Shared computational mechanism for tilt compensation accounts for biased verticality percepts in motion and pattern vision

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.

The Effect of Roll Circular Vection on Roll Tilt Postural Responses and Roll Subjective Postural Horizontal of Healthy Normal Subjects

Background: Falls and related injuries are critical issues in several disease states, as well as aging, especially when interactions between vestibular and visual sensory inputs are involved. Slow support surface tilt (0.6 deg/s) followed by subjective postural horizontal (SPH) assessments have been proposed as a viable method for assessing otolith contributions to balance control. Previous assessments of perceived body alignment to vertical, including subjective visual vertical, have suggested that visual inputs are weighted more when vestibular information is near the threshold and less reliable during slow body tilt. To date, no studies have examined the influence of visual stimuli on slow roll-tilt postural responses and the SPH. Therefore, this study investigated how dynamic visual cues, in the form of circular vection (CV), influence postural responses and the perception of the horizontal during and after support surface tilt. Methods: Ten healthy young adults (6 female, mean age 23) wore a head-mounted display while standing on a tilting platform. Participants were asked to remain upright for 30 s, during which (1) the visual scene rotated, inducing roll CV clockwise (CW) or counter-clockwise (CCW) at 60°/s; (2) the platform only (PO) rotated in roll to test SPH (0.6°/s, 2°, CW or CCW); (3) a combination of both; or (4) neither occurred. During SPH trials, participants used a hand-held device to reset the position of the platform to 0.8°/s to their perceived SPH. The angular motion of body segments was measured using pairs of light-emitting diodes mounted on the head, trunk and pelvis. Segment motion, prior to platform motion, was compared to that at peak body motion induced by platform motion and when SPH had been set. Results: When the support surface was tilted 2°, peak upper body tilt significantly increased for congruent CV and platform tilt and decreased at the pelvis for incongruent CV when compared to PO, leading to significant differences across body segments for congruent and incongruent conditions (p ≤ 0.008). During PO, participants' mean SPH deviated from horizontal by 0.2°. The pelvis deviated 0.2°, the trunk 0.3°, and the head 0.5° in the direction of initial platform rotation. When platform tilt and CV directions were congruent or incongruent, only head tilt at SPH reset under congruent conditions was significantly different from the PO condition (1.7° vs. 0.5°). Conclusions: Roll CV has a significant effect on phasic body responses and a less significant effect on tonic body responses to lateral tilt. The SPH of the support surface was not altered by CV. Responses during tilt demonstrated enhanced reactions for congruent and reduced reactions for incongruent CV, both different from responses to CV alone. Tonic body displacements associated with SPH were changed less than those during tilt and were only slightly larger than displacements for CV alone. This study supports the hypothesis of weighted multisensory integration during dynamic postural tasks being highly dependent on the direction of visual cues during tilt and less dependent on tonic SPH offsets. These techniques could be used to examine vestibular and visual interactions within clinical populations, particularly those with visual vertigo and dizziness.

Visuo-vestibular conflicts within the roll plane modulate multisensory verticality perception

The integration of visuo-vestibular information is crucial when interacting with the external environment. Under normal circumstances, vision and vestibular signals provide corroborating information, for example regarding the direction and speed of self-motion. However, conflicts in visuo-vestibular signalling, such as optic flow presented to a stationary observer, can change subsequent processing in either modality. While previous studies have demonstrated the impact of sensory conflict on unisensory visual or vestibular percepts, here we investigated whether visuo-vestibular conflicts impact sensitivity to multisensory percepts, specifically verticality. Participants were exposed to a visuo-vestibular conflicting or non-conflicting motion adaptor before completing a Vertical Detection Task. Sensitivity to vertical stimuli was reduced following visuo-vestibular conflict. No significant differences in criterion were found. Our findings suggest that visuo-vestibular conflicts not only modulate processing in unimodal channels, but also broader multisensory percepts, which may have implications for higher-level processing dependent on the integration of visual and vestibular signals.

Body Pitch Together With Translational Body Motion Biases the Subjective Haptic Vertical

Accurate perception of verticality is critical for postural maintenance and successful physical interaction with the world. Although previous research has examined the independent influences of body orientation and self-motion under well-controlled laboratory conditions, these factors are constantly changing and interacting in the real world. In this study, we examine the subjective haptic vertical in a real-world scenario. Here, we report a bias of verticality perception in a field experiment on the Hong Kong Peak Tram as participants traveled on a slope ranging from 6° to 26°. Mean subjective haptic vertical (SHV) increased with slope by as much as 15°, regardless of whether the eyes were open (Experiment 1) or closed (Experiment 2). Shifting the body pitch by a fixed degree in an effort to compensate for the mountain slope failed to reduce the verticality bias (Experiment 3). These manipulations separately rule out visual and vestibular inputs about absolute body pitch as contributors to our observed bias. Observations collected on a tram traveling on level ground (Experiment 4A) or in a static dental chair with a range of inclinations similar to those encountered on the mountain tram (Experiment 4B) showed no significant deviation of the subjective vertical from gravity. We conclude that the SHV error is due to a combination of large, dynamic body pitch and translational motion. These observations made in a real-world scenario represent an incentive to neuroscientists and aviation experts alike for studying perceived verticality under field conditions and raising awareness of dangerous misperceptions of verticality when body pitch and translational self-motion come together.

