MRI magnetic field stimulates rotational sensors of the brain

Vertigo in and around magnetic resonance imaging (MRI) machines has been noted for years [1, 2]. Several mechanisms have been suggested to explain these sensations [3, 4], yet without direct, objective measures, the cause is unknown. We found that all of our healthy human subjects developed a robust nystagmus while simply lying in the static magnetic field of an MRI machine. Patients lacking labyrinthine function did not. We use the pattern of eye movements as a measure of vestibular stimulation to show that the stimulation is static (continuous, proportional to static magnetic field strength, requiring neither head movement nor dynamic change in magnetic field strength) and directional (sensitive to magnetic field polarity and head orientation). Our calculations and geometric model suggest that magnetic vestibular stimulation (MVS) derives from a Lorentz force resulting from interaction between the magnetic field and naturally occurring ionic currents in the labyrinthine endolymph fluid. This force pushes on the semicircular canal cupula, leading to nystagmus. We emphasize that the unique, dual role of endolymph in the delivery of both ionic current and fluid pressure, coupled with the cupula's function as a pressure sensor, makes magnetic-field-induced nystagmus and vertigo possible. Such effects could confound functional MRI studies of brain behavior, including resting-state brain activity.     

A theoretical and experimental determination of vestibular dynamics in caloric stimulation

A biophysical model was established for predicting the temperature at the horizontal semicircular canal and the horizontal slowphase velocity (SPV) in response to water or air irrigations of various flow rates and time varying temperatures. This model considered the thermal characteristics and blood flows of the skin and temporal bone. Long-term caloric stimulation with air at 101/min was performed on 16 healthy subjects using step input and pseudo-random temperature variations. The results were in agreement with the predicted static and dynamic SPV responses. The model predictions were also correct in the case of water irrigations performed by other investigations with respect to SPV and the temperatures measured at the semicircular canal. An association of higher responses with lower response time as postulated by the model, was confirmed by the experimental results, and was thought to be due to vasomotor changes in the skin. The most important model parameters were the length of the heat transmission path, the effective heat conductivity of the skin, and the blood flow and heat conductivity of the temporal bone, in addition to the flow rates and thermodynamic properties of the irrigation. These parameters should thus be considered in further caloric stimulation studies for a better understanding of the large variability in vestibular responses.This research was supported in part by Program Project Grant HL 11747.     

A new light on caloric test--what was disclosed by three dimensional analysis of caloric nystagmus?

For better understanding of caloric nystagmus, this phenomenon will be reviewed historically in three stages. 1) The first light on caloric nystagmus was thrown by Barany 1906. Through direct observation of eye movements, Barany established the caloric test as an important tool to determine the side of lesion for vertigo. 2) The second light is shed by electrooculogram (EOG) from the late 1950th. EOG enabled qualitative analysis of caloric nystagmus, and proved Barany's convection theory, but resulted in neglect of vertical and roll eye movements. 3) The third light is gained by 3D recording of eye movements started from the late 1980th. 3D recordings of eye movements enabled us to analyze the spatial orientation of caloric nystagmus, and disclose the close correlation of the nystagmus components in the head vertical and the space vertical planes, suggesting a contribution of the velocity storage integrator. The 3D property of caloric nystagmus will be explained in detail.     

Numerical modeling of the cupular displacement and motion of otoconia particles in a semicircular canal

Balance is achieved and maintained by a balance system called a labyrinth that is composed of three semicircular canals and the otolith organs that sense linear gravity and acceleration. Within each semicircular canal, there is a gelatinous structure called the cupula, which is deformed under the influence of the surrounding endolymph. One of the balance disorders is benign paroxysmal positional vertigo, and one of the pathological conditions that have been identified as possible causes of this syndrome is canalithiasis—disturbance of the endolymph flow and cupular displacement caused by the free-moving otoconia particles within the lumen of the canal. Analysis of phenomena occurring within the semicircular canal can help to explain some balance-related disorders and the response of the vestibular system to external perturbations under various pathological conditions. Numerical simulations allow a study of the influence of a wide range of factors, without the need to perform experiments and clinical examinations. In case of canalithiasis, an accurate explanation and tracking of the motion of otoconia particles in vivo is obviously nearly impossible. In this study, a numerical model was developed to predict the motion of otoconia particles within the semicircular canal and the effect of the endolymph flow and particles on the deformation of the cupula.     

