The visual processing system functions too slowly to terminate saccadic eye movement properly when the movement brings the target image to the fovea. The brain must have an intrinsic neural system which monitors the eye movement and position. In a model proposed by David A. Robinson in 1975, a neural integrator for gaze holding was proposed and assumed to cancel out further movement using negative feedback subtraction from the desired eye position signal encoded with respect to the head centered reference frame. Robinson's model representing such simple and strong logic still serves as a basic frame of investigation. This review tried to present the essence of the model and summarize related issues and models.
The ncural integrator has been at least a conceptual portion of the vestibular system since the earliest vestibular studies. Like others before him, Robinson (e.g. 1975) observed adequate stimulation of the vestibular end organ by angular acceleration results in eye position as the final output of the vestibular-ocular reflex (VOR). Therefore, he reasoned that there must be two levels of integration, one attributable to the mechanics of the cupula and the other to an hypothesized neural integrator. He further suggested that this integrator is common for all subsystems of conjugate eye movement. The results of both single-unit recordings and lesions suggest that the interstitial nucleus of Cajal (INC) and the nucleus prepositus hypoglossi (NPH) of both cats and monkeys are part of the vertical and horizontal oculomotor integrators, respectively. Both structures contain neurons that have discharge components related to velocity and position, as might be expected of a neural integrator. In addition, injury to either structure weakens integrator function. The clearest effect of lesions is on gaze-holding ability. Damage to either the NPH or the INC impairs an animal's ability to maintain eccentric gaze. The INC alone cannot perform the integrating function. Strong reciprocal connections between the INC and the vestibular nuclei play an important role in vertical integration. In addition, the cerebellar flocculus may also be included in the integrator. Some of the oculomotor subsystems may have separate integrators.
A method of quantitatively evaluating discontinuity in pursuit eye movements (PEM) using spectral analysis is proposed. In this method, comparison of the measured values under various stimulus frequencies (FSs) was considered possible. The power ratio (PR) that represented the degree of mixing of discontinuous component into PEM was calculated. The lower limit of the range for spectral analysis was the FS. The upper limit was defined assuming the following model. PEM mixed in the discontinuous components was considered to be a wave composed of synchronized sine and rectangular waves with frequencies equal to FSs. The rectangular wave was composed of the sine wave with elementary frequency and its odd harmonics. The upper limit was defined as follows; on all FSs, the same number of odd harmonics existed within the nyquist frequency. The denominator of PR was the total power of the range for spectral analysis described above. The numerator was the total power between the frequency of the third harmonic and the upper limit of the range for spectrum analysis. The horizontal PEM was measured in six adults aged 22 to 37 years old and six preschool children aged five years old, whose PEM did not reach the adult level. FSs were 0.3-, 0.5-, 0.7-Hz and the amplitude of stimulus movement was ±17.5° visual angle. For both groups, PEM became less smooth with increased FSs, and there was a significant increase in PR. At all FSs, PRs of children were significantly higher than those of adults. This finding suggests the validity of this method.
To evaluate dynamic postural control, the Body Tracking Test (BTT) was developed. This paper evaluated tracking ability in normal subjects. In this test, the subject standing on the stabilometer was asked to track a moving optical target displayed on the screen by changing his/her bodily position. Stabilometric data of his/her body movement was recorded and computer-analyzed. The horizontal and vertical span of the target was 15 cm on a 14-inch color display monitor. The optical target moved in a horizontal or vertical manner. The movement of the optical target was programmed by controlled triangular waves and the optical speed was 0.125 Hz. The sampling time was 50 mseconds for a total of 60 seconds. In BTT, the gain for target and tracking was 2.0. Healthy young adult volunteers were tested. The age range was 21 to 37 years. There were 87 subjects tested (Male 49, Female 38). The mean age was 27.6. Total distance of bodily movements was adjusted by body weight. The ratio of the total distance of bodily movements divided by total target movement was called "Index of BTT movement". The index of BTT movement was 1.40±0.3 on the average, but the index of BTT movement was not appropriate for evaluating tracking. We were able to extract classifications from the data, which were called "5 grades of evaluating tracking ability". Grades A to E were defined as 30% Grade A, 43.7% Grade B, 21.3% Grade C, 5% Grade D, and 0% Grade E. This evaluation was subjective method, but appropriate for evaluating BTT.
