Perception of self-tilt in a true and illusory vertical plane

Underestimation of self-tilt increases in reduced gravity conditions

BACKGROUND

During large angles of self-tilt in the roll plane on Earth, measurements of the subjective visual vertical (SVV) in the dark show a bias towards the longitudinal body axis, reflecting a systematic underestimation of self-tilt.

OBJECTIVE

This study tested the hypothesis that self-tilt is underestimated in partial gravity conditions, and more so at lower gravity levels.

METHODS

The SVV was measured in parabolic flight at three partial gravity levels: 0.25, 0.50, and 0.75 g. Self-tilt was varied amongst 0, 15, 30, and 45 deg, using a tiltable seat. The participants indicated their SVV by setting a linear array of dots projected inside a head mounted display to the perceived vertical. The angles of participants' body and head roll tilt relative to the gravito-inertial vertical were measured by two separate inertial measurement units.

RESULTS

Data on six participants were collected. Per G-level, a regression analysis was performed with SVV setting as dependent variable and head tilt as independent variable. The latter was used instead of chair tilt, because not all the participants' heads were aligned with their bodies. The estimated regression slopes significantly decreased with smaller G-levels, reflecting an increased bias of the SVV towards the longitudinal body axis. On average, the regression slopes were 0.95 (±0.38) at 0.75 g; 0.84 (±0.22) at 0.5 g; and 0.63 (±0.33) at 0.25 g.

CONCLUSIONS

The results of this study show that reduced gravity conditions lead to increased underestimation of roll self-tilt.

Influence of vestibular signals on bodily self-consciousness: Different sensory weighting strategies based on visual dependency

Previous studies showed that the vestibular system is crucial for multisensory integration, however, its contribution to bodily self-consciousness more specifically on full-body illusions is not well understood. Thus, the current study examined the role of visuo-vestibular conflict on a full-body illusion (FBI) experiment that was induced during a supine body position. In a mixed design experiment, 56 participants underwent through a full-body illusion protocol. During the experiment, half of the participants received synchronous visuo-tactile stimulation, and the other half received asynchronous visuo-tactile stimulation, while their physical body was lying in a supine position, but the virtual body was standing. Additionally, the contribution of individual sensory weighting strategies was investigated via the Rod and Frame task (RFT), which was applied both before (pre-FBI standing and pre-FBI supine) and after the full-body illusion (post-FBI supine) protocol. Subjective reports of the participants confirmed previous findings suggesting that there was a significant increase in ownership over a virtual body during synchronous visuo-tactile stimulation. Additionally, further categorization of participants based on their visual dependency (by RFT) showed that those participants who rely more on visual information (visual field dependents) perceived the full-body illusion more strongly than non-visual field dependents during the synchronous visuo-tactile stimulation condition. Further analysis provided not only a quantitative demonstration of full-body illusion but also revealed changes in perceived self-orientation based on their field dependency. Altogether, findings of the current study make further contributions to our understanding of the vestibular system and brought new insight for individual sensory weighting strategies during a full-body illusion.

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.

Quantitative orientation preference and susceptibility to space motion sickness simulated in a virtual reality environment

Orientation preference should appear when variable weightings of spatial orientation cues are used between individuals. It is possible that astronauts' orientation preferences could be a potential predictor for susceptibility to space motion sickness (SMS). The present study was conducted to confirm this relationship on Earth by quantifying orientation preferences and simulating SMS in a virtual reality environment. Two tests were carried out. The first was to quantitatively determine one's orientation preference. Thirty-two participants' vision and body cue preferences were determined by measuring perceptual up (PU) orientations. The ratio of vision and body vector (ROVB) was used as the indicator of one's orientation preference. The second test was to visually induce motion sickness symptoms that represent similar sensory conflicts as SMS using a virtual reality environment. Relationships between ROVB values and motion sickness scores were analyzed, which revealed cubic functions by using optimal fits. According to ROVB level, participants were divided into three groups - body group, vision group, and confusion group - and the factor of gender was further considered as a covariate in the analysis. Consistent differences in motion sickness scores were observed between the three groups. Thus, orientation preference had a significant relationship with susceptibility to simulated SMS symptoms. This knowledge could assist with astronaut selection and might be a useful countermeasure when developing new preflight trainings.

