Bimedial recession in high AC/A esotropia. A long-term follow-up.
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Child↗
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Timing and competition in networks representing ambiguous figures.
The duration of perception for eye rivalry and the Necker cube was obtained during continuous presentation of the stimuli. The frequency histograms support the idea of continuous competition between the neural representations of both percepts. They also suggest that when a percept is installed there is a relative refractoriness for changing the percept that is modeled by a sigmoid equation. This refractoriness lasts a few seconds. The duration of the perceptions was modified by the instruction to the subjects. Results are discussed in terms of a simple competition model, a temporal segmentation hypothesis, and the possible top-down influences. These are considered useful mechanisms for selecting relevant information in a serial processor and for maintaining perception, cognitive evaluation, and action of a given context in the same time epoch.
Adult↗
Unilateral retinitis pigmentosa associated with exfoliation syndrome.
A case of a 71-year-old woman showing pigmentary degeneration of the retina associated with posterior subcapsular cataract in one eye, with a normal fellow eye, is presented. Clinical examinations of the patient were performed in order to rule out the various causes which are known to produce funduscopic features that mimic retinitis pigmentosa. Since these investigations were all negative, the fundus changes were interpreted as a unilateral retinitis pigmentosa. One year later, the patient was re-examined and an exfoliation syndrome was discovered in the affected eye, while the fellow eye was unchanged. An association of unilateral retinitis pigmentosa and exfoliation syndrome in the same eye can be regarded as exceptional. The possibility of a correlation of the pathogenetic mechanisms involved in the development of both conditions is discussed.
Aged↗
The locust's use of motion parallax to measure distance.
1. Locusts perform a visual behavior known as peering before jumping to targets. Peering consists of a side to side translational movement of the head. Wallace (1959) first proposed that this behavior is performed in order to obtain depth information through motion parallax. The present study analyzes more quantitatively the role of peering in the locust's (Schistocerca americana) estimation of distance in an attempt to understand the neural mechanisms involved. 2. Jump velocity was found to be related monotonically to target distance and was used as a measure of the locust's judgement of target distance. 3. By presenting locusts with a target which moved laterally while they peered (artificial parallax), it was possible to simulate the motion parallax of a target at any distance. Jump velocities elicited by means of artificial parallax were the same as jump velocities elicited by the corresponding real distances, demonstrating that locusts use motion parallax as a cue to distance. 4. By moving the target in the same direction but further than the locust's head it was possible to simulate targets whose motion parallax specified a position behind the animal. Locusts jump forward to such paradoxical stimuli with the jump velocity appropriate for targets located at the absolute value of the simulated distance. This suggests that locusts are insensitive to the relation between the direction of head and image motion. 5. Measurement of jump velocity in locusts with one eye occluded revealed an interaction between the parallax signals from the two eyes - locusts appear to sum (or average) the motion perceived in each eye. 6. Jump velocity is predicted more accurately by target distance than by either image displacement or image velocity during peering. This implies that the locust's computation of target distance involves signals concerning its own head motion.
Animals↗
Motion parallax is used to control postural sway during walking.
Three experiments tested the hypothesis that postural sway during locomotion is visually regulated by motion parallax as well as optical expansion. Oscillating displays of three-dimensional scenes were presented to participants walking on a treadmill, while postural sway was recorded. Displays simulated: (a) a cloud, in which parallax and expansion are congruent, (b) a hallway, (c) the side walls of the hallway, (d) a ground surface, (e) a wall, (f) the wall with a central hole, (g) a hall farther from the observer, and (h) a wall farther from the observer. In contrast to previous results with a hallway, responses with the cloud were isotropic and directionally specific. The other displays demonstrated that motion parallax was more effective than simple horizontal flow in eliciting lateral sway. These results are consistent with the hypothesis that adaptive control of sway during walking is based on congruent expansion and parallax in natural environments.
Adult↗
Congenital idiopathic microcoria.
Five infants had congenital unilateral microcoria. The red reflex was absent and vision was severely reduced in all five eyes preoperatively. A pupil was created surgically in three of the eyes, resulting in a variable improvement in vision. The underlying lens was clear in all three children. The origin of the pupillary anomaly is unknown, but probably is related to remnants of the fetal pupillary membrane. Early surgical treatment and vigorous occlusion therapy can result in useful vision.
Child, Preschool↗
Evidence of foveal splitting in a commissurotomy patient.
A bilateral projection of the central fovea, produced by naso-temporal overlap at the retinal vertical meridian, has been proposed as the anatomical basis of macular sparing. To evaluate this claim, a commissurotomy patient was required to compare target figures presented 1 degrees or less from the retinal midline with reference figures presented 2.5 degrees from the midline in the same or opposite visual field. The subject judged whether the target and reference were the same or different. It was found that the subject's fovea was effectively split with respect to the cerebral hemispheres; targets in the same visual field as the reference were readily matched with the reference, but accuracy dropped to chance for targets in the opposite field. Ways of reconciling this data with anatomical evidence of nasotemporal overlap and reports of macular sparing are discussed.
