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Biomedical subjects

C G Lau

Publications and source records attributed to C G Lau.

11 recordsLinked to original sources
PubMed1992

Optokinetic and vestibular interactions with smooth pursuit: psychophysical responses.

The effect was evaluated in normal subjects of the subjective perception of motion of a small visual target (VT) when combined with the effect of vestibular stimulation produced by different magnitudes of constant angular accelerations in the dark or the effect of optokinetic stimulation produced by different constant velocities of rotation. The visual target appeared to the subject to travel more slowly and for a shorter duration when it moved in the direction of the body's angular acceleration or against that of the optokinetic drum. The perceived error in motion was: (i) in the same direction as the subject's motion sensation produced by either of the two stimuli, and (ii) quantitatively related, although differently, to the magnitude of each of the two stimulus modalities; an heuristic model is proposed to account for these observations.

Acceleration↗
PubMed1979

Modification of constant optokinetic nystagmus by vestibular stimuation.

Experiments were conducted to quantify the effect of a vestibular stimulation of known magnitude on a constant optokinetic nystagmus (OKN). Ten normal human subjects were tested with varying magnitudes of vestibular stimuli that were superimposed on a constant 30 degrees optokinetic stimulus. The gain of the vestibular system in the dark was 0.42 +/- 0.11, and the gain in the light during superimposition testing was 0.12 +/- 0.02. From these results, predictions were made that the degree of vestibular imbalance necessary to produce an asymmetric OKN would generate a spontaneous nystagmus in the dark, which would be equivalent to 20 to 30 degrees. Data from a large group of patients were used for corroboration of the results.

Adult↗
PubMed1979

Neural correlates of nystagmus in abducens nerve.

1. The firing rates of action potentials of abducens nerve single fibers were recorded in the cat's orbit during a variety of vestibular and optokinetic stimulations. 2. Comparison was made of the neural firing rates associated with agonist and antagonist responses during slow and fast components of vestibular and optokinetic nystagmus. It was found that the relationship between the motoneuron firing rates and the eye motion was independent of the reflex with which they were associated--vestibular or optokinetic, or the type of response--agonist or antagonist. No neurons were observed that responded only during the fast or only during the slow nystagmus phase. Motoneuron firing rates were proportional to both velocity and position of the eye in a ratio of 1 (spikes/s)/(deg/s) to 7.2 (spikes/s)/deg. The behavior of the motoneurons was compatible with the hypothesis that thier firing rates are sufficient to overcome both elastic and viscous forces by which the muscles and ligaments hold the eye in the orbit. 3. For low-frequency head rotations, eye displacement and neural responses showed a small phase angle difference. At higher frequencies, however, while the eyes maintained a fixed relationship to the head rotation, the neural responses showed an increasing phase lead. One component of this phase lead compensated for the phase lag introduced by the orbital mechanics. The other was modeled as a constant delay of approximately 70 ms, which may be accounted for by neuromuscular transmission and transduction.

Abducens Nerve↗
PubMed1979

Visual-vestibular interaction and cerebellar atrophy.

The vestibular and optokinetic ocular control systems were studied in 10 patients with cerebellar atrophy and in 10 normal subjects using (1) constant velocity optokinetic stimulation, (2) sinusoidal rotation in the dark, and (3) sinusoidal rotation in the light with a surrounding fixed optokinetic drum. The gain (maximum slow component velocity/maximum head or drum velocity) of induced nystagmus was calculated from electro-oculographic recordings. Optokinetic nystagmus was abnormal in seven patients and the average optokinetic gain in the patients was significantly (p less than 0.01) less than that of the normal group. Three patients with "clinically pure" cerebellar atrophy had increased vestibular responses, and one patient with clinical signs of peripheral neuropathy had decreased responses, probably due to associated vestibular nerve disease. The average vestibulo-ocular reflex gain in patients did not differ significantly from controls (p greater than 0.05). Three patients had normal vestibular and optokinetic responses when tested independently, but had abnormal visual-vestibular interaction. These patients probably had selective disorders of the midline cerebellar pathways that mediate visual-vestibular interaction. By studying each system, both independently and during interaction, all patients were identified as abnormal, and a more precise anatomic localization of the atrophy was obtained.