Natural statistics of head roll: implications for Bayesian inference in spatial orientation

We previously proposed a Bayesian model of multisensory integration in spatial orientation (Clemens IAH, de Vrijer M, Selen LPJ, van Gisbergen JAM, Medendorp WP. J Neurosci 31: 5365-5377, 2011). Using a Gaussian prior, centered on an upright head orientation, this model could explain various perceptual observations in roll-tilted participants, such as the subjective visual vertical, the subjective body tilt (Clemens IAH, de Vrijer M, Selen LPJ, van Gisbergen JAM, Medendorp WP. J Neurosci 31: 5365-5377, 2011), the rod-and-frame effect (Alberts BBGT, de Brouwer AJ, Selen LPJ, Medendorp WP. eNeuro 3: ENEURO.0093-16.2016, 2016), as well as their clinical (Alberts BBGT, Selen LPJ, Verhagen WIM, Medendorp WP. Physiol Rep 3: e12385, 2015) and age-related deficits (Alberts BBGT, Selen LPJ, Medendorp WP. J Neurophysiol 121: 1279-1288, 2019). Because it is generally assumed that the prior reflects an accumulated history of previous head orientations, and recent work on natural head motion suggests non-Gaussian statistics, we examined how the model would perform with a non-Gaussian prior. In the present study, we first experimentally generalized the previous observations in showing that also the natural statistics of head orientation are characterized by long tails, best quantified as a t-location-scale distribution. Next, we compared the performance of the Bayesian model and various model variants using such a t-distributed prior to the original model with the Gaussian prior on their accounts of previously published data of the subjective visual vertical and subjective body tilt tasks. All of these variants performed substantially worse than the original model, suggesting a special value of the Gaussian prior. We provide computational and neurophysiological reasons for the implementation of such a prior, in terms of its associated precision-accuracy trade-off in vertical perception across the tilt range.NEW & NOTEWORTHY It has been argued that the brain uses Bayesian computations to process multiple sensory cues in vertical perception, including a prior centered on upright head orientation which is usually taken to be Gaussian. Here, we show that non-Gaussian prior distributions, although more akin to the statistics of head orientation during natural activities, provide a much worse explanation of such perceptual observations than a Gaussian prior.

Perceptual Biases as the Side Effect of a Multisensory Adaptive System: Insights from Verticality and Self-Motion Perception

Perceptual biases can be interpreted as adverse consequences of optimal processes which otherwise improve system performance. The review presented here focuses on the investigation of inaccuracies in multisensory perception by focusing on the perception of verticality and self-motion, where the vestibular sensory modality has a prominent role. Perception of verticality indicates how the system processes gravity. Thus, it represents an indirect measurement of vestibular perception. Head tilts can lead to biases in perceived verticality, interpreted as the influence of a vestibular prior set at the most common orientation relative to gravity (i.e., upright), useful for improving precision when upright (e.g., fall avoidance). Studies on the perception of verticality across development and in the presence of blindness show that prior acquisition is mediated by visual experience, thus unveiling the fundamental role of visuo-vestibular interconnections across development. Such multisensory interactions can be behaviorally tested with cross-modal aftereffect paradigms which test whether adaptation in one sensory modality induces biases in another, eventually revealing an interconnection between the tested sensory modalities. Such phenomena indicate the presence of multisensory neural mechanisms that constantly function to calibrate self-motion dedicated sensory modalities with each other as well as with the environment. Thus, biases in vestibular perception reveal how the brain optimally adapts to environmental requests, such as spatial navigation and steady changes in the surroundings.

Efficacy of lateral truncal tilt training with a wedge on postural vertical and activities of daily living in recovery phase after stroke: A randomized crossover trial

BACKGROUND: The subjective postural vertical (SPV) is affected by training that requires participants to maintain balance; training to achieve sitting balance may also help SPV recovery and activities of daily living (ADLs). OBJECTIVE: This study aimed to assess the medium-term effects of balance training on the postural vertical (PV) and ADLs in patients with stroke. METHODS: Thirty-three patients with stroke were enrolled in this randomized crossover trial. The outcomes were SPV, SPV with eyes opened (SPV-EO), and the Functional Independent Measure (FIM) after the 14-day intervention of experimental or control training. Experimental training consisted of a maximum lateral truncal tilt to the paretic side at 0.25 Hz with or without a 10° wedge, repeated 60 times. Repeated-measures two-way analysis of variance was performed with two factors: intervention and the intervention period. RESULTS: There was no interaction between the control condition and the PV, but there was interaction between the intervention period and using a wedge for SPV variability errors. FIM showed an interaction between the intervention period and the use of a wedge. CONCLUSIONS: Balance training while sitting with a wedge significantly improved the SPV and FIM. This improved cognition and perception, which facilitate difficult dynamic tasks in ADLs.

