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At least 703 records · Page 39Linked to original sources
PubMed2000

Asymmetry of the human visual field in magnetic response to apparent motion.

Predominance of the lower visual field has been shown in various visual tasks, but whether the upper visual field is involved in a specific neural process is unknown. We used magnetoencephalography to study the effect of orientation and direction on the responses of five subjects to apparent motion from the human extrastriate cortex. The first magnetic response always was the largest, and the peak latency of about 200 ms did not change with the stimulus conditions. Amplitudes of the first responses were highest when motions were oriented at the horizontal meridian, decreasing with the degree of the angle between motion orientation and the horizontal meridian. There was no difference in amplitude between the two directions in the lower visual field, whereas the value of the response to downward motion in the upper visual field was significantly larger than that to upward motion. These amplitude changes are not due to differences in the anatomical distribution of neural activities because the estimated origins for the first responses always were in the same cortical area (around the occipito-parieto-temporal region) and the directions of the current vectors did not change with the stimulus conditions, and the estimated current strength changed with the stimulus conditions as did the response amplitude. These findings suggest that the human extrastriate cortex has a directional preference for downward versus upward motion in the upper visual field.

Adult↗
PubMed2000

Photoreceptor topography of the retina in the New World monkey Cebus apella.

The number and topographical distribution of photoreceptors was studied in whole-mounted retinas of Cebus apella. It was estimated a total of 48 million rods and 3.8 million cones. The average peak foveal cone density and the Nyquist Limit at the foveola were estimated as 169, 127 cells/mm(2) and 46.77+/-7.98 cyc/deg, respectively. A cone-enriched rim was found near the ora serrata, more noticeable in the nasal retina. Rod distribution was asymmetrical along horizontal and vertical meridians with a higher density in the dorsal retina. The rod/cone ratio was variable and asymmetrical along both meridians.

Animals↗
PubMed2001

Disconjugate oculomotor learning caused by feeble image-size inequality: differences between secondary and tertiary positions.

In order to examine the minimum value of image-size inequality capable of inducing lasting disconjugacy of the amplitude of saccades, six normal emmetropic subjects were exposed for 16 min to 2% image size inequality. Subjects were seated at 1 m in front of a screen where a random-dot pattern was projected and made saccades of 7.5 and 15 deg along the horizontal and vertical principal meridians and to tertiary positions in the upper and lower field. During the training period, compensatory disconjugacy of the amplitude of the saccades occurred for the principal horizontal and vertical meridians; such increased disconjugacy persisted after training, suggesting learning. In contrast, for horizontal saccades to or from tertiary positions made in the upper and lower field, no consistent changes in the disconjugacy occurred, either during training or after the training condition. In an additional experiment, three subjects read sequences of words with the 2% magnifier in front of their dominant eye: in such a task, horizontal saccades to or from tertiary positions at the upper or lower field showed appropriate and lasting disconjugacy for two of the three subjects. We conclude that even a 2% image size inequality stimulates oculomotor learning, leading to persistent disconjugacy of saccades. The small disparity created by the image-size inequality is thus compensated by the oculomotor system rather than tolerated by the sensory system (e.g. by enlarging the Panum's area).

Aniseikonia↗
PubMed2001

Plasticity of convergence-dependent variations of cyclovergence with vertical gaze.

Binocular alignment of foveal images is facilitated by cross-couplings of vergence eye movements with distance and direction of gaze. These couplings reduce horizontal, vertical and cyclodisparities at the fovea without using feedback from retinal image disparity. Horizontal vergence is coupled with accommodation. Vertical vergence that aligns tertiary targets in asymmetric convergence is thought to be coupled with convergence and horizontal gaze. Cyclovergence aligns the horizontal retinal meridians during gaze elevation in symmetrical convergence and is coupled with convergence and vertical gaze. The latter vergence-dependent changes of cyclovergence have been described in terms of the orientation of Listing's plane and have been referred to as the binocular extension of Listing's law. Can these couplings be modified? Plasticity has been demonstrated previously for two of the three dimensions of vergence (horizontal and vertical). The current study demonstrates that convergence-dependent changes of the orientation of Listing's plane can be adapted to either exaggerate or to reduce the cyclovergence that normally facilitates alignment of the horizontal meridians of the retinas with one another during gaze elevation in symmetrical convergence. The adaptability of cyclovergence demonstrates a neural mechanism that, in conjunction with the passive forces determined by biomechanical properties of the orbit, could play an active role in implementing Listing's extended law and provide a means for calibrating binocular eye alignment in three dimensions.