Adolescent↗
PubMed1979

Slow build-up of optokinetic nystagmus associated with downbeat nystagmus.

Eye movement recordings in two patients with downbeat nystagmus demonstrated an unusual finding of severely impaired smooth pursuit and relatively unimpaired optokinetic nystagmus (OKN). OKN was characterized by a remarkable, slow build-up of slow-component velocity, similar to that found in afoveate animals. Optokinetic after-nystagmus (OKAN), or transient persistence of nystagmus after cessation of visual stimulation, typical of the optokinetic response of normal human subjects, was also preserved in these patients. These observations suggest that the normal contribution of smooth pursuit to the ocular motor response to rotation of the visual environment can be selectively removed by a lesion at the level of the craniocervical junction.

Adult↗
PubMed1978

Linear model for visual-vestibular interaction.

The results of experiments are evaluated in terms of a simple model for the interaction of eye movement responses to simultaneous optokinetic and vestibular stimuli. The model predictions agree with the results of these experiments and explain many clinical observations concerning the effect of vision on nystagmus. The model accounts for the dominance of the visual system's response over the vestibular system's response at low frequencies. It also accounts for the inability of patients with decreased smooth pursuit system response to suppress the vestibulo-ocular reflex during simultaneous optokinetic and vestibular stimulations. The model provides useful information for the design of combined optokinetic and vestibular stimuli for test vestibulo-ocular reflexes.

Eye Movements↗
PubMed1978

Vestibular-optokinetic interactions in normal subjects and in patients with peripheral vestibular dysfunction.

The vestibulo-ocular reflex (VOR) during rotation, optokinetic nystagmus (OKN), and interactions between the vestibulo-ocular and optokinetic systems were studied quantitatively in 10 normal human subjects, 15 patients with unilateral horizontal semicircular canal paralysis (UP), and 11 patients with bilateral horizontal semicircular canal paralysis (BP). The OKN gain (eye velocity/drum velocity) was not significantly different between the normal subjects and the patient groups. During tests in which rotatory and visual stimuli were presented simultaneously, the contribution to the observed eye movements by the VOR was only one-fourth to one-third of its gain during rotation in the dark in the normal subjects and UP patients. These interactive tests did not differentiate UP patients from normal subjects but did separate BP patients from normal subjects.

Adult↗
PubMedDate not supplied

Implications of Ewald's second law for evaluation of vestibular function.

The significance of Ewald's second law in the evaluation of the vestibulo-ocular reflex (VOR) was investigated using the transfer characteristics of the vestibular and VOR systems in normal rabbits and rabbits in which one horizontal semicircular canal had been blocked. The transfer characteristics of the vestibular system were derived from the experimental results reported by Goldberg and Fernández in 1971. A comparison was made of the properties of the bilateral and monolateral VOR systems with the predictions of a piecewise linear model of the vestibular system. The data received quantitatively collaborate the prediction of Ewald's second law as it applies to the VOR responses.

Animals↗
PubMedDate not supplied

Implications of Ewald's second law for diagnosis of unilateral labyrinthine paralysis.

The feasibility of replacement of the caloric test with sinusoidal rotatory stimulations of differing frequencies was investigated in ten normal subjects and seven patients with unilateral labyrinthine paralysis. Testing was performed at frequencies ranging from 0.0125 to 0.2 Hz at 60 degrees/sec and at 0.05 Hz at peak velocities of 30 degrees/sec to 180 degrees/sec. Gain, phase, and asymmetry of the nystagmic responses were measured in these groups. Results indicated that the rotatory test was not reliable enough to be used as the sole measure for identifying patients with unilateral labyrinthine lesions.

Analog-Digital Conversion↗