Influence of perceived egocentric coordinates on the subjective visual vertical

The majority of previous studies which have explored the mechanisms underlying perception of the direction of gravity in static roll tilt have proposed that the tendency to estimate the subjective visual vertical (SVV) as tilted towards body tilt (‘Aubert effect’) arises from an underestimation of perceived body tilt. The present study has evaluated an alternative assumption that erroneous estimates of verticality may be related to the ability to estimate the orientation of external objects with respect to the observer's perceived body Z-axis. Experiments showed that Aubert effects and the overestimations of 30–90° angles from the body Z-axis in the roll plane were both related to errors made in adjusting a visual rod parallel to the body Z-axis. The results suggest that errors in providing visual estimates of the observer's own body Z-axis reference are implicated in Aubert effect.     

Perceived versus actual head-on-trunk orientation during arm movement control

Static roll head tilt induces bias in the trajectory of upper limb voluntary movements. The aim of the experiment was to investigate whether this bias is dependant on the perception of body configuration rather than on its actual configuration. We used the ‘return’ phenomenon as a method to produce dissociation between perceived and actual head tilt. Static roll head tilt in supine subjects was sustained for 15 min during which subjects were periodically required to estimate verbally the tilt of their head respective to their trunk and draw, with their right index finger, straight lines aligned with their trunk. After 15 min, subjects’ head were realigned with the trunk, and subjects continued to give verbal estimate of head position and perform the motor task. Results showed that the initial angular deviation of the lines in the direction opposite to head tilt gradually diminished. The adaptation was noticeable within the first 3–5 min of tilt and subsequently diminished. Verbal estimates confirmed the return phenomenon, i.e. subjects perceived their head as slowly returning towards its neutral position after a few minutes of sustained tilt. When realigned with the trunk, subjects experienced the illusion that their head was tilted in the opposite direction to the initial head tilt and a line deviation in the opposite direction to those made on initial exposure was observed (after-effect). These results indicate that the angular deviation in motor production observed in condition of static head tilt were largely related to the perceived body configuration and therefore favour the hypothesis that the conscious perception of body configuration plays a key role in organising sensorimotor tasks.     

Spatial coding of eye movements relative to perceived earth and head orientations during static roll tilt

This purpose of this study was to examine the spatial coding of eye movements during static roll tilt (up to ±45°) relative to perceived earth and head orientations. Binocular videographic recordings obtained in darkness from eight subjects allowed us to quantify the mean deviations in gaze trajectories along both horizontal and vertical coordinates relative to the true earth and head orientations. We found that both variability and curvature of gaze trajectories increased with roll tilt. The trajectories of eye movements made along the perceived earth-horizontal (PEH) were more accurate than movements along the perceived head-horizontal (PHH). The trajectories of both PEH and PHH saccades tended to deviate in the same direction as the head tilt. The deviations in gaze trajectories along the perceived earth-vertical (PEV) and perceived head-vertical (PHV) were both similar to the PHH orientation, except that saccades along the PEV deviated in the opposite direction relative to the head tilt. The magnitude of deviations along the PEV, PHH, and PHV corresponded to perceptual overestimations of roll tilt obtained from verbal reports. Both PEV gaze trajectories and perceptual estimates of tilt orientation were different following clockwise rather than counterclockwise tilt rotation; however, the PEH gaze trajectories were less affected by the direction of tilt rotation. Our results suggest that errors in gaze trajectories along PEV and perceived head orientations increase during roll tilt in a similar way to perceptual errors of tilt orientation. Although PEH and PEV gaze trajectories became nonorthogonal during roll tilt, we conclude that the spatial coding of eye movements during roll tilt is overall more accurate for the perceived earth reference frame than for the perceived head reference frame. Received: 22 April 1997 / Accepted: 18 December 1997     

Vertical (Z-axis) acceleration alters the ocular response to linear acceleration in the rabbit