Accommodation, Ocular↗
PubMed2002

The relationship of visual threshold and reaction time to visual field eccentricity with conventional automated perimetry.

To investigate the relationship between reaction time (RT), stimulus intensity and visual field eccentricity. We generated frequency of seeing (FOS) curves and measured RTs by testing 10 perimetrically experienced normal subjects with a Humphrey perimeter controlled by a custom program. Subjects were tested from 10 degrees to 50 degrees eccentricity along the nasal horizontal meridian in 10 degrees increments. A range of 20 dB, centered on threshold, was tested in 1 dB steps along with 60 and 0 dB intensities as catch trials. Twenty repetitions for each intensity at each location were used. Linear regression showed a significant increase in suprathreshold RT (to the 0 dB stimulus) with increasing eccentricity. The RT at the calculated FOS 50% threshold was prolonged by about 200 ms compared with the RT using the 0 dB target at the equivalent eccentricities. Also, when the difference between the RT at 0 dB stimulus and the RT at threshold was regressed against visual field eccentricity there was a significant decrease with eccentricity. When the RT(pi) (RT prolongation from threshold relative to the 0 dB stimulus) was plotted as a function of decreasing stimulus attenuation, the results fit the function RT(pi)=a+bi(3) (i=stimulus intensity) with r(2)>0.94 at all eccentricities. However, the slope of the function flattened with increasing eccentricity. Using conventional automated perimetry stimuli in perimetrically experienced young subjects, suprathreshold RT increases but threshold RT prolongation decreases with increasing visual field eccentricity. RT fits a power function with decreasing stimulus attenuation but the slope flattens with eccentricity. This relationship found along the nasal horizontal meridian may allow use of RT to cross-check threshold results or to define response windows for reliability indices of conventional automated perimetry.

Adult↗
PubMed2002

The spatial distribution of selective attention assessed using the multifocal visual evoked potential.

To determine the manner in which attention is distributed among numerous locations in the visual space, we used a multifocal recording technique that allowed simultaneous recordings of evoked cortical activity from 12 visual field areas out to 23.6 degrees. We found that multifocal visual evoked potential (mfVEP) amplitude was larger when a region of visual space was attended than when it was not attended. The magnitude of this effect was inversely related to visual field eccentricity and there was no attention-related modulation of VEP amplitude for the most eccentric region. In addition, we found that mfVEP amplitudes in the regions contiguous to the attended region could also be larger, depending upon their spatial relationship to the attended region. Specifically, amplitudes in more central regions on the 'meridian of attention' were larger when the subject attended anywhere along that meridian.

Attention↗
PubMed2003

The distribution of preferred orientations in the peripheral visual field.

Orientation discrimination was measured for line targets in the fovea and in locations along four meridians (vertical, horizontal, 45 degrees and 135 degrees ) in the peripheral visual field. In all locations, the performance for vertical and horizontal contours was on average almost twice as good as for oblique ones. In addition, especially in peripheral locations along oblique meridians, thresholds tend to be better for radially-oriented contours compared to tangential ones. These effects are robust to length of the test lines and to small changes in position and cannot therefore be due to cortical singularities (pinwheels). The results suggest that the advantage of horizontal and vertical contours, as well as possibly of radial ones, is a manifestation of a characteristic structural property of the organization of the visual pathway.

Adult↗
PubMed2003

Comparison of the time courses of concomitant and nonconcomitant vertical phoria adaptation.

Vertical phoria adaptation was measured before, during, and after 1 h of training with either a prism or magnifying lens. With the prism (concomitant adaptation) a single vertical disparity was presented at primary position. With the magnifier (nonconcomitant adaptation) two vertical disparities of opposite sign were presented along the vertical meridian. Following adaptation, binocular vision was prevented with an eye patch, and vertical phorias were measured periodically along the primary vertical meridian over the course of 8 h. Despite individual variation, adaptation followed approximately exponential time courses. The average time constants for the decay of concomitant and nonconcomitant adaptation were 31 and 83 min, respectively. There was no consistent relationship between the rates of acquisition and decay nor was there a strong relationship between the gains of the adaptive responses and the rates of decay although there was a general trend for the gains of the nonconcomitant responses to be higher and the rate of decay slower than the concomitant responses. The results support the notion that concomitant and nonconcomitant phoria adaptation involve different mechanisms but not the contention that adaptation to prisms is easier or more robust than adaptation to lenses.