Adult↗
Specialization of the right hemisphere for visuomotor control.
The accuracy of saccades directed towards the remembered positions of targets in left (LVF) or right (RVF) visual hemifield was measured. The majority of right-handed subjects were found to be more accurate at directing their gaze to locations in the LVF than in the RVF, suggesting that the right hemisphere is superior to the left in oculomotor control. Even after completion of a corrective saccade following the primary saccade, subjects systematically undershot target direction and overshot target depth, suggesting that visual feedback normally plays an important role in the fine guidance of gaze after the completion of a primary saccade.
Adult↗
Functional separation of global and local stereopsis investigated by cross-adaptation.
Reports that brain lesions may differentially affect global and local stereopsis suggest that anatomically separate mechanisms underlie these two functions. We demonstrate here that adaptation to global stereograms induces an after-effect on local stereopsis as well as on global stereopsis, showing that the two mechanisms cannot be entirely separate. We suggest that global stereopsis depends upon an interaction between primary visual area V1 and higher brain areas such as inferotemporal cortex.
Adolescent↗
The relationship between eye dominance and head tilt in humans.
Previous data suggest that vertical imbalances between the eyes produced by lateral head tilt are linked to ocular dominance, but no studies have heretofore objectively measured head tilt in addressing this relationship. Photographic measurements of the head tilt of 70 subjects were made, along with measures of sighting dominance and vertical misalignment during sighting. The analyses revealed that photographically measured head tilt is significantly associated with vertical imbalances during sighting and with sighting dominance. These data support the theory that motoric dominance in humans is partly related to mechanisms underlying postural asymmetry.
Adolescent↗
Oblique muscle palsies fixating with the paretic eye.
Palsy of the superior oblique muscle is one of the most commonly occurring entities in strabismus; the clinical characteristics are easily recognizable. Isolated inferior oblique muscle palsy, although anatomically enigmatical, is also known to ophthalmologists. When a patient with an oblique muscle palsy chooses to fixate with the paretic eye, characteristic patterns of motility may be obscured. Patients with superior oblique muscle palsy or isolated inferior oblique muscle palsy who habitually fixate with the paretic eye, may present with limited elevation or depression respectively. In each case, limited motility exists secondary to decreased innervational input to the contralateral antagonist of the paretic muscle, or to a mechanical restriction caused by prolonged contracture of the yoke of the paretic muscle. Inhibitional palsy of the contralateral antagonists and the fallen and rising eye syndromes may present diagnostic dilemmas unless the underlying oblique muscle palsy is recognized. Proper diagnosis may be obtained with three clinical tests; the 3-step test, the comparison of ductions to versions, and forced ductions.
Contracture↗
Compound binocular rivalry.
Binocular rivalry was examined with random dot patterns consisting of three colours: red, green and grey. The microstructure of the patterns was defined by the individual dots, and the correspondence between the microstructures in the two eyes was manipulated. The macrostructures were defined by the distributions of red, green and grey dots over the displays, so that they consisted of orthogonally striped patterns. The degree of correspondence between the microstructures was varied in Expt 1, together with the spatial frequency of the microstructure. Rivalry periods of the macrostructures were briefer when the microstructures were in correspondence, In Expt 2 the spatial frequencies of the macrostructures were varied. The lower spatial frequency predominated for longer than the higher. The results are discussed in terms of independent pathways for corresponding and rivalry stimulation. In addition a stimulus pairing that produces clear dichoptic colour mixtures is presented.
Dominance, Cerebral↗
Global stereopsis in human albinos.
The presence of global stereopsis was examined in 18 clinically diagnosed albinos; four non-albino controls were also tested including two observers with congenital nystagmus. Stereopsis was evaluated with standard clinical stereo tests and with TV generated random dot stereograms. The latter test involved electrophysiological measures of vertical eye movement tracking in response to a stimulus target. For either test procedure, global stereopsis could be demonstrated in a significant number of albinos across varying phenotypes. These results are of interest in view of electrophysiological investigations in albino animal models which indicate a paucity of binocularly driven cortical neurons in visual areas 17, 18 and 19. While stereopsis may be mediated in our albinos via residual appropriately projecting retino-geniculo-cortical fibres, we suggest that inter and intra cortical communication via corpus callosal connections may play a primary role in providing the adequate neural substrate for albino binocularity.
Adolescent↗
Binocular motion rivalry in macaque monkeys: eye dominance and tracking eye movements.
