Pitch Tilt Research Articles

Open-Loop Control Co-Design of Semisubmersible Floating Offshore Wind Turbines Using Linear Parameter-Varying Models

Abstract This paper discusses a framework to design elements of the plant and control systems for floating offshore wind turbines in an integrated manner using linear parameter-varying models. Multiple linearized models derived from aero-elastic simulation software in different operating regions characterized by the incoming wind speed are combined to construct an approximate low-fidelity model of the system. The combined model is then used to generate open-loop, optimal control trajectories as part of a nested control co-design strategy that explores the system’s power production and stability using the platform pitch tilt as a proxy in the context of crucial plant and control design decisions. The radial distance between the central and outer columns and the diameter of the outer columns of the semisubmersible platform are the plant design variables. The platform stability and power production are studied for different plant design decisions. The effect of plant decisions on subsequent power production and stability response of the floating wind turbine is quantified in terms of the levelized cost of energy. The results show that the inner-loop constraints and the plant design decisions affect the turbine’s power and, subsequently, the cost of the system.

A modified two-dimensional sensory organization test that assesses both anteroposterior and mediolateral postural control.

The Sensory Organization Test (SOT) was designed to measure changes in postural control in response to unreliable visual and/or proprioceptive feedback. However, secondary to the manipulation of sensory cues in only the sagittal plane, the SOT is capable of only describing postural control in a single direction. The present study aimed to characterize postural responses to a modified SOT designed to concurrently challenge both anteroposterior and mediolateral postural control. Twenty-one healthy adult volunteers (30.6 ± 10.2 years) completed the standard anteroposterior one-dimensional (1D) SOT, in addition to a modified SOT with the support surface sway-referenced to both anteroposterior and mediolateral postural sway (two-dimensional, 2D). Our primary analysis concerned a comparison of mediolateral, as well as anteroposterior postural sway measured during the standard one-dimensional (i.e., pitch tilt) and the novel two-dimensional (i.e., roll and pitch tilt) sway-referenced paradigms. Here, postural sway was quantified by calculating the root mean square distance (RMSD) of the center of pressure (CoP) during each trial. Our data showed that the 2D sway-referenced conditions yielded a selective increase in mediolateral postural sway relative to the standard 1D conditions for both wide (η2 = 0.66) and narrow (η2 = 0.78) stance conditions, with anteroposterior postural sway being largely unaffected (η2 = 0.001 to 0.103, respectively). The ratio between mediolateral postural sway in the sway-referenced conditions and postural sway in the corresponding stable support surface conditions was greater for the 2D (2.99 to 6.26 times greater) compared to 1D paradigms (1.25 to 1.84 times greater), consistent with a superior degradation of viable proprioceptive feedback in the 2D paradigm. A modified 2D version of the SOT was shown to provide a greater challenge to mediolateral postural control relative to the standard 1D SOT protocol, putatively as a result of a superior capacity to degrade proprioceptive feedback in the mediolateral direction. Given these positive findings, future studies should investigate the clinical utility of this modified SOT as a means by which to better characterize sensory contributions to postural control in the presence of various sensorimotor pathologies, including vestibular hypofunction.

Tilt perception is different in the pitch and roll planes in human.

Neurophysiological tests probing the vestibulo-ocular, colic and spinal pathways are the gold standard to evaluate the vestibular system in clinics. In contrast, vestibular perception is rarely tested despite its potential usefulness in professional training and for the longitudinal follow-up of professionals dealing with complex man-machine interfaces, such as aircraft pilots. This is explored here using a helicopter flight simulator to probe the vestibular perception of pilots. The vestibular perception of nine professional helicopter pilots was tested using a full flight helicopter simulator. The cabin was tilted six times in roll and six times in pitch (-15°, -10°, -5°, 5°, 10° and 15°) while the pilots had no visual cue. The velocities of the outbound displacement of the cabin were kept below the threshold of the semicircular canal perception. After the completion of each movement, the pilots were asked to put the cabin back in the horizontal plane (still without visual cues). The order of the 12 trials was randomized with two additional control trials where the cabin stayed in the horizontal plane but rotated in yaw (-10° and +10°). Pilots were significantly more precise in roll (average error in roll: 1.15 ± 0.67°) than in pitch (average error in pitch: 2.89 ± 1.06°) (Wilcoxon signed-rank test: p< 0.01). However, we did not find a significant difference either between left and right roll tilts (p= 0.51) or between forward and backward pitch tilts (p= 0.59). Furthermore, we found that the accuracies were significantly biased with respect to the initial tilt. The greater the initial tilt was, the less precise the pilots were, although maintaining the direction of the tilt, meaning that the error can be expressed as a vestibular error gain in the ability to perceive the modification in the orientation. This significant result was found in both roll (Friedman test: p< 0.01) and pitch (p< 0.001). However, the pitch trend error was more prominent (gain=0.77 vs gain=0.93) than roll. This study is a first step in the determination of the perceptive-motor profile of pilots, which could be of major use for their training and their longitudinal follow-up. A similar protocol may also be useful in clinics to monitor the aging process of the otolith system with a simplified testing device.