Whether ocular orientation to gravity is produced solely by linear acceleration in the horizontal plane of the head or depends on both horizontal and vertical components of the acceleration of gravity is controversial. Here, we compared orienting eye movements of rabbits during head tilt to those produced by centrifugation that generated centripetal acceleration along the naso-occipital (X-), bitemporal (Y-) and vertical (Z-) axes in a constant gravitational field. Sensitivities of ocular counter-pitch and vergence during pitch tilts were ≈25°/g and ≈26°/g, respectively, and of ocular counter-roll during roll tilts was ≈20°/g. During X-axis centripetal acceleration with 1 g of gravity along the Z-axis, pitch and vergence sensitivities were reduced to ≈13°/g and ≈16°/g. Similarly, Y-axis acceleration with 1g along the Z-axis reduced the roll sensitivity to ≈16°/g. Modulation of Z-axis centripetal acceleration caused sensitivities to drop by ≈6°/g in pitch, ≈2°/g in vergence, and ≈5°/g in roll. Thus, the constant 1g acceleration along the Z-axis reduced the sensitivity of ocular orientation to linear accelerations in the horizontal plane. Orienting responses were also modulated by varying the head Z-axis acceleration; the sensitivity of response to Z-axis acceleration was linearly related to the response to static tilt. Although the sign of the Z-axis modulation is opposite in the lateral-eyed rabbit from that in frontal-eyed species, these data provide evidence that the brain uses both the horizontal and the vertical components of acceleration from the otolith organs to determine the magnitude of ocular orientation in response to linear acceleration.

Percept-related changes in horizontal optokinetic nystagmus at different body orientations in space

Large-field motion of the visual environment is a powerful stimulus to induce the perception of contra-directional self-motion in a stationary observer. We investigated the interrelations between horizontal optokinetic nystagmus and subjective states of motion perception under variation of subjects' orientation with respect to gravity. Subjects were tested sitting upright and lying supine, and signalled transitions between object- and self-motion perception whilst viewing an optokinetic stimulus rotating about the subjects' longitudinal axis at a range of angular velocities. Optokinetic stimulation in the supine condition resulted in subjects perceiving a graviceptive conflict and the illusory perception of whole body tilt in a direction opposite to optokinetic stimulus rotation, whereas during upright viewing the axis of stimulus rotation was aligned with the direction of gravity and thus did not result in a conflict or perception of tilt. In both postures, self-motion perception coincided with an increased deviation of mean horizontal gaze position in the perceived direction of heading with a concurrent reduction in optokinetic nystagmus slow-phase gain. Slow-phase gain was also significantly reduced in the supine position as well as at increasing stimulus velocities. The results demonstrate that spontaneous transitions between the perception of object-motion and that of self-motion consistently coincide with spatial attentional and orientational strategies, shifting from passive monitoring to active oculomotor exploration and anticipation.     

Content-Based Estimation of Brain MRI Tilt in Three Orthogonal Directions

In a general scenario, the brain images acquired from magnetic resonance imaging (MRI) may experience tilt, distorting brain MR images. The tilt experienced by the brain MR images may result in misalignment during image registration for medical applications. Manually correcting (or estimating) the tilt on a large scale is time-consuming, expensive, and needs brain anatomy expertise. Thus, there is a need for an automatic way of performing tilt correction in three orthogonal directions (X, Y, Z). The proposed work aims to correct the tilt automatically by measuring the pitch angle, yaw angle, and roll angle in X-axis, Z-axis, and Y-axis, respectively. For correction of the tilt around the Z-axis (pointing to the superior direction), image processing techniques, principal component analysis, and similarity measures are used. Also, for correction of the tilt around the X-axis (pointing to the right direction), morphological operations, and tilt correction around the Y-axis (pointing to the anterior direction), orthogonal regression is used. The proposed approach was applied to adjust the tilt observed in the T1- and T2-weighted MR images. The simulation study with the proposed algorithm yielded an error of 0.40 ± 0.09°, and it outperformed the other existing studies. The tilt angle (in degrees) obtained is ranged from 6.2 ± 3.94, 2.35 ± 2.61, and 5 ± 4.36 in X-, Z-, and Y-directions, respectively, by using the proposed algorithm. The proposed work corrects the tilt more accurately and robustly when compared with existing studies.

