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Biomedical subjects

R V Kenyon

Publications and source records attributed to R V Kenyon.

At least 19 recordsLinked to original sources
PubMed1993

The effects of background visual roll stimulation on postural and manual control and self-motion perception.

The effects of background visual roll stimulation on postural control, manual control, and self-motion perception were investigated in this study. In the main experiment, 8 subjects were exposed to wide field-of-view background scenes that were tilted and static, continuously rotating, or sinusoidally rotating at frequencies between 0.03 and 0.50 Hz, as well as a baseline condition. The subjects performed either a postural control task (maintain an upright stance) or a manual control task (keep an unstable central display horizontally level). Root-mean square (RMS) error in both the postural and manual control tasks was low in the static tilt condition and extremely high in response to continuous rotation. Although the phases of the postural and manual responses were highly similar, the power and RMS error generated by the sinusoidal visual background stimulation peaked at a lower frequency in the postural task. Vection ratings recorded at the end of the postural and manual trials somewhat paralleled the frequency tuning differences between tasks, which a subsequent experiment showed to be the result of the differential motion of the central display rather than the differential positioning of the subject. In general, these results show that the dynamic characteristics of visual orientation systems vary according to the specific motor and/or perceptual system investigated.

Adult↗
PubMed1988

Streptomycin in the chick embryo: post-hatching vestibular behavior and morphology.

Developing chick embryos were exposed to streptomycin injected on days 5 through 13 of the 21 day developmental period. Histological and behavioral abnormalities were found almost exclusively in chicks exposed after day 7. The nature of the behavioral deficits included abnormal head posture, head tremor, and inability to compensate for applied vestibular stimuli. Head movement measurements showed that the head tremor had frequencies of oscillations from 10-35 Hz. The amplitude of the tremor was a large as 10 degrees. Histology showed damage to the secretory dark cells of the membranous labyrinth in those chicks that showed behavior changes. Even with increased dosages chicks exposed prior to day 7 rarely showed abnormal vestibular behavior but instead experienced increased mortality. Further tests examining tissue levels of streptomycin showed little or no streptomycin in embryos until day 10. These results are discussed in terms of their utility as an alternative model to surgical manipulation of the vestibular system in developing embryos. Behavioral consequences are compared to other work with drugs and to the effects of weightlessness and unusual environments on vestibular orientation and behavior.

Animals↗
PubMed1986

M.I.T./Canadian vestibular experiments on the Spacelab-1 mission: 1. Sensory adaptation to weightlessness and readaptation to one-g: an overview.

Experiments on human spatial orientation were conducted on four crewmembers of Space Shuttle Spacelab Mission 1. This introductory paper presents the conceptual background of the project, the relationship among the experiments and their relevance to a "sensory reinterpretation hypothesis". Detailed experiment procedures and results are presented in the accompanying papers in this series. The overall findings are discussed in this article as they pertain to the following aspects of hypothesized sensory reinterpretation in weightlessness: utricular otolith afferent signals are reinterpreted as indicating head translation rather than tilt, sensitivity of reflex responses to footward acceleration is reduced, and increased weighting is given to visual and tactile cues in orientation perception and posture control. Three subjects developed space motion sickness symptoms, which abated after several days. Head movements, as well as visual and tactile cues to orientation influenced symptoms in a manner consistent with the sensory-motor conflict theory of space motion sickness. Six short duration tests of motion sickness susceptibility, conducted pre-flight, failed to predict sickness intensity in weightlessness. An early otolith-spinal reflex, measured by electromyography from the gastrocnemius-soleus muscles during sudden footward acceleration, was inhibited immediately upon entering weightlessness and declined further during the flight, but was unchanged from pre-flight when measured shortly after return to earth. Dynamic visual-vestibular interaction was studied by measuring subjective roll self-motion created by looking into a spinning drum. Results suggest increased weighting of visual cues and reduced weighting of graviceptor signals in weightlessness. Following the 10 day flight, erect posture with eyes closed was disturbed for several days.(ABSTRACT TRUNCATED AT 250 WORDS)

Adaptation, Physiological↗
PubMed1986

M.I.T./Canadian vestibular experiments on the Spacelab-1 mission: 5. Postural responses following exposure to weightlessness.

The four science crewmembers of Spacelab-1 were tested for postural control before and after a 10 day mission in weightlessness. Previous reports have shown changes in astronaut postural behavior following a return to earth's 1-g field. This study was designed to identify changes in EMG latency and amplitudes that might explain the instabilities observed post-flight. Erect posture was tested by having the subject stand on a pneumatically driven posture platform which pitched rapidly and unexpectedly about the ankles causing dorsi- and plantarflexion. Electromyographic (EMG) activity from the tibialis anterior and the gastrocnemiussoleus muscles was measured during eyes open and eyes closed trials. The early (pre 500 ms) EMG response characteristics (latency, amplitude) in response to a disturbance in the posture of the subject were apparently unchanged by the 10 days of weightlessness. However, the late (post 500 ms) response showed higher amplitudes than was found pre-flight. General postural control was quantitatively measured pre- and post-flight by a "sharpened Romberg Rails test". This test showed decrements in standing stability with eyes closed for several days post-flight.

