Journal of Vestibular Research - Volume 4, issue 5

Article Type: Other

DOI: 10.3233/VES-1994-4510

Citation: Journal of Vestibular Research, vol. 4, no. 5, pp. I-II, 1994

Authors: Fuchs, Albert

Article Type: Editorial

DOI: 10.3233/VES-1994-4501

Citation: Journal of Vestibular Research, vol. 4, no. 5, pp. 325-326, 1994

Authors: Straube, Andreas | Helmchen, Christoph | Robinson, Farrel | Fuchs, Albert | Büttner, Ulrich

Article Type: Research Article

Abstract: Some of the clinical hallmarks of lateral medullary infarction — Wallenberg's syndrome — are saccadic dysmetria, smooth pursuit deficit, and lateropulsion of the body. Similar movement disorders are seen in monkeys after local unilateral injection of GABAergic drugs in the caudal fastigial nucleus of monkeys. These include an ipsilateral saccadic hypermetria and a contralateral saccadic hypometria as well as cogwheel smooth pursuit eye movements toward the contralateral side and an ipsiversive lateropulsion of the body. It was previously suggested that the lateral medullary infarction causes a lesion of climbing fibers to the cerebellum. This lack of climbing fiber input increases …the activity of ipsilateral Purkinje cells, which consequently provide too much inhibition of the deep cerebellar nuclei. Show more

Keywords: lateral medullary infarction, saccadic lateropulsion, caudal fastigial nucleus, local muscimol injection, climbing fibers

DOI: 10.3233/VES-1994-4502

Citation: Journal of Vestibular Research, vol. 4, no. 5, pp. 327-333, 1994

Authors: Dell'Osso, L.F.

Article Type: Research Article

Abstract: Current models of the ocular motor system are usually presented in their most reduced form, are unilateral in architecture, and precise yoking is presumed. Although this simplifies the models, it does not accurately simulate the actual neuroanatomy and limits the models to simple, stereotyped responses. Studies of normal humans and monkeys have demonstrated striking disconjugacies in normal responses. Normal saccades may be disconjugate, or 1 eye may exhibit a dynamic overshoot. Asymmetric vergence can result in disconjugate saccades, unequal magnification spectacles cause differential saccadic gain adjustment, and saccades to unequal disparities also cause unequal saccades in the 2 eyes. In …strabismus, deviated eyes typically do not mimic the movements of the fixating eye nor do their latent or congenital nystagmus waveforms duplicate those of the fixating eye. In spasmus nutans, each eye oscillates independently of the other. In achiasmatic dogs, uni-ocular saccades and uni-ocular nystagmus waveforms are seen; the same may be true in human achiasma. These data from both normals and those with abnormalities suggest that current models for ocular motor control are inadequate representations of the actual system. The inability of unilateral, yoked control (or even bilateral, yoked control) system models to duplicate the ocular motor responses of binocular mammals suggests that their ocular motor systems evolved from the bilateral, independent control systems seen in chameleons. One need only postulate a yoking overlay superimposed on two independent control systems to achieve conjugacy (bilateral, yoked, independent control) of the eyes. Abnormalities producing grossly disconjugate eye movements may then be simulated using the independent control of each eye released by a deficiency in the yoking overlay. Independent control of each eye coupled with the essential bilateral brain stem architecture implies that each individual muscle is driven by independent populations of neurons (burst cells, neural integrator cells, etc.). The agonist muscles of each eye are usually coordinated (yoked) but may function independently if the task dictates or if binocularity did not develop. Models based on the above architecture would be robust and could duplicate the many responses (both normal and abnormal) possible from the neurophysiological system. Show more

Keywords: ocular motor control, eye muscle control, models, yoking

DOI: 10.3233/VES-1994-4503

Citation: Journal of Vestibular Research, vol. 4, no. 5, pp. 335-345, 1994

Authors: Leigh, R. John | Huebner, William P. | Gordon, Jacob L.

