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At least 1,243 records · Page 69Linked to original sources
PubMed1995

Reduction of infant myopia: a longitudinal cycloplegic study.

Changes of cycloplegic retinoscopy refraction from 8.5 to 38.5 months of age were compared in two infant groups in the Cambridge population: "infant myopes", having at least one myopic axis (0 to -3.5 D inclusive), and a second, "control" group with low hyperopia (< or = +3.5 D). Cycloplegia eliminated the variable accommodation of infants. The myopic group showed a significant emmetropization of the mean spherical equivalent towards low hyperopia by 3 yr. There was no significant change in the control group's mean spherical equivalent power. Both groups showed a significant reduction in astigmatism with age. Analysis of the vertical and horizontal powers showed significant "emmetropization" of these meridians, in both groups, towards low hyperopia from 8.5 to 38.5 months. These meridional emmetropization changes were significant for both With-the-Rule and Against-the-Rule astigmatism.

Aging↗
PubMed1995

Stereopsis, spatial frequency and retinal eccentricity.

Stereoscopic depth discrimination thresholds increase with retinal eccentricity and distance from the horopter. However, in contrast to spatial resolution, the effects of spatial frequency on stereo-thresholds in the periphery are unknown. For spatial vision, it is generally assumed that the retina is comprised of a series of overlapping spatial filter mechanisms and that there is a commensurate increase in spatial scale as a function of retinal eccentricity. If the same holds true for mechanisms sensitive to stereoscopic depth, then stereo-thresholds for low spatial frequency stereoscopic stimuli may remain relatively invariant across the visual field, while thresholds for relatively high spatial frequency stimuli would increase. To further understand the role of the disparity sensitive mechanisms involved in depth discrimination, increment depth discrimination thresholds for both crossed and uncrossed disparities were measured as a function of eccentricity for retinal locations up to 10.0 deg along the horizontal meridian. We found that stereoscopic depth discrimination thresholds, as a function of distance from the horopter, increased in an exponential manner irrespective of spatial frequency. Stereo-thresholds also increased as a function of retinal eccentricity, however, the rate of increase depended on the spatial frequency composition of the stimuli. Best stereo-thresholds for stimuli composed of low spatial frequencies remained relatively invariant for retinal eccentricities up to 10.0 deg, while thresholds for the high spatial frequency stimuli increased with eccentricity.

Depth Perception↗
PubMed1994

Retinal eccentricity and the latency of eye saccades.

The latency-eccentricity function, for the first saccade towards a small green or red target on the horizontal hemi-retinal meridian, is bowl-shaped with a central latency peak, a minimum plateau from 0.75 to 12 degrees, and a gradual increase in latency towards the periphery. The function is highly reproducible and the central peak is a robust finding. For our step sizes and criteria its height is around 35-75 msec. Manipulations of target intensity and colour, or the state of adaptation by a retinal bleach, show that sensory contributions to the central peak are generally small (5-15 msec) for adequately suprathreshold targets. Beyond 35 degrees in the temporal retina (nasal visual field) latencies become erratic, even for bright green targets, and there are direction errors. Frequent direction errors occur over a much wider range of eccentricities for red targets that are 1 log above the foveal threshold for perception.

Adolescent↗
PubMed1994

The architecture of perceptual spatial interactions.

Lateral interactions between spatial filters were explored with a lateral masking paradigm. Contrast sensitivity (two-alternative forced-choice) for a Gabor signal in the presence of two flanking high contrast Gabor signals (masks) was measured. When the target to mask distance was less than 2 target wavelengths the contrast sensitivity decreased up to a factor of two relative to a no mask condition. At larger separations, up to eight wavelengths, an increase in contrast sensitivity occurred. This increase was maximal at separation distances of 2-3 wavelengths, where sensitivity increased by a factor of two. However, the enhancement magnitude and range was dependent on the offset between the Gabor signal orientation and the direction defined by the virtual line connecting the two masks (global orientation). Maximal effects occurred when this offset was zero (100% increase in sensitivity) and 90 deg (50% increase). A 45 deg offset yielded only a small enhancement (20%). The enhancement dependence on spatial arrangement was found to be invariant across different global orientations (meridian). This pattern of interactions may be involved in grouping colinear line segments into smooth curves.

