PubMed Health⌕ Search

SEARCH · PubMed Health

Results for “FIXATION, OCULAR”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

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

At least 289 records · Page 16Linked to original sources
PubMed1990

Selective nonconjugate binocular adaptation of vertical saccades and pursuits.

Hering's law describes the equal and symmetrical rotation of the two eyes. It is possible to calibrate the binocular yoking of the two eyes in response to disparate size and/or motion of the two ocular images. It is unclear if the ratio of movements by the two eyes is modified by selective adaptation of each versional system (i.e. saccades and pursuits) or if there is an apparent adaptation of Hering's law that results from a single underlying process. The latter could be accomplished by vergence (prism) adaptation, which could interact with all versional systems. In this investigation, binocularly stimulated saccades and pursuits were adapted separately for 2 hr to unequal vertical target displacements. Three adaptation paradigms were used; each included a 10% binocular gradient disparity. The adapting stimulus for the pursuit system was 0.25 Hz vertical triangular motion of 20 deg, peak to peak. Two saccade adaptation paradigms included one which emphasized correcting vertical disparity during the pulse component of the saccade, the other minimized the influence of disparity prior to, during and immediately after saccades (vergence paradigm). Yoking ratios (YRs) for vertical pursuits and saccades were compared before and after adaptation. The pursuit paradigm produced marked adaptation of the pursuit YRs while it had negligible effect on saccade YRs. The pulse saccade paradigm adapted the saccade YRs twice as much as the pursuit YRs whereas the vergence paradigm resulted in little adaptation of YRs for either saccades or pursuits. Pursuits adapted to the first paradigm in 15-30 min whereas saccades adapted to the second paradigm in 1.5-2 hr. These results indicate that there is not a single common nonconjugate adaptation mechanism for vertical pursuits and saccades. Results of the vergence paradigm demonstrate that feedback during or immediately after eye movements is necessary in order to stimulate the binocular versional adaptation mechanism. Versional adaptation may be considered as a calibration of Hering's law.

Adaptation, Ocular↗
PubMed1991

Sensory interactions during human fusional response.

The nature of sensory interactions during human fusional response was investigated using band-limited, 10th-derivative-of-Gaussian patterns. Experiments were performed to investigate the effects of spatial separation, disparity magnitude, and stimulus spatial frequency on horizontal sensory fusional amplitudes (SFA). When the disparities in the inducing and probe regions were in the same direction the SFA for the probe increased; when the disparities were in opposite directions the SFA was reduced. Increases and decreases in the SFA for the probe caused by the inducing stimulus were designated enhancement and inhibition respectively. Both enhancement and inhibition increased with increasing inducing disparity. They were evident over a range of spatial frequencies from 0.75 to 3.0 c/deg and among retinal regions separated by as much as 1.5 deg arc. The SFA for the probe was not inhibited when the peak spatial frequency of the inducing stimulus was 2 octaves higher than that of the probe. The observed changes in SFA were found to be due to shifts in location rather than changes in the extent of the fusional range for the probe.

Convergence, Ocular↗
PubMed1995

A reflectometric technique for assessing photoreceptor alignment.

Clinical studies of photoreceptor orientation are limited by the fact that psychophysical methods for measuring the Stiles-Crawford effect are time consuming and require excellent co-operation from the subject. We have developed a novel instrument, the photoreceptor alignment reflectometer (PAR), that determines photoreceptor alignment by measuring the distribution in the pupil of light reflected by one retinal location. This determination is accomplished in a measurement time of 4 sec and requires minimal co-operation from the subject. The technique is not significantly affected by reflections at the limiting membrane, or by changes in entrance and exit pupil configuration, or by location of bleaching light entry. The PAR was used to measure the orientation of foveal photoreceptors, their directionality, and the ratio of directional to diffuse flux in 20 normal subjects ranging in age from 20 to 60 yr.

Adaptation, Ocular↗
PubMed1995

Modulation of motion aftereffect by surround motion and its dependence on stimulus size and eccentricity.

