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At least 253 records · Page 14Linked to original sources
PubMed1998

The effect of forced vergence on retinal correspondence.

The spatial characteristics of the changes in retinal correspondence produced by forced convergence were studied. The vertical extent of lateral shifts in binocular correspondence were quantified by comparing the convergence of the eyes measured with binocular search coils to the convergence of the eyes as determined using nonius lines having vertical separations (gaps) between the nonius lines of 0.5-4.8 degrees. Lateral shifts in binocular correspondence only occurred for nonius gaps < 3-4 degrees. The effects of horizontal retinal eccentricity on lateral shifts in correspondence were determined by measuring the nonius horopter of the subject under forced convergence, using 11 nonius line eccentricities between 4.5 degrees left and right. The nonius horopter was shifted toward the fusion target maximally near the fixation point. There was no shift beyond 3 degrees of eccentricity. We conclude that the nonius horopter is 'dimpled' vertically and horizontally, facilitating local fusion by shifting the line horopter and the region of single binocular vision toward the point of regard over a region of 3 degrees around the fixation point.

Adult↗
PubMed2006

What do eye-fixation patterns tell us about unilateral spatial neglect?

PURPOSE: Eye-fixation patterns, which include ocular searching and fixation, may change with tasks, stimuli, and instructions. This article reviews our studies over 18 years on eye-fixation patterns of neglect patients and aims to elucidate the visuospatial processing of unilateral spatial neglect. METHODS: We recorded eye-fixation patterns when patients with neglect bisected a line in various conditions. RESULTS: Patients with neglect rarely searched to the left side when bisecting a line of the ordinary length (e.g., 200 mm). They persisted in fixating a right-side point, at which they later marked the subjective midpoint. They made no effective comparison between the leftward and rightward extents not only for a whole line but also for its explored right segment. Where they 'favored' to fixate as the subjective midpoint depended strongly upon the location of the right endpoint in space. Their representational image of a line was also estimated with modified line bisection tasks performed on a touch-panel display. CONCLUSIONS: For patients with neglect, the representational image of a line may be formed on the basis of the attended segment between the right endpoint and the favored point of fixation. The line bisection task, if combined with recording of eye-fixation, would further contribute to elucidation of the mechanisms underlying neglect.

Cues↗
PubMed1985

Power spectra for ocular drift and tremor.

Eye position during fixation has been measured without contact and the power spectrum of drift and tremor eye movements has been estimated. In the 0-40 Hz frequency range power declines with frequency roughly as 1/f2. In the 40-100 Hz frequency range the dominant spectral component is a broad spectral peak with a peak amplitude of about 6 arc sec. The shape of the broad spectral peak is dependent on the fixation direction. A model that explains qualitatively the shape of the power spectra function is presented. It is suggested that tremor eye movements are by-products of the clock-like firing of motor neurons.

Eye Movements↗
PubMed1975

"Saccadic nystagmus" in cerebellar cortical atrophy.

An ocular dyskinesia designated "saccadic nystagmus" was observed in a patient with cerebellar cortical atrophy. Saccadic nystagmus is a sustained ocular dyskinesia present during visual fixation and abolished by eye closure. It is difficult to distinguish visually from either pendular or jerk nystagmus without eye movement recordings. The oscillations are horizontal and rapid and may be influenced by direction of gaze. Caloric nystagmus (eye closed) and optokinetic nystagmus were normal in our patient. Visual fixation abolished caloric nystagmus, which was replaced by saccadic nystagmus.

Caloric Tests↗
PubMedDate not supplied

Evaluation of the vestibulo-ocular reflex by gaze function.

The relationship between the vestibulo-ocular reflex (VOR) and gaze fixation was investigated in normal adults, normal children, patients with cerebellar lesion and patients with labyrinthine dysfunction. Large VOR gains were found in patients with cerebellar lesion and normal children, both presenting poor suppression by gaze. Pathological decline in the VOR gain with labyrinthine lesion impaired spatial gaze fixation; however, it did not affect fixation-induced suppression. Gaze effect on the VOR, whether amplifying or suppressing, decreased linearly and rapidly at higher frequencies so that VOR seemed to be no more modified in the light at 2-3 Hz in any subject group. The present study suggested that VOR is controlled even in the dark by both mechanisms to obtain spatial gaze fixation and to gaze at a spatially moving target.

Adult↗
PubMed1996

A bilateral model integrating vergence and the vestibulo-ocular reflex.

