.

Modeling orientation perception adaptation to altered gravity environments with memory of past sensorimotor states

Transitioning between gravitational environments results in a central reinterpretation of sensory information, producing an adapted sensorimotor state suitable for motor actions and perceptions in the new environment. Critically, this central adaptation is not instantaneous, and complete adaptation may require weeks of prolonged exposure to novel environments. To mitigate risks associated with the lagging time course of adaptation (e.g., spatial orientation misperceptions, alterations in locomotor and postural control, and motion sickness), it is critical that we better understand sensorimotor states during adaptation. Recently, efforts have emerged to model human perception of orientation and self-motion during sensorimotor adaptation to new gravity stimuli. While these nascent computational frameworks are well suited for modeling exposure to novel gravitational stimuli, they have yet to distinguish how the central nervous system (CNS) reinterprets sensory information from familiar environmental stimuli (i.e., readaptation). Here, we present a theoretical framework and resulting computational model of vestibular adaptation to gravity transitions which captures the role of implicit memory. This advancement enables faster readaptation to familiar gravitational stimuli, which has been observed in repeat flyers, by considering vestibular signals dependent on the new gravity environment, through Bayesian inference. The evolution and weighting of hypotheses considered by the CNS is modeled via a Rao-Blackwellized particle filter algorithm. Sensorimotor adaptation learning is facilitated by retaining a memory of past harmonious states, represented by a conditional state transition probability density function, which allows the model to consider previously experienced gravity levels (while also dynamically learning new states) when formulating new alternative hypotheses of gravity. In order to demonstrate our theoretical framework and motivate future experiments, we perform a variety of simulations. These simulations demonstrate the effectiveness of this model and its potential to advance our understanding of transitory states during which central reinterpretation occurs, ultimately mitigating the risks associated with the lagging time course of adaptation to gravitational environments.

Postural adjustment as a function of scene orientation

Visual orientation plays an important role in postural control, but the specific characteristics of postural response to orientation remain unknown. In this study, we investigated the relationship between postural response and the subjective visual vertical (SVV) as a function of scene orientation. We presented a virtual room including everyday objects through a head-mounted display and measured head tilt around the naso-occipital axis. The room orientation varied from 165° counterclockwise to 180° clockwise around the center of display in 15° increments. In a separate session, we also conducted a rod adjustment task to record the participant's SVV in the tilted room. We applied a weighted vector sum model to head tilt and SVV error and obtained the weight of three visual cues to orientation: frame, horizon, and polarity. We found significant contributions for all visual cues to head tilt and SVV error. For SVV error, frame cues made the largest contribution, whereas polarity contribution made the smallest. For head tilt, there was no clear difference across visual cue types, although the order of contribution was similar to the SVV. These findings suggest that multiple visual cues to orientation are involved in postural control and imply different representations of vertical orientation across postural control and perception.

Visuospatial orientation: Differential effects of head and body positions

To orientate in space, the brain must integrate sensory information that encodes the position of the body with the visual cues from the surrounding environment. In this process, the extent of reliance on visual information is known as the visual dependence. Here, we asked whether the relative positions of the head and body can modulate such visual dependence (VD). We used the effect of optokinetic stimulation (30°/s) on subjective visual vertical (SVV) to quantify VD as the average optokinetic-induced SVV bias in clockwise and counter-clockwise directions. The VD bias was measured in eight subjects with a head-on-body tilt (HBT) where only the head was tilted on the body, and also with a whole-body tilt (WBT) where the head and body were tilted together. The VD bias with HBT of 20° was in the same direction of the head tilt position (left tilt VD -1.35 ± 0.1.2°; right VD 1.60 ± 0.9°), whereas the VD bias with WBT of 20° was in a direction away from the body tilt position (left tilt VD 2.5 ± 1.1°; right tilt VD -2.1 ± 0.9°). These findings show differential effects of relative head and body positions on visual cue integration, a process which could facilitate optimal interaction with the surrounding environment for spatial orientation.

Panoramic Uncertainty in Vertical Perception

Judgments of the orientation of a visual line with respect to earth vertical are affected by panoramic visual cues. This is illustrated by the rod-and-frame effect (RFE), the finding that the perceived orientation of a luminous rod is biased by the orientation of a surrounding squared frame. In this study, we tested how the uncertainty of frame orientation affects the RFE by asking upright or tilted participants to psychometrically judge the orientation of a briefly flashed rod contained within either a circular frame, a squared frame, or either of two intermediate frame forms, called squircles, presented in various orientations. Results showed a cyclical modulation of frame-induced bias across the range of the square and squircular frame orientations. The magnitude of this bias increased with increasing squaredness of the frame, as if the more unequivocal the orientation cues of the frame, the larger the reliance on them for rod orientation judgments. These findings are explained with a Bayesian optimal integration model in which participants flexibly weigh visual panoramic cues, depending on their orientation reliability, and non-visual cues in the perception of vertical.

Assessing the contribution of active somatosensory stimulation to self-acceleration perception in dynamic driving simulators

In dynamic driving simulators, the experience of operating a vehicle is reproduced by combining visual stimuli generated by graphical rendering with inertial stimuli generated by platform motion. Due to inherent limitations of the platform workspace, inertial stimulation is subject to shortcomings in the form of missing cues, false cues, and/or scaling errors, which negatively affect simulation fidelity. In the present study, we aim at quantifying the relative contribution of an active somatosensory stimulation to the perceived intensity of self-motion, relative to other sensory systems. Participants judged the intensity of longitudinal and lateral driving maneuvers in a dynamic driving simulator in passive driving conditions, with and without additional active somatosensory stimulation, as provided by an Active Seat (AS) and Active Belts (AB) integrated system (ASB). The results show that ASB enhances the perceived intensity of sustained decelerations, and increases the precision of acceleration perception overall. Our findings are consistent with models of perception, and indicate that active somatosensory stimulation can indeed be used to improve simulation fidelity.

Pupil responses associated with the perception of gravitational vertical under directional optic flows

This study assessed the pupil responses in the sensory integration of various directional optic flows during the perception of gravitational vertical. A total of 30 healthy participants were enrolled with normal responses to conventional subjective visual vertical (SVV) which was determined by measuring the difference (error angles) between the luminous line adjusted by the participants and the true vertical. SVV was performed under various types of rotational (5°/s, 10°/s, and 50°/s) and straight (5°/s and 10°/s) optic flows presented via a head-mounted display. Error angles (°) of the SVV and changes in pupil diameters (mm) were measured to evaluate the changes in the visually assessed subjective verticality and related cognitive demands. Significantly larger error angles were measured under rotational optic flows than under straight flows (p < 0.001). The error angles also significantly increased as the velocity of the rotational optic flow increased. The pupil diameter increased after starting the test, demonstrating the largest diameter during the final fine-tuning around the vertical. Significantly larger pupil changes were identified under rotational flows than in straight flows. Pupil changes were significantly correlated with error angles and the visual analog scale representing subjective difficulties during each test. These results suggest increased pupil changes for integrating more challenging visual sensory inputs in the process of gravity perception.