     

Pitch body orientation influences the perception of self-motion direction induced by optic flow

We studied the effect of static pitch body tilts on the perception of self-motion direction induced by a visual stimulus. Subjects were seated in front of a screen on which was projected a 3D cluster of moving dots visually simulating a forward motion of the observer with upward or downward directional biases (relative to a true earth horizontal direction). The subjects were tilted at various angles relative to gravity and were asked to estimate the direction of the perceived motion (nose-up, as during take-off or nose-down, as during landing). The data showed that body orientation proportionally affected the amount of error in the reported perceived direction (by 40% of body tilt magnitude in a range of ±20°) and these errors were systematically recorded in the direction of body tilt. As a consequence, a same visual stimulus was differently interpreted depending on body orientation. While the subjects were required to perform the task in a geocentric reference frame (i.e., relative to a gravity-related direction), they were obviously influenced by egocentric references. These results suggest that the perception of self-motion is not elaborated within an exclusive reference frame (either egocentric or geocentric) but rather results from the combined influence of both.     

Perception of longitudinal body axis in microgravity during parabolic flight

It has been proposed that an internal representation of body vertical has a prominent role in spatial orientation. This investigation investigated the ability of human subjects to accurately locate their longitudinal body axis (an imaginary straight body midline running from head to toes) while free-floating in microgravity. Subjects were tested in-flight, as well as on ground in normal gravity in both the upright and supine orientations to provide baseline measurements. The subjects wore a goggle device and were in total darkness. They used knobs to rotate two luminous lines until they were parallel to the subjective direction of their longitudinal body axis, in the roll (right/left) and the pitch (forward/backward) planes. Results showed that the error between the perceived and the objective direction of the longitudinal body axis was significantly larger in microgravity than in normal gravity. This error in this egocentric frame of reference is presumably due to the absence of somatosensory cues when free-floating. Mechanical pressure on the chest using an airbag reduced the error in perception of the longitudinal body axis in microgravity, thus improving spatial orientation.     

Influence of gravitoinertial force level on the subjective vertical during recumbent yaw axis body tilt

We tilted recumbent subjects at various angles about their yaw (foot to head) axis and had them indicate the direction of their subjective vertical and apparent head midline about the same axis. One set of tests was conducted during parabolic flight maneuvers where the background gravitoinertial acceleration varied from 0 to 1.8g. The blindfolded subjects (n = 6) were tested supine and at tilts of 60° and 30° left and right about their horizontal long body axis. They used a gravity neutral joystick to indicate their subjective vertical or their head midline continuously from the high force through the 0g portions of parabolas. In 0g, all subjects felt supine and oriented the joystick perpendicular to their body when indicating the subjective vertical. This points to strong influences of the symmetric somatic touch and pressure cues from the apparatus on orientation when the otolith organs are unloaded. In contrast to the settings in 0g, settings of the subjective vertical in 1g and 1.8g varied as a function of body orientation. However, the settings did not differ between 1g and 1.8g test conditions. Subjective vertical judgments were also made by subjects (n = 11) in the Brandeis slow rotation room, with the room stationary and rotating at a speed that produced a 2g resultant of gravitational and centrifugal acceleration. There were no differences between settings of the subjective vertical made in 1g and 2g. The similarity of 1g and hyper-g settings during recumbent yaw tilts, both in parabolic flight and in the rotating room, contrasts with the previously observed, strong influence which force levels above 1g have on settings of the subjective vertical during tilt of the body in pitch or roll. The findings for all three axes are consistent with a recently developed model of static spatial orientation.     

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.     

Assessment of the perception of verticality and horizontality with self-paced saccades

We investigated the ability of human subjects (Ss) to make self-paced saccades in the earth-vertical and horizontal directions (space-referenced task) and in the direction of the head-vertical and horizontal axis (self-referenced task) during whole body tilts of 0°, 22.5°, 45° and 90° in the frontal (roll) plane. Saccades were recorded in the dark with computerised video-oculography. During space-referenced tasks, the saccade vectors did not fully counter-rotate to compensate for larger angles of body tilt. This finding is in agreement with the ’A’ effect reported for the visual vertical. The error was significantly larger for saccades intended to be space-horizontal than space-vertical. This vertico-horizontal dissociation implies greater difficulty in defining horizontality than verticality with the non-visual motor task employed. In contrast, normal Ss (and an alabyrinthine subject tested) were accurate in orienting saccades to their own (cranio-centric) vertical and horizontal axes regardless of tilt indicating that cranio-centric perception is robust and apparently not affected by gravitational influences. Received: 9 October 1996 / Accepted: 23 January 1998     

Roles of gravitational cues and efference copy signals in the rotational updating of memory saccades