Comparing Vestibular Responses to Linear and Angular Whole-Body Accelerations in Real and Immersive Environments.

The vestibular end organs differ in terms of anatomical and physiological characteristics. Sensory modalities' stimuli including visual stimuli and vestibular sensation can influence these organs differently. This paper explores differences between vestibular responses to axial tilts in physical and virtual environments. Four passive whole-body movements (linear: up-down, and angular: yaw, pitch, and roll) were applied to twenty-seven healthy participants once using a hydraulic chair (physical) and once visually using a head-mounted display (virtual). Electrovestibulography (EVestG) was used as the outcome measure to investigate the magnitude of vestibular-response-change in both ears for physical and virtual stimuli. Three features including average action potential (AP) area, AP amplitude, and mean detected firing rate change were used as indices of response. The results show that for both physical and virtual stimuli (1) generally the pitch and roll tilts produce the largest EVestG changes compared to other tilts (2) roll and pitch tilt responses are not significantly different from each other and (3) right side and left side roll tilts' responses are not significantly different. The findings indicate although visually- and physically-induced vestibular responses are different in terms of afferent activity, visual stimuli can still result in distinct responses when exposed to different axial tilts.

Human vestibular perceptual thresholds for pitch tilt are slightly worse than for roll tilt across a range of frequencies.

Vestibular perceptual thresholds measure vestibular sensory and perceptual noise by quantifying how small of a passive self-motion an individual is able to reliably perceive. Vestibular thresholds have clinical and operational relevance, as they are elevated in vestibular migraine patients, and even healthy individuals with higher (i.e., worse) thresholds have degraded balance. Vestibular thresholds have been quantified across a range of frequencies (motion durations) for rotations and translations, with differences identified for different motion directions (e.g., up/down thresholds are higher than those for left/right motions). While roll tilt thresholds have been well quantified, pitch tilt thresholds have not. Here we aim to quantify pitch tilt thresholds across a range of frequencies and test whether they are higher than in those for roll tilt. In ten normal subjects, we found pitch tilt thresholds at 0.15, 0.2, 0.5, and 1Hz averaged 1.66, 1.61, 0.99, 0.51 degrees, respectively. Using a general linear model, we found subjects' pitch tilt thresholds were slightly, but significantly, higher than their roll tilt thresholds across all frequencies tested. These differences were approximately 10% at 0.15, 0.2, and 1Hz and 3% at 0.5Hz. Pitch tilt thresholds exhibited a similar frequency response as in roll tilt (decreasing a higher frequencies). They also had substantial inter-individual variability, which correlated across pitch tilt frequencies and between pitch and roll tilt thresholds. We discuss why pitch tilt thresholds might be higher, including the pitched-up orientation of the utricular plane of the otoliths, compared to previous studies, and discuss functional implications.

Tilt thresholds for acceleration rendering in driving simulation

The tilt coordination technique is used in driving simulation for reproducing a sustained linear horizontal acceleration by tilting the simulator cabin. If combined with the translation motion of the simulator, this technique increases the acceleration rendering capabilities of the whole system. To perform this technique correctly, the rotational motion must be slow to remain under the perception threshold and thus be unnoticed by the driver. However, the acceleration to render changes quickly. Between the slow rotational motion limited by the tilt threshold and the fast change of acceleration to render, the design of the coupling between motions of rotation and translation plays a critical role in the realism of a driving simulator. This study focuses on the acceptance by drivers of different configurations for tilt restitution in terms of maximum tilt angle, tilt rate, and tilt acceleration. Two experiments were conducted, focusing respectively on roll tilt for a 0.2 Hz slaloming task and on pitch tilt for an acceleration/deceleration task. The results show what thresholds have to be followed in terms of amplitude, rate, and acceleration. These results are far superior to the standard human perception thresholds found in the literature.

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.

Modeling human perception of orientation in altered gravity.

