The anatomical configuration of the eye ball in the orbit is similar to that of a ball-and-socket joint, and the arrangement of extra-ocular muscles gives the globe three rotational degrees of freedom (DOF). If one describes a three-dimensional (3D) eye position using three consecutive rotations about the horizontal, vertical, and torsional eye-fixed axes, the value of torsion, so-called false torsion, depends on the order of rotation. This is a result of the non-commutativity of rotatory movements. The problem of false torsion can be avoided by representing a 3D eye position by a single axis rotation from a chosen reference position to the current eye position, e. g. rotation vectors (Haustein, 1989). According to Listing's law, all axes about which the eye rotates from the reference position to other positions lie in a plane, so-called Listing's plane, provided the head is erect and stationary (Helmholtz, 1867). If the reference position coincides with the primary position, Listing's plane is perpendicular to the direction of gaze in primary position. Listing's law is a remarkable example, of how a motor system reduces DOF (from 3 to 2) to simplify multidimensional motor control. Characteristic changes in the orientation of Listing's plane have been reported under several conditions: Static tilts in the frontal plane of the head induce ocular counterrolling, i.e. parallel shifts of Listing's plane along the torsional axis. Static tilts in the sagittal plane counter-rotate Listing's plane about the vertical axis. Convergence leads to a temporal rotation of Listing's plane about the horizontal axis. During sleep, Listing's plane is not preserved, which implies neural implementation of Listing's law.
A paramedian area of the mesodiencephalic junction, referred to as the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) or Forel's field H, is important in the genesis of vertical saccadic eye movements. Vertical medium-lead burst neuron (MLBN) exhibits a high frequency burst of spikes that are induced 5-15 msec prior to vertical saccades. These MLBNs are located in the dorsomedial part of the Forel's field H. These MLBNs directly project to the oculomotor nucleus, forming direct synaptic connections with vertical eye movement-related motoneurons in the oculomotor nucleus. Omnipause neurons (OPNs) in the paramedian pontine reticular formation form direct inhibitory connections with vertical MLBNs. Based on these results, we postulated that vertical saccades are generated when the disynaptic excitatory input from the superior colliculus reaches the vertical MLBNs, which form direct connections with vertical motoneurons. The high frequency spike burst of the MLBNs during vertical saccades is presumed to be created by this excitatory input when the MLBNs are released from tonic inhibition caused by OPN activity.
The present study investigated the immunocytochemical localization of putative neurotransmitters/neuromodulators and their receptors in the vestibular periphery. Excitatory amino acids, such as glutamate and aspartate are abundant in hair cells, indicating that these amino acids may be candidates for hair cell transmitters. The level of glutamate is significantly higher in both types of hair cells, in contrast with the glutamine level which is significantly higher in supporting cells only. This complementary localization pattern is consistent with the existence of a glutamate-glutamine cycle between hair cells and supporting cells. The existence of NMDAR1 and AMPA type, GluR2/3, 4 glutamate receptors in the vestibular ganglion cells further supports the hypothesis that glutamate may serve as a hair cell transmitter. Primary afferent neurons (vestibular ganglion cells) can be divided into at least two chemically distinct subpopulations based on the consentrations of a neuropeptide (substance P). Substance P-immunoreactive small ganglion cells may receive information from the peripheral region of the endorgans. In addition to acetylcholine, a known efferent neurotransmitter, CGRP and GABA are also distributed in vestibular efferent neurons. The localization of CGRP- and GABA-like immunoreactivities found in the efferent nerve fibers varied according to species. This difference in the distribution patterns of neuroactive substances, found in the efferent system, may indicate that each species has a chemically (and probably functionally) distinct efferent system related to the specific environment and/or evolution.
The visual system is considered to make a limited contribution to the maintenance of the upright posture, but is available for elaboration of postural stability, that is, for minimizing body oscillations while standing. These experiments investigated the roles of the visual framework in maintaining an upright posture. Body oscillations were measured as the deviation of the center of gravity using a gravicorder. The results were as follows. Both the area and the total length of the trace of body sway increased with the distance between the body and the wall. The standing posture also became unstable as an upper part of the wall inclined in the direction of the subject. Women were more affected than men under both conditions. These results indicate that the visual framework perceived peripherally plays an important role in postural stability while standing upright.