How Tilting the Head Interferes With Eye-Hand Coordination: The Role of Gravity in Visuo-Proprioceptive, Cross-Modal Sensory Transformations

To correctly position the hand with respect to the spatial location and orientation of an object to be reached/grasped, visual information about the target and proprioceptive information from the hand must be compared. Since visual and proprioceptive sensory modalities are inherently encoded in a retinal and musculo-skeletal reference frame, respectively, this comparison requires cross-modal sensory transformations. Previous studies have shown that lateral tilts of the head interfere with the visuo-proprioceptive transformations. It is unclear, however, whether this phenomenon is related to the neck flexion or to the head-gravity misalignment. To answer to this question, we performed three virtual reality experiments in which we compared a grasping-like movement with lateral neck flexions executed in an upright seated position and while lying supine. In the main experiment, the task requires cross-modal transformations, because the target information is visually acquired, and the hand is sensed through proprioception only. In the other two control experiments, the task is unimodal, because both target and hand are sensed through one, and the same, sensory channel (vision and proprioception, respectively), and, hence, cross-modal processing is unnecessary. The results show that lateral neck flexions have considerably different effects in the seated and supine posture, but only for the cross-modal task. More precisely, the subjects’ response variability and the importance associated to the visual encoding of the information significantly increased when supine. We show that these findings are consistent with the idea that head-gravity misalignment interferes with the visuo-proprioceptive cross-modal processing. Indeed, the principle of statistical optimality in multisensory integration predicts the observed results if the noise associated to the visuo-proprioceptive transformations is assumed to be affected by gravitational signals, and not by neck proprioceptive signals per se. This finding is also consistent with the observation of otolithic projections in the posterior parietal cortex, which is involved in the visuo-proprioceptive processing. Altogether these findings represent a clear evidence of the theorized central role of gravity in spatial perception. More precisely, otolithic signals would contribute to reciprocally align the reference frames in which the available sensory information can be encoded.

Psychophysical Haptic Measurement of Vertical Perception: Elucidating a Hand Sensory Bias

The primary sensory modality for probing spatial perception can vary among psychophysical paradigms. In the subjective visual vertical (SVV) task, the brain must account for the position of the eye within the orbit to generate an estimate of a visual line orientation, whereas in the subjective haptic vertical (SHV) task, the position of the hand is used to sense the orientation of a haptic bar. Here we investigated whether a hand sensory bias can affect SHV measurement. We measured SHV in 12 subjects (6 left-handed and 6 right-handed) with a forced-choice paradigm using their left and right hands separately. The SHV measurement was less accurate than the SVV measurements (-0.6 ± 0.7) and it was biased in the direction of the hand used in the task but was not affected by handedness; SHV left hand -6.8 ± 2.1° (left-handed -7.9 ± 3.6°, right-handed -5.8 ± 2.5°) and right hand 9.8 ± 1.5° (left-handed 7.4 ± 2.2°, right-handed 12.3 ± 1.8°). SHV measurement with the same hand was also affected by the haptic bar placement on the left or right side versus midline, showing a side effect (left vs midline -2.0 ± 1.3°, right vs midline 3.8 ± 1.7°). Midline SHV measures using the left and right hands were different, confirming a laterality effect (left hand -4.5 ± 1.7°, right hand 6.4 ± 2.0°). These results demonstrate a sensory bias in SHV measurement related to the effects of both hand-in-body (i.e., right vs left hand) and hand-in-space positions. Such modality-specific bias may result in disparity between SHV and SVV measurements, and therefore cannot be generalized to vertical or spatial perception.

Contributions of Body-Orientation to Mental Ball Dropping Task During Out-of-Body Experiences

