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At least 793 records · Page 44Linked to original sources
PubMed2006

Binding, relational memory, and recall of naturalistic events: a developmental perspective.

This research was an investigation of children's performance on a task that requires memory binding. In Experiments 1 and 2, 4-year-olds, 6-year-olds, and adults viewed complex pictures and were tested on memory for isolated parts in the pictures and on the part combinations (combination condition). The results suggested improvement in memory for the combinations between the ages of 4 and 6 years but not in memory for the isolated parts. In Experiments 2 and 3, the authors also examined the developmental relationship between performance in the combination condition and free recall of a naturalistic event, finding preliminary evidence that performance on a memory task that requires binding is positively related to performance in episodic memory.

Adolescent↗
PubMed1998

Electric fish measure distance in the dark.

Distance determination in animals can be achieved by visual or non-visual cues. Weakly electric fish use active electrolocation for orientation in the dark. By perceiving self-produced electric signals with epidermal electroreceptors, fish can detect, locate and analyse nearby objects. Distance discrimination, however, was thought to be hardly possible because it was assumed that confusing ambiguity could arise with objects of unknown sizes and materials. Here we show that during electrolocation electric fish can measure the distance of most objects accurately, independently of size, shape and material. Measurements of the 'electric image' projected onto the skin surface during electrolocation revealed only one parameter combination that was unambiguously related to object distance: the ratio between maximal image slope and maximal image amplitude. However, slope-to-amplitude ratios for spheres were always smaller than those for other objects. As predicted, these objects were erroneously judged by the fish to be further away than all other objects at an identical distance. Our results suggest a novel mechanism for depth perception that can be achieved with a single, stationary two-dimensional array of detectors.

Animals↗
PubMedDate not supplied

Sensitivity for vertical retinal image differences.

The detection of horizontal disparities between the right and left retinal images is accepted as an important component in the visual perception of depth. It has occasionally been pointed out that a disparity will also occur in the vertical when a target is nearer to one eye than the other and could be used as a second component. Here it is shown that while it is possible to detect size differences between the retinal images in the vertical direction, the sensitivity is at least one order of magnitude less than the horizontal differences. It had been generally accepted that vertical disparities cannot be detected (literature summarized by Tschermak), but there are clear reports of the ability of subjects to respond to relative differences in the vertical magnification of the images of the two eyes, the so-called induced size effect. It manifests itself in the appearance of a visual scene as if there had been an opposite horizontal magnification and is, therefore, in the correct direction if it is to be utilized in asymmetrical convergence.

Depth Perception↗
PubMed1998

Premotor commands encode monocular eye movements.

Binocular coordination of eye movements is essential for stereopsis (depth perception) and to prevent double vision. More than a century ago, Hering and Helmholtz debated the neural basis of binocular coordination. Helmholtz believed that each eye is controlled independently and that binocular coordination is learned. Hering believed that both eyes are innervated by common command signals that yoke the eye movements (Hering's law of equal innervation). Here we provide evidence that Hering's law is unlikely to be correct. We show that premotor neurons in the paramedian pontine reticular formation that were thought to encode conjugate velocity commands for saccades (rapid eye movements) actually encode monocular commands for either right or left eye saccades. However, 66% of the abducens motor neurons, which innervate the ipsilateral lateral rectus muscle, fire as a result of movements of either eye. The distribution of sensitivity to ipsilateral and contralateral eye movements across the abducens motor neuron pool may provide a basis for learning binocular coordination in infancy and adapting it throughout life.

Abducens Nerve↗
PubMed1990

Functions of the colour-opponent and broad-band channels of the visual system.

The colour-opponent and broad-band channels of the primate visual system originate in the retina and remain segregated through several neural stations in the visual system. Until now inferences about their function in vision have been based primarily on studies examining single-cell receptive field properties which have shown that the colour-opponent retinal ganglion cells have small receptive fields, produce sustained responses and receive spatially segregated inputs from different cone types; the broad-band cells have large receptive fields, respond transiently and receive cone inputs that are not spatially separated. We have now examined the visual capacities of rhesus monkeys before and after interrupting either of these channels with small lesions at the lateral geniculate nucleus. Here we report that the colour-opponent channel is essential for the processing of colour, texture, fine pattern and fine stereopsis, whereas the broad-band channel is crucial for the perception of fast flicker and motion. Little or no deficits were found in brightness and coarse-shape discrimination, low spatial frequency stereopsis and contrast sensitivity after the disruption of either of the channels.

