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S H Seidman

Publications and source records attributed to S H Seidman.

9 recordsLinked to original sources
PubMed1992

The stability of human eye orientation during visual fixation.

Using the magnetic search coil technique, gaze stability in the horizontal, vertical and torsional planes was measured binocularly in human subjects during visual fixation. Horizontal and vertical eye rotations exhibited a mixture of slow drifts and resetting microsaccades yielding an average standard deviation of 0.11 and 0.10 deg, respectively. In contrast, torsional rotations showed unsystematic smooth drifts with fewer saccades yielding an average standard deviation of 0.18 deg. The lower precision of gaze control in the torsional plane may reflect (i) a discrepancy between the encoding of retinal images in two dimensions but of ocular motor control signals in three dimensions, and (ii) the visual consequences of ocular drifts in the torsional plane, which differ from those in the horizontal and vertical planes.

Adult↗
PubMed1992

High-frequency vestibuloocular reflex as a diagnostic tool.

During locomotion, the head is subject to rotational perturbations with fundamental frequencies in the range 0.5-5.0 Hz, and significant harmonics up to 20 Hz. Patients who have lost labyrinthine function complain of oscillopsia and visual impairment during locomotion. Measurements of head movements during walking and running in place in such patients indicate that head stability is similar to that in normal subjects. Therefore, head stability is mainly guaranteed by mechanical, not neurogenic, factors. On the other hand, the visual symptoms of such patients can be ascribed to instability of gaze. Thus, it seems that other mechanisms such as visual following, the cervicoocular reflex, or anticipatory eye movements cannot compensate for loss of the VOR during locomotion (though they may do so for lower-frequency or active head rotations). The indispensable role of the VOR during locomotion is probably a reflection of its short latency (16 mseconds or less in the horizontal and vertical planes), which guarantees short phase lags during high-frequency head rotations. Our results indicate that laboratory testing of patients with vestibular symptoms should employ stimuli that correspond to those occurring during locomotion.

Aged↗
PubMed1992

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↗
PubMed1992

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↗
PubMed1992

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↗
PubMed1991

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↗
PubMed1989

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↗
PubMed1989

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↗