Out-of-body experiences (OBEs) provide fascinating insights into our understanding of bodily self-consciousness and the workings of the brain. Studies that examined individuals with brain lesions reported that OBEs are generally characterized by participants experiencing themselves outside their physical body (i.e., disembodied feeling) (Blanke and Arzy, 2005). Based on such a characterization, it has been shown that it is possible to create virtual OBEs in immersive virtual environments (Ehrsson, 2007; Ionta et al., 2011b; Bourdin et al., 2017). However, the extent to which body-orientation influences virtual OBEs is not well-understood. Thus, in the present study, 30 participants (within group design) experienced a full-body ownership illusion (synchronous visuo-tactile stimulation only) induced with a gender-matched full-body virtual avatar seen from the first-person perspective (1PP). At the beginning of the experiment, participants performed a mental ball dropping (MBD) task, seen from the location of their virtual avatar, to provide a baseline measurement. After this, a full-body ownership illusion (embodiment phase) was induced in all participants. This was followed by the virtual OBE illusion phase of the experiment (disembodiment phase) in which the first-person viewpoint was switched to a third-person perspective (3PP), and participants' disembodied viewpoint was gradually raised to 14 m above the virtual avatar, from which altitude they repeated the MBD task. During the experiment, this procedure was conducted twice, and the participants were allocated first to the supine or the standing body position at random. Results of the MBD task showed that the participants experienced increased MBD durations during the supine condition compared to the standing condition. Furthermore, although the findings from the subjective reports confirmed the previous findings of virtual OBEs, no significant difference between the two postures was found for body ownership. Taken together, the findings of the current study make further contributions to our understanding of both the vestibular system and time perception during OBEs.

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.

Watching the Effects of Gravity. Vestibular Cortex and the Neural Representation of "Visual" Gravity

Gravity is a physical constraint all terrestrial species have adapted to through evolution. Indeed, gravity effects are taken into account in many forms of interaction with the environment, from the seemingly simple task of maintaining balance to the complex motor skills performed by athletes and dancers. Graviceptors, primarily located in the vestibular otolith organs, feed the Central Nervous System with information related to the gravity acceleration vector. This information is integrated with signals from semicircular canals, vision, and proprioception in an ensemble of interconnected brain areas, including the vestibular nuclei, cerebellum, thalamus, insula, retroinsula, parietal operculum, and temporo-parietal junction, in the so-called vestibular network. Classical views consider this stage of multisensory integration as instrumental to sort out conflicting and/or ambiguous information from the incoming sensory signals. However, there is compelling evidence that it also contributes to an internal representation of gravity effects based on prior experience with the environment. This a priori knowledge could be engaged by various types of information, including sensory signals like the visual ones, which lack a direct correspondence with physical gravity. Indeed, the retinal accelerations elicited by gravitational motion in a visual scene are not invariant, but scale with viewing distance. Moreover, the "visual" gravity vector may not be aligned with physical gravity, as when we watch a scene on a tilted monitor or in weightlessness. This review will discuss experimental evidence from behavioral, neuroimaging (connectomics, fMRI, TMS), and patients' studies, supporting the idea that the internal model estimating the effects of gravity on visual objects is constructed by transforming the vestibular estimates of physical gravity, which are computed in the brainstem and cerebellum, into internalized estimates of virtual gravity, stored in the vestibular cortex. The integration of the internal model of gravity with visual and non-visual signals would take place at multiple levels in the cortex and might involve recurrent connections between early visual areas engaged in the analysis of spatio-temporal features of the visual stimuli and higher visual areas in temporo-parietal-insular regions.

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.

Neuroanatomical correlates of the perception of body axis orientation during body tilt: a voxel-based morphometry study

Accurate perception of the orientations of the body axis and gravity is essential for actions. The ability to perceive these orientations during head and body tilt varies across individuals, and its underlying neural basis is unknown. To address this, we investigated the association between inter-individual differences in local gray matter (GM) volume and inter-individual differences in the ability to estimate the directions of body longitudinal axis or gravity during whole-body tilt using voxel-based morphometry (VBM) analysis in 50 healthy adults (20–46 years, 25 men and 25 women). Although no anatomical regions were identified relating to performance requiring estimates of gravitational direction, we found a significant correlation between the GM volume in the right middle occipital gyrus and the ability to estimate the body axis orientation. This finding provides the first evidence on neuroanatomical substrates of the perception of body axis orientation during body tilt.

Prolonged Static Whole-Body Roll-Tilt and Optokinetic Stimulation Significantly Bias the Subjective Postural Vertical in Healthy Human Subjects

Background: Prolonged static whole-body roll-tilt has been shown to bias estimates of the direction of gravity when assessed by static paradigms such as the subjective visual vertical and the subjective haptic vertical.

Objective: We hypothesized that these shifts are paradigm-independent and thus predicted a post-tilt bias as well for self-adjustments along perceived vertical (subjective postural vertical, SPV). Likewise, rotatory optokinetic stimuli, which have been shown to shift the SPV when presented at the time of adjustments, may have an lasting effect on the SPV, predicting a shift in the perceived direction of gravity in the direction of the optokinetic rotatory stimulation.

Methods: Self-adjustments along perceived vertical by use of a motorized turntable were recorded at baseline and after 5 min of static whole-body roll-tilt (orientation = ±90°, adaptation period) in 10 healthy human subjects. During adaptation subjects were either in darkness (no OKN stimulation) or were presented a full-field rotatory optokinetic stimulus (velocity = ±60°/s). Statistical analysis of adjustment errors for the different conditions was performed using a generalized linear model.

