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B N Segal

Publications and source records attributed to B N Segal.

9 recordsLinked to original sources
PubMed1996

Objective identification of dizzy patients by vestibulo-ocular and vestibulospinal testing.

OBJECTIVE: To evaluate how well two new objective tests of vestibular function might help identify truly dizzy patients. DESIGN: A prospective study of dizzy patients. SETTING: Department of Otolaryngology, Sir Mortimer B. Davis-Jewish General Hospital, Montreal, Quebec. METHODS: Vestibular function (in 39 dizziness clinic patients and 30 normals) was characterized as follows: 1. While subjects sat on a rotatable chair trying to look at a wall target, vestibulo-ocular function was quantified by accuracy of ocular stabilization during brief chair turns imposed during 1-second periods of darkness. 2. While subjects stood feet together rotating their head and torso together from the knees, vestibulospinal function was quantified by the fastest comfortable torsal-head movement made without exacerbating dizziness. RESULTS: After plotting ocular stabilization accuracy against fastest comfortable torsal-head velocity, patients and normals tended to separate into two groups. Statistical analysis permitted most dizzy patients (sensitivity: 87%), and most normals (specificity: 83%) to be identified. CONCLUSION: Results are promising, since approximately 40% of patients had no abnormal vestibular signs.

Adolescent↗
PubMed1988

Goal-directed vestibulo-ocular function in man: gaze stabilization by slow-phase and saccadic eye movements.

Vestibular function was examined during passive head movements having profiles that approximated low-to-intermediate range of natural self-generated movements (10-220 degrees/s peak velocity, about 0.5 s duration). A seated subject looked at a point target on the wall, the lights were extinguished and the chair was briefly turned while the subject tried to "look" at the just-viewed point. The chair was stopped, the lights were turned on again and the target was re-fixated, if necessary. Ocular stabilization was characterized (1) by "net stabilization" that was due to the combined effects of both slow-phase and rapid (saccadic or quick-phase) eye movements, (2) by "cumulative-slow-phase stabilization" that was due to slow-phase eye movements, and (3) by "cumulative-saccadic stabilization" that was due to effects of all rapid eye movements. It was found that both slow-phase and saccadic eye movements tended to keep the eyes on the actual unseen target. During repeatedly applied head movements, net and cumulative-slow-phase stabilization tended to be almost perfect. However, the average magnitude of the error in net stabilization (i.e., deviation from perfection) was always less than the corresponding error in slow-phase stabilization. This occurred because in a given turn, saccadic movements tended to supplement deficient slow-phase movements and to decrement excessive slow-phases. In 4 of 5 subjects, cumulative-saccadic stabilization tended to equal the error in cumulative-slow-phase stabilization. All results were unaffected by head velocities up to +/- 220 degrees/s. It was concluded that these saccades tended to stabilize gaze (eye + head) in space during head movements in total darkness.

Eye Movements↗
PubMed1988

Unilateral loss of peripheral vestibular function in patients: degree of compensation and factors causing decompensation.

Unilateral surgical ablation of peripheral vestibular function has been suggested for the treatment of a number of diseases that involve vestibular dysfunction. The postoperative distressing symptoms usually subside with time, whereupon the patient is said to have clinically compensated. However, even in well-compensated patients, the initial symptoms may reappear--under certain conditions that are briefly discussed (decompensation)--and, in addition, vestibular gaze stabilization deficits, (apparently permanent) appear whenever moderately rapid head movements are imposed. Thus, surgical ablation of unilateral peripheral vestibular function should not be considered "a treatment of choice," and should be performed in only carefully selected cases.

Animals↗
PubMed1985

Modulation of human velocity storage sampled during intermittently-illuminated optokinetic stimulation.

