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PubMed1994

Disparity vergence eye movements exhibit preprogrammed motor control.

To determine the control strategy used to mediate disparity vergence eye movements, an optical technique was employed which eliminated the visual feedback normally associated with a vergence movement. The resulting "open-loop" vergence responses from four subjects exhibited multiple step-like movements. This finding augments previous demonstrations of multiple-step tracking strategies in vergence eye movements and supports the theory that the initial portion of the vergence response is under preprogrammed control. Additional evidence for preprogramming is given by the similarity of the initial dynamics between open- and closed-loop responses as evidenced by similar ratios of peak velocity to amplitude (i.e. "main sequence" plots).

Convergence, Ocular↗
PubMed1996

Temporal and spatial frequency tuning of the flicker motion aftereffect.

The motion aftereffect (MAE) was used to study the temporal and spatial frequency selectivity of the visual system at supra-threshold contrasts. Observers adapted to drifting sine-wave gratings of a range of spatial and temporal frequencies. The magnitude of the MAE induced by the adaptation was measured with counterphasing test gratings of a variety of spatial and temporal frequencies. Independently of the spatial or temporal frequency of the adapting grating, the largest MAE was found with slowly counterphasing test gratings (at approximately 0.125-0.25 Hz). The largest MAEs were also found when the test grating was of similar spatial frequency to that of the adapting grating, even at very low spatial frequencies (0.125 c/deg). These data suggest that MAEs are dominated by a single, low-pass temporal frequency mechanism and by a series of band-pass spatial frequency mechanisms. The band-pass spatial frequency tuning even at low spatial frequencies suggests that the "lowest adaptable channel" concept [Cameron et al. (1992). Vision Research, 32, 561-568] may be an artifact of disadvantaged low spatial frequencies using static test patterns.

Adaptation, Ocular↗
PubMed2005

A binocular rivalry study of motion perception in the human brain.

The relationship between brain activity and conscious visual experience is central to our understanding of the neural mechanisms underlying perception. Binocular rivalry, where monocular stimuli compete for perceptual dominance, has been previously used to dissociate the constant stimulus from the varying percept. We report here fMRI results from humans experiencing binocular rivalry under a dichoptic stimulation paradigm that consisted of two drifting random dot patterns with different motion coherence. Each pattern had also a different color, which both enhanced rivalry and was used for reporting which of the two patterns was visible at each time. As the perception of the subjects alternated between coherent motion and motion noise, we examined the effect that these alternations had on the strength of the MR signal throughout the brain. Our results demonstrate that motion perception is able to modulate the activity of several of the visual areas which are known to be involved in motion processing. More specifically, in addition to area V5 which showed the strongest modulation, a higher activity during the perception of motion than during the perception of noise was also clearly observed in areas V3A and LOC, and less so in area V3. In previous studies, these areas had been selectively activated by motion stimuli but whether their activity reflects motion perception or not remained unclear; here we show that they are involved in motion perception as well. The present findings therefore suggest a lack of a clear distinction between 'processing' versus 'perceptual' areas in the brain, but rather that the areas involved in the processing of a specific visual attribute are also part of the neuronal network that is collectively responsible for its perceptual representation.

Adult↗
PubMed2006

Characteristics of accommodative behavior during sustained reading in emmetropes and myopes.

Accommodation has long been suspected to be involved in the development of myopia because near work, particularly reading, is known to be a risk factor. In this study, we measured several dynamic characteristics of accommodative behavior during extended periods of reading under close-to-natural conditions in 20 young emmetropic and stable myopic subjects. Accommodative responses, errors, and variability (including power spectrum analysis) were analyzed and related to accommodative demand and subject refractive error. All accommodative behaviors showed large inter-subject variability at all of the reading demands. Accommodative lags and variability significantly increased with closer demands for all subjects (ANOVA, p<0.05). Myopes had significantly greater variability in their accommodation responses compared to emmetropes (ANOVA, p<0.05) and had larger accommodative lags at further reading distances (unpaired t test p<0.05). Power spectrum analysis showed a significant increase in the power of accommodative microfluctuations with closer demands (ANOVA, p<0.05) and with increasing myopia at the closest reading demand (ANOVA, p<0.01). The difference in the stability of the accommodative behavior between individuals with different refractive states suggests a possible relationship between variability in accommodation and the development of myopia.

Accommodation, Ocular↗
PubMed2003

Ocular prevalence versus ocular dominance.