Animals↗
PubMed2001

Speed skills: measuring the visual speed analyzing properties of primate MT neurons.

Knowing the direction and speed of moving objects is often critical for survival. However, it is poorly understood how cortical neurons process the speed of image movement. Here we tested MT neurons using moving sine-wave gratings of different spatial and temporal frequencies, and mapped out the neurons' spatiotemporal frequency response profiles. The maps typically had oriented ridges of peak sensitivity as expected for speed-tuned neurons. The preferred speed estimate, derived from the orientation of the maps, corresponded well to the preferred speed when moving bars were presented. Thus, our data demonstrate that MT neurons are truly sensitive to the object speed. These findings indicate that MT is not only a key structure in the analysis of direction of motion and depth perception, but also in the analysis of object speed.

Action Potentials↗
PubMed1988

Art, illusion and the visual system.

The verve of op art, the serenity of a pointillist painting and the 3-D puzzlement of an Escher print derive from the interplay of the art with the anatomy of the visual system. Color, shape and movement are each processed separately by different structures in the eye and brain and then are combined to produce the experience we call perception.

Art↗
PubMed2002

Visual deficits in a patient with 'kaleidoscopic disintegration of the visual world'.

We describe psychophysical, neuropsychological and neuro-ophthalmological studies of visual abilities in a patient who, following a right hemisphere stroke, had difficulty in combining parts of objects into a whole and in reading. Strikingly, her perceptual problems were accentuated when the objects moved or when she moved. Formal testing showed that her main deficits were in depth perception, various tasks of motion and object recognition of degraded stimuli. But low-level detection and discrimination of form and color were normal. Despite her deficits in visual motion and degraded static-object recognition, her visual recognition of 'biological motion' stimuli was normal. Structural magnetic resonance imaging revealed an infarct in the ventro-medial occipito-temporal region, extending ventro-laterally and leading to a 'kaleidoscopic disintegration of visible objects'.

Adrenalectomy↗
PubMed1997

Developing Technology for Suspended Imaging for Minimal Access Surgery.

Some of the problems of performing laparoscopic surgery can be traced to the position and intrinsic qualities of the displayed video image. Conversion of the normal monitor screen image to a suspended image can reduce image-related difficulties. The principles of suspended image production are described and the potential for more flexible image positioning and enhanced depth perception is explained. A recently initiated research program is investigating design possibilities and conducting fundamental research into suspended image appraisal.

Journal Article↗
PubMed1998

Cannabinoid/anandamide system and schizophrenia: is there evidence for association?

Cognitive impairments during psychotic episodes are assumed to be caused not only by one single putative classical neurotransmitter dysfunction but also to be due to an impaired equilibrium of the interaction between different neurobiological generators of cognitive processes. Here, the perceptual abnormalities induced by psychotogenic agents play a major role as tools for understanding model psychoses. The recently discovered cannabinoid receptor system with its endogenous ligand anandamide can be regarded as an extremely relevant regulation system, a dysfunctionality of which may explain at least one subtype of endogenous psychoses. The present paper discusses the possible associations between the endogenous anandamide/cannabinoid system and schizophrenic psychoses. Neuropsychological experiments with the 3-D inversion paradigm were performed in healthy volunteers intoxicated with delta9-Tetrahydrocannabinol (delta9-THC). The 3-D inversion paradigm represents a visual illusion of binocular depth perception. Such an inversion occurs in many cases, especially when objects with a higher degree of familiarity (e.g. photographs of faces) are displayed. It is assumed that cognitive factors override the binocular disparity cues of stereopsis. We tested the hypothesis that, during psychotic and related prepsychotic states, the human CNS is unable to correct implausible perceptual hypotheses. Our study provides evidence of strong similarities between data acquired from patients, suffering from productive schizophrenic psychoses and delta9-THC-intoxicated healthy volunteers, as concerns disturbances in the internal regulation of perceptual processes.

