Difference in the perception of the horizon during true and simulated tilt in the absence of semicircular canal cues

Flying enhances viewing from above bias on ambiguous visual stimuli

The human spatial orientation system is well designed on the ground but is imperfect in the aeronautical three-dimensional (3D) environment. However, human perception systems perform Bayesian statistics based on encountered environments and form shortcuts to improve perceptual efficiency. It is unknown whether our perception of spatial orientation is modified by flying experience and forms perceptual biases. The current study tested pilot perceptual biases on ambiguous visual stimuli, the bistable point-light walkers, and found that flying experiences increased the pilot's tendency to perceive himself as higher than the target and the target as farther away from them. Such perceptual effects due to flight are likely to be attributed to experience of variable vestibular state in a higher position in 3D space, rather than the experience of a higher viewpoint. Our findings suggest that flying experience will modifies our visual perceptual biases, and that more attention should be paid to the enhanced viewing from above bias when flying to avoid overestimating altitude or viewing angle when the visual conditions are ambiguous.

Dynamic arm movements attenuate the perceptual distortion of visual vertical induced during prolonged whole-body tilt

Concurrent body movements have been shown to enhance the accuracy of spatial judgment, but it remains unclear whether they also contribute to perceptual estimates of gravitational space not involving body movements. To address this, we evaluated the effects of static or dynamic arm movements during prolonged whole-body tilt on the subsequent perceptual estimates of visual or postural vertical. In Experiment 1, participants were asked to continuously perform static or dynamic arm movements during prolonged tilt, and we assessed their effects on the prolonged tilt-induced shifts of subjective visual vertical (SVV) at a tilted position (during-tilt session) or near upright (post-tilt session). In Experiment 2, we evaluated how static or dynamic arm movements during prolonged tilt subsequently affected the subjective postural vertical (SPV). In Experiment 1, we observed that the SVV was significantly shifted toward the direction of prolonged tilt in both sessions. The SVV shifts decreased when performing dynamic arm movements in the during-tilt session, but not in the post-tilt session. In Experiment 2, as well as SVV, the SPV was shifted toward the direction of prolonged tilt, but it was not significantly attenuated by the performance of static or dynamic arm movements. The results of the during-tilt session suggest that the central nervous system utilizes additional information generated by dynamic body movements for perceptual estimates of visual vertical.

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.

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.

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.

Otolith signals contribute to inter-individual differences in the perception of gravity-centered space

The aim of the present study was to investigate (1) the relative contribution of the egocentric reference as well as body orientation perception to visual horizon percept during tilt or during increased gravito-inertial acceleration (GiA, hypergravity environment) conditions and (2) the role of vestibular signals in the inter-individual differences observed in these perceptual modalities. Perceptual estimates analysis showed that backward tilt induced (1) an elevation of the visual horizon, (2) an elevation of the egocentric estimation (visual straight ahead) and (3) an overestimation of body tilt. The increase in the magnitude of GiA induced (1) a lowering of the apparent horizon, (2) a lowering of the straight ahead and (3) a perception of backward tilt. Overall, visual horizon percept can be expressed as the combination of body orientation perception and egocentric estimation. When assessing otolith reactivity using off-vertical axis rotation (OVAR), only visual egocentric estimation was significantly correlated with horizontal OVAR performance. On the one hand, we found a correlation between a low modulation amplitude of the otolith responses and straight ahead accuracy when the head axis was tilted relative to gravity. On the other hand, the bias of otolith responses was significantly correlated with straight ahead accuracy when subjects were submitted to an increase in the GiA. Thus, straight ahead sense would be dependent to some extent to otolith function. These results are discussed in terms of the contribution of otolith inputs in the overall multimodal integration subtending spatial constancy.

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.

Influence of multisensory graviceptive information on the apparent zenith

We studied the contribution of vestibular and somatosensory/proprioceptive stimulation to the perception of the apparent zenith (AZ). Experiment 1 involved rotation on a centrifuge and settings of the AZ. Subjects were supine on the centrifuge, and their body position was varied in relation to the rotation axis so that the gravitoinertial resultant force at the otoliths was 1 or 1.2 g with the otolith organs positioned 50 or 100 cm from the axis of rotation. Their legs were also positioned in different configurations, flexed and elevated or extended, to create different distributions of blood and lymph. Experiment 2 involved (a) settings of the AZ for subjects positioned supine with legs fully extended or legs flexed and elevated to create a torsoward shift of blood and (b) settings of the subjective visual vertical for subjects horizontally positioned on their sides with legs extended or bent. Experiment 3 had subjects in the same body configurations as in Experiment 2 indicate when they were horizontal as they were rotated in pitch or roll about an inter-aural or naso-occipital axis. The experimental results for all three experiments demonstrated that both visual localization and apparent body horizontal are jointly determined by multimodal combinations of otolithic and somatosensory/proprioceptive stimulation. No evidence was found for non-overlapping or exclusive mechanisms determining one or the other. The subjective postural horizontal and AZ were affected in similar ways by comparable manipulations.

Kinaesthetic and visual perceptions of orientations

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

Perception of tilt and ocular torsion of vestibular patients during eccentric rotation

Four patients following unilateral vestibular loss and four patients complaining of otolith-dependent vertigo were tested during eccentric yaw rotation generating 1 x g centripetal acceleration directed along the interaural axis. Perception of body tilt in roll and in pitch was recorded in darkness using a somatosensory plate that the subjects maintained parallel to the perceived horizon. Ocular torsion was recorded by a video camera. Unilateral vestibular-defective patients underestimated the magnitude of the roll tilt and had a smaller torsion when the centrifugal force was towards the operated ear compared to the intact ear and healthy subjects. Patients with otolithic-dependent vertigo overestimated the magnitude of roll tilt in both directions of eccentric rotation relative to healthy subjects, and their ocular torsion was smaller than in healthy subjects. Eccentric rotation is a promising tool for the evaluation of vestibular dysfunction in patients. Eye torsion and perception of tilt during this stimulation are objective and subjective measurements, which could be used to determine alterations in spatial processing in the CNS.