How do visual and postural cues combine for self-tilt perception during slow pitch rotations?

Self-orientation perception relies on the integration of multiple sensory inputs which convey spatially-related visual and postural cues. In the present study, an experimental set-up was used to tilt the body and/or the visual scene to investigate how these postural and visual cues are integrated for self-tilt perception (the subjective sensation of being tilted). Participants were required to repeatedly rate a confidence level for self-tilt perception during slow (0.05°·s(-1)) body and/or visual scene pitch tilts up to 19° relative to vertical. Concurrently, subjects also had to perform arm reaching movements toward a body-fixed target at certain specific angles of tilt. While performance of a concurrent motor task did not influence the main perceptual task, self-tilt detection did vary according to the visuo-postural stimuli. Slow forward or backward tilts of the visual scene alone did not induce a marked sensation of self-tilt contrary to actual body tilt. However, combined body and visual scene tilt influenced self-tilt perception more strongly, although this effect was dependent on the direction of visual scene tilt: only a forward visual scene tilt combined with a forward body tilt facilitated self-tilt detection. In such a case, visual scene tilt did not seem to induce vection but rather may have produced a deviation of the perceived orientation of the longitudinal body axis in the forward direction, which may have lowered the self-tilt detection threshold during actual forward body tilt.

Roll aftereffects: influence of tilt and inter-stimulus interval

A theme in sensory perception is that exposure to a stimulus causes perception of subsequent stimuli to be shifted in the opposite direction. Such phenomenon is known as aftereffect and has been extensively described in the visual system as well as recently described for the vestibular system during translation. It is known from aviation studies that after a maneuver in roll, pilots can experience a false perception of roll in the opposite direction. The magnitude and duration of this effect as well as the potential influence of the gravity vector have not previously been defined. In the current paper this roll aftereffect (RAE) is examined in response to whole-body roll about an earth-horizontal axis in eight healthy human subjects. The peak velocity of a 0.5-s-duration roll was varied based on previous responses to find the point where subjects perceived no motion. Without a preceding stimulus, the starting position (upright, 9° left, or 9° right) did not influence roll perception. The RAE was measured in a completely dark room using an adapting (first interval) stimulus consisting of 9° of roll over 1.5 s (peak velocity, 12°/s), delivered 0.5, 3, or 6 s prior to test (second interval) stimulus. A significant RAE was seen in all subjects. Half a second after the adapting stimulus, a test stimulus had to be on average 1.5 ± 0.4°/s in the opposite direction to be perceived as stationary. When the subject remained upright after the adapting stimulus, the RAE diminished with time, although it remained significantly larger at 3 and 6 s when the subject remained tilted after the adapting stimulus. These data demonstrate that roll perception can be influenced by small preceding stimuli and tilt causes a persistence of the RAE.

The perception of verticality in lunar and Martian gravity conditions

Although the mechanisms of neural adaptation to weightlessness and re-adaptation to Earth-gravity have received a lot of attention since the first human space flight, there is as yet little knowledge about how spatial orientation is affected by partial gravity, such as lunar gravity of 0.16 g or Martian gravity of 0.38 g. Up to now twelve astronauts have spent a cumulated time of approximately 80 h on the lunar surface, but no psychophysical experiments were conducted to investigate their perception of verticality. We investigated how the subjective vertical (SV) was affected by reduced gravity levels during the first European Parabolic Flight Campaign of Partial Gravity. In normal and hypergravity, subjects accurately aligned their SV with the gravitational vertical. However, when gravity was below a certain threshold, subjects aligned their SV with their body longitudinal axis. The value of the threshold varied considerably between subjects, ranging from 0.03 to 0.57 g. Despite the small number of subjects, there was a significant positive correlation of the threshold with subject age, which calls for further investigation.