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.

Influence of panoramic cues during prolonged roll-tilt adaptation on the percept of vertical

The percept of vertical, which mainly relies on vestibular and visual cues, is known to be affected after sustained whole-body roll tilt, mostly at roll positions adjacent to the position of adaptation. Here we ask whether the viewing of panoramic visual cues during the adaptation further influences the percept of the visual vertical. Participants were rotated in the frontal plane to a 90° clockwise tilt position, which was maintained for 4-minutes. During this period, the subject was either kept in darkness, or viewed panoramic pictures that were either veridical (aligned with gravity) or oriented along the body longitudinal axis. Errors of the subsequent subjective visual vertical (SVV), measured at various tilt angles, showed that the adaptation effect of panoramic cues is local, i.e. for a narrow range of tilts in the direction of the adaptation angle. This distortion was found irrespective of the orientation of the panoramic cues. We conclude that sustained exposure to panoramic and vestibular cues does not adapt the subsequent percept of vertical to the direction of the panoramic cue. Rather, our results suggest that sustained panoramic cues affect the SVV by an indirect effect on head orientation, with a 90° periodicity, that interacts with a vestibular cue to determine the percept of vertical.

Multisensory contribution in visuospatial orientation: an interaction between neck and trunk proprioception

A coherent perception of spatial orientation is key in maintaining postural control. To achieve this the brain must access sensory inputs encoding both the body and the head position and integrate them with incoming visual information. Here we isolated the contribution of proprioception to verticality perception and further investigated whether changing the body position without moving the head can modulate visual dependence-the extent to which an individual relies on visual cues for spatial orientation. Spatial orientation was measured in ten healthy individuals [6 female; 25-47 years (SD 7.8 years)] using a virtual reality based subjective visual vertical (SVV) task. Individuals aligned an arrow to their perceived gravitational vertical, initially against a static black background (10 trials), and then in other conditions with clockwise and counterclockwise background rotations (each 10 trials). In all conditions, subjects were seated first in the upright position, then with trunk tilted 20° to the right, followed by 20° to the left while the head was always aligned vertically. The SVV error was modulated by the trunk position, and it was greater when the trunk was tilted to the left compared to right or upright trunk positions (p < 0.001). Likewise, background rotation had an effect on SVV errors as these were greater with counterclockwise visual rotation compared to static background and clockwise roll motion (p < 0.001). Our results show that the interaction between neck and trunk proprioception can modulate how visual inputs affect spatial orientation.

Distinct adaptation patterns between grip dynamics and arm kinematics when the body is upside-down

In humans, practically all movements are learnt and performed in a constant gravitational field. Yet, studies on arm movements and object manipulation in parabolic flight have highlighted very fast sensorimotor adaptations to altered gravity environments. Here, we wondered if the motor adjustments observed in those altered gravity environments could also be observed on Earth in a situation where the body is upside-down. To address this question, we asked participants to perform rhythmic arm movements in two different body postures (right-side-up and upside-down) while holding an object in precision grip. Analyses of grip-load force coordination and of movement kinematics revealed distinct adaptation patterns between grip and arm control. Grip force and load force were tightly synchronized from the first movements performed in upside-down posture, reflecting a malleable allocentric grip control. In contrast, velocity profiles showed a more progressive adaptation to the upside-down posture and reflected an egocentric planning of arm kinematics. In addition to suggesting distinct mechanisms between grip dynamics and arm kinematics for adaptation to novel contexts, these results also suggest the existence of general mechanisms underlying gravity-dependent motor adaptation that can be used for fast sensorimotor coordination across different postures on Earth and, incidentally, across different gravitational conditions in parabolic flights, in human centrifuges, or in Space.NEW & NOTEWORTHY During rhythmic arm movements performed in an upside-down posture, grip control adapted very quickly, but kinematics adaptation was more progressive. Our results suggest that grip control and movement kinematics planning might operate in different reference frames. Moreover, by comparing our results with previous results from parabolic flight studies, we propose that a common mechanism underlies adaptation to unfamiliar body postures and adaptation to altered gravity.

Multisensory Interactions in Head and Body Centered Perception of Verticality

Percepts of verticality are thought to be constructed as a weighted average of multisensory inputs, but the observed weights differ considerably between studies. In the present study, we evaluate whether this can be explained by differences in how visual, somatosensory and proprioceptive cues contribute to representations of the Head In Space (HIS) and Body In Space (BIS). Participants (10) were standing on a force plate on top of a motion platform while wearing a visualization device that allowed us to artificially tilt their visual surroundings. They were presented with (in)congruent combinations of visual, platform, and head tilt, and performed Rod & Frame Test (RFT) and Subjective Postural Vertical (SPV) tasks. We also recorded postural responses to evaluate the relation between perception and balance. The perception data shows that body tilt, head tilt, and visual tilt affect the HIS and BIS in both experimental tasks. For the RFT task, visual tilt induced considerable biases (≈ 10° for 36° visual tilt) in the direction of the vertical expressed in the visual scene; for the SPV task, participants also adjusted platform tilt to correct for illusory body tilt induced by the visual stimuli, but effects were much smaller (≈ 0.25°). Likewise, postural data from the SPV task indicate participants slightly shifted their weight to counteract visual tilt (0.3° for 36° visual tilt). The data reveal a striking dissociation of visual effects between the two tasks. We find that the data can be explained well using a model where percepts of the HIS and BIS are constructed from direct signals from head and body sensors, respectively, and indirect signals based on body and head signals but corrected for perceived neck tilt. These findings show that perception of the HIS and BIS derive from the same sensory signals, but see profoundly different weighting factors. We conclude that observations of different weightings between studies likely result from querying of distinct latent constructs referenced to the body or head in space.

The Effects of Visual Parabolic Motion on the Subjective Vertical and on Interception

Observers typically present a strong bias in estimating the orientation of a visual bar when their body is tilted >60° in the roll plane and in the absence of visual background information. Known as the A-effect, this phenomenon likely results from the under-compensation of body tilt. Static visual cues can reduce such bias in the perceived vertical. Yet, it is unknown whether dynamic visual cues would be also effective. Here we presented projectile motions of a visual target along parabolic trajectories with different orientations relative to physical gravity. The aim of the experiment was twofold: First, we assessed whether the projectile motions could bias the estimation of the perceived orientation of a visual bar, measured with a classical subjective visual vertical (SVV) task. Second, we evaluated whether the ability to estimate time-to-contact of the visual target in an interception task was influenced by the orientation of these parabolic trajectories. Two groups of participants performed the experiment, either with their head and body tilted 90° along the roll plane or in an upright position. We found that the perceived orientation of the visual bar in the SVV task was affected by the orientation of the parabolic trajectories. This result was present in the tilted but not in the upright participants. In the interception task, the timing error increased linearly as a function of the orientation of the parabola. These results support the hypothesis that a gravity vector estimated from dynamic visual stimuli contributes to the subjective visual vertical.