Article Type: Research Article

Abstract: In studies over the past 30 years, D.A. Robinson and colleagues established that the dynamic characteristics of smooth pursuit eye movements (SP) are different at the onset from those at the cessation of the response. They proposed that cessation of SP was due to a separate fixation system. During head movements, both fixation and SP may contribute to gaze stabilization. We investigated the relative contributions of fixation and SP to the “visually enhanced” vestibulo-ocular reflex (VVOR) using a paradigm requiring a transition from VVOR to combined eye-head tracking (CEHT). We found, in four normal subjects, that ringing typical of SP …generally did not occur during VVOR, but that it often appeared after the transition to CEHT. The findings were different in two patients with absent peripheral vestibular function; ringing typical of SP occurred always during VVOR but disappeared during the onset of CEHT. These results can be explained by a model in which an internal representation of target velocity serves as input to parallel SP and fixation systems, and as the determinant of which of the two systems will provide the command signal. Interpretation of our data using this model indicates that either fixation or SP systems may “visually enhance” the VOR, depending on the magnitude of retinal error velocity that remains after vestibular eye movements have been generated. Show more

Keywords: smooth pursuit, visual fixation, vestibulo-ocular reflex

DOI: 10.3233/VES-1994-4504

Citation: Journal of Vestibular Research, vol. 4, no. 5, pp. 347-353, 1994

Authors: Demer, Joseph L.

Article Type: Research Article

Abstract: We investigated the effect of aging on vertical smooth pursuit, small-field optokinetic nystagmus (OKN), the vestibulo-ocular reflex (VOR), and visual-vestibular interactions (VVI). Telescopic spectacles were used to magnify the visual effects of head movements and accentuate the demand on VVI. Eleven normally sighted young subjects (age 30 ± 6 y, mean ± SD), and 9 normally sighted elderly subjects (age 70 ± 8 y) were studied. Three types of stimulus motion were used: 1) predictable single sinusoidal frequencies from 0.8 to 3.2 Hz, 2) poorly predictable motion consisting of a sum of multiple harmonics of a 0.4 Hz fundamental, and …3) unpredictable velocity impulses. Tracking gains declined with frequency and stimulus unpredictability but were always greater for young than elderly subjects. Pursuit gain was always less than OKN gain for subjects tested under comparable conditions of frequency and velocity. In both subject groups, pursuit and OKN tracking exhibited phase lags at higher frequencies, particularly for poor predictable motion. Tracking phase lags were greater for pursuit than OKN and were consistently greater in the elderly than in young subjects. In young subjects, stimulus dimming using a 2 log neutral density filter reduced tracking gains and increased phase lags. In both young and elderly subjects, VOR gain was <1.0 at low frequencies and increased slightly with increasing frequency. Visually enhanced VOR gain during normal vision was the ideal value of 1.0 at all frequencies in both subject groups. During wearing of telescopic spectacles, the young subjects exhibited marked gain enhancement that was greatest at lower frequencies and greater during predictable than unpredictable motion. Young subjects were able to suppress the VOR by fixation of real or imaginary targets moving with the head; this suppression was better at the lowest frequencies and for predictable motion. Phase errors were substantial for significant VVI in young subjects but were minimal in the elderly. Visual enhancement of VOR gain, as well as suppression by real and imaginary head-fixed targets, were attenuated in the elderly subjects. During rotations in which young subjects wore 4 × telescopic spectacles, 2 log unit dimming of the visual environment reduced gain and increased phase lags. The deficiencies in VVI exhibited by elderly subjects are attributable both to deficits in the visual tracking component, as well as to nonvisual mechanisms, because even changes in mental set are less effective in modifying gain in older subjects. Tracking phase lags and gain reductions in the elderly can be simulated in young subjects by substantially reducing stimulus luminance, which prolongs afferent visual latency. Show more

Keywords: aging, vertical optokinetic nystagmus, vertical pursuit, vertical visual-vestibular interaction

DOI: 10.3233/VES-1994-4505

Citation: Journal of Vestibular Research, vol. 4, no. 5, pp. 355-370, 1994

Authors: Lefèvre, P. | Missal, M. | Galiana, H.L.

Article Type: Research Article

Abstract: Recent experimental results show evidence for the corrective role of postsaccadic drifts. This paper addresses the modeling of these slow correcting gaze movements (SCMs). Classical arguments to explain drifts are presented, both in the head fixed condition (pulse-step mismatch) and in the head free condition (vestibulo-ocular reflex (VOR)). The most significant behavioral and electrophysiological experimental data related to SCMs are then briefly reported, with the conclusion that SCMs have a clear corrective role, incompatible with classical explanations of drifts. Based on these experimental data, existing models of the saccadic system are then compared. A theoretical comparison of the classical Robinson …model with an alternative model is proposed. Two possible (slow and fast) pathways involved in the control of SCMs are examined, and simulation results are presented. Finally, the discussion addresses the observed species differences in SCMs. The link between natural SCMs and electrical SC stimulations, and the interactions between saccades, VOR, and SCMs are also discussed. Show more

Keywords: saccades, drifts, gaze, modeling

DOI: 10.3233/VES-1994-4506

Citation: Journal of Vestibular Research, vol. 4, no. 5, pp. 371-381, 1994

Authors: Ron, S. | Berthoz, A. | Gur, S.