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

Perception of verticality and cardiovascular responses during short-radius centrifugation

Artificial gravity using short-radius centrifugation has been proposed as an integrative countermeasure during spaceflight. To determine the rotation parameters of a short-radius centrifuge so that subjects rotating in the dark would feel as if they were standing upright. Twelve subjects were lying supine in a nacelle on a 2.8 m-radius centrifuge with their head closer to the axis of rotation and their feet pointing radially outwards. Subjects verbally reported body orientation for 26 combinations of centrifuge rotation rate and nacelle pitch tilt. ECG and respiratory responses were also recorded. Five subjects felt like they were vertical when centrifugation elicited 1 g at their center of mass along their body longitudinal axis, whereas seven subjects felt they were vertical when they experienced about 1 g at ear level, regardless of the nacelle tilt angle. Heart rate variability varied with the subjects' perception of verticality. These results suggest that one group of subject was relying principally on the otolith organs for the perception of verticality, whereas the other group was also relying on extravestibular somatosensory receptors. The crewmember's perception of verticality might be a factor to take into account for the prescription for artificial gravity during spaceflight.

Neural Representation of Orientation Relative to Gravity in the Macaque Cerebellum

A fundamental challenge for maintaining spatial orientation and interacting with the world is knowledge of our orientation relative to gravity, i.e., head tilt. Sensing gravity is complicated because of Einstein's equivalence principle, in which gravitational and translational accelerations are physically indistinguishable. Theory has proposed that this ambiguity is solved by tracking head tilt through multisensory integration. Here we identify a group of Purkinje cells in the caudal cerebellar vermis with responses that reflect an estimate of head tilt. These tilt-selective cells are complementary to translation-selective Purkinje cells, such that their population activities sum to the net gravitoinertial acceleration encoded by the otolith organs, as predicted by theory. These findings reflect the remarkable ability of the cerebellum for neural computation and provide quantitative evidence for a neural representation of gravity, whose calculation relies on long-postulated theoretical concepts such as internal models and Bayesian priors.

Postural configuration affects the perception of earth-based space during pitch tilt

This study investigates the relative contribution of body parts in the elaboration of a whole-body egocentric attraction phenomenon previously observed during earth-based judgments. This was addressed through a particular earth-based task requiring estimating the possibility of passing under a projected line, imagining a forward horizontal displacement. Different postural configurations were tested, involving whole-body tilt, trunk tilt alone or head tilt alone. Two legs positions relative to the trunk were manipulated. Results showed systematic deviations of the subjective “passability” toward the tilt, linearly related to the tilt magnitude. For each postural configuration, the egocentric influence appeared to be highly dependent on the position of trunk and head axes, whereas the legs position appeared not relevant. When compared to the whole-body tilt condition, tilting the trunk alone consistently reduced the amount of the deviation toward the tilt, whereas tilting the head alone consistently increased it. Our results suggest that several specific effects from multiple body parts can account for the global deviation of the estimates observed during whole-body tilt. Most importantly, we support that the relative contribution of the body segments could mainly depend on a reweighting process, probably based on the reliability of sensory information available for a particular postural set.

Analysis of Postural Adjustment Responses to Perturbation Stimulus by Surface Tilts in the Feet-together Position

Base of support (BOS) influences the postural control of upright standing in humans. Since a narrowed BOS in the upright position makes postural control difficult, it is important to evaluate and practice balance with a narrowed BOS. We performed an experimental study to clarify the postural control mechanism in a feet-together position in response to dynamic perturbations. Subjects were 10 healthy young adults. Using an electrically operated platform, tilting perturbation was applied to subjects standing in feet-together position. Perturbation was in four directions: backward, forward, right, and left. Using a motion analysis system and electromyography (EMG), we simultaneously measured the three-dimensional postural change and EMG of seven muscles of the trunk and legs. We obtained joint angular change from the motion analysis and the integrated values of muscle activity and latency of the lower extremities from the EMG. In terms of pitch tilt, the ankle strategy was observed: this involved simultaneous contraction of the ankle muscles as usual in the upright position. In contrast, in terms of roll tilt, ”counterweighting” was observed. We also observed a change in postural control to respond to the narrowed BOS. We found that, unlike the feet-apart position, the feet-together position, narrowing the BOS in the left-right direction causes a prominent change and makes postural control strategy difficult only in response to roll tilt perturbations. In the future, we need to investigate the effectiveness of balance training including roll tilting in the feet-together position in improving balance ability in the left-right direction.