On the Vertigo Due to Static Magnetic Fields

Vertigo is sometimes experienced in and around MRI scanners. Mechanisms involving stimulation of the vestibular system by movement in magnetic fields or magnetic field spatial gradients have been proposed. However, it was recently shown that vestibular-dependent ocular nystagmus is evoked when stationary in homogenous static magnetic fields. The proposed mechanism involves Lorentz forces acting on endolymph to deflect semicircular canal (SCC) cupulae. To investigate whether vertigo arises from a similar mechanism we recorded qualitative and quantitative aspects of vertigo and 2D eye movements from supine healthy adults (n = 25) deprived of vision while pushed into the 7T static field of an MRI scanner. Exposures were variable and included up to 135s stationary at 7T. Nystagmus was mainly horizontal, persisted during long-exposures with partial decline, and reversed upon withdrawal. The dominant vertiginous perception with the head facing up was rotation in the horizontal plane (85% incidence) with a consistent direction across participants. With the head turned 90 degrees in yaw the perception did not transform into equivalent vertical plane rotation, indicating a context-dependency of the perception. During long exposures, illusory rotation lasted on average 50 s, including 42 s whilst stationary at 7T. Upon withdrawal, perception re-emerged and reversed, lasting on average 30 s. Onset fields for nystagmus and perception were significantly correlated (p<.05). Although perception did not persist as long as nystagmus, this is a known feature of continuous SSC stimulation. These observations, and others in the paper, are compatible with magnetic-field evoked-vertigo and nystagmus sharing a common mechanism. With this interpretation, response decay and reversal upon withdrawal from the field, are due to adaptation to continuous vestibular input. Although the study does not entirely exclude the possibility of mechanisms involving transient vestibular stimulation during movement in and out of the bore, we argue these are less likely.     

A model of Alexander's law of vestibular nystagmus

The observation that the amplitude of vestibular nystagmus grows as gaze is increased in the direction of the nystagmus fast phase and diminished with gaze in the opposite direction is known as Alexander's law. We have developed an analog computer model to simulate Alexander's law in nystagmus secondary to dysfunction of a semicircular canal. The model utilizes relevant brainstem anatomy and physiology and includes gaze modulation of vestibular signals and push-pull integration to create eye positition commands. When simulating normally functioning semicircular canals, the model produced no nystagmus. When simulating total impairment of the canal on one side with gaze directed maximally in the opposite direction, the model produced a large amplitude nystagmus with linear slow phases directed toward the affected side. As gaze was changed from far contralateral to ipsilateral, the nystagmus gradually diminished to zero. When simulating partial impairment of one canal, the nystagmus was smaller in amplitude and absent in ipsilateral gaze.     

The bony labyrinth of Neanderthals

This paper presents a comprehensive comparative analysis of the Neanderthal bony labyrinth, a structure located inside the petrous temporal bone. Fifteen Neanderthal specimens are compared with a Holocene human sample, as well as with a small number of European Middle Pleistocene hominins, and early anatomically modern and European Upper Palaeolithic humans. Compared with Holocene humans the bony labyrinth of Neanderthals can be characterized by an anterior semicircular canal arc which is smaller in absolute and relative size, is relatively narrow, and shows more torsion. The posterior semicircular canal arc is smaller in absolute and relative size as well, it is more circular in shape, and is positioned more inferiorly relative to the lateral canal plane. The lateral semicircular canal arc is absolutely and relatively larger. Finally, the Neanderthal ampullar line is more vertically inclined relative to the planar orientation of the lateral canal. The European Upper Palaeolithic and early modern humans are most similar, although not fully identical to Holocene humans in labyrinthine morphology. The European Middle Pleistocene hominins show the typical semicircular canal morphology of Neanderthals, with the exception of the arc shape and inferiorly position of the posterior canal and the strongly inclined ampullar line. The marked difference between the labyrinths of Neanderthals and modern humans can be used to assess the phylogenetic affinities of fragmentary temporal bone fossils. However, this application is limited by a degree of overlap between the morphologies. The typical shape of the Neanderthal labyrinth appears to mirror aspects of the surrounding petrous pyramid, and both may follow from the phylogenetic impact of Neanderthal brain morphology moulding the shape of the posterior cranial fossa. The functionally important arc sizes of the Neanderthal semicircular canals may reflect a pattern of head movements different from that of modern humans, possibly related to aspects of locomotor behaviour and the kinematic properties of their head and neck.     

