Journal of Vestibular Research - Volume 1, issue 4

Authors: Mettens, P. | Godaux, E. | Cheron, G.

Article Type: Research Article

Abstract: We studied the effects of ketamine, an antagonist of the N-methyl-D-aspartate receptors, on (1) the spontaneous saccades, (2) the vestibulo-ocular reflex (VOR), and (3) the optokinetic nystagmus (OKN) in 8 cats. Ketamine was given intramuscularly at four dosages (1, 2, 8, and 16 mg/kg). Eye movements were measured using the magnetic field-search coil technique. Ketamine did not prevent the occurrence of saccades, but each of them was followed by a centripetal postsaccadic drift. The time-constant of the drift induced by ketamine was 1.0 s when the given dosage was 1 mg/kg and 0.35 s when the given dosage was 16 …mg/kg. Post-saccadic drift caused by a low dosage of ketamine may reflect only a mismatch between the pulse and the step commands that create saccades. The highest used dosages of ketamine aggravated the post-saccadic drift probably by disturbing the oculomotor neural integrator. To elicit the horizontal VOR, the head was submitted either to sinusoidal rotations (± 20∘ ; 0.05 to 1 Hz) or to a rotation at a constant velocity (100∘ /s during 40 s). In darkness, the VOR step gain was reduced by ketamine in a dosage-dependent manner. VOR phase lead at 0.10 Hz oscillation in darkness increased from 4.0∘ ± 2.4∘ to 51.6∘ ± 7.5∘ after administration of ketamine at 16 mg/kg. This suggests that ketamine, at least at higher dosages, induces a failure of the neural integrator. Chemical blockade of the vestibular commissure by ketamine may also be responsible for the reduction of the VOR gain. Horizontal OKN was tested using a step stimulus (30∘ /s during 40 s). When ketamine was given at 1 mg/kg, the average steady-state gain of the OKN diminished from 0.6 ± 0.2 to 0.3 ± 0.1. After administration of ketamine at 2 mg/kg, the OKN was abolished. The sensitivity of OKN to ketamine is explained at least partly by the fact that ketamine acts against the visual pathways in the retina, in the geniculate nucleus, and in the visual cortex. The time course of the optokinetic afternystagmus (OKAN) and that of the decrease of the perrotatory and postrotatory VOR were not reduced by ketamine administered at 1 or 2 mg/kg. This shows that ketamine does not affect the velocity-storage mechanism at these dosages. Show more

Keywords: ketamine, ocular movement, oculomotor neural integrator, cat, N-methyl-D-aspartate

DOI: 10.3233/VES-1991-1401

Citation: Journal of Vestibular Research, vol. 1, no. 4, pp. 325-338, 1991

Authors: Briner, W. | Linthicum Jr., F.H. | Gadre, A.

Article Type: Research Article

Abstract: We used computer techniques to reconstruct in three dimensions 12 vestibular nuclear complexes (VNC) from serially sectioned brains of 7 patients. Specimens were from 4 patients with vestibular disorders and 3 patients considered vestibular normal. Despite the variety of the patient’s otologic histories all the nuclei demonstrated the same shape and structural characteristics.

Keywords: vestibular nucleus, human, three-dimensional anatomy, vertigo

DOI: 10.3233/VES-1991-1402

Citation: Journal of Vestibular Research, vol. 1, no. 4, pp. 339-345, 1991

Authors: Lafortune, S.H. | Ireland, D.J. | Jell, R.M.

Article Type: Research Article

Abstract: The effects of static tilts about the roll (anterior-posterior) axis on human horizontal optokinetic afternystagmus (HOKAN) were examined. Static tilts in roll, with subjects lying on their left side, produced significant tilt-dependent HOKAN suppression. Only the slow (indirect pathway) component time constant (1/D) of the double exponential model for human HOKAN decreased with angle of roll tilt. The effect was direction specific in that suppression occurred only following a leftward-going stimulus. These findings provide further support for the postulate that otolith-organ-mediated activity can couple to the horizontal velocity storage mechanism in humans. A slight trend towards a tilt-dependent reduction of …coefficient A (initial slow phase velocity of fast component decay) was revealed, suggesting the possibility that otolith-organ-mediated activity could couple to direct (pursuit-mediated?) pathways as well. No horizontal-to-vertical cross-coupling occurred, indicating that this aspect of the 3-dimensional model for velocity storage proposed by Raphan & Cohen (1988) may not completely apply to humans. Show more

Keywords: velocity storage, integrator, vestibular, man, oculomotor, otolith organs

DOI: 10.3233/VES-1991-1403

Citation: Journal of Vestibular Research, vol. 1, no. 4, pp. 347-355, 1991

Authors: Hine, Trevor

Article Type: Research Article

Abstract: The kinematics of the horizontal VOR for near fixation demand that VOR gain should change dependent on the target distance and the orientation of the head with respect to the target, or, equivalently, the amount of ocular vergence and the asymmetry of this vergence. Across two experiments, the gain of the VOR was measured in the right eye of humans who rotated their heads to the right or left while viewing a target placed either 22, 32.5 or 200 cm from the center of head rotation, in conditions with and without visual feedback. When tbe eye was in-line with the …target, the measured VOR gain was up to 43% greater than when the eye was in an eccentric position. However, in the eccentric position, higher VOR gains were achieved with visual feedback of the target than without feedback, indicative of a visual component in the compensatory eye movement. Also, by changing the posture of the left eye but keeping the right eye constant, the VOR gain in the right eye was changed substantially during a subsequent head rotation. Hence, the positions of both eyes in their orbits determine the gain of the VOR in each eye. Show more

Keywords: active VOR, asymmetric vergence, visual feedback

DOI: 10.3233/VES-1991-1404

Citation: Journal of Vestibular Research, vol. 1, no. 4, pp. 357-371, 1991

Authors: Fetter, Michael | Diener, Hans-Christoph | Dichgans, Johannes

Article Type: Research Article

Abstract: Postural control during stance was investigated using the EQUITEST® system in 10 patients during recovery after an acute unilateral vestibular lesion and was compared to the time course of recovery of the static and dynamic vestibulo-ocular imbalance. During the acute phase the patients showed a characteristic pattern with normal upright stance as long as at least one accurate sensory input (visual or somatosensory) was provided and severe postural disturbances when they had to rely primarily on vestibular afferences. Both static vestibulo-ocular and vestibulo-spinal balance recovered very fast, showing basically normal results on postural testing within about 2 weeks after …the lesion. Thereafter, no pathological pattern was detectable during postural testing even in patients with persistent complete unilateral vestibular lesions. Reflexive postural responses to unexpected rapid displacements of the support surface seemed not to be influenced by vestibular imbalance even in the acute phase of the lesion. Show more

Keywords: posture, unilateral vestibular lesion, vestibulo-spinal reflexes, vestibulo-ocular reflex

DOI: 10.3233/VES-1991-1405

Citation: Journal of Vestibular Research, vol. 1, no. 4, pp. 373-383, 1991