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.

A gravity-based three-dimensional compass in the mouse brain

Gravity sensing provides a robust verticality signal for three-dimensional navigation. Head direction cells in the mammalian limbic system implement an allocentric neuronal compass. Here we show that head-direction cells in the rodent thalamus, retrosplenial cortex and cingulum fiber bundle are tuned to conjunctive combinations of azimuth and tilt, i.e. pitch or roll. Pitch and roll orientation tuning is anchored to gravity and independent of visual landmarks. When the head tilts, azimuth tuning is affixed to the head-horizontal plane, but also uses gravity to remain anchored to the allocentric bearings in the earth-horizontal plane. Collectively, these results demonstrate that a three-dimensional, gravity-based, neural compass is likely a ubiquitous property of mammalian species, including ground-dwelling animals.

Head direction neurons constitute the brain’s compass, and are classically known to indicate head orientation in the horizontal plane. Here, the authors show that head direction neurons form a three-dimensional compass that can also indicate head tilt, and anchors to gravity.

Contributions of optostatic and optokinetic cues to the perception of vertical

While it has been well established that optostatic and optokinetic cues contribute to the perception of vertical, it is unclear how the brain processes their combined presence with the nonvisual vestibular cues. Using a psychometric approach, we examined the percept of vertical in human participants (n = 17) with their body and head upright, presented with a visual frame tilted at one of eight orientations (between ±45°, steps of 11.25°) or no frame, surrounded by an optokinetic roll-stimulus (velocity =  ±30°/s or stationary). Both cues demonstrate relatively independent biases on vertical perception, with a sinusoidal modulation by frame orientation of ~4° and a general shift of ~1-2° in the rotation direction of the optic flow. Variability was unaffected by frame orientation but was higher with than without optokinetic rotation. An optimal-observer model in which vestibular, optostatic, and optokinetic cues provide independent sources to vertical perception was unable to explain these data. In contrast, a model in which the optokinetic signal biases the internal representation of gravity, which is then optimally integrated with the optostatic cue, provided a good account, at the individual participant level. We conclude that optostatic and optokinetic cues interact differently with vestibular cues in the neural computations for vertical perception.NEW & NOTEWORTHY Static and dynamic visual cues are known to bias the percept of vertical, but how they interact with vestibular cues remains to be established. Guided by an optimal-observer model, the present results suggest that optokinetic information is combined with vestibular information into a single, vestibular-optokinetic estimate, which is integrated with an optostatically derived estimate of vertical.

Gravito-inertial ambiguity resolved through head stabilization

It has been frequently observed that humans and animals spontaneously stabilize their heads with respect to the gravitational vertical during body movements even in the absence of vision. The interpretations of this intriguing behaviour have so far not included the need, for survival, to robustly estimate verticality. Here we use a mechanistic model of the head/otolith organ to analyse the possibility for this system to render verticality 'observable', a fundamental prerequisite to the determination of the angular position and acceleration of the head from idiothetic, inertial measurements. The intrinsically nonlinear head-vestibular dynamics is shown to generally lack observability unless the head is stabilized in orientation by feedback. Thus, our study supports the hypothesis that a central function of the physiologically costly head stabilization strategy is to enable an organism to estimate the gravitational vertical and head acceleration during locomotion. Moreover, our result exhibits a rare peculiarity of certain nonlinear systems to fortuitously alter their observability properties when feedback is applied.

Body orientation contributes to modelling the effects of gravity for target interception in humans

KEY POINTS

It is known that interception of targets accelerated by gravity involves internal models coupled with visual signals. Non-visual signals related to head and body orientation relative to gravity may also contribute, although their role is poorly understood. In a novel experiment, we asked pitched observers to hit a virtual target approaching with an acceleration that was either coherent or incoherent with their pitch-tilt. Initially, the timing errors were large and independent of the coherence between target acceleration and observer's pitch. With practice, however, the timing errors became substantially smaller in the coherent conditions. The results show that information about head and body orientation can contribute to modelling the effects of gravity on a moving target. Orientation cues from vestibular and somatosensory signals might be integrated with visual signals in the vestibular cortex, where the internal model of gravity is assumed to be encoded.

ABSTRACT

Interception of moving targets relies on visual signals and internal models. Less is known about the additional contribution of non-visual cues about head and body orientation relative to gravity. We took advantage of Galileo's law of motion along an incline to demonstrate the effects of vestibular and somatosensory cues about head and body orientation on interception timing. Participants were asked to hit a ball rolling in a gutter towards the eyes, resulting in image expansion. The scene was presented in a head-mounted display, without any visual information about gravity direction. In separate blocks of trials participants were pitched backwards by 20° or 60°, whereas ball acceleration was randomized across trials so as to be compatible with rolling down a slope of 20° or 60°. Initially, the timing errors were large, independently of the coherence between ball acceleration and pitch angle, consistent with responses based exclusively on visual information because visual stimuli were identical at both tilts. At the end of the experiment, however, the timing errors were systematically smaller in the coherent conditions than the incoherent ones. Moreover, the responses were significantly (P = 0.007) earlier when participants were pitched by 60° than when they were pitched by 20°. Therefore, practice with the task led to incorporation of information about head and body orientation relative to gravity for response timing. Instead, posture did not affect response timing in a control experiment in which participants hit a static target in synchrony with the last of a predictable series of stationary audiovisual stimuli.

Age-related reweighting of visual and vestibular cues for vertical perception

As we age, the acuity of our sensory organs declines, which may affect our lifestyle. Sensory deterioration in the vestibular system is typically bilateral and gradual, and could lead to problems with balance and spatial orientation. To compensate for the sensory deterioration, it has been suggested that the brain reweights the sensory information sources according to their relative noise characteristics. For rehabilitation and training programs, it is important to understand the consequences of this reweighting, preferably at the individual subject level. We psychometrically examined the age-dependent reweighting of visual and vestibular cues used in spatial orientation in a group of 32 subjects (age range: 19-76 yr). We asked subjects to indicate the orientation of a line (clockwise or counterclockwise relative to the gravitational vertical) presented within an oriented square visual frame when seated upright or with their head tilted 30° relative to the body. Results show that subjects' vertical perception is biased by the orientation of the visual frame. Both the magnitude of this bias and response variability become larger with increasing age. Deducing the underlying sensory noise characteristics, using Bayesian inference, suggests an age-dependent reweighting of sensory information, with an increasing weight of the visual contextual information. Further scrutiny of the model suggests that this shift in sensory weights is the result of an increase in the noise of the vestibular signal. Our approach quantifies how noise properties of visual and vestibular systems change over the life span, which helps to understand the aging process at the neurocomputational level. NEW & NOTEWORTHY Perception of visual vertical involves a weighted fusion of visual and vestibular tilt cues. Using a Bayesian approach and experimental psychophysics, we quantify how this fusion process changes with age. We show that, with age, the vestibular information is down-weighted whereas the visual weight is increased. This shift in sensory reweighting is primarily due to an age-related increase of the noise of vestibular signals.

