The role of ocular torsion in visual measures of vestibular function

This brief review highlights problems in the interpretation of results about perceived postural roll-tilt of human subjects undergoing roll-tilt around their naso-occipital axis, when visual stimuli are used as a means of indicating perception. The otolithic stimulus, which causes the changes in perceived posture, also causes the eyes to roll (or tort). In turn, the altered torsional position of the eye causes the perceived orientation of visual stimuli to change. Consequently, indicators of postural perception, which rely on visual stimuli, are a confounded combination of two factors; the person's perceived postural roll-tilt, and the effect of the otolithic stimulus on ocular torsional position. Consequently, setting of a visual stimulus do not permit direct unambiguous interpretation of a subject's perceived postural roll-tilt.     

Habituation and adaptation of the vestibuloocular reflex: a model of differential control by the vestibulocerebellum

Summary We habituated the dominant time constant of the horizontal vestibuloocular reflex (VOR) of rhesus and cynomolgus monkeys by repeated testing with steps of velocity about a vertical axis and adapted the gain of the VOR by altering visual input with magnifying and reducing lenses. After baseline values were established, the nodulus and ventral uvula of the vestibulocerebellum were ablated in two monkeys, and the effects of nodulouvulectomy and flocculectomy on VOR gain adaptation and habituation were compared. The VOR time constant decreased with repeated testing, rapidly at first and more slowly thereafter. The gain of the VOR was unaffected. Massed trials were more effective than distributed trials in producing habituation. Regardless of the schedule of testing, the VOR time constant never fell below the time constant of the semicircular canals (5 s). This finding indicates that only the slow component of the vestibular response, the component produced by velocity storage, was habituated. In agreement with this, the time constant of optokinetic after-nystagmus (OKAN) was habituated concurrently with the VOR. Average values for VOR habituation were obtained on a per session basis for six animals. The VOR gain was adapted by natural head movements in partially habituated monkeys while they wore ×2.2 magnifying or ×0.5 reducing lenses. Adaptation occurred rapidly and reached about ±30%, similar to values obtained using forced rotation. VOR gain adaptation did not cause additional habituation of the time constant. When the VOR gain was reduced in animals with a long VOR time constant, there were overshoots in eye velocity that peaked at about 6–8 s after the onset or end of constant-velocity rotation. These overshoots occurred at times when the velocity storage integrator would have been maximally activated by semicircular canal input. Since the activity generated in the canals is not altered by visual adaptation, this finding indicates that the gain element that controls rapid changes in eye velocity in the VOR is separate from that which couples afferent input to velocity storage. Nodulouvulectomy caused a prompt and permanent loss of habituation, returning VOR time constants to initial values. VOR gain adaptation, which is lost after flocculectomy, was unaffected by nodulouvulectomy. Flocculectomy did not alter habituation of the VOR or of OKAN. Using a simplified model of the VOR, the decrease in the duration of vestibular nystagmus due to habituation was related to a decrement in the dominant time constant of the velocity storage integrator (1/h ₀). Nodulouvulectomy, which reversed habituation, would be effected by decreasing h ₀, thereby increasing the VOR time constant. Small values of h ₀ would cause velocity storage to approach an ideal integrative process, leading the system to become unstable. By controlling the VOR time constant through habituation, the nodulus and uvula can stabilize the slow component of the VOR. VOR gain adaptation was related to a modification of the direct vestibular path gain g ₁, without altering the coupling to velocity storage g ₀ or its time constant (1/h ₀). The mismatched direct- and indirect-pathway gains simulated the overshoots in the dynamic response to a step in velocity, that were observed experimentally. We conclude that independent distributed elements in the VOR modify its dynamic response, under control of separate parts of the vestibulocerebellum.     