A simple technique for thermal stimulation of single semicircular canals.

Two simple thermodes for stimulating single semicircular canals have been designed in order to elicit eye nystagmus. The first, for warm stimulation, is based on the Joule effect: a silver needle is warmed up by means of a coiled nickrome wire. The temperature at the thermode tip is function of current intensity. The thermode for cold stimulation is a device in which a circulating freezing mixture cools a silver needle. The temperature at the thermode tip is proportional to the temperature of the freezing mixture.     

Oculomotor areas in the rabbit's midbrain and pretectum

The role of some meso- and diencephalic structures in eye movements was investigated by ablation and stimulation experiments. Optokinetic nystagmus was abolished by small lesions in the lateral pretectum, but not by complete removal of the superior colliculi. Stimulation of the superior colliculus and other visual centers was effective in eliciting nystagmus (slow phase ipsilateral), but the most efficient trigger zones are found in the lateral pretectum and the midbrain tegmentum. Only from these areas could nystagmus still be elicited after degeneration of the primary optic fibers. The lateral pretectal trigger zone is probably identical with the nucleus of the optic tract. It is postulated that this nucleus is an essential station for horizontal optokinetic reactions. Saccades were obtained by stimulation of the mesencephalic central grey, but not for any visual centers such as the superior colliculus.     

Oculomotor areas in the rabbits midbrain and pretectum.

The role of some meso- and diencephalic structures in eye movements was investigated by ablation and stimulation experiments. Optokinetic nystagmus was abolished by small lesions in the lateral pretectum, but not by complete removal of the superior colliculi. Stimulation of the superior colliculus and other visual centers was effective in eliciting nystagmus (slow phase ipsilateral), but the most efficient trigger zones are found in the lateral pretectum and the midbrain tegmentum. Only from these areas could nystagmus still be elicited after degeneration of the primary optic fibers. The lateral pretectal trigger zone is probably identical with the nucleus of the optic tract. It is postulated that this nucleus is an essential station for horizontal optokinetic reactions. Saccades were obtained by stimulation of the mesencephalic central grey, but not for any visual centers such as the superior colliculus.     

The primate subarcuate fossa and its relationship to the semicircular canals part II: adult interspecific variation

Studies have reported an empirical link between the size of the semicircular canals and locomotor agility across adult primates. In this paper, we investigate the possibility that this relationship does not follow from the function of the semicircular canals to sense head rotations, but rather reflects spatial constraints imposed by the subarcuate fossa. The latter sits among the three canals and contains the petrosal lobule of the cerebellar paraflocculus, a structure involved in neural processing of locomotion-related eye movements. Hence, it is feasible that agility-related variations of lobule and fossa size affect the arc size of the surrounding semicircular canals. The present study tests such hypothetical correlations by evaluating canal size, fossa size, and agility among extant adult primates. Phylogenetically informed multivariate regression analyses show that, after controlling for body mass, the size of the subarcuate fossa has a significant positive effect on the overall size of the anterior canal and the width of the posterior canal. Multivariate regressions involving the height of the posterior canal and overall size of the lateral canal are not significant. Further bivariate analyses confirm that fossa size is unlikely to play a role in the previously reported link between agility and the size of the posterior and lateral canals. However, fossa size, especially its opening though the arc of the anterior canal, cannot be excluded as a factor that influences the size of the anterior canal more than agility. The findings show that the most reliable functional signals pertaining to locomotion in species that possess a patent subarcuate fossa are likely to come from the lateral canal and are least likely to come from the anterior canal.     

Heat exchange in the cryosurgery of Menière's disease: Experimental and clinical studies

The temperature course in the lateral semicircular canal and in the facial canal was studied in experiments during freezing of the semicircular canal. The course of the temperature was measured with thermocouples. Concurrently, the heat flow was measured, and also the total heat exchange was measured throughout the freezing period by a thermoelectric heat flowmeter incorporated in the cryotip. The measurements showed correlation between the total amount of heat exchanged, the freezing time, and the temperature in the semicircular canal. This correlation was utilized to assess and calculate (the temperature of the lateral semicircular canal) the course of the cryoprocess in vivo, where it is possible to measure the heat flow and the total heat exchange during the freezing period only.

