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Biomedical subjects

H Mittelstaedt

Publications and source records attributed to H Mittelstaedt.

At least 19 recordsLinked to original sources
PubMed2000

Triple-loop model of path control by head direction and place cells.

Arthropods as well as mammals are able to return straight home after a random search excursion under conditions that are designed to exclude all external cues. After a brief clarification of the terminology, two principal systems of information processing that can achieve this performance are introduced and analysed: Polar versus Cartesian path integration. The different demands and achievements of the two systems are confronted with neurophysiological findings on the functioning of the hippocampus, and with a recent comprehensive model of how the hippocampal place cells perform path integration. To connect the neurophysiological findings with the behavior of the animal, a new model is developed. It achieves three functionally diverse performances: maintenance and control of a compass direction, navigation by path integration, and formation of goals by connecting non-spatial features with their location. This is done by three interconnected feedback loops, set by a common reference variable. Their information-processing structure enables the animal not only to home but also to go straight from any stored goal to any other, without explicit representation of the distance between them, and without a topological arrangement of the store. The model explains behaviors not yet understood and predicts still undiscovered performances. Because it allows the isolation of orienting from storing functions yet also shows how they can be connected, the model may help to reconcile conflicting views on the function of the hippocampus.

Animals↗
PubMed1999

The role of the otoliths in perception of the vertical and in path integration.

The role of the otoliths in essential performances of human orientation is analyzed. The following interactions of the otoliths are considered: 1. The otoliths cooperate with graviceptors in the trunk in the perception of body posture. The truncal graviceptors turn out to yield on average 60% of the total gain. 2. The otoliths cooperate with proprioceptors in the head-to-trunk coordinate transformation. However, under static conditions, proprioceptors in the legs, although effective in the control of posture, neither affect the perception of posture nor of the visual vertical. 3. In contrast to the perception of posture, the perception of the visual vertical (SVV) receives the necessary gravity information exclusively from the otoliths. However, their output appears to be affected by a central nervous component that tends to rotate the SVV into the z-axis of head and trunk. A theory of vectorial summation of this component, the "idiotropic vector," with the otolithic vector is able to explain the cause of the A- and E- effects, the increase of the variance of the SVV with the tilt angle, and the asymmetrical effect of rotatory visual flow. 4. Finally, it is shown that the otoliths, by the separation of the effects of tilt from those of translation, play an essential role in navigation by path integration.

Body Image↗
PubMed1998

Origin and processing of postural information.

This contribution surveys the sources and the processing of spatial information about posture, that is, about the orientation of the body and its parts with respect to the vertical (whereas 'position' designates their orientation to each other). Postural information is, to a considerable extent, gained by sense organs in the head. Hence information gained by the mobile eyes and the pitched-up labyrinths is first transformed from a retinal and otolithic into a head-fixed frame of reference, then from head- to trunk-fixed coordinates, and, finally, from a trunk-fixed to an exocentric frame of reference. To that end the position of eyes and otoliths to the head, of the head to the trunk, and of the trunk to the rest of the world must be known, deduced by efference copies or measured by proprioceptors. It is shown that the perceived relation of the visual world to the vertical is exclusively determined by sense organs in the head, whereas body posture is also directly measured by recently discovered graviceptors in the human trunk. It appears that the proprioceptors mediate perception of position, but not, or only indirectly, of posture.

Gravitation↗
PubMed1998

Perception of spatial orientation in microgravity.

Experiments during space and parabolic flights have shown that human spatial orientation in microgravity differs to a significant extent from its performance on earth. Due to the missing reference of gravitational force, unusual perceptual phenomena are observed, from inversion illusions to errors of perceived motion and position with respect to the spacecraft. This article gives an overview of results collected from space missions and parabolic flight campaigns, and proposes new lines of research about the perceptual phenomena of spatial orientation in microgravity. It is shown that most of the disorientation phenomena can be explained by the existence of an internal estimate of the gravitational vertical. In microgravity it is still maintained, but incorrectly updated, and thus alters the processing of sensory information in the central nervous system. This in turn leads to the observed illusions, and probably also facilitates space motion sickness.