Primates are able to localize a briefly flashed target despite intervening movements of the eyes, head, or body. This ability, often referred to as updating, requires extraretinal signals related to the intervening movement. With active roll rotations of the head from an upright position it has been shown that the updating mechanism is 3-dimensional, robust, and geometrically sophisticated. Here we examine whether such a rotational updating mechanism operates during passive motion both with and without inertial cues about head/body position in space. Subjects were rotated from either an upright or supine position, about a nasal-occipital axis, briefly shown a world-fixed target, rotated back to their original position, and then asked to saccade to the remembered target location. Using this paradigm, we tested subjects' abilities to update from various tilt angles (0, +/-30, +/-45, +/-90 degrees), to 8 target directions and 2 target eccentricities. In the upright condition, subjects accurately updated the remembered locations from all tilt angles independent of target direction or eccentricity. Slopes of directional errors versus tilt angle ranged from -0.011 to 0.15, and were significantly different from a slope of 1 (no compensation for head-in-space roll) and a slope of 0.9 (no compensation for eye-in-space roll). Because the eyes, head, and body were fixed throughout these passive movements, subjects could not use efference copies or neck proprioceptive cues to assess the amount of tilt, suggesting that vestibular signals and/or body proprioceptive cues suffice for updating. In the supine condition, where gravitational signals could not contribute, slopes ranged from 0.60 to 0.82, indicating poor updating performance. Thus information specifying the body's orientation relative to gravity is critical for maintaining spatial constancy and for distinguishing body-fixed versus world-fixed reference frames.     

Tactile recalibration of auditory spatial representations

In the well-known spatial ventriloquism effect, auditory stimuli are mislocalized towards the location of synchronous but spatially disparate visual stimuli. Recent studies have demonstrated a similar influence of tactile stimuli on auditory localization, which predominantly operates in an external coordinate system. Here, we investigated whether this audio-tactile ventriloquist illusion leads to comparable aftereffects in the perception of auditory space as have been observed previously for audiovisual stimulation. Participants performed a relative sound localization task in which they had to judge whether a brief sound was perceived at the same or a different location as a preceding tactile stimulus (“Experiment 1”) or to the left or right of a preceding visual stimulus (“Experiment 2”). Sound localization ability was measured before and after exposure to synchronous audio-tactile stimuli with a constant spatial disparity. After audio-tactile adaptation, unimodal sound localization was shifted in the direction of the tactile stimuli during the preceding adaptation phase in both tasks. This finding provides evidence for the existence of an audio-tactile ventriloquism aftereffect and suggests that auditory space (rather than specific audio-tactile connections) can be rapidly recalibrated to compensate for audio-tactile spatial disparities.     

Antihysteresis of perceived longitudinal body axis during continuous quasi-static whole-body rotation in the earth-vertical roll plane

Estimation of subjective whole-body tilt in stationary roll positions after rapid rotations shows hysteresis. We asked whether this phenomenon is also present during continuous quasi-static whole-body rotation and whether gravitational cues are a major contributing factor. Using a motorized turntable, 8 healthy subjects were rotated continuously about the earth-horizontal naso-occipital axis (earth-vertical roll plane) and the earth-vertical naso-occipital axis (earth-horizontal roll plane). In both planes, three full constant velocity rotations (2°/s) were completed in clockwise and counterclockwise directions (acceleration = 0.05°/s², velocity plateau reached after 40 s). Subjects adjusted a visual line along the perceived longitudinal body axis (pLBA) every 2 s. pLBA deviation from the longitudinal body axis was plotted as a function of whole-body roll position, and a sine function was fitted. At identical whole-body earth-vertical roll plane positions, pLBA differed depending on whether the position was reached by a rotation from upright or by passing through upside down. After the first 360° rotation, pLBA at upright whole-body position deviated significantly in the direction of rotation relative to pLBA prior to rotation initiation. This deviation remained unchanged after subsequent full rotations. In contrast, earth-horizontal roll plane rotations resulted in similar pLBA before and after each rotation cycle. We conclude that the deviation of pLBA in the direction of rotation during quasi-static earth-vertical roll plane rotations reflects static antihysteresis and might be a consequence of the known static hysteresis of ocular counterroll: a visual line that is perceived that earth-vertical is expected to be antihysteretic, if ocular torsion is hysteretic.     

