General recommendations.
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Aerospace Medicine↗
Biomedical subjects
S Glasauer
Publications and source records attributed to S Glasauer.
Pathological torsional eye deviation during voluntary saccades: a violation of Listing's law.
BACKGROUND: Under normal conditions, there are no torsional eye movements during voluntary saccades when the head is stationary (Listing's law). METHODS AND RESULTS: Using dual search coils for three dimensional eye movement recordings, a patient is reported who had direction specific rapid deviations of torsional eye position (up to 10.5 degrees) during voluntary saccades followed by a slow exponential torsional drift after the end of the saccade ("blip") towards the initial torsional eye position. In the absence of spontaneous nystagmus, this transient torsion means a violation of Listing's law for voluntary saccades and was associated with a lesion involving the cerebellar vermis, its deep nuclei, and the dorsolateral medulla. Amplitudes of the blip were larger for ipsilesional (hypermetric) than contralesional (hypometric) horizontal saccades. For comparison transient torsion during and after saccades was also examined in six normal subjects. Using the same in vivo calibration, there were no blips larger than 1.2 degrees in any of them. CONCLUSION: Transient torsion with large amplitudes can be clinically seen on bedside examination and might thus be a new clinical sign in the diagnosis of saccadic disorders.
Adult↗
Rostral fastigial nucleus activity in the alert monkey during three-dimensional passive head movements.
The fastigial nucleus (FN) receives vestibular information predominantly from Purkinje cells of the vermis. FN in the monkey can be divided in a rostral part, related to spinal mechanisms, and a caudal part with oculomotor functions. To understand the role of FN during movements in space, single-unit activity in alert monkeys was recorded during passive three-dimensional head movements from rostral FN. Seated monkeys were rotated sinusoidally around a horizontal earth-fixed axis (vertical stimulation) at different orientations 15 degrees apart (including roll, pitch, vertical canal plane and intermediate planes). In addition, sinusoidal rotations around an earth-vertical axis (yaw stimulus) included different roll and pitch positions (+/-10 degrees, +/-20 degrees). The latter positions were also used for static stimulation. One hundred fifty-eight neurons in two monkeys were modulated during the sinusoidal vertical search stimulation. The vast majority showed a uniform response pattern: a maximum at a specific head orientation (response vector orientation) and a null response 90 degrees apart. Detailed analysis was obtained from 111 neurons. On the basis of their phase relation during dynamic stimulation and their response to static tilt, these neurons were classified as vertical semicircular canal related (n = 79, 71.2%) or otolith related (n = 25; 22.5%). Only seven neurons did not follow the usual response pattern and were classified as complex neurons. For the vertical canal-related neurons (n = 79) all eight major response vector orientations (ipsilateral or contralateral anterior canal, posterior canal, roll, and nose-down and nose-up pitch) were found in Fn on one side. Neurons with ipsilateral orientations were more numerous and on average more sensitive than those with contralateral orientations. Twenty-eight percent of the vertical canal-related neurons also responded to horizontal canal stimulation. None of the vertical canal-related neurons responded to static tilt. Otolith-related neurons (n = 25) had a phase relation close to head position and were considerably less numerous than canal-related neurons. Except for pitch, all other response vector orientations were found. Seventy percent of these neurons responding during dynamic stimulation also responded during static tilt. The sensitivity during dynamic stimulation was always higher than during static stimulation. Sixty-one percent of the otolith-related neurons responded also to horizontal canal stimulation. These results show that in FN, robust vestibular signals are abundant. Canal-related responses are much more common than otolith-related responses. Although for many canal neurons the responses can be related to single canal planes, convergence between vertical canals but also with horizontal canals is common.
Animals↗
Updating an object's orientation and location during nonvisual navigation: a comparison between two processing modes.
In the present study, we compared the effects of two processing modes on the updating of the location and orientation of a previously viewed object in space during a guided walk without vision. In Experiment 1, in order to measure the error for initial perception of object's orientation, 12 subjects rotated a miniature model until it matched the memorized orientation of its counterpart object in space. In Experiment 2, they attempted either to keep track of the object continuously (in the object-centered [OC] task) or to estimate the object's perspective only at the terminal vantage point given the trajectory they walked (in the trajectory-centered [TC] task). Subjects indicated the location of the object by facing it, and then rotated the model in order to indicate its orientation from the new vantage point. Results showed that, with respect to the TC mode, the OC mode induced a slow-down of the subjects' self-paced locomotion velocity for both linear and angular movements, and a decrease of the latencies as well as smaller absolute errors for the orientation-of-the-object response. Mean signed errors on object's orientation were equivalent for both processing modes, suggesting that the latter induced different allocations of processing resources on a common representation of space updated by "path integration."
Adult↗
Perception of spatial orientation in different g-levels.
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Adaptation, Physiological↗
The predictive brain: anticipatory control of head direction for the steering of locomotion.