Results: After 5 min of static whole-body roll-tilt in darkness, we observed significant (p < 0.001) shifts in the SPV averaging −2.8° (adaptation position: −90°) and 3.1° (+90°), respectively. Adding an optokinetic rotatory stimulus resulted in an additional, significant shift of SPV adjustments toward the direction of the previously presented optokinetic rotation (optokinetic clockwise rotation: 1.4°, p = 0.034; optokinetic counter-clockwise rotation: −1.3°, p = 0.037). Trial-to-trial variability of turntable adjustments was not significantly affected by adaptation.

Conclusions: Prolonged static roll-tilt results in a significant post-tilt bias of the perceived direction of gravity when assessed by the SPV, confirming previous findings from other vision-dependent and vision-independent paradigms. This finding emphasizes the impact of recent whole-body roll orientations relative to gravity. Such adaptational shifts in verticality estimates may be explained in the context of Bayesian optimal observer theory with a bias of prior knowledge (i.e., expectation biased by experience). Our findings also have clinical implications, as the observed post-tilt bias may contribute to postural instability when standing up in the morning with an increasing risk for falls and fall-related injuries in humans with preexisting balance disorders.

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.

Which way is down? Visual and tactile verticality perception in expert dancers and non-experts

Gravity provides an absolute verticality reference for all spatial perception, allowing us to move within and interact effectively with our world. Bayesian inference models explain verticality perception as a combination of online sensory cues with a prior prediction that the head is usually upright. Until now, these Bayesian models have been formulated for judgements of the perceived orientation of visual stimuli. Here, we investigated whether judgements of the verticality of tactile stimuli follow a similar pattern of Bayesian perceptual inference. We also explored whether verticality perception is affected by the postural and balance expertise of dancers. We tested both the subjective visual vertical (SVV) and the subjective tactile vertical (STV) in ballet dancers and non-dancers. A robotic arm traced downward-moving visual or tactile stimuli in separate blocks while participants held their head either upright or tilted 30° to their right. Participants reported whether these stimuli deviated to the left (clockwise) or right (anti-clockwise) of the gravitational vertical. Tilting the head biased the SVV away from the longitudinal head axis (the classical E-effect), consistent with a failure to compensate for the vestibulo-ocular counter-roll reflex. On the contrary, tilting the head biased the STV toward the longitudinal head axis (the classical A-effect), consistent with a strong upright head prior. Critically, tilting the head reduced the precision of verticality perception, particularly for ballet dancers' STV judgements. Head tilt is thought to increase vestibular noise, so ballet dancers seem to be surprisingly susceptible to degradation of vestibular inputs, giving them an inappropriately high weighting in verticality judgements.

Time Course of Sensory Substitution for Gravity Sensing in Visual Vertical Orientation Perception following Complete Vestibular Loss

Loss of vestibular function causes severe acute symptoms of dizziness and disorientation, yet the brain can adapt and regain near to normal locomotor and orientation function through sensory substitution. Animal studies quantifying functional recovery have yet been limited to reflexive eye movements. Here, we studied the interplay between vestibular and proprioceptive graviception in macaque monkeys trained in an earth-vertical visual orientation (subjective visual vertical; SVV) task and measured the time course of sensory substitution for gravity perception following complete bilateral vestibular loss (BVL). Graviceptive gain, defined as the ratio of perceived versus actual tilt angle, decreased to 20% immediately following labyrinthectomy, and recovered to nearly prelesion levels with a time constant of approximately three weeks of postsurgery testing. We conclude that proprioception accounts for up to 20% of gravity sensing in normal animals, and is re-weighted to substitute completely perceptual graviception after vestibular loss. We show that these results can be accounted for by an optimal sensory fusion model.

The gravitational imprint on sensorimotor planning and control

Humans excel at learning complex tasks, and elite performers such as musicians or athletes develop motor skills that defy biomechanical constraints. All actions require the movement of massive bodies. Of particular interest in the process of sensorimotor learning and control is the impact of gravitational forces on the body. Indeed, efficient control and accurate internal representations of the body configuration in space depend on our ability to feel and anticipate the action of gravity. Here we review studies on perception and sensorimotor control in both normal and altered gravity. Behavioral and modeling studies together suggested that the nervous system develops efficient strategies to take advantage of gravitational forces across a wide variety of tasks. However, when the body was exposed to altered gravity, the rate and amount of adaptation exhibited substantial variation from one experiment to another and sometimes led to partial adjustment only. Overall, these results support the hypothesis that the brain uses a multimodal and flexible representation of the effect of gravity on our body and movements. Future work is necessary to better characterize the nature of this internal representation and the extent to which it can adapt to novel contexts.