Several benign paroxysmal positional vertigo (BPPV) treatments, including Particle Repositioning Maneuver (PRM) and Liberatory Maneuver (LM), are based on the "canalolithiasis" theory, and the usefulness of these treatments has been confirmed. We examined whether the accuracy of prognosis can be improved by checking the direction of nystagmus during treatment (secondary nystagmus). When secondary nystagmus occurred in the same direction as the provoked initial position, the efficacy of treatment was 90.9%. When secondary nystagmus was reversed, the efficacy was 30.8%. When there was no nystagmus appearing after the initial position, all cases improved. These results support the canalolithiasis theory and the spontaneous improvement rate seems to be about 30%. It is thus important to check secondary nystagmus to predict the prognosis of the BPPV after positioning treatment.
The maintenance of an upright posture in humans requires information from vision, proprioception, labyrinth and plantar mechanoreceptors. To evaluate the role of plantar mechanoreceptors, stabilometry was performed in ten normal subjects whose plantar mechanoreceptors were anaesthetized by hypothermia. After anaesthesia, area of sway (A) and accumulated shift distance length (L) increased significantly. However, L/A decreased significantly. These results were very similar to those of patients with severe bilateral labyrinthine disorders.
In 7 healthy subjects, horizontal smooth pursuit eye movements (SPEM) and horizontal optokinetic ocular reflex (OKR) were measured in the dark under the passive attitude which subjects tried to fixate at the center of a screen (passive SPEM and passive OKR). A sinusoidal spot target oscillation and a sinusoidal narrow-band (7 deg bandwidth in vertically) pattern oscillation were employed to elicit passive SPEM and passive OKR, respectively. The pattern consisted of randomly arranged fine dots. For the control, SPEM was elicited by following the spot target actively (active SPEM). Nine sinusoidal oscillations with frequencies ranging from 0.2 to 1.6 Hz and peak velocities ranging from 19 to 150 deg/s were employed. Passive OKR was clearly demonstrated, and the velocity gain of passive OKR was slightly lower than that of active SPEM under frequencies less than 0.8 Hz and was similar at 1.6 Hz. Common frequency-phase profiles were observed between passive OKR and active SPEM. However, the velocity gain of passive SPEM was much lower than that of passive OKR and depended on the amplitude of target oscillation i.e. the lower amplitude provided a higher gain. We concluded that poor response of the passive SPEM and amplitude dependency of the passive SPEM response was caused by the absence of assistance from the voluntary saccade in the passive attitude. The SPEM response for a small moving target may depend on the voluntary saccade function which maintains a target at the fovea while supplementing SPEM.
Vestibular evoked myogenic potentials (VEMPs) have been reported as a clinical test on the vestibular system. We examined VEMPs in three patients with cerebel-lopontine angle tumors (CPAT) and in six normal subjects. To record VEMPs, a positive electrode was placed on the sternocleidomastoid muscle (SCM), and a reference electrode was placed on the lateral end of the upper sternum. During the recording, subjects were instructed to rotate their heads in the direction opposite to the stimulated side to activate the SCM. Electromyographic signals from each side were amplified and bandpass filtered (20-2000 Hz). Clicks (0.1 ms, 95 dBnHL) were presented through a headphone. The stimulation rate was 5 Hz, the analysis time was 50 ms. The responses to 200 stimuli were averaged. The patients with CPAT also underwent pure tone audiometry (PTA), ABR, and caloric tests. In all normal subjects, VEMPs, a positive peak at about 13 ms and a negative peak at about 23 ms after presentation of click were recorded on the SCM ipsilateral to the stimulated side. All patients with CPAT showed abnormal VEMPs on the affected side. Two patients showed absence of VEMPs although caloric responses were present. One patient showed decreased VEMPs, but his caloric responses were normal. VEMPs in this patient disappeared after neurosurgery, although pure tone hearing and ABRs were not changed. Results of our study and a review of the literature suggested that VEMPs could reflect a function different from those of ABRs and caloric responses.