The torsional vestibuloocular reflex can be canceled but not enhanced by visual stimuli.
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Eye Movements↗
Biomedical subjects
S H Seidman
Publications and source records attributed to S H Seidman.
Eye movements during motion after-effect.
Using the magnetic search coil technique, we measured torsional eye movements in four male subjects during and after rotation of a visual display around the line of sight. During rotation of the display, subjects developed a torsional nystagmus with slow-phases in the direction of target rotation that had a typical gain of less than 0.01. Upon cessation of display motion, subjects experienced a motion after-effect (MAE) in the direction opposite prior target rotation, which persisted for greater than 15 sec. During this MAE, slow-phase eye movements of low velocity were in the same direction as the MAE, but did not persist as long as perceptual effects. In separate experiments, horizontal eye movements were recorded during horizontal stimulus motion; during MAE, no eye movements occurred due to stronger fixation mechanisms. We conclude that MAE is not caused by retinal slip of images, but MAE and the accompanying eye movements might be produced by shared or similar mechanisms.
Adult↗
Comparison of predictable smooth ocular and combined eye-head tracking behaviour in patients with lesions affecting the brainstem and cerebellum.
We compared the ability of eight normal subjects and 15 patients with brainstem or cerebellar disease to follow a moving visual stimulus smoothly with either the eyes alone or with combined eye-head tracking. The visual stimulus was either a laser spot (horizontal and vertical planes) or a large rotating disc (torsional plane), which moved at one sinusoidal frequency for each subject. The visually enhanced vestibulo-ocular reflex (VOR) was also measured in each plane. In the horizontal and vertical planes, we found that if tracking gain (gaze velocity/target velocity) for smooth pursuit was close to 1, the gain of combined eye-hand tracking was similar. If the tracking gain during smooth pursuit was less than about 0.7, combined eye-head tracking was usually superior. Most patients, irrespective of diagnosis, showed combined eye-head tracking that was superior to smooth pursuit; only two patients showed the converse. In the torsional plane, in which optokinetic responses were weak, combined eye-head tracking was much superior, and this was the case in both subjects and patients. We found that a linear model, in which an internal ocular tracking signal cancelled the VOR, could account for our findings in most normal subjects in the horizontal and vertical planes, but not in the torsional plane. The model failed to account for tracking behaviour in most patients in any plane, and suggested that the brain may use additional mechanisms to reduce the internal gain of the VOR during combined eye-head tracking. Our results confirm that certain patients who show impairment of smooth-pursuit eye movements preserve their ability to smoothly track a moving target with combined eye-head tracking.
Adult↗
Experimental tests of a superposition hypothesis to explain the relationship between the vestibuloocular reflex and smooth pursuit during horizontal combined eye-head tracking in humans.
1. We used a modeling approach to test the hypothesis that, in humans, the smooth pursuit (SP) system provides the primary signal for cancelling the vestibuloocular reflex (VOR) during combined eye-head tracking (CEHT) of a target moving smoothly in the horizontal plane. Separate models for SP and the VOR were developed. The optimal values of parameters of the two models were calculated using measured responses of four subjects to trials of SP and the visually enhanced VOR. After optimal parameter values were specified, each model generated waveforms that accurately reflected the subjects' responses to SP and vestibular stimuli. The models were then combined into a CEHT model wherein the final eye movement command signal was generated as the linear summation of the signals from the SP and VOR pathways. 2. The SP-VOR superposition hypothesis was tested using two types of CEHT stimuli, both of which involved passive rotation of subjects in a vestibular chair. The first stimulus consisted of a "chair brake" or sudden stop of the subject's head during CEHT; the visual target continued to move. The second stimulus consisted of a sudden change from the visually enhanced VOR to CEHT ("delayed target onset" paradigm); as the vestibular chair rotated past the angular position of the stationary visual stimulus, the latter started to move in synchrony with the chair. Data collected during experiments that employed these stimuli were compared quantitatively with predictions made by the CEHT model. 3. During CEHT, when the chair was suddenly and unexpectedly stopped, the eye promptly began to move in the orbit to track the moving target. Initially, gaze velocity did not completely match target velocity, however; this finally occurred approximately 100 ms after the brake onset. The model did predict the prompt onset of eye-in-orbit motion after the brake, but it did not predict that gaze velocity would initially be only approximately 70% of target velocity. One possible explanation for this discrepancy is that VOR gain can be dynamically modulated and, during sustained CEHT, it may assume a lower value. Consequently, during CEHT, a smaller-amplitude SP signal would be needed to cancel the lower-gain VOR. This reduction of the SP signal could account for the attenuated tracking response observed immediately after the brake. We found evidence for the dynamic modulation of VOR gain by noting differences in responses to the onset and offset of head rotation in trials of the visually enhanced VOR.(ABSTRACT TRUNCATED AT 400 WORDS)
Adult↗
Superior oblique myokymia. Quantitative characteristics of the eye movements in three patients.