Effect of dynamic visual motion on perception of postural vertical through the modulation of prior knowledge of gravity

To internally estimate gravitational direction and body orientation, the central nervous system considers several sensory inputs from the periphery and prior knowledge of gravity. It is hypothesized that the modulation of visual inputs, supplying indirect information of gravity, affects the prior knowledge established internally by other sensory inputs from vestibular and somatosensory systems, leading to the alteration of perceived body orientation relative to gravity. In order to test the hypothesis, we examined the effect of presenting a visual motion stimulus during a whole-body static tilt on the subsequent evaluation of the perceived postural vertical. Fifteen subjects watched a target moving along the body longitudinal axis directing from head to feet with constant downward acceleration (CA condition) or constant velocity (CV condition), or they did not receive any visual stimulation (NV condition) during the whole-body static tilt. Subsequently, the direction of the subjective postural vertical (SPV) was evaluated. The result showed that the SPV in the CA condition was significantly tilted toward the direction of the preceding tilt compared to that in the NV condition while those in the CV and NV conditions were not significantly different. The present result suggests that dynamic visual motion along body longitudinal axis with downward acceleration can modulate prior knowledge of gravity, and in turn this affects the perception of body verticality.

Spatial orientation: Model-based approach to multi-sensory mechanisms

Perception of spatial orientation is generated through multimodal sensory integration. In this process, there are systematic errors with changes in the head or body position, which reflect challenges for the brain in maintaining a common sensory reference frame for spatial orientation. Here, we focus on this multisensory aspect of spatial orientation. We review a Bayesian spatial perception model that can be used as a framework to study sensory contributions to spatial orientation during lateral head tilts and probe neural networks involved in this process.

The A-Effect and Global Motion

When the head is tilted, an objectively vertical line viewed in isolation is typically perceived as tilted. We explored whether this shift also occurs when viewing global motion displays perceived as either object-motion or self-motion. Observers stood and lay left side down while viewing (1) a static line, (2) a random-dot display of 2-D (planar) motion or (3) a random-dot display of 3-D (volumetric) global motion. On each trial, the line orientation or motion direction were tilted from the gravitational vertical and observers indicated whether the tilt was clockwise or counter-clockwise from the perceived vertical. Psychometric functions were fit to the data and shifts in the point of subjective verticality (PSV) were measured. When the whole body was tilted, the perceived tilt of both a static line and the direction of optic flow were biased in the direction of the body tilt, demonstrating the so-called A-effect. However, we found significantly larger shifts for the static line than volumetric global motion as well as larger shifts for volumetric displays than planar displays. The A-effect was larger when the motion was experienced as self-motion compared to when it was experienced as object-motion. Discrimination thresholds were also more precise in the self-motion compared to object-motion conditions. Different magnitude A-effects for the line and motion conditions—and for object and self-motion—may be due to differences in combining of idiotropic (body) and vestibular signals, particularly so in the case of vection which occurs despite visual-vestibular conflict.

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.

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.

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.

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.

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.

Effects of Optokinetic Stimulation on Verticality Perception Are Much Larger for Vision-Based Paradigms Than for Vision-Independent Paradigms

Introduction

Verticality perception as assessed by the subjective visual vertical (SVV) is significantly biased by a rotating optokinetic stimulus. The underlying mechanisms of this effect remain open. Potentially, the optokinetic stimulus induces a shift of the internal estimate of the direction of gravity. This hypothesis predicts a shift of perceived vertical using other, non-vision dependent, paradigms as well. Alternatively, an optokinetic stimulus may only induce a shift of visual orientation, and so would be task specific.

Methods

To test this prediction, both vision-dependent SVV and vision-independent [subjective haptic vertical (SHV)] paradigms were applied. In 12 healthy human subjects, perceived vertical was measured in different whole-body roll positions (up to ±120°, steps = 30°) while watching a clockwise or counterclockwise rotating optokinetic stimulus. For comparison, baseline trials were collected in darkness. A generalized linear model was applied for statistical analysis.

Results

A significant main effect for optokinetic stimulation was noted both for the SVV paradigm (p < 0.001) and the SHV paradigm (p = 0.013). However, while pairwise comparisons demonstrated significant optokinetic-induced shifts (p ≤ 0.035) compared to baseline in all roll-tilted orientations except 30° and 60° left-ear-down position and counterclockwise optokinetic stimulation for the SVV paradigm, significant shifts were found in only 1 of the 18 test conditions (120° left-ear-down roll orientation, counterclockwise optokinetic stimulation) for the SHV paradigm. Compared to the SHV, the SVV showed significantly (p < 0.001) larger shifts of perceived vertical when presenting a clockwise (15.3 ± 16.0° vs. 1.1 ± 5.2°, mean ± 1 SD) or counterclockwise (−12.6 ± 7.7° vs. −2.6 ± 5.4°) rotating optokinetic stimulus.