Vestibular effects on cerebral blood flow

BackgroundHumans demonstrate a number of unique adaptations that allow for the maintenance of blood pressure and brain blood flow when upright. While several physiological systems, including cerebral autoregulation, are involved in this adaptation the unique role the vestibular system plays in helping to maintain brain blood flow is just beginning to be elucidated. In this study, we tested the hypothesis that stimulation of the vestibular system, specifically the otoliths organs, would result in changes in cerebral blood flow.ResultsTo test our hypothesis, we stimulated the vestibular organs of 25 healthy subjects by pitch tilt (stimulates both canals and otoliths) and by translation on a centrifuge (stimulates otoliths and not the canals) at five frequencies: 0.5, 0.25, 0.125 and 0.0625 Hz for 80 sec and 0.03125 Hz for 160 sec. Changes in cerebral flow velocity (by transcranial Doppler) and blood pressure (by Finapres) were similar during both stimuli and dependent on frequency of stimulation (P < 0.01). However, changes in cerebral blood flow were in opposition to changes in blood pressure and not fully dependent on changes in end tidal CO2.ConclusionThe experimental results support our hypothesis and provide evidence that activation of the vestibular apparatus, specifically the otolith organs, directly affects cerebral blood flow regulation, independent of blood pressure and end tidal CO2 changes.

An automated sensor-based method of simple behavioural classification of sheep in extensive systems

Automated sensor-based behaviour classification systems are already developed and used for cattle in an agricultural context. To develop a similar tool for sheep we investigated the use of pitch and roll tilt sensors to provide information about sheep behaviour. The aim was to determine if different behaviour types associated with grazing can be accurately identified using tilt sensor data. We collected data from two adjacent enclosures at two contrasting sites, one in the Southern Uplands grazed by Cheviot ewes and one in the Western Highlands of Scotland grazed by Scottish Blackface ewes. In addition to our observations of ewes on hill pasture we also observed collared ewes in flat and gently sloped fields and in a shed to evaluate the tilt data. The data was analysed using three classification methods, a linear discriminant analysis, a classification tree method, and a manually developed decision tree consisting of four “if then” loops. We found that we can distinguish between the two behaviour categories “active” and “inactive”, even if only pitch tilt data is used. All three methods provide very good classification predictions with more than 90% correct results. However, the classification tree method was less robust than the other methods. Using the manually developed decision tree, we produced results of activity that were robust and credible. By using the behaviour classification we are able to collect data on different genotypes, systems or management options in sheep farming, and in combination with GPS we will be able to improve management strategies and gain information about grazing ecology. It can also be seen as a first step to support farmers with a viable system to comply with welfare regulations in the light of European Union Common Agricultural Policy and as an integral part for future developments regarding virtual fencing technology for sheep.

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 approximately 25 degrees /g and approximately 26 degrees /g, respectively, and of ocular counter-roll during roll tilts was approximately 20 degrees /g. During X-axis centripetal acceleration with 1 g of gravity along the Z-axis, pitch and vergence sensitivities were reduced to approximately 13 degrees /g and approximately 16 degrees /g. Similarly, Y-axis acceleration with 1g along the Z-axis reduced the roll sensitivity to approximately 16 degrees /g. Modulation of Z-axis centripetal acceleration caused sensitivities to drop by approximately 6 degrees /g in pitch, approximately 2 degrees /g in vergence, and approximately 5 degrees /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.