The control of head direction in humans walking along planned circular trajectories was investigated in this study. Five healthy volunteers were asked to walk at a constant speed along circular trajectories in the light and while blindfolded. Head and walking directions were analysed on a real-time basis. Head direction systematically anticipated changes in the direction of locomotion (by about 200 ms). The anticipation interval depended on the curvature of the circle. In the light, head orientation was deviated with respect to the walking direction, toward the inner concavity of the performed trajectory. The results suggest that head direction is controlled on a step-by-step basis in a predictive fashion. A 'go where you look' strategy seems to underlie steering along circular trajectories.
Adult↗
A simplified calibration method for three-dimensional eye movement recordings using search-coils.
An algorithm is presented which allows the calibration of three-dimensional eye movements in two magnetic fields with two search-coils in one eye, whose relative orientation does not have to be known. Prior to recordings a calibration cube is placed in front of the eye to measure real and apparent crosstalk, produced by imprecisely oriented horizontal or vertical magnetic fields, and to obtain a first approximation of offset voltages. For calibration it suffices if one point is fixated and, in addition, spontaneous eye movements for 30-60 sec are performed. Additional fixation points can be used to improve the calibration. Position quaternions are applied for computing eye movement recordings from man and monkey and to determine Listing's plane.
Algorithms↗
Contralesionally beating torsional nystagmus in a unilateral rostral midbrain lesion.
The rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) and the interstitial nucleus of Cajal (iC) are involved in the generation of vertical and torsional saccades and gaze holding. Midbrain lesions involving either riMLF or iC produce a tonic torsional ocular deviation to the contralesional side. Lesions of the iC are associated with an ipsilesional torsional nystagmus (TN). For the first time, we describe a patient with a unilateral midbrain lesion showing a contralesionally beating torsional nystagmus. This nystagmus was probably caused by a lesion involving the riMLF.
Brain Diseases↗
Linear acceleration perception: frequency dependence of the hilltop illusion.
Periodical stimulation of the otoliths during linear horizontal displacement leads not only to motion and displacement perception but also to the "Hilltop Illusion", which is the perception of being transported not on an earth-horizontal path, but over a small hill. This phenomenon is explained by the partial reinterpretation of the linear acceleration to a tilt of the gravitational vertical. The dynamics of the illusion and its role in the perception of spatial orientation were investigated for eight frequencies. The subjects were exposed to sinusoidal linear acceleration and had to dynamically adjust a luminous line display to their subjective vertical. This served as a quantitative indicator of the amount of perceived tilt with respect to gravity. The high frequency part (0.1-0.33 Hz) of the experiment was performed on a conventional linear acceleration device (sled), the low frequency part (0.0083-0.05 Hz) on the sled centrifuge (a rotating sled). The amplitude ratio of the settings of the subjective vertical was found to be similar to that of a lowpass with a 15 s time constant. However, the phase shifts never exceeded a 40 lag, which together with other experimental results led to the conclusion that a non-linear, possibly predictive type of information processing must be at work.
Acceleration↗
Otolithic thresholds influence the perception of passive linear displacement.
The vestibular sensors are the necessary source of information for estimating self-displacement during passive linear transport. Displacement has to be computed from the otolith signal by means of an integration process (path integration), which provides a measure of linear acceleration. However, the onset of self-motion perception is delayed due to perceptual thresholds. We investigated the effect of these thresholds on the estimation of passive displacement. Subjects seated on a linear acceleration device (ESA-SLED) were displaced to their left or right. Two tasks were performed: (1) subjects pressed a button when they perceived self-motion; (2) subjects pressed a button when they thought they had reached a previously seen visual target located above the sled rail. Displacement estimation (task 2) was found to depend upon the acceleration magnitude and on the individual motion perception threshold measured in task 1. The results can be explained by assuming that self-displacement computation starts at the onset of self-motion perception and is initialised by a value that is independent of the amplitude of acceleration, thereby compensating for the displacement information lost due to the thresholds.
Acceleration↗
Goal-directed linear locomotion in normal and labyrinthine-defective subjects.
When a subject is walking blindfolded straight ahead towards a previously seen target, the brain must update an internal representation with respect to the environment. This study examines whether the information given by the vestibular system is necessary for this simple path integration task and gives a quantitative description of locomotor behaviour during the walk by comparing ten normal and seven bilateral labyrinthine-defective (LD) subjects. Each subject performed 20 blindfolded walks (EC) and ten walks with eyes open (EO) towards a target attached to the floor 4 m in front of them; these walks were made at different velocities. The positions of head, trunk and feet were recorded using a 3D motion analysis system. No significant difference was found between normal and LD groups in terms of the distance error of reaching the target, while LD subjects showed a larger lateral error. Path curvature, expressed as the standard deviation of the angle between the direction of one step and straight ahead, was found to be significantly larger for LD subjects in the EC condition, demonstrating their instability when walking without vision. Mean walking velocity was lower for LD subjects than for normal subjects in both EC and EO conditions. Both groups walked faster with eyes open; LD subjects increased their velocity by increasing step length, normal subjects by increasing step frequency. Head stabilisation in the frontal plane during locomotion was not significantly different between LD and normal subjects, whereas both head and trunk rotation were slightly larger in LD subjects during blindfolded walking. The results show that bilateral LD subjects are able to perform linear goal-directed locomotion towards memorised targets. Thus, the vestibular system does not appear to be necessary for active linear path integration.