Effects of neck muscles vibration on the perception of the head and trunk midline position

The present study focused on the influence of neck vibration on the perception of the head and trunk midline position (orientation and localization). The orientation of the head and trunk was investigated by the rolling adjustment of a rod on their midline while their localization was investigated by the adjustment of the position of a visual dot as being straight-ahead the eyes or the sternum. The first experiment investigated whether a head–trunk dissociation was induced by the unilateral vibration of neck muscles in upright and restrained subjects. Results showed that the subjective orientation and localization of whole-body midline were shifted toward the vibrated side. The second experiment determined the effect of the neck muscles vibration when the subjects were lying on their side. The effect of vibration disappeared when the side of vibration was opposed to the side of postural inclination and it was stronger than in the upright position when the side of vibration and the side of postural inclination were congruent. Whereas, results suggested that the input from neck muscle proprioceptors participates directly to the elaboration of the egocentric space, the question may be raised as to how the sensory cues interacted in their contribution to the neural generation of an egocentric, body centred coordinate system.     

Localization of the subjective vertical during roll, pitch, and recumbent yaw body tilt

Localization of the subjective vertical during body tilt in pitch and in roll has been extensively studied because of the relevance of these axes for aviation and control of posture. Studies of yaw orientation relative to gravity are lacking. Our goal was to perform the first thorough evaluation of static orientation in recumbent yaw and to collect as efficiently as possible roll and pitch orientation data which would be consistent with the literature, using the same technique as our yaw tests. This would create the first comprehensive, coherent data set for all three axes suitable for quantitative tri-dimensional modeling of spatial orientation. We tested localization of the vertical for subjects tilted in pitch (-100 degrees to +130 degrees ), in roll (-90 degrees to +90 degrees ), and in yaw while recumbent (-80 degrees to +80 degrees ). We had subjects point a gravity-neutral probe to the gravitational vertical (haptically indicated vertical) and report verbally their perceived tilt. Subjects underestimated their body tilts in recumbent yaw and pitch and overestimated their tilts in roll. The haptic settings for pitch and roll were consistent with data in the literature obtained with haptic and visual indications. Our data constitute the first tri-dimensional assessment of the subjective vertical using a common measurement procedure and provide the basis for the tri-axial modeling of vestibular function presented in the companion paper.     

Spatial orientation of optokinetic nystagmus and ocular pursuit during orbital space flight

On Earth, eye velocity of horizontal optokinetic nystagmus (OKN) orients to gravito-inertial acceleration (GIA), the sum of linear accelerations acting on the head and body. We determined whether adaptation to microgravity altered this orientation and whether ocular pursuit exhibited similar properties. Eye movements of four astronauts were recorded with three-dimensional video-oculography. Optokinetic stimuli were stripes moving horizontally, vertically, and obliquely at 30°/s. Ocular pursuit was produced by a spot moving horizontally or vertically at 20°/s. Subjects were either stationary or were centrifuged during OKN with 1 or 0.5 g of interaural or dorsoventral centripetal linear acceleration. Average eye position during OKN (the beating field) moved into the quick-phase direction by 10° during lateral and upward field movement in all conditions. The beating field did not shift up during downward OKN on Earth, but there was a strong upward movement of the beating field (9°) during downward OKN in the absence of gravity; this likely represents an adaptation to the lack of a vertical 1-g bias in-flight. The horizontal OKN velocity axis tilted 9° in the roll plane toward the GIA during interaural centrifugation, both on Earth and in space. During oblique OKN, the velocity vector tilted towards the GIA in the roll plane when there was a disparity between the direction of stripe motion and the GIA, but not when the two were aligned. In contrast, dorsoventral acceleration tilted the horizontal OKN velocity vector 6° in pitch away from the GIA. Roll tilts of the horizontal OKN velocity vector toward the GIA during interaural centrifugation are consistent with the orientation properties of velocity storage, but pitch tilts away from the GIA when centrifuged while supine are not. We speculate that visual suppression during OKN may have caused the velocity vector to tilt away from the GIA during dorsoventral centrifugation. Vertical OKN and ocular pursuit did not exhibit orientation toward the GIA in any condition. Static full-body roll tilts and centrifugation generating an equivalent interaural acceleration produced the same tilts in the horizontal OKN velocity before and after flight. Thus, the magnitude of tilt in OKN velocity was dependent on the magnitude of interaural linear acceleration, rather than the tilt of the GIA with regard to the head. These results favor a filter model of spatial orientation in which orienting eye movements are proportional to the magnitude of low frequency interaural linear acceleration, rather than models that postulate an internal representation of gravity as the basis for spatial orientation.Abbreviations A_g Acceleration of gravity - A_c Centripetal acceleration - CCW Counterclockwise - CW Clockwise - FD- X Flight day X - g Gravity - GIA Gravito-inertial acceleration - H Horizontal - LED Left-ear-down - LEO Left-ear-out - LOB Lying-on-back - L- X Launch minus X days - NCM No-chair-motion - ND Nose-down - NU Nose-up - OCR Ocular counter-colling - OKAN Optokinetic after-nystagmus - OKN Optokinetic nystagmus - OKS Optokinetic stimulus - pos Position - REO Right-ear-out - R+ X Recovery plus X days - T Torsional - V Vertical - vel Velocity     