Contrast Sensitivity↗
PubMed1994

Do peripheral non-informative cues induce early facilitation of target detection?

It has been reported that simple reaction time (RT) to a peripheral visual target is faster if the target is presented within about 200 msec from the onset of a non-informative cue flashed at the same location, as compared with RT to a target presented at an uncued location. This period of facilitation is followed by a period of inhibition during which RT is longer if cue and target are shown at the same location or at different locations within the same hemifield, as opposed to contralateral cues and targets. Early facilitation has been explained by an automatic covert orienting towards the cue, while the following inhibition has been regarded as a consequence of such covert orienting. In a series of four experiments, we have investigated the dependency of these effects on the temporal and spatial relationships between cue and target. Normal, right-handed subjects responded to a target displayed for 16 msec simultaneously with, or following at stimulus-onset asynchronies (SOAs) of 60, 130, 300 or 900 msec, the onset of a non-informative cue. Both cues and targets could appear at random in one of four locations (Expts 1-3) or in one of two locations (Expt 4) disposed symmetrically across the fixation point along the horizontal meridian. Duration of the cue varied between experiments. In Expt 1 it was 16 msec. In Expt 2 the cue remained on view throughout the period of the SOA and terminated 300 msec after target onset. In the remaining two experiments cue duration was 130 msec. In the first experiment, at all cue-target SOAs RTs to target flashed either at the same location or in the same hemifield as the cue were significantly slower than RTs to contralateral cue-target combinations (RT inhibition). In the other experiments, there was no RT inhibition with targets in cued locations if the cue remained on during target presentation and outlasted target offset. Since at no SOA was RT to targets in cued locations shorter than RT to targets contralateral to cues, there was no direct evidence for facilitation. However, the facilitatory influence of these cues could be inferred from the fact that they countered and masked inhibition. RT to uncued targets ipsilateral to cues was consistently inhibited in all experimental conditions. These results show that at each cue-target SOA the consequences of a peripheral non-informative cue depend on whether or not the cue remains visible during target processing.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1994

Mechanisms of vertical phoria adaptation revealed by time-course and two-dimensional spatiotopic maps.

The spatial spread of short term phoria adaptation was measured in response to either a single vertical disparity presented at a single eye position, or, vertical disparities of opposite sign presented at two different locations along either the primary vertical or horizontal meridians or along an oblique axis. The spread of adaptation to eye positions not specifically adapted was assessed by measuring phoria across a two-dimensional surface. The change in phoria was uniform across the field in response to a single disparity. With two disparities, adaptation conformed to the stimulus demand in the direction in which the disparity varied but was uniform in the orthogonal direction. The time-course of the adaptation indicated the presence of two mechanisms, a global one which shifted the phoria uniformly across the field and a local one which selectively adjusted the phoria to the position dependent demands of the disparity stimulus.

Adaptation, 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↗
PubMed1996

Stereoscopic depth perception and vertical disparity: neural mechanisms.

The additivity assumption relates to the various stereo-disparity components in the vertical and horizontal meridians, each of which is assumed to be independent of the other, with the total disparity in each dimension being the linear sum of the separate components. Information about the position of the eyes provided by the corollary discharge leads to compensatory changes in the lateral geniculate nuclei whereby the angle of gaze disparity component at retinal level is offset by equal and opposite changes at geniculate level. These geniculate changes concern only eye position. Changes in the retinal images such as those produced by lenses (i.e. induced effect) are passed on to the cortex without modification at the geniculate level. Discrimination of the local depth disparity component can be achieved by subtracting the local vertical eccentricity component from the total horizontal disparity.

Depth Perception↗
PubMedDate not supplied

Acupuncture and related treatment modalities. Part I: Theoretical background.