Subjective visual vertical in erect/supine subjects and under microgravity: effects of lower body negative pressure

Perception of the subjective visual vertical (SVV) is mainly based on the contributions from the visual, vestibular, and proprioceptive systems, and participates to the process of spatial orientation in relation to the surrounding environment and to the gravito-inertial force. The SVV can be significantly influenced by the presence of a displaced visual field, as in the case of the rod and frame test (RFT). A series of studies showed the effects of haematic mass shifts to and from the lower limbs on SVV, due to visceral mechanoreceptors (VM) located at the level of the kidneys and of the thorax. These sensors may be artificially activated with a lower body negative pressure (LBNP) device. In this study, the role of visual and VM cues to orientation perception have been evaluated using the RFT and the LBNP devices under a microgravity environment. A preliminary investigation was conducted in a sample of military pilots to develop a RFT protocol to be used in microgravity environments. This protocol was adopted to evaluate the contribution of VM to the SVV in a cosmonaut before, during and after a 10 day space flight, with and without concurrent activation of LBNP. The same test sequence, including LBNP exposure, was repeated a few months later on Earth on the same subject. As expected, the influence of the frame on rod positioning was statistically significant in all test conditions. During the in-flight experimental step, a substantial lack of significant changes compared to the pre-flight condition was observed. Moreover, substantially no effects due to LBNP were observed. A mild rod displacement from the body axis was detected under microgravity compared to the pre-flight recording. Such a finding was in part reduced during LBNP. The same findings were observed during the post-flight repetition of the experiment. Our results showed an absence in this subject of significant effects on the RFT due to microgravity. In conclusion, no effects from his VM on the RFT and minor changes in the SVV could be detected.

Influence of visual contextual cues on haptic discrimination of orientations in 5-month-old infants

The present research addressed the question of the influence of visual contextual cues on the manual discrimination of spatial orientations in 5-month-old infants. Different types of visual contextual cues were proposed during the haptic discrimination task: congruent-informative, non congruent-informative or noninformative. A familiarization (with a 60-s fixed-duration)/reaction to novelty procedure was used in three experiments. In Experiment 1, a congruent-informative visual context (a visual context composed of alternate black and white stripes tilted to 20 degrees -left) was present and a haptic discrimination between a 20 degrees -left rod and a 30 degrees -left rod was observed (this discrimination was absent without visual context in Gentaz and Streri's study, 2004). In Experiment 2, the visual context cues (composed of black dots) were noninformative, and infants could not discriminate these two oblique rods. In Experiment 3, the presence of a non congruent visual context (a visual context composed of alternate black and white stripes tilted to 20 degrees -left, as in Experiment 1) disturbed the gravitational vertical perception usually observed: infants could not discriminate the vertical rod from the 10 degrees -left rod. These results showed that only the informative (congruent and non congruent) visual contextual cues influenced the haptic discrimination of spatial orientations in 5-month-old infants. These results are discussed in relation with data observed in adults and with the current models of the multisensorial integration and attentional level.

Perceived self-orientation in allocentric and egocentric space: effects of visual and physical tilt on saccadic and tactile measures

Do physical tilt and tilt of the visual environment affect perception of allocentric and egocentric space? We addressed this question using two perceptual-motor tasks: alignment of a tactile rod (ROD) and saccadic eye movements (EM). Nine participants indicated the vertical axis of their heads (egocentric task), as well as the direction of gravity (allocentric task). Head orientation (+/-60 degrees and 0 degrees) and visual environment orientation (+/-120 degrees, +/-60 degrees and 0 degrees) were independently manipulated in the fronto-planar roll plane. ROD and EM estimates of both allocentric and egocentric reference directions varied with head and room orientation. Physical tilt dominated allocentric estimates in the dark where overestimates of physical tilt were noted up to 11 degrees using both measures. Allocentric ROD and EM estimates were significantly correlated across all head orientations (r=.70, p<.01) but only when upright for egocentric estimates (r=.38, p<.01). The relative contributions of the visual environment, gravity's direction and long-body axis to the estimation of allocentric and egocentric directions were determined by vector modeling. This modeling found that vision determined about 14% of the allocentric ROD and EM estimates, that the long-axis body reference played no discernible role, and that the largest factor was gravity, the effective direction of which was non-veridical. For egocentric estimates, vision contributed about 3% with the largest factor being the body reference. We conclude that perception of allocentric and egocentric space is likely influenced by multiple senses that define common egocentric and allocentric frames of reference accessible for saccadic and tactile estimates of perceived self-orientation.