Optokinetic nystagmus in the rabbit and its modulation by bilateral microinjection of carbachol in the cerebellar flocculus

Summary 1. In the alert, pigmented rabbit, eye movements were recorded during optokinetic nystagmus (OKN) and during optokinetic afternystagmus (OKAN). These responses were elicited by steps in surround-velocity ranging from 5–110°/s during binocular as well as monocular viewing. 2. In the baseline condition, OKN showed an approximately linear build-up of eye velocity to a steady-state, followed by a linear decay of eye velocity during OKAN after the lights were turned off. Build-up during binocular viewing was characterized by a constant, maximum eye-acceleration (about 1°/s²) for stimulus velocities up to 60°/s. OKAN, instead, was characterized by a fixed duration (about 10 s) for stimulus velocities up to 20°/s. Steady-state eye velocity saturated at about 50°/s. 3. Monocular stimulation in the preferred (nasal) direction elicited a build-up that was on average twice as slow as during binocular stimulation. Steady-state velocity during monocular stimulation saturated at about 20°/s. OKAN was of equal duration as during binocular stimulation. In the non-preferred direction, a very irregular nystagmus was elicited without velocity build-up. The stronger response to binocular stimulation, compared to the responses under monocular viewing condition in either nasal and temporal direction suggests potentiation of the signals of either eye during binocular viewing. 4. OKN and OKAN were re-assessed after intra-floccular microinjection of the nonselective cholinergic agonist carbachol. In the binocular viewing condition, eye-acceleration during build-up was strongly enhanced from 1°/s² before to 2.5°/s² after injection. The saturation level of steady-state eye velocity was also increased, from 50°/s before to more than 60°/s after carbachol. The duration of OKAN, however, was shortened from 10 s before to 6 s after injection. The response to monocular stimulation in the preferred direction revealed similar changes. 5. The flocculus appears to be involved in the control of the dynamics of OKN in the rabbit. Cholinergic mechanisms affect the floccular control of the rate at which slow-phase velocity can be built up and the rate of decay of eye velocity during OKAN. Cholinergic stimulation of the flocculus enhances the dynamics of OKN, while velocity storage is shortened.     

Eye movements and vestibular-nerve responses produced in the squirrel monkey by rotations about an earth-horizontal axis

Summary The eye movements produced by constant-speed rotations about an earth-horizontal axis (EHA) are similar in the alert squirrel monkey to those observed in other species. During EHA rotations, there are persistent eye movements, including a nonreversing nystagmus at lower rotation speeds and either a direction-reversing nystagmus or sinusoidal eye movements at higher rotation speeds. Horizontal eye movements are produced by barbecuespit (yaw) rotations, vertical eye movements by head-over-heels (pitch) rotations. The responses can be viewed as composed of a bias component, reflected in the nonreversing nature of the nystagmus, and a cyclic component, reflected in the periodic modulation of slow-phase eye velocity as head position varies. Vestibular-nerve recordings in the barbiturate-anesthetized monkey indicate that neither semicircular-canal nor otolith afferents give rise to a directionally specific dc signal which can account for the bias component. Apparently the appropriate dc signal has to be constructed centrally from a sinusoidal or ac peripheral input. The otolith organs are a likely source of this peripheral input, although contributions from the semicircular canals and from somatosensory receptors must also be considered. Our results suggest that the directional information required to distinguish rotation direction, rather than being contained in the discharge of individual otolith afferents, is encoded across a population of afferents. Possible sources of such information are the phase differences in the sinusoidal responses of otolith afferents differing in their functional polarization vectors.Supported by Grants NS 01330 from the National Institutes of Health and NGR-14-001-225 from the National Aeronautics and Space Administration     

The interaction of conflicting retinal motion stimuli in oculomotor control

Summary Oculomotor response has been assessed in humans during the presentation of conflicting retinal motion stimuli. In the majority of experiments a background stimulus was made to move with a constant velocity ramp in one direction followed by rapid resets at regular intervals. In the absence of an adequate fixation target this ramp-reset stimulus induced a nystagmus with a slow-phase velocity and saccadic frequency which remained almost constant as reset frequency was increased from 2 to 5 Hz. Moreover, the induced eye velocity could be considerably increased if the subject attempted active matching of display velocity. During both active and passive responses eye velocity gain reached a peak when display velocity was between 2°/s and 5°/s. The presence of small stationary targets induced a suppression of the passive ramp-reset response which was modified by target eccentricity and by tachistoscopic target illumination. When subjects pursued a sinusoidally oscillating target against a stationary structured background, eye velocity gain was significantly less than for pursuit against a blank background. The degree of interaction between conflicting stimuli was found to be dependent on their relative size, peripheral location and velocity. However, it appears that the human observer is able selectively to enhance feedback gain from one particular source in order to dominate stimuli from other unwanted sources.     