Acceleration↗
Illusions of verticality in weightlessness.
In weightlessness most subjects feel themselves, and see the visual surroundings, in either an upright or an upside-down orientation although the gravitational force vector is missing. According to a theory of gravity perception, these illusions of positional and visual verticality are assumed to be caused by the force-independent z-axis bias of vestibular and somatic graviceptors. This hypothesis is tested by comparison of measurements of the joint bias in normal gravity with reports of probands in space flight. The expected correlations between the sign of the biases and the occurrence of the respective illusions appear in fact to exist, as well as a negative correlation to incidences of space sickness. If confirmed in a larger sample, the presumed dependency may eventually afford a predictive test of both phenomena.
Humans↗
Estimation of passive horizontal linear whole-body displacement in humans.
1. Passive linear self-motion estimation along the X and Y axes was investigated in human subjects. 2. A target was viewed from a distance of 0.8 or 2.4 m from the starting point. Subjects were then blindfolded and transported toward the target on a distance of 3.2 m and back to the start. Acceleration was constant: +/- 0.2 or +/- 1 m/s2. The subjects pushed a button on both outward and return paths, when they passed the previously seen target. 3. The results showed anticipation of the target on the outward path, and not on the return. This was identical for both axes and both accelerations. 4. The data are in accord with a model of double integration of the otolith signal, suggesting that linear path integration is a basic sensory mechanism.
Acceleration↗
Crucial effects of weightlessness on human orientation.
This contribution examines the consequences of two remarkable experiences of subjects in weightlessness, 1) the missing of sensations of trunk tilt and of the respective concomitant reflexes when the head is tilted with respect to the trunk, and 2) the persistence of a perception of "up" and "down," that is, of the polarity of the subjective vertical (SV) in the absence of, as well as in contradiction to, visual cues. The first disproves that the necessary head-to-trunk coordinate transformation be achieved by adding representations of the respective angles gained by utricles and neck receptors, but corroborates an extant model of cross-multiplication of utricular, saccular, and neck receptor components. The second indicates the existence of force-independent components in the determination of the SV. Although the number of subjects is still small and experimental conditions are not as homogeneous as desired, measurements and/or reports on the ground, in parabolic, and in space flight point to the decisive role of the saccular z-bias, that is, of a difference of the mean resting discharges of saccular units polarized in the rostrad and the caudad (+/- z-) direction.
Humans↗
Interaction of semicircular canals and otoliths in the processing structure of the subjective zenith.
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Afferent Pathways↗
Determinants of orientation in microgravity.
During two parabolic flight campaigns, one with the NASA-KC-135, the second with the ESA Caravelle, human spatial orientation in an altered gravitational environment was studied by measuring the subjective visual vertical (SVV) by means of a luminous line, and by asking the subjects to give a report, with eyes closed, about their orientation to apparent vertical. The inflight data are compared with baseline data measurements of the subjective horizontal body position (SHP) at normogravity (1g) and at 2g. Pertinent theoretical alternatives to modelling subjective static orientation are developed and compared to the data. It turns out that a good fit to the baseline results and a satisfactory prediction of the perceived orientation in microgravity can be obtained if the otolithic output is assumed to be normalized, but that of the somatic gravity sensors is not.
Aerospace Medicine↗
How to explain a constant subjective vertical at constant high speed rotation about an earth-horizontal axis.
When rotated in darkness about an earth-horizontal axis at speeds above 0.2-0.5 Hz, subjects, instead of feeling rotated, experience a constant (though extrapersonally diverse) position in space and a constant visual vertical (SV). Computer simulation shows that this phenomenon cannot be explained by the extant models of Mayne (1) and Ormsby (2) about the interaction of otoliths and semicircular canals. It follows, however, from a static theory of the SV (3) if, as in the presently proposed dynamic model, the otolith afference is processed by a low-pass filter. At high speed rotation this filter can only be passed by the force-independent, temporally invariant components of the otolith information. Such force-independent components are bound to result from biassed resting discharges, and have previously been shown to affect the SV and the self-adopted horizontal position. The interaction of otoliths and canals proposed by the model does provide a veridical vertical in a working range of angular frequencies and hence a basis for inertial navigation.
Acceleration↗
Spatial orientation during locomotion [correction of locomation] following space flight.
To investigate changes in spatial orientation ability and walking performance following space flight, 7 astronaut subjects were asked pre- and post-flight to perform a goal directed locomotion paradigm which consisted of walking a triangular path with and without vision. This new paradigm, involving inputs from different sensory systems, allows quantification of several critical parameters, like orientation performance, walking velocities and postural stability, in a natural walking task. The paper presented here mainly focuses on spatial orientation performance quantified by the errors in walking the previously seen path without vision. Errors in length and reaching the corners did not change significantly from pre- to post-flight, while absolute angular errors slightly increased post-flight. The significant decrease in walking velocity and a change in head-trunk coordination while walking around the corners of the path observed post-flight may suggest that during re-adaptation to gravity the mechanisms which are necessary to perform the task have to be re-accomplished.
Electronic Data Processing↗