An introduction to the therapeutic applications, history, and theory of acupuncture and several related treatment modalities is presented. The practices of acupuncture, moxibustion, acupressure, and shiatsu are described. The underlying concept of treatment of imbalances of ch'i, or life energy, is presented along with the flow of ch'i in meridians (pathways), and the theories of yin and yang, Five Elements, and Eight Principle Patterns.

Acupuncture Therapy↗
PubMed1982

The contribution of the corpus callosum to receptive fields in the lateral suprasylvian visual areas of the cat.

In both ordinary cats and 'Boston' Siamese cats the visual areas in the lateral parts of the middle and posterior suprasylvian gyri (LSA) contain an extensive representation of the ipsilateral half of the visual field. In addition, in both groups of cats the overwhelming majority of neurons in LSA can be driven from both eyes. In Siamese cats this binocular interaction is in marked contrast with what is found in area 17 where neurons are almost exclusively activated through the contralateral eye. Transection of the posterior 1/3 to 1/2 of the corpus callosum had a different effect on the physiological organization of LSA in the two types of cats. In ordinary cats it caused the loss of the ipsilateral hemifield representation in the eye ipsilateral to the side of recording and reduced this representation in the other eye. However, after the section of the corpus callosum LSA neurons remained binocular. In Siamese cats the callosal transection left the representation of the ipsilateral hemifield in LSA unaffected, both totally abolished the input from the ipsilateral eye. These findings suggest that the visual callosal input to LSA has a different functional significance in ordinary and Siamese cats. In the former cats it may be related to perceptual equivalence across the vertical meridian of the visual field, whereas in the latter cats it may subserve interocular equivalence.

Animals↗
PubMed1983

Peripheral vision and optokinetic nystagmus in children with unilateral congenital cataract.

The vision of cats which were monocularly deprived during early infancy, of kittens, and of young human infants shares two limitations: detection in the nasal visual field is far poorer than detection in the temporal visual field, and optokinetic nystagmus (OKN) is difficult to elicit when a pattern moves nasally to temporally. Here we report similar limitations on the vision of children who had a dense central cataract in one eye during early infancy. Extensive static perimetry with one of these children whose visual acuity was good in both eyes revealed that her threshold for detection all along the horizontal meridian was higher in her aphakic than in her normal eye, with this difference much more pronounced in the nasal visual field than in the temporal visual field. Three children who developed cataracts after 6 months of age showed no such discrepancy between thresholds in the temporal and nasal fields. We tested the symmetry of OKN in 12 children treated for unilateral congenital cataract. In every test of an aphakic (n = 4) or normal eye (n = 12), OKN occurred significantly more often when stripes moved temporally to nasally than when they moved nasally to temporally. In contrast, no asymmetry was observed in any of 13 children treated for traumatic cataracts incurred after 3 years of age. We conclude that children treated for unilateral congenital cataract, like young human infants and monocularly deprived cats, show asymmetric OKN and relatively poor detection in the nasal visual field.

Animals↗
PubMed1984

Binocularity in the visual cortex of the adult cat does not depend on the integrity of the corpus callosum.

The importance of the corpus callosum for binocular interaction in areas 17 and 18 of the adult cat is still a matter of controversy, since its specific role in integrating information from the two eyes has been suggested by some and questioned by others. We have reanalyzed the problem by assessing binocular interaction for single neurons in areas 17 and 18 of adult cats submitted to section of the posterior two-thirds of the corpus callosum. In 5 cats this interhemispheric disconnection was performed from 10 days to 7 weeks before the electrophysiological recordings; in another cat callosal afferents to the recording sites were at first partially eliminated by an acute lesion of corresponding cortical zones in the other hemisphere, and thereafter completely interrupted by a posterior callosal section performed in the same recording session. Recordings were mainly aimed at the callosal zone of areas 17 and 18, which coincides with the border between these two areas and corresponds to visual field regions bordering the vertical meridian. Electrophysiological recordings were carried out in awake, unanesthetized animals in which all nociceptive pathways were previously interrupted by a midpontine pretrigeminal transection. The results indicate that the interhemispheric disconnection, whether acute or chronic, does not disrupt binocularity in areas 17 and 18; moreover, the analysis of the ocular dominance for binocular neurons did not reveal any imbalance between the inputs from the two eyes, since at all levels of eccentricity the majority of binocular neurons was equally activated by both eyes. Since in previous experiments on anesthetized cats, section of the corpus callosum apparently reduced binocular interaction in areas 17 and 18, we suggest that such an effect, which was lacking in our unanesthetized cats, was probably due to an interaction or summation between callosotomy and anesthesia.