As a mechanism to detect differential motion, we have proposed a model of 'a motion contrast detector' and have shown that it can explain the perceptual change from motion capture to induced motion with increasing stimulus size and decreasing eccentricity. To further test the feasibility of the model, we examined the effect of surround motion on the motion aftereffect (MAE) elicited in the center. Using a drifting grating surrounded by another drifting grating, the duration of MAE in the center after adaptation was measured for various surround velocities (Expt 1). MAE was stronger when the surround moved oppositely to, than together with, the center. This finding was consistent with some previous reports. Using similar stimuli, MAE was measured at various stimulus sizes and eccentricities by the cancellation technique (Expt 2). The effect of surround modulation turned out to vary with both size and eccentricity. We examined if the apparent dependence on eccentricity could reflect a simpler effect of cortical size when the data were rescaled according to a linear scaling factor. We interpret our results in terms of motion contrast detectors, possibly located in the area MT.

Adaptation, Ocular↗
PubMed1995

Rotation of Listing's plane by horizontal, vertical and oblique prism-induced vergence.

We examined the changes in Listing's plane resulting from prismatically induced vergence. The three-dimensional angular positions of the two eyes were compared in normal subjects wearing search coils and gazing at targets 1.9 m away with and without prisms. For horizontal base-out prisms each degree of convergence in one eye yielded 0.72 deg of temporal rotation of Listing's plane in that eye. The results from vertical prisms were not what was expected from the horizontal results. A base-up prism on the right eye induced a downward and temporal rotation of Listing's plane. A base-down prism on the right eye induced an upward and nasal rotation of Listing's plane. The effects of oblique prisms were those expected from combining the effects of horizontal and vertical prisms. Thus in addition to producing a horizontal or vertical misalignment of the gaze line, prisms induce an unexpected position-dependent torsional disparity.

Convergence, Ocular↗
PubMed1995

An adaptable association between vertical and horizontal vergence.

Vertical phoria (vergence error under monocular viewing conditions) can be trained to vary with conjugate eye position. The adaptive response controls the vertical alignment of the two eyes in the absence of binocular disparity and is used to compensate for binocular changes of the oculomotor system induced by developmental and environmental factors. Vertical phoria was associated with horizontal disparity vergence by adapting vertical vergence to two vertically disparate targets separated along the depth axis. This association was primarily dependent on the horizontal vergence as opposed to monocular eye position or binocular conjugate eye position. Following this adapted association with horizontal disparity vergence, vertical phoria aftereffects were also evoked by accommodative vergence. Previous reports have demonstrated an adapted association between vertical phoria and conjugate eye position. The current report examines the difference in the vertical phoria resulting from adaptation to vertically disparate targets separated along either the vertical axis or depth axis. The amplitude of the vertical vergence aftereffect was approximately 4 times greater for targets separated along the depth axis than in the vertical meridian. The association between vertical phoria with conjugate eye position and horizontal vergence is proposed to result from a cross-coupling of vertical vergence with supranuclear regions that control conjugate and horizontal vergence eye movements. A selective interaction would enable the oculomotor system to correct disturbances in specific supranuclear regions as they interface with vertical vergence.

Accommodation, Ocular↗
PubMed1995

Theoretical explanations of Listing's law and their implication for binocular vision.

We shall discuss three theoretical explanations of Listing's law for conjugate eye movements with the head fixed: the original argument by Helmholtz, which is "sensorimotor" in its attempt to optimize vision by using internal feedback from the oculomotor system, and two comparatively simple recent explanations based on either visual or oculomotor performance. These geometrical demonstrations shed some light on recent generalizations of Listing's law to vergent eye movements.

Convergence, Ocular↗
PubMed1996

Disparity-evoked vergence is driven by interocular correlation.

Disparity-evoked vergence is studied in stereograms showing one or two depth planes which are defined by isolated dots of varying density and contrast. Vergence position immediately after stimulus presentation was measured using dichoptic nonius lines. Since the stimulus was not visible after the onset of the vergence movement, the experiment accesses the initiation of vergence rather than its eventual result. In the unequivocal stimuli (one depth plane), elicited vergence tends to reduce disparity. Disparities of 0.5-1 deg are most effective which is in accordance with earlier findings. If two depth planes are presented, elicited vergence lies between the two planes, approaching the plane with higher dot density and/or dot contrast. In quantitative measurements, we show that the depth-averaging mechanism uses signal power per depth plane as a weight. Therefore, the relative pulling strength of dot density compared with dot contrast follows a power law with exponent 2. We propose a population code for vergence control based on disparity-tuned pools of units.

