Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “FIXATION, OCULAR”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 595 records · Page 33Linked to original sources
PubMed1998

Canal-otolith interactions in the squirrel monkey vestibulo-ocular reflex and the influence of fixation distance.

Natural head movements include angular and linear components of motion. Two classes of vestibulo-ocular reflex (VOR), mediated by the semicircular canals and otoliths (the angular and linear VOR, or AVOR and LVOR, respectively), compensate for head movements and help maintain binocular fixation on targets in space. In this study, AVOR/LVOR interactions were quantified during complex head motion over a broad range of fixation distances at a fixed stimulus frequency of 4.0 Hz. Binocular eye movements were recorded (search-coil technique) in squirrel monkeys while fixation distance (assessed by vergence) was varied using brief presentations of earth-fixed targets at various distances. Stimuli consisted of rotations around an earth-vertical axis and therefore always activated the AVOR. Horizontal and vertical AVORs were assessed when the head was centered over the axis of rotation and oriented upright (UP) and right-side-down (RD), respectively. AVOR gains increased slightly with increasing vergence in darkness, as expected given the small anterior position of the eyes in the head. Combined AVOR/LVOR responses were recorded when subjects were displaced eccentrically from the rotation axis. Eccentric rotations activated the AVOR just as when the head was centered, but added a translational stimulus which generated an LVOR component in response to interaural (IA) or dorsoventral (DV) tangential accelerations, depending on whether the head was UP or RD, respectively. When the head was eccentric and facing nose-out, the AVOR and LVOR produced ocular responses in the same plane and direction (coplanar and synergistic), and response magnitudes increased with increasing vergence. With the head facing nose-in, AVOR and LVOR response components were oppositely directed (coplanar and antagonistic). The AVOR dominated the response when fixation distance was far, and phase was compensatory for head rotation. As fixation distance decreased toward the rotation axis, responses declined to near zero, and when fixation distance approached even closer, the LVOR component dominated and response phase inverted. The same pattern was observed for both horizontal (head UP) and vertical (head RD) responses. The LVOR was recorded directly by rotating subjects eccentrically but in the nose-up (NU) orientation. The AVOR then generated torsional responses to head roll, coexistent with either horizontal or vertical LVOR responses to tangential acceleration when the subject was oriented head-out or right-side-out, respectively. Only the LVOR response components were modulated by vergence. A vectorial analysis of AVOR, LVOR, and combined responses supports the conclusion that AVOR and LVOR response components combine linearly during complex head motion.

Animals↗
PubMed2001

Contextual abnormalities of saccadic inhibition in children with attention deficit hyperactivity disorder.

Abnormalities of executive function are observed consistently in children with attention-deficit hyperactivity disorder (ADHD), and it is hypothesised that these arise because of disruption to a behavioural inhibition system. Executive and inhibitory functions were compared between unmedicated and medicated children with ADHD (combined type), age-matched healthy children and healthy adults. Executive functions were measured using a test of spatial working memory shown previously to be sensitive to ADHD and to stimulant medication. Inhibitory functions were measured using an ocular motor paradigm that required individuals to use task context to control the release of fixation. Context was set according to the probability that a target would appear at either of the two locations. In one block, targets appeared on 80% of trials. In the other block, targets appeared on 20% of trials. The ability to control the release of fixation was inferred from the fixation offset effect (FOE), or the difference in saccade latency when the current fixation is offset 200 ms prior to the onset of the saccade target (gap condition), compared with when there is no offset (overlap condition). Although the healthy children made more errors on the spatial working memory task than the healthy adults, there was no difference between the two groups in their ability to control fixation using context. Both showed a larger FOE when target probability was low. As expected, the unmedicated ADHD group made more errors on the spatial working memory test than the healthy children, although spatial working memory performance was normal in the medicated ADHD group. However, both the unmedicated and medicated ADHD groups were unable to modulate the FOE according to context, and this was due to their inability to voluntarily inhibit saccades when there was a low target probability. These data suggest that the context-based modulation of fixation release is not controlled by the same systems that control executive function. Furthermore, deficits in executive function and inhibitory control appear independent in children with ADHD.

Attention Deficit Disorder with Hyperactivity↗
PubMed2006

[Diagrammatic presentation of the results of strabismus examination in the various diagnostic positions of gaze need standardization].

We have found that there is no consistent opinion about the recording of the results of ocular deviations in various diagnostic positions of gaze, as well as for head tilt tests to the right and to the left in textbooks, manuscript and hospital medical records in China. For the sake of convenience in analysis and diagnosis of ocular deviations, it is necessary to have a unified opinion in recording these results. In accordance with the habitual diagrammatic presentation of the results of ocular deviations in the various diagnostic positions of gaze, as well as for head tilt tests to the right and to the left at home and abroad, we recommend two recording methods: the examiner's view and the patient's view. The patient's view is used in the synoptophore readings, Worth four-dot test, Maddox rod and double Maddox rod test, diplopia test, Hess screen and Lancaster screen test. The examiner's view is used in the prism cover test and arrangement for pictures in nine ocular positions of gaze.

Fixation, Ocular↗
PubMed1978

[Electrooculographic examination in paralytic and concomitant squint (author's transl)].