Animals↗
PubMed1988

Cerebral asymmetries and interhemispheric processes.

Those who attempt to characterize the functions of the cerebral hemispheres tend, broadly speaking, to do so either in terms of structural specializations or of different information-processing modes. Often little attention is paid to the possible importance, in determining the outcome of experiments in this field, of interhemispheric processes. An experiment is described which concurrently studies hemispheric response differences and interhemispheric processes. Sets of dot patterns are learned, each made up of an original and three distortions of the original. The degree of distortion is systematically changed and is quantified in terms of information theory. Subjects then examine pairs of patterns and decide whether they belong to the 'same' family, i.e. an original and one of its distortions, or 'different', i.e. a pattern previously learned and a completely new one. Manual response times are recorded for 'same' and 'different' responses and functions plotted of response latencies against degree of pattern distortion. The pairs of patterns are presented under three different conditions. Either both patterns in one visual field (unilateral condition), one pattern in each visual field (bilateral condition) or the patterns one above the other straddling the vertical meridian (central condition). Response latencies are shortest for the bilateral condition, next shortest for the unilateral condition and slowest for the central condition. Models which may account for these results are examined and one crucially involving changed information transmission rates with increasing distortion of patterns to be compared is found to provide the most parsimonious explanation of the results from all three experimental conditions.

Adolescent↗
PubMed1988

Detection of visual stimuli after lesions of the superior colliculus in the rat; deficit not confined to the far periphery.

The visual neglect which follows lesions of the superior colliculus in the rat is well documented, although its nature has been somewhat contentious. Recently, using large, bright stimuli, Overton et al. [Exp. Brain Res., 59 (1985) 559-569] presented evidence that collicular lesions produce a detection deficit in the far peripheral visual field (120 degrees), whilst the central field is unaffected. However, some simple tests for visual neglect by other workers suggest that a detection deficit may also exist in the central field, if the stimuli are dim (and small). This possibility was investigated psychophysically. Using a signal detection paradigm, sensitivity to small (2.5 degrees v.a.), transient (modal duration 200 ms) stimuli of varying luminance was determined at 40 degrees on either side of the vertical meridian, before and after collicular lesions or control lesions of the striate cortex. Despite extensive preoperative training, collicular animals showed a decrease in sensitivity of around 50% for all stimulus intensities postoperatively. This deficit was significantly more severe than that produced by striate lesions. Furthermore, it proved very difficult to retrain the animals to a preoperative criterion. These data suggest that collicular lesions do indeed cause a detection deficit which is not confined to the far periphery, and that this deficit may specifically affect the detection of small stimuli. The proposed detection deficit is related to recent anatomical findings in the rat.

Animals↗
PubMed1990

The ocular dominance and receptive field properties of visual cortex cells of cats following long-term transection of the optic chiasm and monocular deprivation during adulthood.

Plasticity-induced interhemispheric transfer of visual information to cortical cells was studied in adult cats. The direct contralateral visual pathway was surgically eliminated permitting binocularity only by callosal transfer. In order to enhance the interhemispheric transfer, one hemisphere was made less visually active by depriving it chronically from visual input. Single cell recording was made in areas 17-18 boundary, the callosal projection zone, of operated (OC), operated and deprived (OCMD), and normal control cats. In the OCMD cats, greater than 90% of the cells in each hemisphere reacted ipsilaterally to the deprived or non-deprived eye. Only 3.1% of the cells in both hemispheres of the OCMD cats and 3.9% in the OC cats had contralateral input via the corpus callosum. The two hemispheres were similar in the selectivity of their cells to stimulus orientation and direction. The average receptive field area of the OCMD cats was also similar for the ipsilaterally driven cells in the two hemispheres; it was 1.2 degrees 2 for the deprived eye and 1.1 degrees 2 for the normal eye. The receptive fields (greater than 95%) of both eyes of the OCMD cats were found in the nasal visual hemifields and greater than 70% of them were at eccentricities of less than 5 degrees from the vertical meridian. The disappearance of the temporal (contralateral) hemifields in these cats and the physiological properties of their cortical cells were determined merely by the chiasm transection which had thus induced nearly complete interhemispheric separation. No effect of the monocular deprivation, in normal adult cats or in cats with chiasm transection was found, even after long periods (greater than 7 months). Therefore, plasticity-induced interhemispheric transfer of visual information was not found during adulthood.