The haptic perception of spatial orientations

This review examines the isotropy of the perception of spatial orientations in the haptic system. It shows the existence of an oblique effect (i.e., a better perception of vertical and horizontal orientations than oblique orientations) in a spatial plane intrinsic to the haptic system, determined by the gravitational cues and the cognitive resources and defined in a subjective frame of reference. Similar results are observed from infancy to adulthood. In 3D space, the haptic processing of orientations is also anisotropic and seems to use both egocentric and allocentric cues. Taken together, these results revealed that the haptic oblique effect occurs when the sensory motor traces associated with exploratory movement are represented more abstractly at a cognitive level.

Body-tilt and visual verticality perception during multiple cycles of roll rotation

To assess the effects of degrading canal cues for dynamic spatial orientation in human observers, we tested how judgments about visual-line orientation in space (subjective visual vertical task, SVV) and estimates of instantaneous body tilt (subjective body-tilt task, SBT) develop in the course of three cycles of constant-velocity roll rotation. These abilities were tested across the entire tilt range in separate experiments. For comparison, we also obtained SVV data during static roll tilt. We found that as tilt increased, dynamic SVV responses became strongly biased toward the head pole of the body axis (A-effect), as if body tilt was underestimated. However, on entering the range of near-inverse tilts, SVV responses adopted a bimodal pattern, alternating between A-effects (biased toward head-pole) and E-effects (biased toward feet-pole). Apart from an onset effect, this tilt-dependent pattern of systematic SVV errors repeated itself in subsequent rotation cycles with little sign of worsening performance. Static SVV responses were qualitatively similar and consistent with previous reports but showed smaller A-effects. By contrast, dynamic SBT errors were small and unimodal, indicating that errors in visual-verticality estimates were not caused by errors in body-tilt estimation. We discuss these results in terms of predictions from a canal-otolith interaction model extended with a leaky integrator and an egocentric bias mechanism. We conclude that the egocentric-bias mechanism becomes more manifest during constant velocity roll-rotation and that perceptual errors due to incorrect disambiguation of the otolith signal are small despite the decay of canal signals.

The effect of visuo-haptic congruency on haptic spatial matching

Eye-hand coordination is crucial for everyday visuo-haptic object-manipulation. Noninformative vision has been reported to improve haptic spatial tasks relying on world-based reference frames. The current study investigated whether the degree of visuo-haptic congruity systematically affects haptic task performance. Congruent and parametrically varied incongruent visual orientation cues were presented while participants manually explored the orientation of a reference bar stimulus. Participants were asked to haptically match this reference orientation by turning a test bar either to a parallel or mirrored orientation, depending on the instruction. While parallel matching can only be performed correctly in a world-based frame, mirror matching (in the mid-sagittal plane) can also be achieved in a body-centered frame. We revealed that visuo-haptic incongruence affected parallel but not mirror matching responses in size and direction. Parallel matching did not improve when congruent visual orientation cues were provided throughout a run, and mirror matching even deteriorated. These results show that there is no positive effect of visual input on haptic performance per se. Tasks, which favor a body-centered frame are immune to incongruent visual input, while such input parametrically modulates performance on world-based haptic tasks.

Verticality perception during off-vertical axis rotation

During prolonged rotation about a tilted yaw axis, often referred to as off-vertical axis rotation (OVAR), a percept of being translated along a conical path slowly emerges as the sense of rotation subsides. Recently, we found that these perceptual changes are consistent with a canal-otolith interaction model that attributes the illusory translation percept to improper interpretation of the ambiguous otolith signals. The model further predicts that the illusory translation percept must be accompanied by slowly worsening tilt underestimates. Here, we tested this prediction in six subjects by measuring the time course of the subjective visual vertical (SVV) during OVAR stimulation at three different tilt-rotation speed combinations, in complete darkness. Throughout the 2-min run, at each left-ear-down and right-ear-down position, the subject indicated whether a briefly flashed line deviated clockwise or counterclockwise from vertical to determine the SVV with an adaptive staircase procedure. Typically, SVV errors indicating tilt underestimation were already present at rotation onset and then increased exponentially to an asymptotic value, reached at about 60 s after rotation onset. The initial error in the SVV was highly correlated to the response error in a static tilt control experiment. The subsequent increase in error depended on both rotation speed and OVAR tilt angle, in a manner predicted by the canal-otolith interaction model. We conclude that verticality misjudgments during OVAR reflect a dynamic component linked to canal-otolith interaction, superimposed on a tilt-related component that is also expressed under stationary conditions.