Humans↗
PubMed1997

Shifts in blood volume alter the perception of posture.

Recent experiments have shown that somatic graviceptors exist in humans. Traditionally, extravestibular gravity information has been thought to originate from mechanoreceptors in the joints, muscles and skin. Experiments with normal, paraplegic and nephrectomized subjects revealed that the kidneys and the cardiovascular system are involved in providing truncal gravity information. The present study intends to determine the influence of shifts in body fluid, especially of the distribution of blood along the subjects' spinal (Z-) axis, on the perception of posture. To this end, the distribution of body fluids was altered by means of the technique of lower body negative and positive pressure (LBNP and LBPP). LBNP leads to venous pooling of blood in the legs, whereas LBPP prevents venous blood from pooling, increasing central volume. Changes in blood distribution were measured by segmental impedance cardiography for four body segments: the upper torso (thoracic cavity), lower torso (abdominal and pelvic region), thigh and calf. Seventeen healthy subjects (mean age: 27.3 years) participated in the experiment. They were positioned on the side (right-ear-down head position) on a tilt table which the subjects and the experimenter could tilt via remote control around an axis parallel to the subjects' visual (X-) axis. The experimenter set the initial tilt in total darkness to arbitrary angles while strictly alternating between head-up and head-down tilts. Subjects were then asked to rotate the board until they felt they were in a horizontal posture. Means and variances of eight pairs of settings were taken as a measure of the subjective horizontal posture (SHP). During LBNP (-30 mmHg), subjects perceived being tilted head-up, whereas LBPP (+30 mmHg) led them to feel tilted head-down. The results corroborate the hypothesis of an effect of the blood's mass on graviception and also indicate supplementary contributions of other visceral afferences.

Adult↗
PubMed1996

Somatic graviception.

Psychophysical experiments show that the perception of posture is to a large degree affected by hitherto unknown graviceptors in the human trunk. By remote control subjects move themselves radially along their spinal axis over the horizontal platform of a rotating centrifuge until they feel horizontal. Normal subjects then set the centrifuge axis on average at 22-28 cm caudal of the meatus, neuromectomized subjects at 45-55 cm. Hence the mass centroid of these receptors should be situated near the last ribs. Evaluation of the residual faculties of paraplegic patients lead to the conclusion that somatic graviception is mediated by two distinctly localized inputs, the first entering the spinal cord at the 11th thoracic segment, and the second reaching the brain cranial of the 6th cervical segment, presumably via the N. phrenicus or the N. vagus. The effect of the first named input is abolished after bilateral nephrectomy. This proves that the kidneys affect gravity perception. But whether they function like statoliths or in another way cannot yet be decided. For the second input, however, the results show unequivocally that it yields gravity information through the inertia of a mass in the body. It is hypothesized that this mass may be that of the blood in the large vessels. This is corroborated by the effect of shifting blood craniad by means of positive pressure to the legs. It is inferred that the inertial forces are measured by mechanoreceptors in the structures that mechanically support the large vessels, rather than by baroreceptors.

Afferent Pathways↗
PubMed1995

Evidence of somatic graviception from new and classical investigations.

Recent psychophysical experiments have provided evidence of an influence of graviceptors in the human trunk on the perception of body posture that equals or even surpasses the contribution of the otoliths. A reinterpretation of classical results on mammals, notably of the Utrecht school, leads to the conclusion that somatic graviceptors also partake in controlling the posture of eyes, neck and limbs.

Animals↗
PubMed1995

New diagnostic tests for the function of utricles, saccules and somatic graviceptors.

Clinical tests that allow us to discriminate between utricular, saccular and somatic effects on gravity perception and control are desirable but wanting. A new test battery is presented which combines four experimental paradigms based on the subjective horizontal body position (SHP), namely, a test on a tiltable board and on a sled centrifuge under varied leg position, with two paradigms based on the subjective visual vertical (SVV). It is shown by a combination of experiments and deductions, that, why, and how these tests can separate the effects of otoliths from those of somatic graviceptors, the effects of the utricles from those of the saccules, and the effects of the constituents of the somatic graviception from each other. The present study demonstrates the capabilities of the tests as well as their limitations.