Visually-induced tilt during parabolic flights

Summary A helmet-mounted visual display system was used to study visually induced sensations of self-motion (vection) about the roll, pitch and yaw axes under normal gravity condition (1g) and during the microgravity and hypergravity phases of parabolic flights aboard the NASA KC-135 aircraft. Under each gravity condition, the following parameters were investigated: (1) the subject's perceived body vertical with eyes closed and with eyes open gazing at a stationary random dot display; (2) the magnitude of sensations of body tilt with respect to the subjective vertical, while the subject viewed displays rotating about the roll, pitch and yaw axes; (3) the magnitude of vection; (4) latency of vection. All eleven subjects perceived a definite up and down orientation throughout the course of the flight. During the microgravity phase, the average magnitudes of perceived body tilt and self-motion increased significantly, and there was no significant difference in vection latency. These results show that there is a rapid onset of increased dependence on visual inputs for perception of self-orientation and self-motion in weightlessness, and a decreased dependence on otolithic and somatosensory graviceptive information. Anti-motion sickness drugs appear not to affect the parameters measured.     

Effects of superior colliculus ablation on the air-righting reflex in the rat

To examine how the superior colliculus, the motor center of orientation and avoidance, could interact with postural reflexes, we investigated effects of unilateral and bilateral ablations on air-righting reflex movements in otherwise intact rats. Superior colliculus ablations variously modified righting movements: After falling from the supine position, the rats sometimes showed dorsiflexion instead of normal ventriflexion; the motor sequence of rotation from the fore- to the hindquarter was often modified to simultaneous rotation; lateral turn from supine to prone position was occasionally insufficient; body direction that was normally kept constant during falling was often changed; final posture sometimes deviated from the horizontal position. The first three abnormalities occurred almost twice in frequency as lesions increased from unilateral to bilateral ablation, and in unilaterally ablated rats, did so in righting contraversive to the lesions. Multiple influences of tectoreticular input to the air-righting reflex center are discussed.     

Vestibular perception and action employ qualitatively different mechanisms. II. VOR and perceptual responses during combined Tilt&Translation

To compare and contrast the neural mechanisms that contribute to vestibular perception and action, we measured vestibuloocular reflexes (VOR) and perceptions of tilt and translation. We took advantage of the well-known ambiguity that the otolith organs respond to both linear acceleration and tilt with respect to gravity and investigated the mechanisms by which this ambiguity is resolved. A new motion paradigm that combined roll tilt with inter-aural translation ("Tilt&Translation") was used; subjects were sinusoidally (0.8 Hz) roll tilted but with their ears above or below the rotation axis. This paradigm provided sinusoidal roll canal cues that were the same across trials while providing otolith cues that varied linearly with ear position relative to the earth-horizontal rotation axis. We found that perceived tilt and translation depended on canal cues, with substantial roll tilt and inter-aural translation perceptions reported even when the otolith organs measured no inter-aural force. These findings match internal model predictions that rotational cues from the canals influence the neural processing of otolith cues. We also found horizontal translational VORs that varied linearly with radius; a minimal response was measured when the otolith organs transduced little or no inter-aural force. Hence, the horizontal translational VOR was dependent on otolith cues but independent of canal cues. These findings match predictions that translational VORs are elicited by simple filtering of otolith signals. We conclude that internal models govern human perception of tilt and translation at 0.8 Hz and that high-pass filtering governs the human translational VOR at this same frequency.