Since our visual sense performs analysis best when images remain steady on the retina, the eyes need to move. To move the eyes, optokinetic and vestibular reflexes have evolved to stabilize images on the retina during head movements. The vestibular and optokinetic systems work together to maintain clear vision during head movements. Because natural head movements are of high frequency, the visual system is impeded by relatively slow retinal processing (about 70 msec), and cannot act rapidly enough to produce compensatory eye movements that can hold images steady on the retina. In contrast, the semicircular-ocular reflex (ScOR) has a latency of less than 16 msec. The ScOR promptly produces slow phase eye movements to compensate for head rotations. To evaluate the ScOR, caloric and rotational tests are routinely used. Bárány first described the velocity step test in 1907. In 1948, Van Egmond et al. described an elaboration of this test, which they called cupulometry. These tests are rarely performed today. They have proved to be highly unreliable and not sensitive enough in the detection of lesions, because of the difficulty in producing repeatable stimuli and making accurate response measurements. The modern era of rotational testing began in the 1960s. Today all aspects of rotational testing including stimulus generation, response measurement, and data analysis are controlled by computer. The vestibular system also contains otolithic receptors that respond to linear accelerations of the head. The otolith-ocular reflex (OOR) becomes important when head translations cause a slip in the image on the retina. Off-vertical axis rotation (OVAR) is a stimulus wherein persons are rotated while the axis about which they are rotating is tilted with respect to gravity. Thus the OVAR can be used to assess the OOR. In this article, the principle and clinical application of rotational tests are introduced.
We have previously reported on the effectiveness of our in-hospital rehabilitation program for patients with treatment-resistant dizziness that have been referred to us by other hospitals. For this study, we undertook a comparative review of the respective benefits of dizziness rehabilitation in 37 ambulatory patients and 58 inpatients with Benign Paroxysmal Positional Vertigo(BPPV). We compared the therapeutic efficacy of dizziness rehabilitation in 37 ambulatory BPPV patients and 58 BPPV inpatients in the group therapy program. The Dizziness Handicap Inventory (DHI), posturography, the Short Form 8 Health Survey (SF-8; Japanese version), Self-rating Depression Scale (SDS), State-trait Anxiety Inventory (Form JY-2), Profile of Mood States (brief Japanese version) and a the period during which nystagmus disappeared captured on a CCD camera were used to evaluate the physiological and psychological effects of the respective therapy regimens. (1) A comparative review of therapy outcomes for the 37 ambulatory BPPV patients and the 58 BPPV patients receiving inpatient treatment revealed inpatient therapy to be markedly more effective in treating dizziness. (2) The results suggest that the rehabilitative effects to be gained from an in-hospital program of group therapy for dizziness both improved dizziness and provided clear evidence for a group therapy effect, thus offering both physiological and psychological improvements for patients with the symptoms of dizziness.
Appearance of the dominant peak-frequency in body sways during standing was retrospectively investigated in patients with spinocerebellar degeneration (SCD). Subjects tested were 25 patients with SCD (16 men and 9 women, ranging in age from 25 to 80; mean 55.8±15.4 yr). The disease types comprised SCA3 (n=5), SCA 6 (n=3), MSA-C (n=12), and an unknown type (n=5). Stabilometry was performed in each patient, who was asked to stand upright with a closed stance, with eyes open and eyes closed, using Anima's stabilometer G6100. The sampling time was 50 ms (20Hz) and the correcting time was 60 s. The peak-frequency was measured based on the power spectrum using the maximum entropy method (MEM). Total locus length, envelope area and velocity-vector of movement of the center of foot-pressure were also measured. The first or main peak appeared in a low frequency range between 0.10 and 0.59Hz in most of the cases with eyes open and eyes closed. It was detected in the lateral direction in 24 patients of the total number (96.0%) and in the anterior-posterior direction in 22 patients of the total (88.0%). The body sway of a frequency of 3Hz was detected in 4 patients (16.0%). Among them, the peak in the lateral direction was observed in 2 (8.0%) with eyes open or closed (one with SCA 6 and one with an unknown disease type). The peak in the anterior-posterior direction was detected in 3 (12.0%) with eyes open or closed (one each with SCA 6, MSA-C and an unknown disease type); in these subjects, the values of the total locus length divided by envelope area were more than 40. The dominant peak-frequency of 3Hz in body sways during standing did not frequently appear in patients with SCD, but it is suggested to be one of characteristic balance disorders in the disease.
An equilibrium exercise performed by a physical therapist for a patient with intractable dysequilibrium A thirty-seven-year-old woman diagnosed as having progressive bilateral vestibular dysfunction due to autoimmune inner ear disease was given equilibrium exercises by a physical therapist. The patient mainly complained of dizziness and unsteadiness in walking. She started her rehabilitation program for her vestibular dysfunction at 4 years after the onset of her complaints. She lay on a bed with a posture like a bow, curved to the left. She sat on a chair tilting to the left and recognized the line which deviated to the left as being perpendicular. The deviation at sitting might be caused due to subjective vertical deviation. We instructed her to be aware that her perpendicularity was deviated to the left and to remain aware of that deviation whenever she did anything during her activities of daily living. She was given a rolling exercise on a mat. She also performed a sitting exercise on a chair at the corner of the room, touching the walls with both her shoulders. As she sensed the walls with her shoulders, she could sit perpendicularly by using her superficial and deep feelings. She was additionally given a standing and walking exercise. In order to relearn how to stand and walk without deviation, she stood between two panels which were situated in parallel with the width between them just a little wider than the breadth of her shoulders. She could stand upright and walk straight by using her superficial and deep feelings through touching the panels with her shoulders. She was advised to watch her toes when stepping forward in order to walk straight. She was instructed to be aware of the shift of the pressure on the soles of her feet while walking. She was able to stand and walk without deviation at the time of hospital discharge.