The majority of previous modelling studies of vergence and the vestibulo-ocular reflex (VOR) have postulated arbitrary structures mainly on the basis of input-output behavioural relationships. Such models were developed following traditional schemes of oculomotor organization, based upon the notion of independence between different oculomotor subsystems. This impedes the simulation of complex binocular interactions and associated central activities. In contrast to preceding studies, the mathematical model for binocular control presented here was developed fully on physiological and anatomical grounds which reflect the organization and functional properties of known vergence and VOR premotor centres. Computer simulations show the model properly simulates the main observed characteristics in the discharge of several premotor and motor nuclei during slow vergence and the VOR in the dark. In particular, the model reproduces the activity profiles of abducens internuclear neurons, secondary vestibular cells, tonic prepositus hypoglossi neurons and ocular motoneurons during vergence and the VOR. It also simulates the activity of mesencephalic neurons whose discharge is modulated by vergence parameters alone. It is shown that given recent neurophysiological and behavioural findings, ocular reflexes cannot be properly modelled as separate independent subsystems whereas a single, unified modelling approach can produce results consistent with observed data. This study also shows how changes in the functional activity of shared pathways in a single two-sided structure produce vergence and conjugate integrators whose function relies on coupled loops across the brainstem: separate, dedicated operators are not necessary to replicate data. This provides evidence that challenges previous studies supporting the existence of separate vergence and conjugate integrators to transform velocity to position signals in the brainstem. A major implication of this study is that it questions the validity of testing conjugate and vergence systems independently, neglecting potential interactions.

Computer Simulation↗
PubMed1984

Purkinje cell activity in the primate flocculus during optokinetic stimulation, smooth pursuit eye movements and VOR-suppression.

Purkinje cell (PC) activity in the flocculus of trained monkeys was recorded during: 1) Vestibular stimulation in darkness. 2) Suppression of the vestibulo-ocular reflex (VOR-supp) by fixation of a small light spot stationary with respect to the monkey. 3) Visual-vestibular conflict (i.e. the visual surround moves together with the monkey during vestibular stimulation), which leads to attenuation or suppression of vestibular nystagmus. 4) Smooth pursuit eye movements. 5) Optokinetic nystagmus (OKN). 6) Suppression of nystagmus during optokinetic stimulation (OKN-supp) by fixation of a small light spot; whereby stimulus velocity corresponds then to image slip velocity. Results were obtained from PCs, which were activated with VOR-supp during rotation to the ipsilateral side. The same PCs were also modulated during smooth pursuit and visual-vestibular conflict. No tonic modulation during constant velocity OKN occurred with slow-phase nystagmus velocities below 40-60 deg/s. Tonic responses were only seen at higher nystagmus velocities. Transient activity changes appeared at the beginning and end of optokinetic stimulation. PCs were not modulated by image slip velocity during OKN-supp. The results show that in primates the same population of floccular PCs is involved in different mechanisms of visual-vestibular interaction and that smooth pursuit and certain components of OKN slow-phase velocity share the same neural pathway. It is argued that the activity of these neurons can neither be related strictly to gaze, eye or image slip velocity; instead, their activity pattern can be best interpreted by assuming a modulation, which is complementary to that of central vestibular neurons of the vestibular nuclei, in the control of slow eye movements.

Animals↗
PubMed2004

[Dissociated near reflex and accommodative convergence excess].

We report on an 8-year-old boy whose near reflex could be elicited exclusively when the left eye was fixing (LF) but not when the right eye was fixing (RF). With RE +1.25/-1.25/169 degrees and LE +1.0/-0.75/24 degrees, the visual acuity was 1.0 OU at 5 m and RE 0.5, LE 1.0 at 0.3 m improving to 1.0 OU by a near addition of 3.0 D. Stereopsis was 100 degrees (Titmus test). The prism and cover test revealed an esophoria of 4 degrees at 5 m. At 3 m there was an esophoria of 6 degrees (RF) and an esotropia of 28 degrees (LF), compensating to an esophoria of 3 degrees (RF/LF) with a near addition of 3.0 D. Accommodation and the pupillary near reaction (OU) were hardly elicitable during RF. During LF, retinoscopy revealed an accommodation of 8 D (OU) and the pupils constricted normally. Correction by bifocal glasses yielded orthotropia with random dot stereopsis at near.

Accommodation, Ocular↗