UNLABELLED: Ocular dominance manifests itself in tests that contain stereo-objects with a disparity beyond Panum's area, e.g. in pointing a finger. These tests force subjects to decide in favour of one or the other eye. In contrast, ocular prevalence is determined using stereo-targets imaged within Panum's areas. These tests allow a graded quantification of the balance between the eyes. Here we present the computer-based Freiburg Ocular Prevalence Test in which stereo-disparate targets have to be aligned, and compare it with the Haase Stereo-balance Test that requires an estimation of the horizontal distance between stationary stereo-disparate objects. In addition, we compare ocular prevalence with ocular dominance. METHODS: (1) We measured the influence of a neutral-grey filter in front of one eye to assess the suitability of the Freiburg and the Haase Tests in revealing graded amounts of ocular prevalence. (2) About 20 subjects with equal vision of their two eyes underwent the Freiburg and the Haase Tests for ocular prevalence, and Parson's Monoptoscope Test for ocular dominance. RESULTS: (1) In both the Freiburg and the Haase Tests, the neutral-grey filter shifted ocular prevalence by about 50%. (2) An ocular prevalence of more than 10% occurred in 13 of the 20 subjects using the Freiburg, and in 14 using the Haase Test. On average, the ocular prevalence was 24.1+/-3.8% in the Freiburg and 32.0+/-8.2% in the Haase Test. The dominant eye coincided with the prevalent eye in 15 of the 20 subjects. DISCUSSION: The effect of the neutral-grey filter indicated that both the Freiburg and the Haase Tests can be used to measure fractions of ocular prevalence, although the Freiburg Test carries a higher reproducibility. Spontaneous ocular prevalence occurs frequently in persons with equal vision of their two eyes. This suggests that ocular prevalence does not represent a condition that requires treatment. Rather, partial suppression of one eye, the correlate of ocular prevalence, may play a physiological role in that it helps to disregard double images at stereo-disparities close to the limits of Panum's area.

Adult↗
PubMed1998

Dynamics of horizontal vergence movements: interaction with horizontal and vertical saccades and relation with monocular preferences.

We studied the dynamics of pure vergence shifts and vergence shifts combined with vertical and horizontal saccades. It is known from earlier studies that horizontal saccades accelerate horizontal vergence. We wanted to obtain a more complete picture of the interactions between version and vergence. Therefore we studied pure version (horizontal and vertical), pure vergence (divergence and convergence) and combinations of both in five adult subjects with normal binocular vision and little phoria (< 5 degrees). The visual targets were LED's in isovergence arrays presented at two distances (35 and 130 cm) in a dimly lit room. Two targets were continuously lit during each trial and gaze-shifts were paced by a metronome. The two subjects with a strong monocular preference made vergence eye movements together with small horizontal saccades during pure vergence tasks. The other subjects, who did not have a strong monocular preference, made pure vergence movements (without saccades). These findings, suggest that monocular preferences influence the oculomotor strategy during vergence tasks. Vergence was facilitated by both horizontal and vertical saccades but vergence peak-velocity during horizontal saccades was higher than during vertical saccades.

Adult↗
PubMed1991

[Vertical accommodative vergence].

Five patients showed vertical accommodative vergence during which the strabismic eye deviated either up- or downwards. Additional components of strabismus included intermittent or constant exotropia, dissociated vertical divergence, and various incomitancies. There were no signs of misdirected regeneration after oculomotor palsy. We interpret vertical accommodative vergence as a supranuclear abnormality, most likely congenital.

Adult↗
PubMed2002

Correlation between hand preference and distance of focusing points of two eyes in the horizontal plane.

Relationships among hand preference, ocular dominance, and the degree of ocular shifting were studied in 78 right-handed and 16 left-handed subjects. Ocular dominance was assessed with the Miles test. The shifting degree of eye was assessed using a modified Miles test. The shifting distance of the right-eye was marginally greater, although significant, in the left-handers as compared to the right-handers. The shifting distance of the left-eye was greater in the right-handers than in the left-handers. The distance of focusing points of two eyes in the horizontal plane was greater in the right-handers than in the left-handers. In the total sample, there was a significant negative Pearson correlation between hand-preference and the shifting distance of the right-eye, and there was a positive correlation between hand-preference and the shifting distance of the left-eye. In addition, there was a significant positive correlation between hand preference and the distance of focusing points of two eyes. These results suggest that hand preference may be related to the degree of ocular asymmetry.

Adolescent↗
PubMed1982

Graphical analysis of prism adaptation, convergence accommodation, and accommodative convergence.