2. Results upon Vertigo

     

15.

Semicircular duct and ampulla dimensions in cat,guinea pig and man     

I. S. Curthoys  C. H. Markham  E. J. Curthoys 《Journal of morphology》1977,151(1):17-34

Predictions from the classic theory of semicircular canal operation, the torsion pendulum model, depend upon labyrinthine dimensions and the physical properties of the endolymph. The dimensions of the semicircular canal, duct and ampulla in cat, guinea pig and man were determined from measurements of magnified sections of decalcified temporal bones. Estimates of the effect of shrinkage were obtained from measures in fresh material and it appears shrinkage is probably only a fairly small factor. The dimensions so obtained were used to provide new estimates of the short time constant and other mechanical parameters of the torsion pendulum model in the three species.     

16.

An analysis of the mechanical forces in each semicircular canal of the cat under single and combined rotations     

Maximo Valentinuzzi 《Bulletin of mathematical biology》1967,29(2):267-289

The vector equation for the general motion of a body in an inertial system is used to analyze the accelerations in the semicircular canals of the cat when the head undergoes rotation about a vertical axis only, rotation about the naso-occipital axis only, and both rotations simultaneously. The corresponding mean forces and mean pressures in the endolymph are calculated by means of a closed line integral along each canal circumference. The importance of the area of the semicircular canal and of its orientation in space become evident. One can see through this mathematical analysis that the input pattern received by the labyrinthine system depends on a set of well-specified geometrical and mechanical conditions, which must be precisely evaluated in order to interpret the nystagmic outputs.     

17.

On the predominance of anti-compensatory eye movements in vestibular nystagmus     

R. Schmid  F. Lardini 《Biological cybernetics》1976,23(3):135-148

The predominance of anti-compensatory eye movements in vestibular nystagmus recorded during sinusoidal and post-rotational tests is interpreted in terms of a mathematical model of the vestibulo-ocular system. Namely, a direct pathway between the vestibular nuclei and the saccadic mechanism is assumed. In the range of frequencies of natural head movements this pathway carries on a signal proportional to head angular velocity. Therefore, during active head movements the saccadic mechanism is forced to produce quick eye rotations in the direction of head movement and, thus, to cooperate in the task of picking up visual targets outside the visual field. During passive head movements giving rise to nystagmus the assumed pathway contributes to reduce the error in eye resetting due to the saccadic delay. Analytical considerations and simulation results seem to prove the adequacy of the proposed model.Work supported by the National Research Council (C.N.R.), Rome, Italy     

18.

Morphology of the mammalian vestibulo-ocular reflex: the spatial arrangement of the human fetal semicircular canals and extraocular muscles     

Cox PG  Jeffery N 《Journal of morphology》2007,268(10):878-890

The vestibulo-ocular reflex is the system of compensatory ocular movements in response to stimulation of the kinetic labyrinth seen in all vertebrates. It allows maintenance of a stable gaze even when the head is moving. Perhaps the simplest influence on the VOR is the spatial orientation of the planes of the semicircular canals relative to the extraocular muscles. It is hypothesized that the extraocular muscles are in parallel alignment with their corresponding semicircular canals in order to reduce the amount of neural processing needed and hence keep reflex times to a minimum. However, despite its obvious importance, little is known of this spatial arrangement. Moreover, nothing is known about any ontogenetic changes in the relative orientations of the extraocular muscles and semicircular canals. The morphologies of fetal and adult specimens of Homo sapiens were examined using magnetic resonance (MR) images. Three-dimensional co-ordinate data were taken from the images and used to calculate vector equations of the extraocular muscles and planes of best fit for the semicircular canals. The relative orientations of the muscles and canals were then calculated from the vectors and planes. It was shown that there are significant correlations between both the anterior and lateral semicircular canals and their corresponding extraocular muscles during ontogeny. In the case of the lateral canal with the medial rectus, the lateral canal with the lateral rectus, and the anterior canal with the inferior oblique, the trend is towards, though never reaching, alignment, whereas the anterior canal and the superior rectus muscle move out of alignment as age increases. Furthermore, it was noted that none of the six muscle-canal pairs is in perfect alignment, either during ontogeny or in adulthood. It was also shown that the three semicircular canals are not precisely orthogonal, but that the anterior and posterior canals form an angle of about 85 degrees , while the anterior and lateral canals diverge by approximately 100 degrees . Overall, it was shown that there is significant reorientation of the extraocular muscles and semicircular canals during ontogeny, but that, in most cases, there is little realignment beyond the fetal period.     