Article Type: Research Article

Abstract: Tight coupling between eye and head movements have been observed in response to a single visual target offset or a single flash. On this basis, when the visual stimulus consists of two successive steps or flashes in the same (horizontal) direction, either increasing in eccentricity or decreasing in eccentricity, gaze should be due to concomitant eye and head angular displacement. The main findings were that in response to increasing eccentricity stimulus, the majority of the gaze movement had a staircase pattern, and to decreasing eccentricity stimulus, a pulse-step pattern. Some responses were one gaze movement to the final target (or …flash) offset. In all these responses, eye and head were tightly coupled. In some cases, however, in response to two flashes or two successive steps decreasing in eccentricity, the initial eye and head movements were dissociated: eye and head were directed to different offsets. Current eye-head models assume a strong coupling between the head displacement and the eye saccade. We have modified the Laurutis and Robinson model to accommodate the new findings of saccade-VOR cooperation and eye-head dissociated responses; We suggest that when gaze falls short of the target offset, the retinal error signal is not only fed to the saccadic system but also partially suppresses the VOR gain control. This allows for “corrective VOR” movements after the gaze saccade has occurred. To accommodate eye-head dissociation movements, we introduced a switch and a delay in the path to the saccadic system and in the path to the head system. The switches and the delays are controlled by higher functions, and their coordination depends on the subject's motor strategy. Show more

Keywords: eye-head, gaze, coupling, saccade, model

DOI: 10.3233/VES-1994-4507

Citation: Journal of Vestibular Research, vol. 4, no. 5, pp. 383-390, 1994

Authors: McConville, K. | Tomlinson, R.D. | King, W.M. | Paige, G. | Na, E.-Q.

Article Type: Research Article

Abstract: Recordings from neurons in the vestibular nuclei indicate that the cells that carry eye position signals encode the position of a single eye (either ipsilateral or contralateral) during both conjugate and vergence eye movements. The fact that the vestibular nuclei are aware of the positions of each eye is not surprising as the otolith-based linear vestibulo-ocular reflex is known to change its behaviour as a function of uniocular eye position. This result suggests that the signal coming from the oculomotor velocity-to-position integrator specifies the position of each eye during vergence movements and thus must receive a vergence velocity input along …with its conjugate velocity inputs. As there is no vergence system in laterally eyed animals, we have proposed two possible models of integrator arrangement that could have developed from conjugate directional (rather than uniocular) integrators in lower animals without frontally mounted eyes. Both of these models explain the existence of near-response cells and produce the required bidirectional gaze paretic nystagmus following unilateral lesions of one integrator. The models also make specific and different predictions concerning the effects of unilateral integrator lesions on the behaviour of the vergence system and thus make suggestions for further experiments. Show more

Keywords: PVP cells, linear VOR, eye position signals, integrator

DOI: 10.3233/VES-1994-4508

Citation: Journal of Vestibular Research, vol. 4, no. 5, pp. 391-400, 1994

Authors: King, W. Michael | Zhou, Wu | Tomlinson, R. David | McConville, Kathleen M.V. | Page, William K. | Paige, Gary D. | Maxwell, James S.

Article Type: Research Article

Abstract: Many neurons in oculomotor pathways encode signals related to eye position. For example, motoneurons in the third, fourth, and sixth cranial nuclei discharge at highly regular rates during fixation intervals. During fixations of far targets, their tonic discharge is linearly related to conjugate eye position. Previous studies provided evidence that premotor cells in brain stem pathways also encoded conjugate eye position. McConville et al. (this volume), however, measured eye movements during binocular fixations when the eyes were converged and concluded that the position signal encoded by premotor position-vestibular-pause (PVP) cells in the vestibular nuclei is related to monocular (right or …left) eye position rather than to conjugate eye position. This surprising relationship would not have been noticed in earlier studies that measured the movements of only one eye (using a single eye coil) or that measured only the conjugate movements of the two eyes (using bitemporal EOG electrodes). How general a feature of oculomotor signal processing is this finding? In this paper, we re-examine the eye position signal in abducens and oculomotor neurons when the movements of the two eyes are conjugate and when they are disjunctive and therefore disassociated. The data suggest that abducens neurons (AMNs and AINs) and oculomotor neurons (putative medial rectus motoneurons), unlike PVP cells, are not monocular but encode mixtures of right and left eye position signals. Show more

Keywords: vestibulo-ocular reflex, vergence, eye movements

DOI: 10.3233/VES-1994-4509

Citation: Journal of Vestibular Research, vol. 4, no. 5, pp. 401-408, 1994