Using the magnetic search coil technique, we measured horizontal, vertical, and torsional rotations of both eyes of two patients with idiopathic superior oblique myokymia, and of the affected eye in a third patient. Superior oblique myokymia was strictly monocular and consisted of an initial intorsion and depression of the affected eye and subsequent oscillations with torsional and vertical components. The peak-to-peak torsional and vertical amplitudes of the oscillations were less than 1 degree, but peak velocities frequently exceeded 4 degrees/sec in both planes. Fourier analysis indicated two features: (1) a broad range of frequencies up to about 50 Hz, indicating irregular oscillations; and (2) a superimposed larger-amplitude oscillation in the range from 1.5 to 6 Hz. Taken with electromyographic data from other studies, these results indicate that superior oblique myokymia reflects spontaneous discharge of trochlear motor neurons that have undergone regenerative changes.
Adult↗
The human torsional vestibulo-ocular reflex during rotation about an earth-vertical axis.
Using the magnetic search coil technique, we have measured the gain and time constant (Tvor) of the torsional vestibulo-ocular reflex (VOR) in 4 subjects who were rotated about an earth-vertical axis with their necks extended and faces supine. Following a 1-min period of rotation in darkness at 50 degrees/s, the post-rotational response to a velocity off-step had a group mean gain of 0.43 and Tvor of 3.7 s. Following a 1-min period of rotation in the light at 50 degrees/s, the post-rotational response in darkness had a group mean gain of 0.29 and Tvor of 4.1 s. Following rotation in darkness with the neck flexed and head prone, the post-rotational response, measured in two subjects, had a mean gain of 0.39 and Tvor of 5.7 s. Similar results were obtained with 100 degrees/s stimuli. In all subjects, the gain and Tvor of the torsional VOR were smaller than corresponding values for their horizontal VOR; these smaller values can be related to the different visual demands made of the torsional VOR.
Adult↗
Behavior of human horizontal vestibulo-ocular reflex in response to high-acceleration stimuli.
We studied the horizontal vestibulo-ocular reflex (VOR) during transient, high-acceleration (1900-7100 degrees/s2) head rotations in 4 human subjects. Such stimuli perturbed the angle of gaze and caused illusory movement of a viewed target (oscillopsia). The disturbance of gaze could be attributed to the latency of the VOR (which ranged from 6-15 ms) and inadequate compensatory eye rotations (median VOR gain ranged from 0.61-0.83).
Acceleration↗
The cervico-ocular reflex of normal human subjects in response to transient and sinusoidal trunk rotations.
We used the magnetic search coil technique to measure the horizontal cervico-ocular reflex (COR) of 8 subjects in response to transient or sinusoidal (0.1-1.0 Hz) trunk rotations while their heads were firmly immobilized. Although we were able to resolve eye rotations of < 0.05 degrees, the COR was hardly measurable (gain was always < 0.07). This finding, made with the most precise measurement technique used to date, suggests that the COR makes a negligible contribution to the stability of gaze in normal subjects during natural activities.
Adult↗