Human perception of whole body roll-tilt orientation in a hypogravity analog: underestimation and adaptation

Overestimation of roll tilt in hypergravity ("G-excess" illusion) has been demonstrated, but corresponding sustained hypogravic conditions are impossible to create in ground laboratories. In this article we describe the first systematic experimental evidence that in a hypogravity analog, humans underestimate roll tilt. We studied perception of self-roll tilt in nine subjects, who were supine while spun on a centrifuge to create a hypogravity analog. By varying the centrifuge rotation rate, we modulated the centripetal acceleration (G_C) at the subject's head location (0.5 or 1 G_C) along the body axis. We measured orientation perception using a subjective visual vertical task in which subjects aligned an illuminated bar with their perceived centripetal acceleration direction during tilts (±11.5-28.5°). As hypothesized, based on the reduced utricular otolith shearing, subjects initially underestimated roll tilts in the 0.5 G_C condition compared with the 1 G_C condition (mean perceptual gain change = -0.27, P = 0.01). When visual feedback was given after each trial in 0.5 G_C, subjects' perceptual gain increased in approximately exponential fashion over time (time constant = 16 tilts or 13 min), and after 45 min, the perceptual gain was not significantly different from the 1 G_C baseline (mean gain difference between 1 G_C initial and 0.5 G_C final = 0.16, P = 0.3). Thus humans modified their interpretation of sensory cues to more correctly report orientation during this hypogravity analog. Quantifying the acute orientation perceptual learning in such an altered gravity environment may have implications for human space exploration on the moon or Mars. NEW & NOTEWORTHY Humans systematically overestimate roll tilt in hypergravity. However, human perception of orientation in hypogravity has not been quantified across a range of tilt angles. Using a centrifuge to create a hypogravity centripetal acceleration environment, we found initial underestimation of roll tilt. Providing static visual feedback, perceptual learning reduced underestimation during the hypogravity analog. These altered gravity orientation perceptual errors and adaptation may have implications for astronauts.

Relationship between subjective visual vertical and balance in individuals with multiple sclerosis

BACKGROUND

Subjective visual vertical (SVV) deviations have been correlated to abnormal cerebellar function in individuals diagnosed with multiple sclerosis (MS). It has been shown that individuals with MS have increased incidence of SVV abnormalities, yet this is not routinely tested in this population during physical therapy evaluation.

OBJECTIVE

This study aims to determine if there is a relationship between SVV and balance performance in people with MS who have cerebellar involvement. We hypothesize that individuals with greater SVV deviations will have worse balance performance.

METHODS

Fifteen females and five males (mean age 54.5 years [±7.03 SD]) with the diagnosis of MS and cerebellar involvement participated. Computerized SVV testing included rod and rod-and-frame conditions. None of the balance outcomes were correlated with the rod-only condition. Because there was a difference in magnitude of results within the rod-and-frame condition, based on whether the frame was rotated clockwise (CW) or counterclockwise (CCW), they were analysed independently.

RESULTS

For all six of the balance outcomes, there was a statistically significant moderate correlation with SVV deviations when the frame was tilted CCW: Barthel Index (r = -0.47, p = 0.018), Berg Balance Score (r = -0.59, p = 0.003), gait velocity (r = -0.52, p = 0.010), International Cooperative Ataxia Rating Scale (r = 0.56, p = 0.006), Scale for the Assessment and Rating of Ataxia (r = 0.62, p = 0.002), and Timed Up and Go (r = 0.58, p = 0.003). Interestingly, the Barthel Index was the only outcome that had statistical significance with a moderate correlation (r = -0.66, p = 0.001) when the frame was rotated CW. In this cohort, greater deviations during the rod-and-frame condition of SVV testing correlated with worse functional outcomes, especially when the frame was tilted CCW.

CONCLUSION

Individuals with MS who demonstrate decreased balance performance may rely more heavily on visual backgrounds. Implementation of SVV assessment for individuals with MS may provide clinicians with valuable information to identify clinical interventions.

Cerebellar Degeneration Increases Visual Influence on Dynamic Estimates of Verticality

Our perception of verticality relies on combining sensory information from multiple sources. Neuronal recordings in animals implicate the cerebellum in the process, yet disease of the human cerebellum was not found to affect this perception. Here we show that a perceptual disturbance of verticality is indeed present in people with a genetically determined and pure form of cerebellar degeneration (spinocerebellar ataxia type 6; SCA 6), but is only revealed under dynamic visual conditions. Participants were required to continuously orient a visually displayed bar to vertical while the bar angle was perturbed by a low-frequency random signal and a random dot pattern rotated in their visual periphery. The random dot pattern was rotated at one of two velocities (4°/s and 16°/s), traveling with either coherent or noisy motion. Perceived vertical was biased by visual rotation in healthy participants, particularly in a more elderly group, but SCA 6 participants were biased more than both groups. The bias was reduced by visual noise, but more so for SCA 6 participants than young controls. Distortion of verticality by visual rotation stems from the stimulus creating an illusion of self-rotation. We modeled this process using a maximum-likelihood sensory cue-combination model operating on noisy visual- and vestibular-rotation signals. The observed effects of visual rotation and visual noise could be compellingly explained by cerebellar degeneration, and to a lesser extent aging, causing an increase in central vestibular noise. This is consistent with the human cerebellum operating on dynamic vestibular signals to inform the process that estimates which way is up.

Variability in the Vestibulo-Ocular Reflex and Vestibular Perception

The vestibular system enables humans to estimate self-motion, stabilize gaze and maintain posture, but these behaviors are impacted by neural noise at all levels of processing (e.g., sensory, central, motor). Despite its essential importance, the behavioral impact of noise in human vestibular pathways is not completely understood. Here, we characterize the vestibular imprecision that results from neural noise by measuring trial-to-trial vestibulo-ocular reflex (VOR) variability and perceptual just-noticeable differences (JNDs) in the same human subjects as a function of stimulus intensity. We used head-centered yaw rotations about an Earth-vertical axis over a broad range of motion velocities (0-65°/s for VOR variability and 3-90°/s peak velocity for JNDs). We found that VOR variability increased from approximately 0.6°/s at a chair velocity of 1°/s to approximately 3°/s at 65°/s; it exhibited a stimulus-independent range below roughly 1°/s. Perceptual imprecision ("sigma") increased from 0.76°/s at 3°/s to 4.7°/s at 90°/s. Using stimuli that manipulated the relationship between velocity, displacement and acceleration, we found that velocity was the salient cue for VOR variability for our motion stimuli. VOR and perceptual imprecision both increased with stimulus intensity and were broadly similar over a range of stimulus velocities, consistent with a common noise source that affects motor and perceptual pathways. This contrasts with differing perceptual and motor stimulus-dependent imprecision in visual studies. Either stimulus-dependent noise or non-linear signal processing could explain our results, but we argue that afferent non-linearities alone are unlikely to be the source of the observed behavioral stimulus-dependent imprecision.