Visual-vestibular interaction in the flocculus of the alert monkey

Summary The activity of Purkinje cells (P-cells) was recorded in the flocculus of alert Rhesus monkeys under different conditions of visual-vestibular stimulation. Stimulus conditions were vestibular, optokinetic, combined and conflicting. About 10–20% of all P-cells were activated in their simple spike activity during conflicting stimulation to the recording side (type I) and gave no response or much less during vestibular stimulation. About half of these P-cells were also activated during optokinetic stimulation to the recording side at velocities above 40–60 deg/s. Simple and complex spike activity behaved in a reciprocal way with overlapping but not identical working ranges. Simple spike modulation was unidirectional, complex spike activity always bidirectional. Modulation of simple spike activity cannot be related to one single parameter of the sensory input or the oculomotor output. The hypothesis is put forward that the vestibular nuclei and the flocculus behave in a complementary fashion in processing visual-vestibular information, the flocculus being specialized for high velocity optokinetic nystagmus and suppression of vestibular nystagmus.Supported by a grant from the Swiss National Foundation for Scientific Research 3.343-2.78     

Vestibular nerve activity in the alert monkey during vestibular and optokinetic nystagmus

Summary Activity of vestibular nerve fibers and eye movements were recorded in the alert monkey during natural stimulation. The animal was rotated about a vertical axis in the dark with velocity trapezoids (vestibular), or a striped cylinder was rotated around the stationary monkey (optokinetic), or these stimuli were combined.After velocity steps in the dark, neuronal activity declined with a dominant time constant of 5–6 s. The time constant of nystagmus recorded simultaneously was always longer, on average 23 s. Vestibular nerve activity was not influenced by optokinetic patterns or additional visual stimuli during combined visualvestibular stimulation. Thus, in contrast to vestibular nuclei neurons, vestibular nerve activity in the alert monkey is only determined by head acceleration and cannot be related to the nystagmus response or visual stimuli.Supported by a grant from the Swiss National Foundation for Scientific Research 3.343-2.78     

脑干病变引起眼球运动障碍、眼球震颤的临床分析

目的：探讨眼球运动障碍、眼球震颤对脑干病变定位的临床意义。方法：对３９例经ＣＴ、ＭＲＩ证实为脑干病变的患者行眼球运动障碍、眼球震颤分析。结果：眼球垂直注视麻痹、会聚麻痹对中脑病变有定位意义;核间性眼肌麻痹对中脑、脑桥病变有定位意义;眼球水平运动障碍对脑桥病变有定位意义;延髓病变无眼球运动障碍;眼球震颤可发生在脑干各种部位病变。结论：眼球运动障碍对脑干病变定位有临床意义;眼球震颤尤其是垂直性、旋转性     

先天性眼球震颤药物疗效的眼震电图评定

为评定 Ａ 型肉毒毒素治疗先天性眼球震颤的疗效,于治疗前后对 ６２ 例患者行眼震电图检查。结果表明,用 Ａ 型肉毒毒素治疗前后眼震电图差异有高度显著性（ Ｐ＜ ０．００１）,用 Ａ 型肉毒毒素治疗眼震总有效率为 ９０．３％ 。眼震电图为临床研究先天性眼震所必需,可根据眼震电图的描记对治疗效果作出评价。     

Listing's law in strabismus and amblyopia: a preliminary report

We investigated whether Listing's law applies in patients with diminished or no stereopsis due to strabismus or amblyopia. Eye movements of normal subjects and patients with strabismus and/or amblyopia were recorded during monocular and binocular fixation; from these data the shape and relative orientation of displacement planes were calculated. In normal subjects, monocular or binocular fixation did not influence the thickness and relative orientation of displacement planes. No differences were found between normals and the patient with amblyopia due to anisometropia. In one patient with strabismus but without amblyopia, the orientation of displacement planes depended on the fixation conditions; a coupling between horizontal vergence effort and plane orientation was observed. Patients with amblyopia and strabismus showed abnormally shaped and/or abnormally orientated displacement planes, which depended on the fixation conditions. Differences between both eyes in the shape of the planes were also observed. These results show that normal Listing behavior can be present in subjects with diminished stereopsis. They also show that normal stereopsis does not necessarily mean normal Listing behavior, suggesting that Listing's law is mainly a result of motor strategy.