When a stationary human subject is suddenly exposed to constant-velocity full-field optokinetic stimulation, slow-phase eye velocity rapidly approaches stimulus velocity without the gradual build-up ("velocity storage") readily seen in other species. Subsequent to extinguishing illumination, the presence of velocity storage is suggested by the persistence of a gradually-declining component of optokinetic after-nystagmus (OKAN). In the past, characterizing modulation of velocity storage has been tedious. It is now shown that such modulation can be characterized by periodic "sampling" of OKAN using "intermittently-illuminated" optokinetic stimuli (light-on: 9.7 s; darkness: 2 s). Six subjects viewed an intermittently-illuminated optokinetic drum turning with a square wave angular velocity profile of 60 deg/s peak amplitude and 0.01 Hz frequency. The resulting modulation of velocity storage was approximately exponential with time constant, T = 5-11 s, and asymptote, A = 10-17 deg/s. A significant negative correlation was observed between T and A values. In a given subject, T and A values agreed (generally within +/- 20%) with values obtained employing previously-used methods, suggesting that velocity storage behaved linearly during periodic optokinetic stimuli of less than 60 deg/s. The new method of sampling OKAN permits the use of arbitrary stimulus profiles required to observe, or to confidently predict, velocity storage response during natural behavioural movements, which was not feasible with older methods. Sampling also increases the ease and speed (roughly three-fold) of data acquisition.

Electronystagmography↗
PubMed1985

Post-suppression vestibulo-ocular reflex in man: visual and non-visual mechanisms.

The vestibulo-ocular reflex (VOR) was intermittently suppressed by fixating a head-fixed target (light on: 9 s; darkness: 2 s) during whole-body (60-100 deg/s peak; 0.01-0.17 Hz) sinusoidal and triangular oscillation about the vertical axis for about 2 min. Eye movements associated with intervals of darkness at different phases of body oscillation were stimulus-locked averaged, thereby estimating the "post-suppression VOR". The gain ( (eye velocity)/(head velocity) ) of the post-suppression VOR was generally 26-50% of the normal gain in maintained darkness. During sinusoidal body oscillation, the phase lead of the post-suppression VOR equalled or, at low frequencies, appeared to exceed the normal lead in maintained darkness. When the light was permanently extinguished after 2 min of intermittent suppression, an initially reduced VOR rapidly (e.g., 0.4 s) appeared and required 6-30 s to build up to normal again. These observations indicated that visual suppression of the VOR dissipated in darkness with rapid and slow components. During 0.17 Hz, 60 deg/s sinusoidal oscillation, the rapid (perhaps visual) component was responsible for 1/3, and the slow (perhaps non-visual) component for 2/3, of all suppressive effects.

Adaptation, Ocular↗
PubMed1980

Statistical analysis of multipath neural systems.

Peripheral sensory and motor systems may be characterized by models consisting of multiple parallel convergent pathways, each described by the same set of equations, but having different parameter values in each path. Such models, although deterministic, are best analyzed using a statistical approach, which is illustrated here by analysis of several simple multipath models composed of linear dynamic elements and static non-linear elements. Relationships between instantaneous means of signals at different points in such systems are used to show that a multipath system can exhibit behaviour which would not be expected from observations of individual pathways. Mechanisms for linearization of static nonlinearities are briefly described. Important implications for neurophysiologists are discussed.

Animals↗
PubMedDate not supplied

Long-term deficits of goal-directed vestibulo-ocular function following total unilateral loss of peripheral vestibular function.

Brief whole-body movements (+/- 5 to 170 degrees/s peak velocity; approximately equal to 0.5 s duration), applied during 1.44-s intervals of total darkness while subjects "looked" at a just-viewed target, were used to examine vestibulo-ocular function in 3 patients who had compensated to total unilateral loss of peripheral vestibular function. We found that the combined effects of slow-phase and saccadic eye movements both tended to keep the eyes stabilized on the unseen target. Compensatory slow-phases elicited during rapid head movements ipsilateral to (i.e., towards) the lesion were only about 60% as effective as those elicited during rapid contralateral movements. Compensatory gaze-correcting saccades tended to supplement deficient slow-phase movements, especially during rapid ipsilateral head movement. However, the gaze-correcting effect of saccades was only about half of the required for perfect stabilization. Thus, two functional vestibular deficits were observed during rapid ipsilateral head movements: (1) reduced slow-phase stabilization, and (2) reduced saccadic ability to adequately supplement the deficient slow phases. However, overall vestibular functional capability, as assessed by observation of the net effect of both slow phases and saccades, was much better than would be indicated by conventional observation of slow-phase movements alone.

Adult↗