Neck muscle spindle noise biases reaches in a multisensory integration task

Reference frame transformations (RFTs) are crucial components of sensorimotor transformations in the brain. Stochasticity in RFTs has been suggested to add noise to the transformed signal due to variability in transformation parameter estimates (e.g., angle) as well as the stochastic nature of computations in spiking networks of neurons. Here, we varied the RFT angle together with the associated variability and evaluated the behavioral impact in a reaching task that required variability-dependent visual-proprioceptive multisensory integration. Crucially, reaches were performed with the head either straight or rolled 30° to either shoulder, and we also applied neck loads of 0 or 1.8 kg (left or right) in a 3 × 3 design, resulting in different combinations of estimated head roll angle magnitude and variance required in RFTs. A novel three-dimensional stochastic model of multisensory integration across reference frames was fitted to the data and captured our main behavioral findings: 1) neck load biased head angle estimation across all head roll orientations, resulting in systematic shifts in reach errors; 2) increased neck muscle tone led to increased reach variability due to signal-dependent noise; and 3) both head roll and neck load created larger angular errors in reaches to visual targets away from the body compared with reaches toward the body. These results show that noise in muscle spindles and stochasticity in general have a tangible effect on RFTs underlying reach planning. Since RFTs are omnipresent in the brain, our results could have implications for processes as diverse as motor control, decision making, posture/balance control, and perception. NEW & NOTEWORTHY We show that increasing neck muscle tone systematically biases reach movements. A novel three-dimensional multisensory integration across reference frames model captures the data well and provides evidence that the brain must have online knowledge of full-body geometry together with the associated variability to plan reach movements accurately.

High-normal blood glucose levels may be associated with decreased spatial perception in young healthy adults

The negative effects of high normal glucose on cognitive function were previously reported in euglycemic individuals of middle age and the elderly population. This study aimed at examining the effect of baseline blood glucose levels on spatial ability, specifically verticality perception on the computerized rod and frame test (CRFT) in young healthy adults. 63 healthy male medical students (age range from 18-23 years), of whom 30 were non-fasting outside the month of Ramadan and 33 fasting during Ramadan of the year 2016, were recruited in order to create varying degrees of glycemia during which verticality perception was carried out. Baseline blood glucose reading was obtained prior to commencing the CRFT test. Blood glucose levels at the time of testing decreased as the duration between the last meal and testing increased. A blood glucose range of 62-117 mg/dl was achieved among participants for this study. Linear regression analysis showed that blood glucose level at testing correlated positively with all alignment spatial error parameters, indicating a probable reduction of spatial perception ability with higher blood glucose levels. These results are consistent with other cognitive studies in older healthy humans and emphasize the critical impact of early glucose dys-homeostasis on cognitive function. They also indicate that elevated blood glucose may affect cognitive functioning outside of the usual complications of diabetes.

Can pre-screening vestibulocerebellar involvement followed by targeted training improve the outcomes of balance in cerebellar ataxia?

Balance problems and frequent falls are common among clients with Cerebellar Ataxia (CA). CA is not a disease by itself but a collection of symptoms due to the involvement of cerebellum or its pathways. Presently the treatment for balance problems for CA is not standardized. Interventions available to improve balance are not specific to symptoms presentation. Functionally the cerebellum is divided into the spinocerebellum, vestibulocerebellum and corticocerebellum. Each functional zone has a distinct role in maintaining balance. Therefore, the presentation of symptoms will vary according to the functional zone involved. Pre-screening clients with CA for identifying the part of cerebellum involved will facilitate clinicians to provide tailor-made interventions for targeting specific symptoms for better outcomes. Pre-screening clients with CA according to the part of cerebellum involved is not in practice and our study will introduce this concept. We hypothesize pre-screening participants with spinocerebellar ataxia (SCA) for the involvement vestibulocerebellum followed by prescribing vestibulocerebellum targeted exercises will have better outcomes when compared to conventional balance training. We plan to conduct two related studies. In study 1 we will screen participants with CA for the involvement of vestibulocerebellum. In study 2, the effects of vestibulocerebellum targeted balance exercises on balance will be studied. We will assess the Subjective Visual Vertical (SVV) deviation and postural sway pattern to screen participants into people with and without vestibulocerebellar involvement. SVV deviation will be estimated using a computerized Subjective Visual Vertical (cSVV) device and postural sway pattern will be assessed using the limits of stability program of the Bertec© Balance system. The obtained SVV deviation scores will be used to derive at cut-off scores to discriminate clients with and without vestibulocerebellar involvement. The second study will test the treatment effects of conventional exercises plus vestibulocerebellum targeted exercises to improve balance by correcting SVV deviation in SCA with vestibulocerebellar involvement. The intervention is planned as 12 one-to-one sessions over three months period. Participants will be reassessed after the intervention and 3 months post-intervention. The findings of this cutting-edge research are extremely important to the clinicians, researchers and clients with SCA.

Neuronal network-based mathematical modeling of perceived verticality in acute unilateral vestibular lesions: from nerve to thalamus and cortex

Acute unilateral lesions of vestibular graviceptive pathways from the otolith organs and semicircular canals via vestibular nuclei and the thalamus to the parieto-insular vestibular cortex regularly cause deviations of perceived verticality in the frontal roll plane. These tilts are ipsilateral in peripheral and in ponto-medullary lesions and contralateral in ponto-mesencephalic lesions. Unilateral lesions of the vestibular thalamus or cortex cause smaller tilts of the perceived vertical, which may be either ipsilateral or contralateral. Using a neural network model, we previously explained why unilateral vestibular midbrain lesions rarely manifest with rotational vertigo. We here extend this approach, focussing on the direction-specific deviations of perceived verticality in the roll plane caused by acute unilateral vestibular lesions from the labyrinth to the cortex. Traditionally, the effect of unilateral peripheral lesions on perceived verticality has been attributed to a lesion-based bias of the otolith system. We here suggest, on the basis of a comparison of model simulations with patient data, that perceived visual tilt after peripheral lesions is caused by the effect of a torsional semicircular canal bias on the central gravity estimator. We further argue that the change of gravity coding from a peripheral/brainstem vectorial representation in otolith coordinates to a distributed population coding at thalamic and cortical levels can explain why unilateral thalamic and cortical lesions have a variable effect on perceived verticality. Finally, we propose how the population-coding network for gravity direction might implement the elements required for the well-known perceptual underestimation of the subjective visual vertical in tilted body positions.