There are shown some possibilities of electrooculographic diagnostics in different forms of strabismus. Kinetic disturbances in fusion in concomitant squint are usually frequent but not constant and in the majority they are different from incomitances in paralytic squint. In this mind electrooculography represents subtile and perspective method in examination of ocular motility.

Child↗
PubMed1989

Spatial organization of sensitivity regulation in rod vision.

To investigate whether scotopic sensitivity is set locally or in neural "pools", we have tested the spatial variation in sensitivity after bleaching with gratings using 3 different methods. One experiment circumvented the influence of involuntary eye movements by deliberately randomizing the horizontal position of a fine test line on the area bleached by the vertical gratings. The spatial variation of threshold across the bleached area is reflected in the width of the frequency-of-seeing curve. A clear difference between the probability-of-seeing curves following a grating bleach and a uniform bleach was seen only up to between 4.2 and 6.3 c/deg, suggesting that adaptation signals are pooled so as to almost obliterate the contrast in finer gratings than this. In a second experiment the lowest bleaching-grating contrast (for a space-averaged initial rhodopsin bleach of 10%) that produced a patterned afterimage stayed close to the scotopic threshold contrast for frequencies from 1 to 6.4 c/deg, but it rose above the contrast threshold at high spatial frequencies. This slight loss of sensitivity at the high frequencies is more evidence for pooling in adaptation. A third experiment assessed the sensitivity profile at the adapting site without any influence of later stages of neural integration. Bleaching and test gratings of slightly different spatial frequency were flashed successively. If the effect of bleaching is restricted to the bleached rods, the observer will effectively be looking at the test grating through a grid of sensitive and insensitive stripes in his own retina. The two gratings come in and out of register at the difference frequency, and a corresponding low-frequency grating should be visible even when the test and bleaching gratings are not themselves resolved by the later stages. We could not see the difference frequency unless the test and bleach gratings were themselves coarse enough to be resolvable in rod vision. This is very strong evidence against any model in which each rod has its own sensitivity-regulating mechanism, and instead supports (for these conditions) Rushton's view that adaptation is entirely the work of a neural pool. The estimated pool size is about 10 min arc of visual angle.

Adaptation, Ocular↗
PubMed1995

Voluntary binocular gaze-shifts in the plane of regard: dynamics of version and vergence.

We studied the dynamics of voluntary, horizontal, binocular gaze-shifts between pairs of continuously visible, real three-dimensional targets. Subjects were stabilized on a biteboard to allow full control of target angles, which were made to differ only in distance (pure vergence), only in direction (pure version; conjugate saccades) or in both distance and direction (disjunctive saccades). A wide range of changes in vergence (0-25 deg) and version (0-65 deg) was recorded to study the dynamics of disjunctive saccades, described until now for limited ranges, throughout the horizontal oculomotor range within manual working space, and to study the velocity-duration-amplitude relations ("main sequence") of disjunctive vs conjugate saccades. Pure vergence was almost never observed; divergence, especially, was always associated with saccades. Likewise, horizontal saccades were never strictly conjugate, they always contained a transient divergence-convergence sequence. The amplitude and velocity of these transient components varied systematically with saccadic size. In combined version-vergence movements, vergence was, in general, accelerated and shortened as a function of increasing version. This effect was fairly uniform for divergence, which appeared to increase in velocity by about as much as the transient peak divergent velocity of the version saccade. The intrasaccadic fraction of divergence increased from about 50% to close to 100% as a function of increasing version. For convergence, saccades up to about 20 deg were also accelerating; in this case it appeared as if the transient peak convergent velocity of the version saccade was added to the basic convergence velocity. For larger saccades this effect was partly counteracted by the penetration of an initial divergence associated with the saccade. This initial divergence delayed and slowed down convergence. The intrasaccadic fraction of convergence varied between about 40% and 70%. In disjunctive saccades the individual eyes did not follow the main-sequence parameters of conjugate saccades of comparable sizes, except for the eye that moved with the combination "abduction and divergence". For all other combinations of vergence and version, disjunctive saccades had lower peak velocities and longer durations than conjugate saccades. As a consequence, disjunctive version was also slower than conjugate version. Thus, while version accelerates vergence, vergence slows down version: in the generalized case of three-dimensional gaze-shifts, peak velocities and durations are in between those of the limiting cases of pure version and pure vergence. We conclude that, within manual working space, binocular gaze-shifts are effected by the highly integrated action of conjugate and disjunctive mechanisms, both of which are expressed preferentially in fast, saccadic movements.

Adult↗
PubMed2006

Luminosity--a perceptual "feature" of light-emitting objects?

Light-emitting objects are perceived as qualitatively different from light-reflecting objects, and the two categories elicit different cortical activity. However, it is unclear whether object luminosity is treated as an independent visual feature, comparable to orientation, motion or colour. Visual search tasks revealed that light-emitting targets led to efficient search when presented with light-reflecting distractors of similar luminance, but this efficiency was induced by the presence of luminance gradients producing the percept of luminosity rather than by luminosity itself. This implies that luminance gradients (not object luminosity) are encoded as features, questioning the existence of specific sensory mechanisms to detect light-emitting objects.

Adaptation, Ocular↗