Adult↗
PubMed2006

Effect of blur adaptation on blur sensitivity in myopes.

Although blur adaptation in myopia has been investigated, knowledge regarding its effect on blur sensitivity remains unknown. In the present study, changes in three blur thresholds (i.e., noticeable, bothersome, and non-resolvable blur) were assessed monocularly after 1h of blur adaptation in myopes. A Badal optical system was used to present either an isolated 20/50 Snellen E or 20/50 lines of text, with the full text field used in the latter condition for all blur judgments. Eight visually normal adult myopes were tested with paralyzed accommodation. All subjects exhibited blur adaptation, with a significant improvement in group mean visual acuity of -0.16 LogMAR. There was a consistent and concurrent significant decrease of 0.15-0.19 D in all blur thresholds for the isolated 20/50 E. However, there was no significant effect of blur adaptation on blur thresholds for the 20/50 text, with large intersubject variability evident. The enhanced blur sensitivity for the isolated E target may in part be attributed to the increased visual resolution following blur adaptation. Differences found in the blur thresholds for the two targets may be related to a variety of neuroperceptual phenomena, in particular lateral masking.

Accommodation, Ocular↗
PubMed2002

Temporal constraints on lens compensation in chicks.

If the effective focal length of a growing eye is modified by spectacle lenses, the eye compensates by altering its growth, thereby keeping images in focus, a process we presume is similar to normal emmetropization. Using chicks, we have investigated how much visual exposure the eye needs to exhibit the two principal components of ocular compensation: altered rate of elongation (a scleral mechanism) and altered choroidal thickness. We have found that surprisingly small amounts of vision through spectacle lenses can elicit robust scleral and choroidal compensation if other visual feedback is limited by keeping the animals in the dark when not wearing lenses. Furthermore, we have found that the amount of vision necessary to induce these responses can be summarized as three rules: First, several brief daily episodes are more effective than a single or a few longer daily episodes, even if the total amount of vision is the same. Second, extremely brief episodes, even if very frequent, are relatively ineffective. Third, when plus and minus lenses are worn successively on the same eye, the plus lens has the dominant effect, even if the minus lens is worn five times longer than the plus lens. In addition, we have shown that the elongation rate and choroidal thickness responses are dissociable, such that brief, infrequent lens-viewing produces only an elongation response in the case of plus lens-wear and only a choroid response in the case of minus lens-wear. We thus show that the emmetropization system does not integrate defocus in a simple, linear fashion. These non-linearities, if present in children, might explain why, although education and reading show an epidemiological correlation with myopia, the total time spent reading and doing other nearwork by individual children generally does not predict the degree of myopia. It may therefore be necessary to quantify more complex temporal patterns of nearwork over the day in order to measure the impact of nearwork on eye growth.

Adaptation, Ocular↗
PubMed1997

Capture of visual direction: an unexpected phenomenon in binocular vision.

Binocular perception of visual direction is based on laws which were formulated more than 100 years ago. These laws govern the directions in which human beings perceive objects visible to both eyes (binocular objects) and objects visible to only one eye (monocular objects). We report here that the laws do not hold for monocular objects adjacent to binocular objects. The perceived directions of these monocular objects are captured by those of nearby binocular objects. Capture of binocular visual direction is an unexpected phenomenon because it refutes the generally accepted notion that a particular retinal location gives rise to a particular subjective visual direction. The practical consequence is that the subjective techniques for measuring eye position which are widely used in fundamental research and clinical practice are unreliable if they are used in densely structured stimuli. We suggest that capture results from a mechanism of lateral interaction between adjacent visual directions. This mechanism ensures that, despite eye movements, objects have the same spatial order in monocular and binocular vision. This conservation of spatial order also explains why retinal blind spots are not manifest in binocular vision.

