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J S Outerbridge

Publications and source records attributed to J S Outerbridge.

14 recordsLinked to original sources
PubMed1984

A bilateral model for central neural pathways in vestibuloocular reflex.

It is argued that vestibular internuclear commissural pathways are functionally important in the vestibuloocular reflex (VOR), particularly since they appear to be modulated during nystagmus. A bilateral approach to VOR modeling is essential to an effective study of the effects of commissural connections on response dynamics. A bilateral model of the VOR central pathways is proposed, with three main postulates: neural filters (NF) on each side of the brain stem, each linked to tonic cells in the ipsilateral vestibular nuclei in negative feedback loops; strong coupling between these bilateral loops by reciprocal commissural connections that significantly affect response dynamics; and modulation of this coupling by inhibitory burst neurons during fast phases. Mathematical analysis of this model shows that the NF need not be good integrators. During slow-phase operation, commissural pathways provide a positive-feedback effect that improves the effective integration function of the bilateral system beyond that of the NF in each side. Analysis suggests that the time constant of the NF might even be as small as that of the eye plant (approximately 0.24 s), so that the NF might be considered to be internal models of the eye plant rather than pseudointegrators. In the model, modulation of commissural gains by burst cells is shown to be sufficient to cause the system to switch between a compensatory position-tracking mode (slow phases) and an anticompensatory velocity-tracking mode (fast phases) during nystagmus. The model simulates a number of behavioral and neurophysiological findings, such as a) tonic vestibular nuclei (VN) cells have sensitivities and decay times larger than primary vestibular fibers, and their response polarity may reverse after section of superficial commissural fibers; b) effective VOR integration deteriorates after cerebellectomy or commissurectomy; c) peak fast-phase eye velocity is modulated by the vestibular signal as well as by fast-phase amplitude. The model accounts for the modulation of central VN responses during nystagmus and, as a result, simulations strongly imply that envelopes of slow-phase eye velocity or smoothed central firing rates will depend on fast-phase strategy and, hence, may not always yield accurate estimates of VOR dynamics. Similarly, the model predicts that "apparent" disassociation between central and ocular responses may occur because of interactions during nystagmus, despite appropriate behavior within slow-phase segments (since VN responses are not simple estimates of eye velocity).(ABSTRACT TRUNCATED AT 400 WORDS)

Eye↗
PubMed1981

Eye oscillations in strobe reared cats.

Cats reared from birth in stroboscopic illumination develop abnormal spontaneous eye oscillations of low amplitude. The present experiments were undertaken to define these eye movements as recorded in the dark, in stroboscopic light of various frequencies, after exposure to normal light and after attenuation of the vestibulo-ocular reflex (VOR) gain by optical reversal of vision. The interaction of spontaneous eye oscillations with voluntary saccadic eye movements, and optokinetic tracking (OKN), were also studied. Two cats, reared from birth to 18 months in 8 Hz strobe light, and one normally reared control animal, were used. Horizontal movement of the right eye was measured by the scleral eye coil method. The frequency content of eye movement records was determined by power spectral analysis. VOR gain was estimated in the dark, by rotating the animals sinusoidally at 1/8 Hz and 5 degrees/sec velocity amplitude. In the dark, both strobe reared cats had abnormal spontaneous eye oscillations at a frequency close to 8 Hz, with peak-to-peak amplitudes of 0.5--1.0 degrees. These abnormal eye movements did not interfere with, nor were they abolished by, normal oculomotor activity. The introduction of strobe light modified the spontaneous eye movements by entraining the oscillations at a given 'forcing' frequency, and by producing a number of harmonics or sub-harmonics. In one of the strobe reared animals, the effect of normal light was to reduce the characteristic 'dark' value of 9 Hz, to a new maintained 'light' value of 2.7 Hz. Adaptive attenuation of the VOR gain caused the abolition of regular spontaneous eye oscillations in the dark; nevertheless, transient oscillations to single strobe flashes could still be elicited in the VOR adapted condition. The results are interpreted as representing an organised attempt by the developing oculomotor system to attain the goal of stable visual perception in a new visual environment.

Animals↗
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↗
PubMed1978

Mechanical properties of succinylcholine activated muscle fibers in the inferior oblique muscle of the cat.

To determine the mechanical properties of the multi-innervated fibres in inferior oblique muscle of the cat, these fibers were selectively activated by means of an intravenous injection of succinylcholine chloride (Sch). For small sinusoidal length changes (+/- 1 mm) of low frequency, tension in the Sch activated muscle exhibited a component that was sensitive to the direction of stretch but was independent of the velocity. At higher frequencies of length change (16 Hz) a form of negative friction was observed. Negative friction had not been previously noted in mammalian muscles, although a similar friction was observed both in the muscles of insects and amphibians. These properties were expressed in the form of a simple mechanical model.

Animals↗
PubMed1975

Optokinetic nystagmus during selective retinal stimulation.

Nystagmic eye movements in response to selective optokinetic stimulation of different parts of the retina were studied in normal human subjects by two methods: 1. a digital computer controlled by the eye movement signal was used to generate an optokinetic display which stimulated only the peripheral retina, simulating a central scotoma, and 2. a single dot of 0.6 degrees in diameter was used as the stimulus during maintained forward gaze. The results show that stimulation of the central or peripheral retina alone can produce optokinetic nystagmus in man, and that essentially the same type of nystagmus is produced in both cases. The slow phase velocity of nystagmus evoked from the peripheral retina falls off rapidly with distance from the fovea but can be facilitated by attention. Results are compared with other findings and a possible explanation is offered for the observed variation in slow phase speed which occurs during constant velocity optokinetic stimulation.

Adolescent↗
PubMedDate not supplied

Nystagmus of paroxysmal positional vertigo.

During attacks of paroxysmal positional vertigo in 12 patients, nystagmus in different gaze positions was observed visually and recorded by electronystagraphy (eyes open, fixating). Visually observed eye movement was similar in all cases, with oblique (upward and horizontal) and rotatory (clockwise during leftward movement) components in fast phase; rotation dominated on ipsilateral gaze (toward the lower ear) and oblique movement dominated on contralateral gaze. However, ENG recordings showed greater variability and were often inconsistent with visual observation; the horizontal component often reversed with change in gaze position, and dissociated eye movements, as well as down-beating nystagmus, were sometimes seen. More sophisticated measurement and strict attention to gaze position is required to resolve these discrepancies.

Adult↗