Adaptation, Physiological↗
PubMed2003

Visual field anisotropy revealed by perceptual filling-in.

Four experiments were performed to investigate how the time required for perceptual filling-in varies with the position of the target in the visual field. Conventional studies have revealed that filling-in is facilitated by a target with greater eccentricity, while no systematic studies have examined the effect of polar angle. Experiment 1 examined the effect of polar angle when the target and surround differed in luminance. Filling-in was facilitated as the target position changed from the horizontal to the vertical meridian. This dependency was more prominent in the upper field than in the lower, although no asymmetry was found between the left and right visual fields. These features were observed in both monocular and binocular viewing. These results were replicated in a modified stimulus configuration, in which the surround was a circular region concentric with the target (Experiment 2). Moreover, it was confirmed that the asymmetry was not due to fluctuation in the retinal image (i.e., eye movement) (Experiment 3). Finally, Experiment 4 examined whether this anisotropy was observed when two differently oriented gratings were presented in the target and surround regions. Again, filling-in was facilitated for a target close to the vertical meridian, irrespective of the relationship between the target and surround orientations. The underlying mechanism of this anisotropy is discussed from the viewpoints of cortical magnification and neural connections in the visual cortex.

Analysis of Variance↗
PubMed2003

Meridional anisotropy in visual processing: implications for the neural site of the oblique effect.

The contention is examined that the oblique effect, i.e., the well-known performance deficit in detecting orientation difference in oblique lines as compared to vertical and horizontal ones, has its origin in a relative deficiency of neurons with obliquely-oriented receptive fields in the primary visual cortex. Psychophysical observations demonstrate a prominent oblique effect also in visual tasks involving widely-separated elements and other stimuli that would elicit little or no response in oriented neurons in the visual cortex. Conversely, some tasks, e.g. position discrimination, exhibit no oblique effect even with short, high-contrast lines. When the comparison with the reference can be accomplished during a single brief exposure rather than sequential ones, thresholds for orientation differences between adjacent contours in oblique meridians are also elevated compared to those in the vertical and horizontal, but to a lesser extent. In one particular texture discrimination task some but not all observers have a conspicuous oblique effect. The discrimination only of the direction of streaming random dots, not of their speed, is poorer for motions in oblique meridians. The findings imply that the neural locus for the oblique effect is more central than the primary visual cortex.

Contrast Sensitivity↗
PubMed2003

Astigmatism in infant monkeys reared with cylindrical lenses.

To determine whether developing primate eyes are capable of growing in a manner that eliminates astigmatism, we reared infant monkeys with cylindrical spectacle lenses in front of one or both eyes that optically simulated with-the-rule, against-the-rule, or oblique astigmatism (+1.50-3.00x90, x180, x45 or x135). Refractive development was assessed by retinoscopy, keratometry and A-scan ultrasonography. In contrast to control monkeys, the cylinder-lens-reared monkeys developed significant amounts of astigmatism. The astigmatism was corneal in nature, bilaterally mirror symmetric and oblique in axis, and reversible. The ocular astigmatism appeared to be due to a reduction in the rate of corneal flattening along the steeper meridian while the other principal meridian appeared to flatten at a more normal rate. However, regardless of the orientation of the optically imposed astigmatism, the axis of the ocular astigmatism was not appropriate to compensate for the astigmatic error imposed by the treatment lenses. Our results indicate that visual experience can alter corneal shape, but there was no evidence that primates have an active, visually regulated "sphericalization" mechanism.

Adaptation, Physiological↗
PubMed1997

Specificity of saccadic adaptation in three-dimensional space.