A new form of graphical case analysis is described which quantifies static interactions between accommodative convergence, convergence accommodation, and prism adaptation. A clinical gradient measure of the CA/C ratio is compared to haploscopic measures to demonstrate the validity of the new clinical technique for measuring convergence accommodation. Graphical and computational methods are illustrated which predict the quantitative interactions between accommodation and convergence that occur after the phoria is partially corrected by lenses or prisms. Complete correction of convergence accommodation errors with spherical lenses or elimination of the stimulus to convergence accommodation by full correction of the associated phoria with prism will simplify the correction of binocular disorders by preventing the complex interactions of accommodative convergence with convergence accommodation. The new form of graphical analysis quantifies the predicted stress upon binocular vision induced by convergence accommodation.

Accommodation, Ocular↗
PubMed1991

Influence of ocular gaze and head position on 4 m heterophoria and fixation disparity.

Four-meter heterophoria and fixation disparity measurements were taken from a sample of 40 young adults in primary gaze, 33 degrees left and right lateral gaze, and in 2 head/gaze positions related to golf putting. Comparisons using analysis of variance were made between the mean values in each gaze position and between the test-retest range of values in each gaze position. Mean heterophoria was more eso (p less than 0.05) in the two head/gaze positions related to golf putting than in other gaze positions, whereas mean fixation disparity was more eso (p less than 0.05) in rightward than in primary gaze. Phoria test-retest range was less (p less than 0.05) in all nonprimary gaze positions than in primary gaze. One of the putting-related gaze positions showed smaller (p less than 0.05) fixation disparity ranges than the primary or leftward position of gaze. All measures were highly variable among subjects. Trends present in the data suggested greater esophoria and eso fixation disparity mean values, and lesser heterophoria and fixation disparity test-retest range values in all nonprimary positions relative to primary gaze. The behavior of exophoric and esophoric subjects differed only in regard to changes in heterophoria associated with left and right lateral gaze. These data indicate differences in oculomotor postural tendencies relative to position of gaze. Relevance of the results to athletes is discussed.

Adult↗
PubMed1991

Effects of long-term visual display terminal usage on visual functions.

Since their rapid introduction into the workplace visual display terminals (VDT's) have been the source of a number of studies aimed at detecting effects on office workers. The published studies have been either short-term or cross-sectional, with one exception. The results have shown varying degrees of relation but by not having a comparable control group all have been limited in their value. We set out to monitor at regular intervals over a 2-year period specific visual functions that have been reported to be affected by VDT usage, and to compare VDT and non-VDT users in the same office environment. By assessing the VDT operators from the time the systems were introduced we have an accurate baseline to work from when assessing change due to the introduction of the VDT.

Accommodation, Ocular↗
PubMed1995

The influence of refractive correction upon disorders of vergence and accommodation.

Clinical care routinely includes prescription of lenses that compensate for the distance refractive error. Indeed, refractive correction is so commonly prescribed that we often neglect its potential effects on disorders of binocular vision. We report improvement of binocular function that resulted 1 or more months after prescription of an initial spectacle correction for 143 nonstrabismic patients who had a refractive error and either a vergence anomaly (28%), an accommodative anomaly (8%), or both (64%). Refractive correction was estimated objectively with an autorefractor and subjectively refined without cycloplegia. Most corrections were low to moderate in power, essentially following Orinda Study guidelines. Recovery of normal vergence and accommodative function varied according to refractive error type (79% of hyperopic astigmats recovered; 20% of myopes recovered), direction of astigmatic axes (67% recovered who had against-the-rule; 45% with with-the-rule recovered), age (63% below age 12 years recovered; 41% older than age 13 years recovered), and vergence anomaly (67% of patients with fusional vergence dysfunction recovered; 38% of those with basic exophoria recovered). These results suggest that improvement in acuity is not the only reason for prescription of a refractive correction--prescription of even small corrections should be considered as these can dramatically improve vergence and accommodative function for many patients.

Accommodation, Ocular↗
PubMed1990

A longitudinal study of children with a family history of strabismus: factors determining the incidence of strabismus.

A longitudinal study of ocular refraction, position, and fixation was performed in children with a family history of strabismus. The children were examined at regular intervals between 3 months and 4 years of age, and the results are discussed in terms of changes in refraction between different ages and correlations between refraction and development of strabismus and amblyopia. Six of 34 children (17.6%) developed constant or intermittent esotropia. The strabismus was first noted between 18 and 30 months of age except in one case. All esotropic children were 4 dioptres hypermetropic or more at 6 months, and their hypermetropia remained almost unchanged through the years. Seven additional children were 4 dioptres or more hypermetropic at 6 months but did not develop a squint. In contrast to the squinting children the hypermetropia in these children changed towards emmetropia. This emmetropisation was most pronounced during the first 2 years of age. The implications of these results for an early diagnosis of strabismus amblyopia are discussed.

Aging↗