The superior colliculus (SC) plays an important role in controlling eye and head movements. The neural organization of the pathways from the SC to motoneurons in the horizontal oculomotor system has been well analyzed, but the neural mechanisms in the vertical saccade have not yet been analyzed in detail. This article reviews the current state of knowledge of the neural mechanisms of the horizontal and vertical saccadic eye movement system, and shows that they have common features: both systems contain excitatory and inhibitory burst neurons which receive inputs from the SC and directly project to motoneurons. Our recent study showed the presence of commissural inhibition between the upper and lower fields of motor map of the bilateral SCs. This reciprocal inhibition between the medial upward saccade area in one SC and the lateral downward saccade area in the other SC is very similar to that in the vestibular system. This similarity of the patterns of reciprocal inhibition in the SC system and the semicircular canal system implies that the SC output system may use the coordinate system based on the semicircular canals.
In everyday life, self-movement of an observer causes the image of the visual scene on the retina to slip. Visual acuity is severely impaired when the retinal image of interest moves excessively. Naturally, the vestibular-ocular reflex (VOR) compensates for the observer's own movements through a short neural pathway from the vestibular organs to the eyes. The amplitude of the VOR is almost equal to the observer's head movement, but not perfect, and the residual retinal image motions are compensated for by visually driven ocular tracking systems. Recent behavioral studies on primates have revealed that there are three visual tracking systems that function to stabilize the gaze of the moving observer as visual back-up. One of these systems, the ‘ocular following response’, helps to stabilize gaze when the observer looks off to one side. The other two systems generate ‘vergence eye movements’ to help maintain binocular alignment on objects that lie ahead when the observer looks in the direction in which he or she is heading. One responds to the change in binocular parallax (disparity) and the other to the radial patterns of the optic flow. All three operate in a machine-like fashion to generate eye movements with ultra-short latencies. We conducted electrophysiological and chemical-lesion studies to determine whether the medial superior temporal (MST) area of the cerebral cortex, which is known to participate in visual motion processing, plays roles in eliciting these tracking eye movements or not. Despite their ultra-short latencies, electrophysiological studies in monkeys revealed a close relationship between ocular and neuronal responses in the MST, and lesions of the MST in both hemispheres significantly reduced the initial part of the tracking responses. Overall the results strongly support the hypothesis that the MST area is a primary site for producing the three visual tracking eye movements at ultra-short latencies.
The vestibulo-ocular reflex (VOR) and optokinetic response (OKR) work cooperatively to compensate for the eye position during head movement. The floccular region of the cerebellar cortex regulates the amplitude and timing of these reflexes through the inhibitory synaptic outputs of Purkinje neurons. Both the VOR and OKR undergo adaptive changes, when the retinal slip causing the blur of visual image occurs continuously. Long-term depression (LTD), a type of plasticity at the synapses between parallel fibers and a Purkinje cell, in the flocculus, has been proposed to contribute to the adaptation of VOR and OKR. The progress in the study on cerebellar LTD has revealed numbers of molecules involved in LTD. Glutamate receptor-like molecule δ2 (GluD2) and delphilin are proteins specifically expressed at the postsynaptic membrane of Purkinje neurons and involved in LTD. GluD2 knockout mice show failure of LTD induction and impaired adaptation of VOR and OKR. On the other hand, delphilin knockout mice show facilitation of the LTD induction and the enhanced adaptation of OKR. These results suggest that the cerebellar LTD is involved in the adaptation of reflex eye movements. In addition, a delayed OKR is observed in the GluD2 knockout mice, which seems to be caused by the abnormal Purkinje neuron activities.
In vivo visualization of neural circuits located in the deep brain area with a micro-endoscope Recent advance in molecular biology enabled us to label specific sets of neurons with several fluorescence proteins. To visualize these labeled cells in vivo, we developed a micro-endoscope system which can be inserted into the brain and can provide circuit images with cellular resolution. The micro-endoscope was made from a commercially available image fiber in which three to six thousand optical fibers are bundled. For functional imaging of neurons, we combined this endoscope with a custom-built confocal laser scanner fitted with a high-speed resonance galvomotor, and obtained confocal endoscopic images at a frame rate of 18-100 fps depending on the number of vertical lines. Our endoscope system visualized fluorescence labeled individual neuron, Ca response in vivo and provided stable images of the neural circuits even in head-restricted alert mice.
We are planning an experiment to determine why and how we perceive tilt in space. Tilt perception was thought to disappear in space, where gravity does not exist as a reference for tilt. Nevertheless, astronauts reported perceived head roll-tilt and ocular counter-rolling (OCR) in a centrifuge experiment in space. To explain these unexpected findings, during long-term space life we will measure the subjective visual body axis (SVBA) and OCR at various body roll-tilt angles relative to the inner-cabin vertical, with and without head roll-tilt relative to the body, with and without visual information. We are also currently performing ground-based experiments to collect control data. In this article, we introduce these space and terrestrial experiments.