When the two eyes are exposed to markedly different patterns, perception becomes unstable, falling into oscillations, so that the image of one eye is seen first and then that from the other. With large stimuli the alternation is piecemeal, whilst when small stimuli are used the whole pattern alternates in unison. The purpose of this study was to determine whether a reliable, objective indicator of the perceptual state during binocular rivalry could be developed in the nonhuman primate. Monkeys (Macaca mulatta) were trained to discriminate direction of motion when presented with vertically drifting gratings moving in opposite directions in the two eyes. A high correlation was found between the direction of the slow phase of the optokinetic nystagmus (OKN) elicited by the drifting gratings during rivalry and the direction of motion reported by the monkey even though the gain of the OKN was reduced during rivalry, and the latency was longer. Behavioral eye dominance during rivalry varied significantly over time, between individuals and as a function of interocular contrast differences. Since the direction of tracking eye movements can be used to reliably monitor perceptual state during binocular motion rivalry, the opportunity exists in nonhuman primates to study the neurophysiological mechanisms underlying motion perception during the perceptually ambiguous condition of binocular rivalry.
Animals↗
Orientation and color processing for partially occluded objects.
The McCollough orientation-contingent color aftereffect could be equally well elicited by either a full test pattern of black and white stripes or a similar test pattern that was largely occluded by white surfaces, provided the latter stripes were made to appear as through continuing under the white surfaces--by means of stereo depth cues. The color aftereffect appeared concentrated around the edges of the stripes that protruded out from under the white surfaces; surfaces that themselves continued to appear a uniform white as shown by color matches. These results suggest that occluded, perceptually-continued edges can elicit the McCollough effect, which is generally thought to occur quite early in the visual pathway.
Adaptation, Ocular↗
Objective evaluation of human and computational stereoscopic visual systems.
Several computational models have been proposed to account for the abilities of human observers in solving the stereo correspondence problem. Such models are often evaluated by counting the number of correct matches that the model makes when presented with a test stimulus, but this measure is difficult to compare with conventional psychophysical measures of human stereo performance. This paper develops an absolute comparison of human and model stereo correspondence by using the concept of statistical efficiency, through which the performance of both humans and models is compared against that of an ideal observer. Some important assumptions need to be examined before the efficiencies of human and model can be compared because these assumptions affect the statistical performance of models. One assumption involves the use of an epipolar constraint to help constrain the matching problem but it is unclear what the resolution (or "height") of an epipolar line should be. Experimental estimates were obtained of the vertical region over which dot position may be varied without disruption of human stereoacuity. These estimates constrained the effective "height" of the zone above and below an epipolar line. Two models of stereopsis were then compared against human performance: the first (due to Pollard, Mayhew & Frisby, Perception, 14, 449-470, 1985) uses local constraints to solve the correspondence problem; the second is a simple area-based correlation model. For nearly all the stimuli tested, both models show strong similarities with human observers. For one particular stimulus, the efficiency of the correlation model is different from that of humans and of the PMF algorithm. This implies that a more complex process than simple correlation may be required to account for human stereo processing and indicates other limitations of correlation models.
Algorithms↗
Perceived texture segregation in chromatic element-arrangement patterns: high intensity interference.
An element-arrangement pattern is composed of two types of elements that differ in the ways in which they are arranged in different regions of the pattern. We report experiments on the perceived segregation of chromatic element-arrangement patterns composed of equal-size red and blue squares as the luminances of the surround, the interspaces and the background (surround plus interspaces) are varied. Perceived segregation was markedly reduced by increasing the luminance of the interspaces. Perceived segregation was approximately constant for constant ratios of interspace luminance to square luminance and increased with the contrast ratio of the squares. Unlike achromatic element-arrangement patterns composed of squares differing in lightness [Beck et al (1991). Vision Research, 32, 719-743] perceived segregation did not decrease when the luminance of the interspaces was below that of the squares. Similar results were obtained for red and yellow, red and green, green and yellow, green and blue, and blue and yellow squares. Perceived segregation based on edge alignment was not interfered with by high intensity interspaces. Stereoscopic cues that caused the squares composing the element-arrangement pattern to be seen in front of the interspaces did not greatly improve perceived segregation. One explanation of the results is in terms of inhibitory interactions among achromatic and chromatic cortical cells tuned to spatial frequency and orientation. Alternately, the results may be explained in terms of how the luminance of the interspaces affects the grouping of the squares for encoding surface representations. Neither explanation accounts fully for the data and both mechanisms may be involved.
Color Perception↗
Stereoscopic depth constancy depends on the subject's task.
Under identical viewing conditions, observers made two types of judgement about the shape of stereoscopically defined surfaces: one required an estimate of viewing distance for correct performance (e.g. setting the depth of a hemi-cylinder to equal its half-height or a dihedral angle to 90 deg), the other did not (matching the depth of, for example, sinusoidal corrugations or hemi-cylinders presented at two distances). Depth constancy for the two types of task was about 75% and 100%, respectively. We argue that observers may use a simple "direct" strategy to perform the depth matching task rather than constructing and comparing a metric representation of each surface.
Convergence, Ocular↗