We report an unusual case of congenital nystagmus in a patient who could both suppress and release her nystagmus at will. A 37-year-old woman complained of blurred vision on right lateral gaze. Initially, she had pendular nystagmus only on right lateral gaze. Nystagmus was considered congenital, because the patient showed unilateral inversion of OKN, the wave form of nystagmus coincided with the asymmetric pendular type of Dell'Osso's classification and no gaze-dependent nystagmus was observed with closed eyes. On follow up, nystagmus also appeared on left lateral gaze and downward gaze. Furthermore, she acquired the ability to control her nystagmus, starting and stopping voluntarily. She could stop her nystagmus by taking the eye position for near fixation. ENG showed left monocular adduction when she stopped her nystagmus. To release the nystagmus, she appeared to focus at a distance with vacant eyes. However, the patient did not show spontaneous nystagmus in the dark, so we speculated that some visual input was needed for the release of nystagmus.
Sixty-one patients complaining of repeated vertigo, dizziness and equilibrium disturbances with cochlear symptoms treated between January 1992 and December 1994 were analyzed statisticaly. We diagnosed 35 patients with Meniere's disease and 14 patients with vertigo of central origin. We compared findings in Meniere's disease with those of vertigo of central origin by inquiry, examination and evaluating progression. The following characteristics indicate vertigo of central origin. 1. As symptoms associated with vertigo, bilateral tinnitis, headache, hot-headed sensation, laguidness and neurological symptom were found more frequently. 2. Regarding categories of vertigo, floating sensations were found more frequently. 3. Regarding duration of vertigo, most episodes were less than 19 minutes. 4. On general examinations, extraordinary blood pressure, hyperlipemia were found more frequently.
Some patients with acute or chronic vestibular disorders continue to complain of dizziness and disequilibrium. Although treated with medication, their activities of daily living (ADL) and occupational activities are restricted. Equilibrium training is applied to reduce the disability of the patients. Methods: Before training is started, the degree of impairment, disability and handicaps of the patients were evaluated. To evaluate subjective and objective disabilities, the ADL were investigated by questionnaire and equilibrium function tests were performed. Three times a day at home, patients performed several somewhat difficult exercises selected from their defective movements. Training continued until their disabilities were reduced. The effects of training were assessed by ADL, equilibrium function tests, stabilometry and gait test. Subjects: The subjects were 8 patients with unilateral vestibular disorders, 4 with postoperative acoustic neurinoma, and 8 with bilateral vestibular disorders. Results: Equilibrium training was effective in all 8 patients with unilateral vestibular dysfunction, in all 4 patients with postoperative acoustic neurinoma, and in 7 of the 8 with bilateral vestibular dysfunction. It was difficult to evaluate the effect of the training in a case of bilateral Meniere's disease. Conclusion: We found many differences in disability among patients with similar impairments, and were able to plan an individual training program suitable for each patient. ADL and equilibrium function tests were useful for evaluating disability and the effect of training. We considered equilibrium training effective for patients with unilateral vestibular disorders who showed chronic disability persisting for more than several weeks after onset, for patients with postoperative acoustic neurinoma who had persistent disability 4 weeks after surgery and for patients with bilateral vestibular disorders with disability persisting for more than 8 weeks.
To observe the effects of aging on equilibrium function, a gait test using normal walking with eyes open and closed was conducted with (A) qualitative and quantitative observation by gross observation of free gait for 10 m, (B) recording by a pen oscillograph on a polygraph of 7 phenomena, namely, head movements (upward-downward: U-D, right-left: R-L, and forward-backward: F-B), activities of the bilateral soleus muscles, and bilateral foot-floor contact sequences, and (C) computer analysis: the power spectra and autocorrelations were calculated from the recordings of head move-ments and integrating activities of the bilateral soleus muscles for younger normal subjects and for elderly subjects. Conclusion 1. Age-related changes in free gait are caused not only by age-related changes in the skeleto-muscular structure but also by the effects of aging on proprioceptive regulation. 2. Certain changes in free gait with eyes closed were more noticeable in the elderly than in the younger group. Some elderly subjects showed no periodicity and no regularity in head movements and activities of the bilateral soleus muscles when walking with eyes closed. However, the rhythm of foot-floor contact with eyes open was generally regular. Age-related changes were not supposed to have any important effect on the central pattern generator of locomotion itself. In the elderly, the role of eyesight was considered more important to stable gait than it was in the younger group.