19.

Convergence rotatory nystagmus under unusual conditions of semicircular canal and otolith stimulation     

O. A. Vorob’ev  S. D. Chistov  T. A. Rybachenko 《Human physiology》2009,35(5):569-575

The characteristics of interaction between two vestibular subsystems (otiliths and semicircular canals) were studied by means of binocular (bilateral) videooculographic recording of eye movements in 43 men aged from 19 to 41 years that had been found healthy upon aviation physical examination. The time course of horizontal vestibular nystagmus was analyzed separately for each eye in subjects who bent forward and straightened up in the sagittal plane while being rotated about the vertical body axis in an electrically driven rotating chair. This combined rotation caused interocular asymmetric nystagmus in 91% of the subjects and convergence rotatory nystagmus in 42% of the subjects. A hypothesis on the mechanism of interocular asymmetric nystagmus caused by the combined rotation and convergence rotatory nystagmus as its special case has been advanced. The hypothesis allows for independent nystagmic mechanisms (subsystems) for the right and left eyes.     

20.

Position and position variations of the canal system of the temporal bone in frontal section   总被引：1，自引：0，他引：1  

J Lang  G St?ber 《Gegenbaurs morphologisches Jahrbuch》1987,133(2):249-289

Estimated are: 1. The axis of the internal acoustic meatus to the horizontal plane in adults and postnatal changes. 2. Eight coronal sections of the temporal bone have been selected to localize the canal systems and structures in the petrous part of the temporal bone and their variations. 3. Described are the different parts of the facial canal, the carotic canal, the auditive tube, the tensor tympani muscle, the major petrosal nerve, and its distances to the carotic canal, the cochlea, the internal acoustic meatus, the supra- and infracochlear cells, the fenestra vestibuli, the fossa jugularis, the canaliculus cochleae, the vestibulum and the semicircular canals. This report includes the development of the supravestibular and other mastoideal cells in the neighbourhood of the canal systems of the petrous bone and the vestibular aqueduct and sac. Estimated are also the distances between the different canal systems. 4. The investigations are discussed with our earlier researches and the results of other researchers and its diagnostic in clinical importance.     

	No Vertigo	Improved	Unchanged
Number of patients	7	5	3

Semicircular duct and ampulla dimensions in cat,guinea pig and man

Predictions from the classic theory of semicircular canal operation, the torsion pendulum model, depend upon labyrinthine dimensions and the physical properties of the endolymph. The dimensions of the semicircular canal, duct and ampulla in cat, guinea pig and man were determined from measurements of magnified sections of decalcified temporal bones. Estimates of the effect of shrinkage were obtained from measures in fresh material and it appears shrinkage is probably only a fairly small factor. The dimensions so obtained were used to provide new estimates of the short time constant and other mechanical parameters of the torsion pendulum model in the three species.     

An analysis of the mechanical forces in each semicircular canal of the cat under single and combined rotations

The vector equation for the general motion of a body in an inertial system is used to analyze the accelerations in the semicircular canals of the cat when the head undergoes rotation about a vertical axis only, rotation about the naso-occipital axis only, and both rotations simultaneously. The corresponding mean forces and mean pressures in the endolymph are calculated by means of a closed line integral along each canal circumference. The importance of the area of the semicircular canal and of its orientation in space become evident. One can see through this mathematical analysis that the input pattern received by the labyrinthine system depends on a set of well-specified geometrical and mechanical conditions, which must be precisely evaluated in order to interpret the nystagmic outputs.     