Psychophysical Evaluation of Sensory Reweighting in Bilateral Vestibulopathy

Perception of spatial orientation is thought to rely on the brain's integration of visual, vestibular, proprioceptive, and somatosensory signals, as well as internal beliefs. When one of these signals breaks down, such as the vestibular signal in bilateral vestibulopathy, patients start compensating by relying more on the remaining cues. How these signals are reweighted in this integration process is difficult to establish, since they cannot be measured in isolation during natural tasks, are inherently noisy, and can be ambiguous or in conflict. Here, we review our recent work, combining experimental psychophysics with a reverse engineering approach, based on Bayesian inference principles, to quantify sensory noise levels and optimal (re)weighting at the individual subject level, in both patients with bilateral vestibular deficits and healthy controls. We show that these patients reweight the remaining sensory information, relying more on visual and other nonvestibular information than healthy controls in the perception of spatial orientation. This quantification approach could improve diagnostics and prognostics of multisensory integration deficits in vestibular patients, and contribute to an evaluation of rehabilitation therapies directed toward specific training programs.

Causal Inference in the Perception of Verticality

The perceptual upright is thought to be constructed by the central nervous system (CNS) as a vector sum; by combining estimates on the upright provided by the visual system and the body's inertial sensors with prior knowledge that upright is usually above the head. Recent findings furthermore show that the weighting of the respective sensory signals is proportional to their reliability, consistent with a Bayesian interpretation of a vector sum (Forced Fusion, FF). However, violations of FF have also been reported, suggesting that the CNS may rely on a single sensory system (Cue Capture, CC), or choose to process sensory signals based on inferred signal causality (Causal Inference, CI). We developed a novel alternative-reality system to manipulate visual and physical tilt independently. We tasked participants (n = 36) to indicate the perceived upright for various (in-)congruent combinations of visual-inertial stimuli, and compared models based on their agreement with the data. The results favor the CI model over FF, although this effect became unambiguous only for large discrepancies (±60°). We conclude that the notion of a vector sum does not provide a comprehensive explanation of the perception of the upright, and that CI offers a better alternative.

The aesthetics of verticality: A gravitational contribution to aesthetic preference

Verticality plays a fundamental role in the arts, portraying concepts such as power, grandeur, or even morality; however, it is unclear whether people have an aesthetic preference for vertical stimuli. The perception of verticality occurs by integrating vestibular-gravitational input with proprioceptive signals about body posture. Thus, these signals may influence the preference for verticality. Here, we show that people have a genuine aesthetic preference for stimuli aligned with the vertical, and this preference depends on the position of the body relative to the gravitational direction. Observers rated the attractiveness of lines that varied in inclination. Perfectly vertical lines were judged to be more attractive than those inclined clockwise or anticlockwise only when participants held an upright posture. Critically, this preference was not present when their body was tilted away from the gravitational vertical. Our results showed that gravitational signals make a contribution to the perception of attractiveness of environmental objects.

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.

Bayesian quantification of sensory reweighting in a familial bilateral vestibular disorder (DFNA9)

DFNA9 is a rare progressive autosomal dominantly inherited vestibulo-cochlear disorder, resulting in a homogeneous group of patients with hearing impairment and bilateral vestibular function loss. These patients suffer from a deteriorated sense of spatial orientation, leading to balance problems in darkness, especially on irregular surfaces. Both behavioral and functional imaging studies suggest that the remaining sensory cues could compensate for the loss of vestibular information. A thorough model-based quantification of this reweighting in individual patients is, however, missing. Here we psychometrically examined the individual patient's sensory reweighting of these cues after complete vestibular loss. We asked a group of DFNA9 patients and healthy control subjects to judge the orientation (clockwise or counterclockwise relative to gravity) of a rod presented within an oriented square frame (rod-in-frame task) in three different head-on-body tilt conditions. Our results show a cyclical frame-induced bias in perceived gravity direction across a 90° range of frame orientations. The magnitude of this bias was significantly increased in the patients compared with the healthy control subjects. Response variability, which increased with head-on-body tilt, was also larger for the patients. Reverse engineering of the underlying signal properties, using Bayesian inference principles, suggests a reweighting of sensory signals, with an increase in visual weight of 20-40% in the patients. Our approach of combining psychophysics and Bayesian reverse engineering is the first to quantify the weights associated with the different sensory modalities at an individual patient level, which could make it possible to develop personal rehabilitation programs based on the patient's sensory weight distribution. NEW & NOTEWORTHY It has been suggested that patients with vestibular deficits can compensate for this loss by increasing reliance on other sensory cues, although an actual quantification of this reweighting is lacking. We combine experimental psychophysics with a reverse engineering approach based on Bayesian inference principles to quantify sensory reweighting in individual vestibular patients. We discuss the suitability of this approach for developing personal rehabilitation programs based on the patient's sensory weight distribution.

Perception of Upright: Multisensory Convergence and the Role of Temporo-Parietal Cortex

We inherently maintain a stable perception of the world despite frequent changes in the head, eye, and body positions. Such "orientation constancy" is a prerequisite for coherent spatial perception and sensorimotor planning. As a multimodal sensory reference, perception of upright represents neural processes that subserve orientation constancy through integration of sensory information encoding the eye, head, and body positions. Although perception of upright is distinct from perception of body orientation, they share similar neural substrates within the cerebral cortical networks involved in perception of spatial orientation. These cortical networks, mainly within the temporo-parietal junction, are crucial for multisensory processing and integration that generate sensory reference frames for coherent perception of self-position and extrapersonal space transformations. In this review, we focus on these neural mechanisms and discuss (i) neurobehavioral aspects of orientation constancy, (ii) sensory models that address the neurophysiology underlying perception of upright, and (iii) the current evidence for the role of cerebral cortex in perception of upright and orientation constancy, including findings from the neurological disorders that affect cortical function.