Tilt and translation motion perception during off-vertical axis rotation

The effect of stimulus frequency on tilt and translation motion perception was studied during constant velocity off-vertical axis rotation (OVAR), and compared to the effect of stimulus frequency on eye movements. Fourteen healthy subjects were rotated in darkness about their longitudinal axis 10 degrees and 20 degrees off-vertical at 45 degrees /s (0.125 Hz) and 20 degrees off-vertical at 180 degrees /s (0.5 Hz). Perceived motion was evaluated using verbal reports and a joystick capable of recording tilt and translation in both sagittal and lateral planes. Eye movements were also recorded using videography. At the lower frequency, subjects reported the perception of progressing along the edge of a cone, whereas at the higher frequency they had the sensation of progressing along the edge of an upright cylinder. Tilt perception and ocular torsion significantly increased as the tilt angle increased from 10 degrees to 20 degrees at the lower frequency, and then decreased at the higher frequency. The phase lag of ocular torsion increased as a function of frequency, while the phase lag of tilt perception did not change. Horizontal eye movements were small at the lower frequency and showed a phase lead relative to the linear acceleration stimulus. While the phase lead of horizontal eye movements decreased at 0.5 Hz, the phase of translation perception did not vary with stimulus frequency and was similar to the phase of tilt perception during all conditions. A second data set was obtained in 12 subjects to compare motion perception phase when using a simple push-button to indicate nose-up orientation, continuous setting of pitch tilt alone, or continuous setting of tilt and translation in both pitch and roll planes as in the first data set. This set of measurements indicated that in the frequency range studied subjects tend to lead the stimulus when using a push-button task while lagging the stimulus when using a continuous setting of tilt with a joystick. Both amplitude and phase of tilt perception using the joystick were not different whether concentrating on pitch tilt alone or attempting a more complex reporting of tilt and translation in both sagittal and lateral planes. During dynamic linear stimuli in the absence of canal and visual input, a change in stimulus frequency alone elicits similar changes in the amplitude of both self-motion perception and eye movements. However, in contrast to the eye movements, the phase of both perceived tilt and translation motion is not altered by stimulus frequency over this limited range. These results are consistent with the hypothesis that neural processing to distinguish tilt and translation stimuli differs between eye movements and motion perception.

How to use body tilt for the simulation of linear self motion

We examined to what extent body tilt may augment the perception of visually simulated linear self acceleration. Fourteen subjects judged visual motion profiles of fore-aft motion at four different frequencies between 0.04-0.33 Hz, and at three different acceleration amplitudes (0.44, 0.88 and 1.76 m/s(2)). Simultaneously, subjects were tilted backward and forward about their pitch axis. The amplitude of pitch tilt was systematically varied. Using a two-alternative-forced-choice paradigm, psychometric curves were calculated in order to determine: 1) the minimum tilt amplitude required to generate a linear self-motion percept in more than 50% of the cases, and 2) the maximum tilt amplitude at which rotation remains sub-threshold in more than 50% of the cases. The results showed that the simulation of linear self motion became more realistic with the application of whole body tilt, as long as the tilt rate remained under the detection threshold of about 3 deg/s. This value is in close agreement with the empirical rate limit commonly used in flight simulation. The minimum required motion cue was inversely proportional to stimulus frequency, and increased with the amplitude of the visual displacement (rather than acceleration). As a consequence, the range of useful tilt stimuli became more critical with increasing stimulus frequency. We conclude that this psychophysical approach reveals valid parameters for motion driving algorithms used in motion base simulators.

The effects of pitch tilt on postrotatory nystagmus in cats

When the head is tilted away from the upright position immediately after termination of earth-vertical axis (EVA) head rotation at a constant velocity, the rotation axis of postrotatory nystagmus (PRN) is gradually directed toward gravity (spatial reorientation). During roll tilt, the extent of the axis shift varies among species. In cats, the reorientation is limited to about 30–45° of head tilt. In the present study, we examined features of PRN during pitch tilt in cats. The animals were rotated about EVA at a constant velocity of 100°/s for 120 s. Within 2 s after stopping EVA rotation, the animals were tilted toward the pitch plane by as much as ±90° (nose-down or nose-up) in steps of 15°. Eye movements were measured with 3D magnetic search coils. The angle of the PRN plane and its slow phase eye velocity (SPV) were measured. The duration of PRN decreased as pitch tilt increased regardless of whether the direction was nose-up or nose-down. The mean time constant of SPV was significantly longer for nose-up than for nose-down. PRN maintained its horizontal orientation without any vertical drift during pitch tilt, but there was little reorientation toward gravity. That is, no significant torsional component of SPV emerged to reorient the eyes according to gravity. Our results indicate that spatial reorientation depends on head orientation. For the PRN responses after pitch tilts in cats, the interactions of semicircular canal and otolith inputs in the central vestibular system might be vestigial.

Perception of tilt and ocular torsion of normal human subjects during eccentric rotation.

The authors hypothesized that a nonvisual measure of the subjective horizontal during eccentric rotation is a reliable method for evaluating the spatial orientation of normal subjects during such rotation. Eccentric rotation is a promising tool for the evaluation of otolithic function in healthy subjects and in patients. Although eye torsion is an objective measurement, it is also affected by angular acceleration/deceleration. Subjective horizontal is more accurately related to the changes in linear acceleration, but visual judgments of orientation are confounded by ocular torsion. 20 subjects were tested during eccentric yaw rotation in both directions, generating centripetal acceleration directed along the interaural axis ranging from 0.38 g to 1 g. Perception of body tilt in roll and in pitch was recorded in darkness using verbal reports and a somatosensory plate that the subjects maintained parallel to the perceived horizon. Torsion of the eyes was recorded by a video camera. Perceived roll tilt was close to the tilt of the gravito-inertial acceleration vector. However, there were differences in perceived roll and pitch tilt between facing-motion and back-to-motion headings, presumably related to the direction of the tangential acceleration. Ocular torsion was dependent on both angular and centripetal accelerations. Measurement of perceived roll and pitch tilt using a somatosensory plate is a reproducible method for quantifying the effects of linear accelerations during eccentric rotation. This method may prove useful for the diagnosis of otolithic dysfunction in dizziness and in patients with vestibular disorders.