Subjective somatosensory vertical during dynamic tilt is dependent on task, inertial condition, and multisensory concordance

To investigate how visual and vestibular cues are integrated for the perception of gravity during passive self-motion, we measured the ability to maintain a handheld object vertical relative to gravity without visual feedback during sinusoidal roll-tilt stimulation. Visual input, either concordant or discordant with actual dynamic roll-tilt, was delivered by a head-mounted display showing the laboratory. The four visual conditions were darkness, visual-vestibular concordance, stationary visual scene, and a visual scene 180 degrees phase-shifted relative to actual tilt. Tilt-indication performance using a solid, cylindrical joystick was better in the presence of concordant visual input relative to the other visual conditions. In addition, we compared performance when indicating the vertical by the joystick or a full glass of water. Subjects indicated the direction of gravity significantly better when holding the full glass of water than the joystick. Matching the inertial characteristics, including fluid properties, of the handheld object to the glass of water did not improve performance. There was no effect of visual input on tilt performance when using the glass of water to indicate gravitational vertical. The gain of object tilt motion did not change with roll-tilt amplitude and frequency (+/-7.5 degrees at 0.25 Hz, +/-10 degrees at 0.16 Hz, and +/-20 degrees at 0.08 Hz), however, the phase of object tilt relative to subject tilt showed significant phase-leads at the highest frequency tested (0.25 Hz). Comparison of the object and visual effects observed suggest that the task-dependent behavior change may be due to an attentional shift and/or shift in strategy.

The plasticity of gravitational reference frame and the subjective vertical: peripheral visual information affects the oblique effect

The experiment examined the human visual perception of orientations and the nature of reference frame in which the oblique effect (lower performance in oblique orientations than in vertical or horizontal ones) was defined. Previous research [M. Luyat, E. Gentaz, Body tilt effect on the reproduction of orientations: studies on the visual oblique effect and subjective orientations, J. Exp. Psychol. Hum. Percept. Perform. 28 (2002) 1002-1011. M. Luyat, E. Gentaz, T.R. Corte, M. Guerraz, Reference frames and haptic perception of orientation: body and head tilt effects on the oblique effect, Percept. Psychophys. 63 (2001) 541-554], using head tilt paradigm to uncouple the gravitational, egocentric and subjective reference frames, showed that the oblique effect was mapped in a subjective gravitational reference frame with the subjective vertical as a cardinal orientation. However, the subjective vertical is not only affected by the tilt of head but also by the tilt of visual context. Then, the tilt of visual oriented cues is another paradigm permitting to evidence the role of the subjective gravitational reference frame. Sixteen participants were asked to reproduce five different orientations of a luminous line: horizontal (0 degrees ), 45 degrees (oblique), 90 degrees (vertical), 135 degrees (oblique) and the subjective vertical. These orientations were reproduced with no visual contextual cues and with tilted visual contextual cues tilted 15 degrees either to the left or to the right. The results showed that the oblique effect decreased with tilted visual contexts but was not completely suppressed. These results proved that this oblique effect is defined in a multimodal reference frame which integrates not only vestibular and proprioceptive cues but also peripheral visual information.

The role of contextual cues in the haptic perception of orientations and the oblique effect

Blindfolded right-handed participants were asked to position, with the right hand, a frontoparallel rod to one of three orientations: vertical (0 degrees) and left 45 degrees and right 45 degrees obliques. Simultaneously, three different backgrounds were explored with the left hand: smooth, congruent stripes (parallel to the orientation to be produced), or incongruent stripes (tilted relative to the orientation to be produced). The analysis of variable errors showed that the oblique effect (higher precision for the vertical orientation than for the oblique orientations) was weakened in the presence of contextual cues, because of an improvement in oblique precision. Moreover, the analysis of constant errors revealed that the perception of orientations erred in the direction of the stripes, similar to the effect that has been found with vision, where visual contextual cues (tilted frame or lines) divert the perception of the vertical. These results are discussed in relation to a patterncentric frame of reference hypothesis or as a congruency effect.