Arachidonic Acids↗
PubMed1989

[Examination of central vision. Visual acuity, contrast sensitivity, color vision].

In the neurophysiological organization of the visual system, form, color, movement, and depth perception are processed separately. Therefore, sensorial examination methods should test each of these basic functions separately, since they may be affected individually or to different extents by pathologic processes. For diagnosis the limit of visual acuity, i.e., the capacity for discrimination must be searched for, using Paliaga's "limits method". Visual acuity can also be tested in infants by the preferential looking method. Contrast sensitivity is tested using sinusoidal grid patterns of varying contrast and spatial frequency. In routine practice, however, this is usually achieved more easily with acuity cards on which contrast is reduced in several stages. The "two-equation method" is a colorimetric test combining two metameric matches, red + green = yellow, and blue + green = cyan, for testing color vision. The test requires an anomaloscope or anomalometer with four light channels. With this method it is possible to test the "red", "green", and "blue" cones and the "red-green" and "blue-yellow" opponents. The test provides a qualitative and quantitative evaluation of color vision disorders. If no colorimeter is available, classic printed test can be used. However, they might never achieve the same qualitative and quantitative precision.

Color Perception Tests↗
PubMed1984

[Pseudophakia--correction of unilateral cataract following penetrating eye injuries in children].

Between 1972 and 1980 extracapsular cataract surgery was performed in 24 children, followed by implantation of an iris clip lens, after primary reconstruction of the anterior segment. Regular follow-ups were carried out at 3-month intervals. Discission of a secondary cataract was necessary in 19 cases. Visual acuity of 0.5 or more was achieved by 19 children. An orthophoria or exophoria was also found in 19 children with positive depth perception after a minimum of 2 and a maximum of 9.5 years' observation.

Adolescent↗
PubMed1995

Risk factors for hip fracture in white women. Study of Osteoporotic Fractures Research Group.

BACKGROUND: Many risk factors for hip fractures have been suggested but have not been evaluated in a comprehensive prospective study. METHODS: We assessed potential risk factors, including bone mass, in 9516 white women 65 years of age or older who had had no previous hip fracture. We then followed these women at 4-month intervals for an average of 4.1 years to determine the frequency of hip fracture. All reports of hip fractures were validated by review of x-ray films. RESULTS: During the follow-up period, 192 women had first hip fractures not due to motor vehicle accidents. In multivariable age-adjusted analyses, a maternal history of hip fracture doubled the risk of hip fracture (relative risk, 2.0; 95 percent confidence interval, 1.4 to 2.9), and the increase in risk remained significant after adjustment for bone density. Women who had gained weight since the age of 25 had a lower risk. The risk was higher among women who had previous fractures of any type after the age of 50, were tall at the age of 25, rated their own health as fair or poor, had previous hyperthyroidism, had been treated with long-acting benzodiazepines or anticonvulsant drugs, ingested greater amounts of caffeine, or spent four hours a day or less on their feet. Examination findings associated with an increased risk included the inability to rise from a chair without using one's arms, poor depth perception, poor contrast sensitivity, and tachycardia at rest. Low calcaneal bone density was also an independent risk factor. The incidence of hip fracture ranged from 1.1 (95 percent confidence interval, 0.5 to 1.6) per 1,000 woman-years among women with no more than two risk factors and normal calcaneal bone density for their age to 27 (95 percent confidence interval, 20 to 34) per 1,000 woman-years among those with five or more risk factors and bone density in the lowest third for their age. CONCLUSIONS: Women with multiple risk factors and low bone density have an especially high risk of hip fracture. Maintaining body weight, walking for exercise, avoiding long-acting benzodiazepines, minimizing caffeine intake, and treating impaired visual function are among the steps that may decrease the risk.

Aged↗
PubMed1986

Snowhole blues: comments on Kinney and Corth.

Empirical observations are reported on illuminance and visibility in overcast snowscapes. Reasons for the improved visibility achieved with yellow goggles are discussed.

Color Perception↗