The saccadic system is known to exhibit a considerable degree of short-term plasticity. Earlier studies have shown that saccadic adaptation, rather than being a global process affecting all saccades equally, has a certain degree of spatial resolution. Its localized nature has become apparent from studies in the frontal plane which have shown that short-term saccadic adaptation, induced along a given meridian, transfers to only a limited range of neighbouring directions. Considering that most natural gaze shifts also have a depth component, we investigated whether the directional specificity of the saccadic adaptive system can be generalized to three-dimensional (3-D) space. Binocular eye movements were recorded in seven subjects while they made saccades to visual stimuli in the horizontal plane of regard. Experiments began by recording baseline saccades, all starting from the same fixation point to either a farther target (far saccades) or an equally eccentric nearer target (near saccades). Next, by displacing the target intra-saccadically in opposite directions in alternating far and near trials, we attempted to simultaneously reduce the gain of the far saccades while increasing the gain of the near saccades. These experiments, aimed at eliciting a state of differential gain, were specifically designed to adapt only the saccadic response, since targets were shifted along corresponding iso-vergence circles. To investigate the effect of varying the radial direction difference, similar differential gain adaptation experiments were conducted in the frontal plane for saccades along two different meridians. Our results show that when the saccadic system is pressured, it is capable of adopting different gains simultaneously for equal-direction saccades to different depth planes. Similarly, opposite gain adaptation can also be achieved in the frontal plane, but only if radial saccade directions are sufficiently separated. The fact that short-term saccadic adaptation can be shown to be directionally specific in two perpendicular planes suggests that the adaptation process is restricted to a limited volume of 3-D oculomotor space.

Adaptation, Ocular↗
PubMed1981

Planned myopic astigmatism as a substitute for accommodation in pseudophakia.

Patients with an implanted intraocular lens can be made nearly spectacle independent by increasing the depth of focus of the uncorrected implanted eye. In simple myopic astigmatism the visual acuity can be above 20/50 from far to near because the size of the blurred retinal image changes less than its shape as an object approaches the eye. For simple myopic astigmatism the change in corneal power must be included in the calculation of intraocular lens power. The change in corneal power induced by surgery is a spherocylinder with no spherical equivalent. The postoperative shape of the cornea can be predicted and intraocular lens power is calculated to make the flatter meridian emmetropic. The stronger meridian of the cornea is then myopic. In a series of 40 successive cases, the refractions and the measured depth of focus illustrate the application of these principles and the advantage of myopic astigmatism for the pseudophakic patient.

Accommodation, Ocular↗
PubMed1987

Corneal topography of pellucid marginal degeneration.

Our computer-based corneal topography analysis system was used to study the keratoscope photographs (keratograms) from two patients with classic pellucid marginal degeneration and a third patient with no inferior corneal thinning, whose keratoscope mire pattern was suggestive of the condition. All three patients showed marked flattening of the central cornea along a vertical axis and marked steepening of the inferior corneal periphery, which also extended into mid-peripheral inferior oblique corneal meridians. The mid-peripheral cornea gradually decreased in power above the inferior oblique meridians.

Astigmatism↗
PubMed1990

An adjustable single running suture technique to reduce postkeratoplasty astigmatism. A preliminary report.

The authors compared postkeratoplasty astigmatism over a 4-month period after surgery in a randomized, prospective study of two groups of patients (total N = 18) who received two different suture techniques. The test group N = 8) had a single running suture with postoperative suture adjustment; on the basis of computer-assisted topographic analysis, the suture was tightened in the flatter meridian and loosened in the steeper meridian in the first month after surgery. The control group (N = 10) had a standard double running suture procedure with no postoperative adjustment; the single running 10-0 nylon suture was removed 3 months after surgery. Four months after penetrating keratoplasty, mean (+/- standard deviation) astigmatism in the test group was 1.7 +/- 0.7 diopters (D), and all patients had less than 2.6 D of astigmatism. In the control group, mean astigmatism was significantly higher (5.4 +/- 2.4 D; range, 0.7-9.0 D; P less than 0.01). The results suggest that postkeratoplasty astigmatism can be reduced with the single running suture technique accompanied by postoperative suture adjustment.

Astigmatism↗
PubMed1994

Color dissociation artifacts in double Maddox rod cyclodeviation testing.