Conclusion

Comparing the effect of optokinetic stimulation on verticality perception in both vision-dependent and vision-independent paradigms, we demonstrated distinct patterns. While significant large and roll-angle dependent shifts were noted for the SVV, offsets were minor and reached significance only in one test condition for the SHV. These results suggest that optokinetic stimulation predominately affects vision-related mechanisms, possibly due to induced torsional eye displacements, and that any shifts of the internal estimate of the direction of gravity are relatively minor.

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.

The Effect of Visual Experience on Perceived Haptic Verticality When Tilted in the Roll Plane

The orientation of the body in space can influence perception of verticality leading sometimes to biases consistent with priors peaked at the most common head and body orientation, that is upright. In this study, we investigate haptic perception of verticality in sighted individuals and early and late blind adults when tilted counterclockwise in the roll plane. Participants were asked to perform a stimulus orientation discrimination task with their body tilted to their left ear side 90° relative to gravity. Stimuli were presented by using a motorized haptic bar. In order to test whether different reference frames relative to the head influenced perception of verticality, we varied the position of the stimulus on the body longitudinal axis. Depending on the stimulus position sighted participants tended to have biases away or toward their body tilt. Visually impaired individuals instead show a different pattern of verticality estimations. A bias toward head and body tilt (i.e., Aubert effect) was observed in late blind individuals. Interestingly, no strong biases were observed in early blind individuals. Overall, these results posit visual sensory information to be fundamental in influencing the haptic readout of proprioceptive and vestibular information about body orientation relative to gravity. The acquisition of an idiotropic vector signaling the upright might take place through vision during development. Regarding early blind individuals, independent spatial navigation experience likely enhanced by echolocation behavior might have a role in such acquisition. In the case of participants with late onset blindness, early experience of vision might lead them to anchor their visually acquired priors to the haptic modality with no disambiguation between head and body references as observed in sighted individuals (Fraser et al., 2015). With our study, we aim to investigate haptic perception of gravity direction in unusual body tilts when vision is absent due to visual impairment. Insofar, our findings throw light on the influence of proprioceptive/vestibular sensory information on haptic perceived verticality in blind individuals showing how this phenomenon is affected by visual experience.

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.

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.

Gravity as a Strong Prior: Implications for Perception and Action

In the future, humans are likely to be exposed to environments with altered gravity conditions, be it only visually (Virtual and Augmented Reality), or visually and bodily (space travel). As visually and bodily perceived gravity as well as an interiorized representation of earth gravity are involved in a series of tasks, such as catching, grasping, body orientation estimation and spatial inferences, humans will need to adapt to these new gravity conditions. Performance under earth gravity discrepant conditions has been shown to be relatively poor, and few studies conducted in gravity adaptation are rather discouraging. Especially in VR on earth, conflicts between bodily and visual gravity cues seem to make a full adaptation to visually perceived earth-discrepant gravities nearly impossible, and even in space, when visual and bodily cues are congruent, adaptation is extremely slow. We invoke a Bayesian framework for gravity related perceptual processes, in which earth gravity holds the status of a so called “strong prior”. As other strong priors, the gravity prior has developed through years and years of experience in an earth gravity environment. For this reason, the reliability of this representation is extremely high and overrules any sensory information to its contrary. While also other factors such as the multisensory nature of gravity perception need to be taken into account, we present the strong prior account as a unifying explanation for empirical results in gravity perception and adaptation to earth-discrepant gravities.

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.

Direction-dependent arm kinematics reveal optimal integration of gravity cues

The brain has evolved an internal model of gravity to cope with life in the Earth's gravitational environment. How this internal model benefits the implementation of skilled movement has remained unsolved. One prevailing theory has assumed that this internal model is used to compensate for gravity's mechanical effects on the body, such as to maintain invariant motor trajectories. Alternatively, gravity force could be used purposely and efficiently for the planning and execution of voluntary movements, thereby resulting in direction-depending kinematics. Here we experimentally interrogate these two hypotheses by measuring arm kinematics while varying movement direction in normal and zero-G gravity conditions. By comparing experimental results with model predictions, we show that the brain uses the internal model to implement control policies that take advantage of gravity to minimize movement effort.

Vestibular stimulation interferes with the dynamics of an internal representation of gravity

The remembered vanishing location of a moving target has been found to be displaced downward in the direction of gravity (representational gravity) and more so with increasing retention intervals, suggesting that the visual spatial updating recruits an internal model of gravity. Despite being consistently linked with gravity, few inquiries have been made about the role of vestibular information in these trends. Previous experiments with static tilting of observers' bodies suggest that under conflicting cues between the idiotropic vector and vestibular signals, the dynamic drift in memory is reduced to a constant displacement along the body's main axis. The present experiment aims to replicate and extend these outcomes while keeping the observers' bodies unchanged in relation to physical gravity by varying the gravito-inertial acceleration using a short-radius centrifuge. Observers were shown, while accelerated to varying degrees, targets moving along several directions and were required to indicate the perceived vanishing location after a variable interval. Increases of the gravito-inertial force (up to 1.4G), orthogonal to the idiotropic vector, did not affect the direction of representational gravity, but significantly disrupted its time course. The role and functioning of an internal model of gravity for spatial perception and orientation are discussed in light of the results.