Animals↗
PubMed1994

Human strabismus: evaluation of the interhemispheric transmission time and hemiretinal differences using a reaction time task.

Experimentally induced strabismus in visually immature cats leads to abnormal development of the posterior corpus callosum. This, in turn, should lead to abnormal interhemispheric integration of unilaterally presented visual information. To test whether strabismus produces deficits in the human commissural visual system, the interhemispheric transmission time (ITT) was compared in strabismic and normal subjects. Simple unimanual reaction times (RT) were tested in 30 subjects in response to a lateralized target presented monocularly at 4 degrees and 35 degrees nasally and temporally from the fovea along the horizontal meridian. This method was also used to examine the effect of strabismus on the central and peripheral portions of each hemiretina. The results showed that in strabismic subjects with or without amblyopia, the ITT did not differ significantly from normals at both eccentricities. In non-amblyopic strabismic patients, RTs in the central and peripheral portions of hemiretina were comparable to normals. However, a reduced speed of response was found in the central visual field (4 degrees) in the amblyopic eye. Our results suggest that the ITT is normal in strabismic subjects and that the longer RTs in the central portion of the nasal and temporal hemiretina of the amblyopic eye may be associated with the severe amblyopic condition.

Adolescent↗
PubMed1994

Interhemispheric transfer and the processing of foveally presented stimuli.

Two arguments are commonly given in favor of a nasotemporal overlap along the vertical meridian of the visual field: anatomical findings and the existence of macular sparing in hemianopia. A review of the literature, however, points to the weakness of the evidence. The anatomical indications are exclusively based on horseradish peroxidase studies, which can not give an unequivocal answer to the amount of overlap in central vision, and which were not supported by a recent study that made use of the more direct [14C]2-deoxy-D-glucose technique. The argument of macular sparing in hemianopia appears to be derived evidence that depends on the validity of the anatomical findings. In addition, behavioral studies consistently failed to find functional confirmation of the overlap. To further test the possibility of bilateral representation in central vision, a new paradigm is proposed. It is argued that if interhemispheric transfer is needed for the processing of foveally presented stimuli, the word-beginning superiority effect should be larger for subjects with left hemisphere dominance than for subjects with right hemisphere dominance. Results are in line with the hypothesis and point to the fact that interhemispheric transfer of visual information may be involved in more processing than usually accepted. It is also noted that transfer time seems to depend on the amount of information that must be transferred, and is significantly shorter than the estimates obtained in visual half field studies.

Adult↗
PubMed1995

Covert orienting to non-informative cues: reaction time studies.

Lateralized, non-informative visual cues lengthen reaction time (RT) to successive targets flashed in the same hemified. Early ipsilateral RT facilitation is limited to the co-occurrence of cues and targets. Inhibition from visual cues has sensory components which do not depend on orienting, as well as attentional components which are limited to one side of the vertical meridian. An inhibition of RT to targets ipsilateral to the cues has been found with somatic or auditory cues and targets, and also when somatic targets follow visual cues or visual targets follow somatic cues. The results reviewed in this paper (1) are best accounted for by directional constraints in motor readiness which are induced by the voluntary suppression of an overt orienting toward the location of the cue; (2) indicate that similar mechanisms of covert orienting operate in the whole peripersonal and near extrapersonal space; and (3) point to a common neural substrate mediating both intramodal and cross-modal effects.

Animals↗