Adaptation, Physiological↗
PubMed1985

A soft contact lens search coil for measuring eye movements.

A five turn coil of magnet wire sandwiched between two soft contact lenses served as a magnetic search coil to measure horizontal and vertical eye movements in humans. The lens was adhered to the eye for 35 min by periodically misting the eye with distilled water; during this time the records of eye position showed that the lens remained firmly attached to the eye. Using this sensor, no topical anesthetic need be applied and the lens can be worn for an extended period of time without increasing intraocular pressure.

Contact Lenses, Hydrophilic↗
PubMed1985

Eye movements during reading: further case reports.

Eye movements during reading and nonreading tasks were measured in four patients having either Friedreich's ataxia, congenital dyslexia, acquired dyslexia with quadrantanopia, or acquired dyslexia with hemianopia. Abnormalities included saccadic intrusions, abnormally large dynamic overshoots, increased fixation duration and number of fixations, and/or reduced reading rate. The results clearly demonstrate the importance of objective eye movement recording for the precise specification of the abnormal reading pattern in patients with neurological dysfunctions.

Adult↗
PubMed1984

Accommodative disfacility presenting as intermittent exotropia.

Accommodative disfacility and intermittent exotropia are two common clinically-observed ocular abnormalities. However, records of individual system dynamics are rare and mechanisms for possible interactive effects between the accommodation and the vergence system remain obscure in cases of system dysfunction. We present objective, quantitative, static and dynamic records of accommodation, vergence and related system responses in a symptomatic patient having accommodation disfacility and intermittent exotropia and propose that these two conditions worked in a pathogenic symbiotic manner.

Accommodation, Ocular↗
PubMed1984

Oculomotor function in Wernicke-Korsakoff's syndrome: I. Saccadic eye movements.

Saccadic eye movements from three patients diagnosed as having Korsakoff's syndrome and one patient who had suffered the acute Wernicke stage of the disorder were examined for changes in saccadic characteristics. The three Korsakoff patients showed increased saccadic latencies; two of the three also had reduced saccadic peak velocities and increased saccadic durations. A higher than expected incidence of hypometric saccades was also found in all four patients; saccadic intrusions were recorded in two Korsakoff patients. These abnormal oculomotor responses were found 2-7 years after the onset of the Korsakoff's syndrome and are consistent with the cerebellar and frontal lobe dysfunction reported in this patient group.

Alcohol Amnestic Disorder↗
PubMed1984

Oculomotor function in Wernicke-Korsakoff's syndrome: II. Smooth pursuit eye movements.

Smooth pursuit eye movements were studied in three patients with alcoholic Korsakoff's syndrome, one with Wernicke's encephalopathy, and an age-matched control. Horizontal smooth pursuit eye movements were abnormal in all patients: peak eye velocity and the ability to sustain smooth eye velocity were reduced. Also, smooth pursuit gain began to decrease at relatively low target velocities (i.e., 8-10 degrees). These data demonstrate a severe disturbance in smooth pursuit function long after the clinically apparent oculomotor abnormalities have passed.

Aging↗
PubMed1983

Saccadic intrusions contributing to reading disability: a case report.

Horizontal eye movements were recorded during tracking and reading in a patient with probable neurological involvement who complained of periods of oscillopsia and severe asthenopia, especially during reading. Saccadic intrusions occurred during all tracking tasks. Single and multiple saccadic intrusions found while reading, frequently accompanied periods of visual disturbance, and we attribute part of the reading disability to the presence of such abnormal eye movements. Various therapeutic measures were attempted and achieved some degree of success.

Adult↗
PubMed1981

Dynamic vergence eye movements in strabismus and amblyopia: asymmetric vergence.

This report investigates line-of-sight asymmetric disparity vergence in patients having either intermittent strabismus, constant strabismus with amblyopia, or amblyopia without strabismus. We find an absence of disparity vergence in all patients with strabismus and in some with amblyopia only. Accommodative vergence and saccades place the dominant eye on the targets moving in depth. These accommodative vergence responses have normal dynamic characteristics, thus indicating a properly functioning vergence motor system. We purpose there is a higher-level central defect in which incoming information of one eye is suppressed, so that the disparity vergence system is rendered inoperable.

Adolescent↗
PubMed1980

An unexpected role for normal accommodative vergence in strabismus and amblyopia.