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.

Does gravity influence the visual line bisection task?

The visual line bisection task (LBT) is sensitive to perceptual biases of visuospatial attention, showing slight leftward (for horizontal lines) and upward (for vertical lines) errors in healthy subjects. It may be solved in an egocentric or allocentric reference frame, and there is no obvious need for graviceptive input. However, for other visual line adjustments, such as the subjective visual vertical, otolith input is integrated. We hypothesized that graviceptive input is incorporated when performing the LBT and predicted reduced accuracy and precision when roll-tilted. Twenty healthy right-handed subjects repetitively bisected Earth-horizontal and body-horizontal lines in darkness. Recordings were obtained before, during, and after roll-tilt (±45°, ±90°) for 5 min each. Additionally, bisections of Earth-vertical and oblique lines were obtained in 17 subjects. When roll-tilted ±90° ear-down, bisections of Earth-horizontal (i.e., body-vertical) lines were shifted toward the direction of the head (P < 0.001). However, after correction for vertical line-bisection errors when upright, shifts disappeared. Bisecting body-horizontal lines while roll-tilted did not cause any shifts. The precision of Earth-horizontal line bisections decreased (P ≤ 0.006) when roll-tilted, while no such changes were observed for body-horizontal lines. Regardless of the trial condition and paradigm, the scanning direction of the bisecting cursor (leftward vs. rightward) significantly (P ≤ 0.021) affected line bisections. Our findings reject our hypothesis and suggest that gravity does not modulate the LBT. Roll-tilt-dependent shifts are instead explained by the headward bias when bisecting lines oriented along a body-vertical axis. Increased variability when roll-tilted likely reflects larger variability when bisecting body-vertical than body-horizontal lines.

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.

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.

Internal Models, Vestibular Cognition, and Mental Imagery: Conceptual Considerations

Vestibular cognition has recently gained attention. Despite numerous experimental and clinical demonstrations, it is not yet clear what vestibular cognition really is. For future research in vestibular cognition, adopting a computational approach will make it easier to explore the underlying mechanisms. Indeed, most modeling approaches in vestibular science include a top-down or a priori component. We review recent Bayesian optimal observer models, and discuss in detail the conceptual value of prior assumptions, likelihood and posterior estimates for research in vestibular cognition. We then consider forward models in vestibular processing, which are required in order to distinguish between sensory input that is induced by active self-motion, and sensory input that is due to passive self-motion. We suggest that forward models are used not only in the service of estimating sensory states but they can also be drawn upon in an offline mode (e.g., spatial perspective transformations), in which interaction with sensory input is not desired. A computational approach to vestibular cognition will help to discover connections across studies, and it will provide a more coherent framework for investigating vestibular cognition.

Diurnal Fluctuations of Verticality Perception - Lesser Precision Immediately after Waking up in the Morning

Internal estimates of direction of gravity are continuously updated by integrating vestibular, visual and proprioceptive input, and prior experience about upright position. Prolonged static roll-tilt biases perceived direction of gravity by adaptation of peripheral sensors and central structures. We hypothesized that in the morning after sleep, estimates of direction of gravity [assessed by the subjective visual vertical (SVV)] are less precise than in the evening because of adaptation to horizontal body position and lack of prior knowledge about upright position. Using a mobile SVV-measuring device, verticality perception was assessed in seven healthy human subjects on 7 days in the morning immediately after waking up and in the evening while sitting upright. Paired t-tests were applied to analyze diurnal changes in SVV trial-to-trial variability. Average SVV variability in the morning was significantly larger than in the evening (1.9 ± 0.6° vs. 0.9 ± 0.3°, p = 0.002). SVV accuracy was not significantly different (−1.2 ± 0.9° vs. −0.4 ± 0.4°, morning vs. evening, p = 0.058) and was within normal range (±2.3°) in all but one subject. A good night’s sleep has a profound effect on the brain’s ability to estimate direction of gravity. Resulting variability was significantly worse after waking up reaching values more than twice as large as in the evening while there was no significant impact on SVV accuracy. We hypothesize that lacking prior knowledge, adaptation of peripheral sensors, and lower levels of arousal and cerebral metabolism contribute to such impoverished estimates. Our observations have considerable clinical impact as they indicate an increased risk for falls and fall-related injuries in the morning.