Perception of the dynamic visual vertical during sinusoidal linear motion

The vestibular system provides information for spatial orientation. However, this information is ambiguous: because the otoliths sense the gravitoinertial force, they cannot distinguish gravitational and inertial components. As a consequence, prolonged linear acceleration of the head can be interpreted as tilt, referred to as the somatogravic effect. Previous modeling work suggests that the brain disambiguates the otolith signal according to the rules of Bayesian inference, combining noisy canal cues with the a priori assumption that prolonged linear accelerations are unlikely. Within this modeling framework the noise of the vestibular signals affects the dynamic characteristics of the tilt percept during linear whole-body motion. To test this prediction, we devised a novel paradigm to psychometrically characterize the dynamic visual vertical-as a proxy for the tilt percept-during passive sinusoidal linear motion along the interaural axis (0.33 Hz motion frequency, 1.75 m/s² peak acceleration, 80 cm displacement). While subjects (n=10) kept fixation on a central body-fixed light, a line was briefly flashed (5 ms) at different phases of the motion, the orientation of which had to be judged relative to gravity. Consistent with the model's prediction, subjects showed a phase-dependent modulation of the dynamic visual vertical, with a subject-specific phase shift with respect to the imposed acceleration signal. The magnitude of this modulation was smaller than predicted, suggesting a contribution of nonvestibular signals to the dynamic visual vertical. Despite their dampening effect, our findings may point to a link between the noise components in the vestibular system and the characteristics of dynamic visual vertical.NEW & NOTEWORTHY A fundamental question in neuroscience is how the brain processes vestibular signals to infer the orientation of the body and objects in space. We show that, under sinusoidal linear motion, systematic error patterns appear in the disambiguation of linear acceleration and spatial orientation. We discuss the dynamics of these illusory percepts in terms of a dynamic Bayesian model that combines uncertainty in the vestibular signals with priors based on the natural statistics of head motion.

Unbounded evidence accumulation characterizes subjective visual vertical forced-choice perceptual choice and confidence

Humans can subjectively yet quantitatively assess choice confidence based on perceptual precision even when a perceptual decision is made without an immediate reward or feedback. However, surprisingly little is known about choice confidence. Here we investigate the dynamics of choice confidence by merging two parallel conceptual frameworks of decision making, signal detection theory and sequential analyses (i.e., drift-diffusion modeling). Specifically, to capture end-point statistics of binary choice and confidence, we built on a previous study that defined choice confidence in terms of psychophysics derived from signal detection theory. At the same time, we augmented this mathematical model to include accumulator dynamics of a drift-diffusion model to characterize the time dependence of the choice behaviors in a standard forced-choice paradigm in which stimulus duration is controlled by the operator. Human subjects performed a subjective visual vertical task, simultaneously reporting binary orientation choice and probabilistic confidence. Both binary choice and confidence experimental data displayed statistics and dynamics consistent with both signal detection theory and evidence accumulation, respectively. Specifically, the computational simulations showed that the unbounded evidence accumulator model fits the confidence data better than the classical bounded model, while bounded and unbounded models were indistinguishable for binary choice data. These results suggest that the brain can utilize mechanisms consistent with signal detection theory-especially when judging confidence without time pressure.NEW & NOTEWORTHY We found that choice confidence data show dynamics consistent with evidence accumulation for a forced-choice subjective visual vertical task. We also found that the evidence accumulation appeared unbounded when judging confidence, which suggests that the brain utilizes mechanisms consistent with signal detection theory to determine choice confidence.

Tactile Flow Overrides Other Cues To Self Motion

Vestibular-somatosensory interactions are pervasive in the brain but it remains unclear why. Here we explore the contribution of tactile flow to processing self-motion. We assessed two aspects of self-motion: timing and speed. Participants sat on an oscillating swing and either kept their hands on their laps or rested them lightly on an earth-stationary surface. They viewed a grating oscillating at the same frequency as their motion and judged its phase or, in a separate experiment, its speed relative to their perceived motion. Participants required the phase to precede body movement (with or without tactile flow) or tactile flow by ~5° (44 ms) to appear earth-stationary. Speed judgments were 4-10% faster when motion was from tactile flow, either alone or with body motion, compared to body motion alone (where speed judgments were accurate). By comparing response variances we conclude that phase and speed judgments do not reflect optimal integration of tactile flow with other cues to body motion: instead tactile flow dominates perceived self-motion - acting as an emergency override. This may explain why even minimal tactile cues are so helpful in promoting stability and suggests that providing artificial tactile cues might be a powerful aid to perceiving self-motion.

Gravity dependence of the effect of optokinetic stimulation on the subjective visual vertical

Accurate and precise estimates of direction of gravity are essential for spatial orientation. According to Bayesian theory, multisensory vestibular, visual, and proprioceptive input is centrally integrated in a weighted fashion based on the reliability of the component sensory signals. For otolithic input, a decreasing signal-to-noise ratio was demonstrated with increasing roll angle. We hypothesized that the weights of vestibular (otolithic) and extravestibular (visual/proprioceptive) sensors are roll-angle dependent and predicted an increased weight of extravestibular cues with increasing roll angle, potentially following the Bayesian hypothesis. To probe this concept, the subjective visual vertical (SVV) was assessed in different roll positions (≤ ± 120°, steps = 30°, n = 10) with/without presenting an optokinetic stimulus (velocity = ± 60°/s). The optokinetic stimulus biased the SVV toward the direction of stimulus rotation for roll angles ≥ ± 30° (P < 0.005). Offsets grew from 3.9 ± 1.8° (upright) to 22.1 ± 11.8° (±120° roll tilt, P < 0.001). Trial-to-trial variability increased with roll angle, demonstrating a nonsignificant increase when providing optokinetic stimulation. Variability and optokinetic bias were correlated (R² = 0.71, slope = 0.71, 95% confidence interval = 0.57-0.86). An optimal-observer model combining an optokinetic bias with vestibular input reproduced measured errors closely. These findings support the hypothesis of a weighted multisensory integration when estimating direction of gravity with optokinetic stimulation. Visual input was weighted more when vestibular input became less reliable, i.e., at larger roll-tilt angles. However, according to Bayesian theory, the variability of combined cues is always lower than the variability of each source cue. If the observed increase in variability, although nonsignificant, is true, either it must depend on an additional source of variability, added after SVV computation, or it would conflict with the Bayesian hypothesis.NEW & NOTEWORTHY Applying a rotating optokinetic stimulus while recording the subjective visual vertical in different whole body roll angles, we noted the optokinetic-induced bias to correlate with the roll angle. These findings allow the hypothesis that the established optimal weighting of single-sensory cues depending on their reliability to estimate direction of gravity could be extended to a bias caused by visual self-motion stimuli.

Visual perception of upright: Head tilt, visual errors and viewing eye

BACKGROUND

Perception of upright is often assessed by aligning a luminous line to the subjective visual vertical (SVV).

OBJECTIVE

Here we investigated the effects of visual line rotation and viewing eye on SVV responses and whether there was any change with head tilt.

METHODS

SVV was measured using a forced-choice paradigm and by combining the following conditions in 22 healthy subjects: head position (20° left tilt, upright and 20° right tilt), viewing eye (left eye, both eyes and right eye) and direction of visual line rotation (clockwise [CW] and counter clockwise [CCW]).

RESULTS

The accuracy and precision of SVV responses were not different between the viewing eye conditions in all head positions (P> 0.05, Kruskal-Wallis test). The accuracy of SVV responses was % significantly different between the CW and CCW line rotations (p ≈ 0.0001; Kruskal-Wallis test) and SVV was tilted in the same direction as the line rotation. This effect of line rotation was however not consistent across head tilts and was only present in the upright and right tilt head positions. The accuracy of SVV responses showed a higher variability among subjects in the left head tilt position with no significant difference between the CW and CCW line rotations (P> 0.05; post-hoc Dunn's test).