Triggering of balance corrections and compensatory strategies in a patient with total leg proprioceptive loss

Triggering of balance corrections may depend on both leg and trunk proprioceptive inputs. To study this issue and to determine how a total proprioceptive loss in the legs (ToLPL) would affect postural reactions in different directions, we investigated the postural control of a patient with a long-standing dorsal root ganglionopathy. This patient had absent stretch reflexes at the ankle and knee joints, delayed reflexes at the hips, but normal muscle strength. Postural control was probed with support-surface movements driven by two different experimental protocols. The first protocol concentrated on leg muscle responses by varying ankle inputs during pitch plane perturbations. The second protocol focussed on the directional sensitivity of upper body responses using combined roll and pitch tilt perturbations. For both protocols, identical techniques were used to record ankle torques, angular velocities of the upper legs and trunk, and surface EMG from leg, hip and trunk muscles. For the first protocol, pitch plane stance perturbations with three different ankle inputs were imposed by a movable support surface. A simultaneous 4-cm rearward translation and 4-deg toe-up rotation produced an 80-deg/s "enhanced ankle input", a simple toe-up rotation gave a 40-deg/s "normal" ankle input and a simultaneous 4-cm rearward translation and 4-deg "toe-down" rotation yielding a 0-deg/s "nulled ankle input". Responses in the ToLPL patient were compared to those of healthy controls and those of patients with lower-leg proprioceptive loss (LLPL). Following normal and enhanced ankle input perturbations, stretch reflexes were absent in ankle and knee joint muscles of the ToLPL patient. Balance correcting responses in the lower legs were diminished and delayed by some 45 ms. In quadriceps, balance-correcting responses were larger than normal, peaked earlier and were not delayed. During the nulled ankle input condition, the ankle muscle responses in the ToLPL patient were again diminished and delayed by 40 ms with respect to both normal subjects and LLPL patients. However, the ToLPL patient again generated an earlier, larger, balance correcting response in quadriceps. For the second protocol, combinations of roll and pitch perturbations were also delivered by a moving support surface. The amplitude was 7.5 deg at 50 deg/s. Eight different directions were applied randomly (pure "toes down", pure "toes up" and directions at 45-deg intervals of roll). As with the first protocol pre-stimulus background muscle activity was excessive in all trunk and most leg muscles. Responses to roll tilt produced several striking changes from normal in the ToLPL patient. First reflexes in gluteus medius were delayed. Second, the trunk roll which commences around 30 ms in normals was in the opposite direction. This roll was accompanied by oppositely directed stretch reflexes in paraspinal muscles. Third, directional sensitivity of balance corrections was far more roll oriented in leg and trunk muscles. Fourth, some tilt directions caused a deactivation response of background activity. This "deactivation strategy" strongly contrasted with the strategy of controls who had low pre-stimulus background activity and activated responses around 100 ms to correct postural instability. These findings provide new insights into the generation of pitch and roll plane directed balance corrections based on the interaction of proprioceptive trigger signals from the ankles, knees and hips. Without proprioceptive input from the ankle and knee, ankle muscle responses are delayed but not absent. Upper leg and trunk responses are not delayed. This suggests that most, if not all, lower leg balance correcting responses are triggered by hip and, possibly, trunk proprioceptive inputs. When leg proprioceptive input is absent, balance correcting responses lose pitch plane sensitivity. The solution used by the patient to overcome these deficits was to markedly raise background muscle activity levels, presumably to provide a stiffer body structure. The lack of trunk flexibility and lateral instability this produced for roll tilts was offset by the ability to compensate by using a hitherto not described "deactivation response" strategy. The patient had a clinical picture usually described as "deafferented"; yet our roll tilt perturbations revealed delayed reflex responses in hip muscles. With vestibulospinal and neck-proprioceptive inputs, these responses may have helped with the development of compensation processes for the total leg proprioceptive deficit.