Animals↗
PubMed1993

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↗
PubMed1993

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↗
PubMed1992

Somatic versus vestibular gravity reception in man.

In order to assess the effect of extravestibular gravity receptors on perception and control of body position against that of the otoliths, the subject (S) is exposed to gravitoinertial forces along the spinal (Z) axis on a tiltable board and on a sled centrifuge. It turns out that (1) both effects, on average, are equally strong, although with considerable variance between Ss; (2) the centroid of the mass(es) governing the somatic receptors lies near the centroid of the body; and (3) somatic gravity reception contains two distinctly different systems. Both appear unimpaired in paraplegic Ss with total bilateral sensory loss (TSL) from the 5th to the 1st lumbar spinal segment. One, the truncal system, is eliminated with TSL from the 11th thoracic segment upwards. Yet another is still functioning with TSL up to and including the 6th cervical segment, with the same effectiveness throughout this range. Hence it must be mediated by vagal or, less likely, sympathetic afference, that is, probably, by the influence of gravity on the cardiovascular system. That the afference of the truncal system appears to enter the cord at the last two thoracic segments supports earlier conjectures about a supererogatory static function of the kidneys. In fact, on the tiltable board, 7 bilaterally nephrectomized Ss behaved like paraplegics with TSL between T11 and C6, yet differed significantly in the predicted direction from the normal controls.

Gravitation↗
PubMed1990

Evaluation of retinal orientation and gaze direction in the perception of the vertical.

The orientation of the median plane of the eye with respect to the head varies with gaze direction according to Listing's Law. The subjective vertical (SV), however, is known to be only partially affected by these involuntary variations of eye orientation. In order to learn more about the compensatory process underlying this finding, six normal-sighted young subjects were tested monocularly and binocularly in eight directions of gaze. The results show that: (1) the SVs, determined monocularly for both eyes, fall on corresponding retinal meridians, the binocular SV-settings generally lie between the monocular ones; (2) the tilt of the SV is not linearly related to the tilt angle of the median plane of the eye, as hitherto assumed. On theoretical considerations, the dependence of ocular tilt on gaze direction may be decomposed into three components, which are all treated differently in the compensation process. We interpret these results in the following way: the SV is determined from the sensorially fused image of both eyes and the tilts of the eyes are accounted for by an extra-retinal signal which is common to both eyes. The characteristics of the compensation mechanism may be explained by an extra-retinal signal which relies on information about gaze direction and Listing's Law. Such a signal might be derived from an efference copy of gaze direction commands.

Eye Movements↗
PubMed1989

The role of the pitched-up orientation of the otoliths in two recent models of the subjective vertical.

Two recent models of the subjective vertical (SV), proposed by Dai, Curthoys and Halmagyi (1989; abbr. DCH-model) and by Mittelstaedt (1983a, b, 1988; abbr. M-model), respectively, are analyzed, experimentally tested and evaluated with regard to the role they attribute to the consequences of the pitched-up orientation of the labyrinth in the human skull. In the DCH-model the response to the resulting pitch shear is, after subtraction from a constant reference and normalization, multiplied with the response to roll shear and thence leads to the well-known Müller- and Aubert-deviations from veridicality. In the M-model the pitched-up orientation is accounted for by a shortcut type of coordinate transformation: subtraction of the saccular afference from the utricular response to pitch yields the head-fixed X-(pitch)component, whereas their addition yields the head-fixed Z-component. The SV will be veridical in a normal range of head positions if the amplitude of the saccular response is related to the utricular one as the sine is to the cosine of the pitched-up angle (approximately 30 degrees). But then, typical deviations must result in other specifiable positions. The DCH- and the M-model are experimentally tested in positions where they predict SV-deviations of opposite sign. The results in 5 partly naive partly well-versed probands are highly significantly different from the DCH-model predictions whereas well compatible with those of the M-model. The implications for the models under scrutiny are discussed as well as for the global modelling of systems when essential internal constituents are unknown or inaccessible.

Ear, Inner↗
PubMed1989

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↗