On the predominance of anti-compensatory eye movements in vestibular nystagmus

The predominance of anti-compensatory eye movements in vestibular nystagmus recorded during sinusoidal and post-rotational tests is interpreted in terms of a mathematical model of the vestibulo-ocular system. Namely, a direct pathway between the vestibular nuclei and the saccadic mechanism is assumed. In the range of frequencies of natural head movements this pathway carries on a signal proportional to head angular velocity. Therefore, during active head movements the saccadic mechanism is forced to produce quick eye rotations in the direction of head movement and, thus, to cooperate in the task of picking up visual targets outside the visual field. During passive head movements giving rise to nystagmus the assumed pathway contributes to reduce the error in eye resetting due to the saccadic delay. Analytical considerations and simulation results seem to prove the adequacy of the proposed model.Work supported by the National Research Council (C.N.R.), Rome, Italy     

Morphology of the mammalian vestibulo-ocular reflex: the spatial arrangement of the human fetal semicircular canals and extraocular muscles

The vestibulo-ocular reflex is the system of compensatory ocular movements in response to stimulation of the kinetic labyrinth seen in all vertebrates. It allows maintenance of a stable gaze even when the head is moving. Perhaps the simplest influence on the VOR is the spatial orientation of the planes of the semicircular canals relative to the extraocular muscles. It is hypothesized that the extraocular muscles are in parallel alignment with their corresponding semicircular canals in order to reduce the amount of neural processing needed and hence keep reflex times to a minimum. However, despite its obvious importance, little is known of this spatial arrangement. Moreover, nothing is known about any ontogenetic changes in the relative orientations of the extraocular muscles and semicircular canals. The morphologies of fetal and adult specimens of Homo sapiens were examined using magnetic resonance (MR) images. Three-dimensional co-ordinate data were taken from the images and used to calculate vector equations of the extraocular muscles and planes of best fit for the semicircular canals. The relative orientations of the muscles and canals were then calculated from the vectors and planes. It was shown that there are significant correlations between both the anterior and lateral semicircular canals and their corresponding extraocular muscles during ontogeny. In the case of the lateral canal with the medial rectus, the lateral canal with the lateral rectus, and the anterior canal with the inferior oblique, the trend is towards, though never reaching, alignment, whereas the anterior canal and the superior rectus muscle move out of alignment as age increases. Furthermore, it was noted that none of the six muscle-canal pairs is in perfect alignment, either during ontogeny or in adulthood. It was also shown that the three semicircular canals are not precisely orthogonal, but that the anterior and posterior canals form an angle of about 85 degrees , while the anterior and lateral canals diverge by approximately 100 degrees . Overall, it was shown that there is significant reorientation of the extraocular muscles and semicircular canals during ontogeny, but that, in most cases, there is little realignment beyond the fetal period.     

Convergence rotatory nystagmus under unusual conditions of semicircular canal and otolith stimulation

The characteristics of interaction between two vestibular subsystems (otiliths and semicircular canals) were studied by means of binocular (bilateral) videooculographic recording of eye movements in 43 men aged from 19 to 41 years that had been found healthy upon aviation physical examination. The time course of horizontal vestibular nystagmus was analyzed separately for each eye in subjects who bent forward and straightened up in the sagittal plane while being rotated about the vertical body axis in an electrically driven rotating chair. This combined rotation caused interocular asymmetric nystagmus in 91% of the subjects and convergence rotatory nystagmus in 42% of the subjects. A hypothesis on the mechanism of interocular asymmetric nystagmus caused by the combined rotation and convergence rotatory nystagmus as its special case has been advanced. The hypothesis allows for independent nystagmic mechanisms (subsystems) for the right and left eyes.     

Position and position variations of the canal system of the temporal bone in frontal section

Estimated are: 1. The axis of the internal acoustic meatus to the horizontal plane in adults and postnatal changes. 2. Eight coronal sections of the temporal bone have been selected to localize the canal systems and structures in the petrous part of the temporal bone and their variations. 3. Described are the different parts of the facial canal, the carotic canal, the auditive tube, the tensor tympani muscle, the major petrosal nerve, and its distances to the carotic canal, the cochlea, the internal acoustic meatus, the supra- and infracochlear cells, the fenestra vestibuli, the fossa jugularis, the canaliculus cochleae, the vestibulum and the semicircular canals. This report includes the development of the supravestibular and other mastoideal cells in the neighbourhood of the canal systems of the petrous bone and the vestibular aqueduct and sac. Estimated are also the distances between the different canal systems. 4. The investigations are discussed with our earlier researches and the results of other researchers and its diagnostic in clinical importance.