CONCLUSIONS

In spite of the challenges to the estimate of upright with head tilt, normal subjects did remarkably well irrespective of the viewing eye. The physiological significance of the asymmetry in the effect of line rotation between the head tilt positions is unclear but it %may suggest suggests a lateralizing effect of head tilt on the visual perception of upright.

A Bayesian Account of Visual-Vestibular Interactions in the Rod-and-Frame Task

Panoramic visual cues, as generated by the objects in the environment, provide the brain with important information about gravity direction. To derive an optimal, i.e., Bayesian, estimate of gravity direction, the brain must combine panoramic information with gravity information detected by the vestibular system. Here, we examined the individual sensory contributions to this estimate psychometrically. We asked human subjects to judge the orientation (clockwise or counterclockwise relative to gravity) of a briefly flashed luminous rod, presented within an oriented square frame (rod-in-frame). Vestibular contributions were manipulated by tilting the subject's head, whereas visual contributions were manipulated by changing the viewing distance of the rod and frame. Results show a cyclical modulation of the frame-induced bias in perceived verticality across a 90° range of frame orientations. The magnitude of this bias decreased significantly with larger viewing distance, as if visual reliability was reduced. Biases increased significantly when the head was tilted, as if vestibular reliability was reduced. A Bayesian optimal integration model, with distinct vertical and horizontal panoramic weights, a gain factor to allow for visual reliability changes, and ocular counterroll in response to head tilt, provided a good fit to the data. We conclude that subjects flexibly weigh visual panoramic and vestibular information based on their orientation-dependent reliability, resulting in the observed verticality biases and the associated response variabilities.

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.

Gravity in the Brain as a Reference for Space and Time Perception

Moving and interacting with the environment require a reference for orientation and a scale for calibration in space and time. There is a wide variety of environmental clues and calibrated frames at different locales, but the reference of gravity is ubiquitous on Earth. The pull of gravity on static objects provides a plummet which, together with the horizontal plane, defines a three-dimensional Cartesian frame for visual images. On the other hand, the gravitational acceleration of falling objects can provide a time-stamp on events, because the motion duration of an object accelerated by gravity over a given path is fixed. Indeed, since ancient times, man has been using plumb bobs for spatial surveying, and water clocks or pendulum clocks for time keeping. Here we review behavioral evidence in favor of the hypothesis that the brain is endowed with mechanisms that exploit the presence of gravity to estimate the spatial orientation and the passage of time. Several visual and non-visual (vestibular, haptic, visceral) cues are merged to estimate the orientation of the visual vertical. However, the relative weight of each cue is not fixed, but depends on the specific task. Next, we show that an internal model of the effects of gravity is combined with multisensory signals to time the interception of falling objects, to time the passage through spatial landmarks during virtual navigation, to assess the duration of a gravitational motion, and to judge the naturalness of periodic motion under gravity.

Vestibular Interactions in the Thalamus

It has long been known that the vast majority of all information en route to the cerebral cortex must first pass through the thalamus. The long held view that the thalamus serves as a simple hi fidelity relay station for sensory information to the cortex, however, has over recent years been dispelled. Indeed, multiple projections from the vestibular nuclei to thalamic nuclei (including the ventrobasal nuclei, and the geniculate bodies)- regions typically associated with other modalities- have been described. Further, some thalamic neurons have been shown to respond to stimuli presented from across sensory modalities. For example, neurons in the rat anterodorsal and laterodorsal nuclei of the thalamus respond to visual, vestibular, proprioceptive and somatosensory stimuli and integrate this information to compute heading within the environment. Together, these findings imply that the thalamus serves crucial integrative functions, at least in regard to vestibular processing, beyond that imparted by a “simple” relay. In this mini review we outline the vestibular inputs to the thalamus and provide some clinical context for vestibular interactions in the thalamus. We then focus on how vestibular inputs interact with other sensory systems and discuss the multisensory integration properties of the thalamus.

Modeling human perception of orientation in altered gravity

Altered gravity environments, such as those experienced by astronauts, impact spatial orientation perception, and can lead to spatial disorientation and sensorimotor impairment. To more fully understand and quantify the impact of altered gravity on orientation perception, several mathematical models have been proposed. The utricular shear, tangent, and the idiotropic vector models aim to predict static perception of tilt in hyper-gravity. Predictions from these prior models are compared to the available data, but are found to systematically err from the perceptions experimentally observed. Alternatively, we propose a modified utricular shear model for static tilt perception in hyper-gravity. Previous dynamic models of vestibular function and orientation perception are limited to 1 G. Specifically, they fail to predict the characteristic overestimation of roll tilt observed in hyper-gravity environments. To address this, we have proposed a modification to a previous observer-type canal-otolith interaction model based upon the hypothesis that the central nervous system (CNS) treats otolith stimulation in the utricular plane differently than stimulation out of the utricular plane. Here we evaluate our modified utricular shear and modified observer models in four altered gravity motion paradigms: (a) static roll tilt in hyper-gravity, (b) static pitch tilt in hyper-gravity, (c) static roll tilt in hypo-gravity, and (d) static pitch tilt in hypo-gravity. The modified models match available data in each of the conditions considered. Our static modified utricular shear model and dynamic modified observer model may be used to help quantitatively predict astronaut perception of orientation in altered gravity environments.

Augmented asymmetrical visual field dependence in asymptomatic diabetics: evidence of subclinical asymmetrical bilateral vestibular dysfunction

BACKGROUND

Diabetes negatively affects the vestibular system in many ways, with vestibular dysfunction (VD), a co-morbidity with a high prevalence in diabetics.

PROCEDURES

The ability to perceive subjective visual vertical (SVV), as a sign of vestibular dysfunction, and visual field dependence was measured using a computerized rod and frame test (CRAF). Alignment errors recorded from 47 asymptomatic Type II diabetics (no vertigo or falls, without peripheral neuropathy or retinopathy) were compared to 29 healthy age matched (46-69years) controls.

FINDINGS

Visual field dependence was significantly larger and more asymmetrical in the diabetics than controls. In the absence of any visual references, or when a vertical reference frame was provided, SVV perception was accurate in both groups, with no significant difference between the controls and diabetics. During tilted frame presentations, the proportion of subjects with either SVV deviations, or an asymmetry index, larger than an upper limit derived from the control data was significantly greater in diabetics than controls.

CONCLUSION

These results suggest that the decreased ability to resolve visuo-vestibular conflict in asymptomatic diabetic patients (free of retinopathy and peripheral neuropathy) compared to controls may be related to diabetic complications affecting vestibular structures and thus causing a decompensation of subclinical vestibular asymmetries.