Accommodative vergence was measured using the Müller experimental paradigm in which target movement along the line of sight of the viewing eye results in large movement of the fellow (covered) eye. Our paper demonstrates an unexpected predominance of accommodative vergence over disparity vergence in patients with constant strabismus amblyopia, intermittent strabismus, and amblyopia without strabismus. Comparison of accommodative vergence responses in these patients with responses from normal subjects using symmetrical and asymmetrical targets under binocular and monocular conditions established the normal character of the patients' accommodative vergence and the absence of disparity vergence in most of them. The intermittent absence of disparity vergence in certain of our patients--some with surgically corrected strabismus, some with moderate amblyopia, and some with amblyopia treated with orthoptics--raises questions of clinical and neurophysiological interest. Preliminary results on the effects of amblyopia on accommodative responses are also presented.

Accommodation, Ocular↗
PubMed1980

Unequal saccades during vergence.

An examination of saccades during vergence eye movements (accommodative and disparity) reveals that binocular saccades have unequal magnitudes in each eye, the smaller saccade of each pair opposing the vergence movement. Furthermore, this inequality cannot be explained by linear summation of the saccadic amplitudes onto the ongoing vergence. Saccadic inequalities are usually greater at the start of the vergence; the amplitude of the smaller saccade may be reduced 70% from that of the larger. Using a sixth-order nonlinear model of the eye movement plant and linear summation of saccadic (pulse-step) and vergence (step) control signals to drive the plant, we have produced a reduction in saccadic amplitudes between the two eyes during vergence that is similar to those found in our subjects. These results suggest that the reduction of saccadic amplitudes is due to an interaction in the muscular system moving the eye, not to a nonlinear neuronal interaction more centrally located.

Accommodation, Ocular↗
PubMed1980

Increased drift in amblyopic eyes.

Reports are conflicting on the presence of increased drift in amblyopic eyes. Furthermore, the individual effects of either amblyopia or strabismus alone on ocular drift have not been systematically investigated. We therefore used a photoelectric method to record horizontal eye position during monocular and binocular fixation in patients having amblyopia without strabismus, intermittent strabismus, or constant strabismus amblyopia. Our principal finding was increased drift amplitude (up to 3.5 degree) and velocity (up to 3.0 degrees per second) in amblyopic eyes during monocular fixation. While increased drift was found 75% of the time in amblyopia without strabismus and 50% of the time in constant strabismus amblyopia, it was found only 20% of the time in intermittent strabismus. Amblyopic drift could be either error-producing or error-correcting in nature. Increased drift was not present during monocular fixation with the dominant eye or during binocular fixation in any of our 16 patients. We therefore conclude that amblyopia and not strabismus is a necessary condition for the presence of markedly increased fixational drift. Increased drift amplitude but not velocity may adversely affect visual acuity in the amblyopic eye.

Adolescent↗
PubMed1980

Dynamic vergence eye movements in strabismus and amblyopia: symmetric vergence.

Dynamic vergence eye movements in response to step target displacements along the midline were measured by an infrared reflection technique in 11 patients having either intermittent strabismus, constants-strabismus amblyopia, or amblyopia without strabismus. We found the absence of normal disparity (fusional) vergence in all patients having strabismus and in some patients having amblyopia without strabismus. A characteristic response consisting of a binocular accommodative vergence component and an early binocular saccadic component was used to foveate the target of interest with the dominant eye. Vergence responses in our control subjects and patients with the nondominant eye occluded were similar to those recorded in our patients during binocular viewing. These results suggest that disparity information is not utilized by patients, probably as a result of long-term, ongoing suppression in the deviated or amblyopic eye. Accommodative vergence with the aid of an early foveating saccade was the primary mechanism for tracking targets in three-dimensional space.

Accommodation, Ocular↗
PubMed1980

Disparity vergence: a proposed name for a dominant component of binocular vergence eye movements.

A proposal to name the dominant aspect of vergence "disparity vergence" is put forward. Support for this proposal is gathered by briefly reviewing the components of vergence: (1) disparity vergence, the visual feedback control system for removing binocular disparity with an appropriate binocular convergence or divergence, (2) accommodative vergence, a synkinesis with accommodative motor control, (3) proximal vergence, and (4) tonic vergence. Sensory fusion is a higher level perceptual process aided by disparity vergence, and historically the term fusional vergence called attention to this relationship. However, it now appears to obscure the fact that disparity is the only direct stimulus to vergence.

Accommodation, Ocular↗
PubMed1979

Saccadic intrusions in strabismus.

Fixational eye movements were studied to determine the presence of and to quantify saccadic intrusions under monocular and binocular viewing conditions in subjects with intermittent strabismus, amblyopia without stabismus, or constant strabismus amblyopia. Saccadic intrusions were present under most test conditions in intermittent strabismus, were rarely observed in amblyopia without strabismus, and were prominent during monocular fixation with the amblyopic eye in constant strabismus anblyopia. This suggests that the presence of saccadic intrusions was related to strabismus and not amblyopia. There was no relationship between saccadic intrusion amplitude and visual acuity. Two possible mechanisms for producing intrusions are abnormally rapid regional visual adaptation and strabismus-induced fixation degradation.

Adolescent↗