Convergence, Ocular↗
PubMed1997

Linking lower and higher stages of motion processing?

The spatial frequency selectivity of motion detection mechanisms can be measured by comparing the magnitude of motion aftereffects (MAEs) as a function of the spatial frequency of the adapting and test gratings. For static test gratings, narrow spatial frequency tuning has been reported in a number of studies. However, for dynamic test patterns, reports have been conflicting. Ashida & Osaka [(1994). Perception, 23, 1313-1320] found no tuning whereas Bex et al. [(1996) Vision Research, 36, 2721-2727] reported a narrow tuning. The main difference between the two studies was the temporal frequency of the test pattern. In this study we measured the spatial frequency tuning of the MAE using test patterns for a range of temporal frequencies. The results confirmed that there was narrow spatial frequency tuning when the test pattern was counterphasing at a low temporal frequency. However, the spatial frequency selectivity broadened as the temporal frequency of the test pattern was increased.

Adaptation, Ocular↗
PubMed1998

Judging distance from ocular convergence.

Subjects misjudge distances considerably when forced to rely on extra-retinal information. Nevertheless, they can reproducibly set a target to the same distance as a reference, or to double or half that distance, even when they have to look back and forth between them because they are prevented from seeing one when looking at the other. Our explanation for this apparent discrepancy is that people have access to reasonably accurate extra-retinal information on changes in ocular convergence, but can only use this information to judge distances if they had reliable information about the orientation of the eyes before the convergence changed.

Convergence, Ocular↗
PubMed1989

Assessing the utility of reliability indices for automated visual fields. Testing ocular hypertensives.

Monocular (right eye) visual fields were recorded with the Humphrey Visual Field Analyzer (30-2 Program) at baseline as well as 6 and 12 months later in 120 patients with established ocular hypertension. Indices of field reliability (fixation loss, less than 20%; false-positives and false-negatives, less than 33%) and field sensitivity (mean deviation [MD] and pattern standard deviation [PSD]) were examined. At baseline, 35% of patients exhibited low reliability (LR) fields, a figure which decreased to approximately 25% at 6 and 12 months, respectively. During this period, over 50% of patients produced at least one LR field, whereas 8.3% were unable to produce even one reliable field. Exhibition of a LR field appeared to be independent of patient age. Fixation errors, the major cause of LR fields, decreased by approximately 10% over the 12-month period; most patients had between 20 and 32% fixation errors. The incidence of significant defects identified by PSD was greater than that for MD; this was true for both reliable and LR fields. It is suggested that increasing the fixation loss criteria for assessing patient reliability to a 33% cutoff might substantially increase the percentage of fields graded reliable with minimal effect on the sensitivity or specificity of the test.

Adult↗
PubMed1992

Adaptive modification of saccadic eye movements.

Experiments are reported in which the target for a saccadic eye movement was displaced during the saccade. Subjects adapted to the displacement by altering the amplitudes of subsequent saccades to compensate for it. Analysis of kinematic details of the saccade trajectories revealed that the adaptation did not arise from a simple remapping of perceived target locations. Instead, the adaptation appeared to be accomplished by a change in the gain of the saccadic system. The gain change arose primarily from a change in the magnitude of the force pulse for the saccade, not a change in the duration of the pulse. These results have implications for the mechanisms that underlie saccades in normal situations. In particular, people can separately adjust the magnitudes and durations of the force pulses used to produce saccades.

Adaptation, Ocular↗
PubMed1988

[Eye position and function following late surgical correction of consecutive strabismus following primary tenotomy].

Even in patients with consecutive strabismus who had a partial or total tenotomy 10 to 50 years ago, reoperation is worthwhile. In 73% of the author's cases the squint angle was reduced to between -5% degrees and +5 degrees postoperatively; adduction or abduction were improved or normalized in 66%, convergence in 20%. In 29% simultaneous vision on a normal or slightly abnormal basis was achieved. Following dissection of scars and adhesions the tonotomied muscle was advanced and in some cases resected. If the squint angle was large, the antagonist was also recessed. In addition to preoperative diagnosis, which included prolonged compensation with prisms, the indication was affected by intraoperative findings.

Adult↗