BACKGROUND: The double Maddox rod test, based on a red Maddox rod in front of one eye and a clear Maddox rod in front of the other, is used to measure cyclodeviation, typically in patients with superior oblique muscle pareses. Discrepant results between the double Maddox rod test and other torsion measures, and reports of "paradoxic" cyclodeviation in the normal eye of some patients with superior oblique paresis, suggest the two-color format of the double Maddox rod test may produce artifactual torsion measures. METHODS: Forty patients with superior oblique paresis were tested twice using the double Maddox rod test, reversing the red and white Maddox rods between eyes for the second test, and 18 were tested further with same-color red or clear Maddox rods in front of both eyes. RESULTS: With the standard double Maddox rod test, 33 (83%) of 40 patients localized their cyclodeviation to the eye viewing through the red Maddox rod, irrespective of laterality of the paresis or fixation preference. In all 33 patients, laterality of the perceived torsion changed between eyes when testing was repeated with red and white Maddox rods interchanged between eyes. With same-color Maddox rods before both eyes, 17 (94%) of 18 patients localized extorsion to the paretic eye. There was 7.6:1 ratio of luminance transmission and a 1.6:1 ratio of grating spatial frequency bandpass in the plano meridian between the clear and red Maddox rods, which appear to be responsible for the double Maddox rod test artifact. CONCLUSION: The traditional double Maddox rod test may produce artifactual cyclodeviation measurements. An alternative version of the test, based on same-color Maddox rods in front of both eyes, is proposed. The relatively high spatial frequency bandpass characteristics of the plano meridian of the Maddox rod (as high as 20/25 Snellen equivalent resolution through the clear Maddox rod) also suggests double Maddox rod testing should be conducted in a dark room to avoid biases from visual environment cues.

Artifacts↗
PubMed1994

Dynamic shifts in corneal topography after radial and transverse keratotomy.

PURPOSE: The authors aimed to quantitate the dynamic patterns of change in corneal topography after multistaged radial and transverse keratotomy using digitized video-keratography. METHODS: Single and paired radial and transverse keratotomies, with videokeratoscopy between each stage and at the end of the procedure, were performed on fresh animal cadaver eyes using an artificial orbit system. RESULTS: All incisions led to central flattening. A single radial keratotomy caused flattening adjacent to the incision, and steepening 180 degrees away. A paired radial keratotomy caused increased flattening in the meridian of the incisions, and less flattening 90 degrees away. A single transverse incision caused steepening adjacent to the incision and diffuse flattening elsewhere. A paired transverse incision caused flattening near the optical center along the meridian bisecting the incisions and steepening 90 degrees away. CONCLUSION: The authors have demonstrated that computerized videokeratography can be used successfully to systematically quantitate dioptric shifts in multiple hemimeridians and measurement zone diameters after refractive surgery.

Animals↗
PubMed1995

Retinal photoreceptor density decreases with age.

PURPOSE: To evaluate the effect of age on the density of retinal photoreceptors in humans. METHODS: Fifty-five normal eyes from human donors, with a mean age of 58.7 +/- 19.1 years and an axial length of less than 27 mm, were examined. After opening of the globes by a 16-mm corneoscleral trephination, 25 retinal tissue samples were obtained in four meridians. The photoreceptors were counted on photographs taken from the photoreceptor inner segments after sonographic removal of the outer segments. The cones in the foveal center could not be evaluated. RESULTS: Outside the foveal center, the photoreceptor density decreased significantly with increasing age. In absolute and relative terms, the decline was more marked for the rods (mean rod loss, 236 +/- 63 cells or 0.37% +/- 0.10%/mm2 and year of a mean density of 62.987 rods/mm2) than for the cones (mean cone loss, 5.90 +/- 0.68 cells or 0.18% +/- 0.02%/mm2 and year of a mean density of 3320 +/- 578 cones/mm2). For both cones and rods, the cell loss was more marked at an eccentricity of approximately 5-8 mm than in the retinal periphery at an eccentricity of 14 to 20 mm. There were no significant differences between the superior, inferior, temporal, or nasal meridian nor between women and men. CONCLUSIONS: The photoreceptors decrease in density with increasing age. The annual cell loss of approximately 0.2% to 0.4% is similar to the age-related loss of retinal ganglion cells and pigment epithelium cells. The decline in photoreceptor count affects more rods than cones. It does not prefer special age groups. It is more pronounced at an eccentricity of 5 to 8 mm than in the retinal periphery at an eccentricity of more than 14 mm. It may be important for an age-related decrease in visual functions and for diseases affecting the photoreceptors in elderly patients.

Adolescent↗