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At least 937 records · Page 52Linked to original sources
PubMed2002

Visual perception without awareness in a patient with posterior cortical atrophy: impaired explicit but not implicit processing of global information.

A patient with progressive posterior cortical atrophy (PCA) was examined on several tests of visual cognition. The patient displayed multiple visual cognitive deficits, which included problems identifying degraded stimuli, attending to two or more stimuli simultaneously, recognizing faces, tracing simple visual stimuli, matching simple shapes, and copying objects. The patient was also impaired in identifying visual targets contained at the global level within global-local stimuli (i.e., smaller letters that compose a larger letter). Although the patient denied any conscious awareness of the global form, he nevertheless displayed a normal pattern of global interference when asked to identify local level targets. Thus, the patient processed the global information despite not being consciously aware of such information. These results suggest that global-local processing can take place in the absence of awareness. Possible neurocognitive mechanisms explaining this dissociation are discussed.

Aged↗
PubMed1990

Visual perception of smoothly curved surfaces from double-projected contour patterns.

This research was designed to examine how human observers are able to perceive the 3-dimensional structure of smoothly curved surfaces from projected patterns of surface contours. Displays were generated by using a method of double projection that made it possible to cover a surface with a wide range of contour patterns of varying geometric structure and to eliminate systematic variations of image shading. The compelling impression of 3-dimensional form from these patterns provides strong evidence that the ability of observers to perceptually interpret surface contours is considerably less restrictive than would be reasonable to expect on the basis of existing computational models. Results suggest that the perceptual analysis of surface contours is able to exploit statistical regularities of contour structure over appropriately large regions of visual space.

Adult↗
PubMed1997

Neural systems shared by visual imagery and visual perception: a positron emission tomography study.

Subjects participated in perceptual and imagery tasks while their brains were scanned using positron emission tomography. In the perceptual conditions, subjects judged whether names were appropriate for pictures. In one condition, the objects were pictured from canonical perspectives and could be recognized at first glance; in the other, the objects were pictured from noncanonical perspectives and were not immediately recognizable. In this second condition, we assume that top-down processing is used to evaluate the names. In the imagery conditions, subjects saw a grid with a single X mark; a lowercase letter was presented before the grid. In the baseline condition, they simply responded when they saw the stimulus, whereas in the imagery condition they visualized the corresponding block letter in the grid and decided whether it would have covered the X if it were physically present. Fourteen areas were activated in common by both tasks, only 1 of which may not be involved in visual processing (the precentral gyrus); in addition, 2 were activated in perception but not imagery, and 5 were activated in imagery but not perception. Thus, two-thirds of the activated areas were activated in common.

Adult↗
PubMed1982

Uniocular Pulfrich phenomenon: an abnormality of visual perception.

We describe a patient with multiple sclerosis who experienced the Pulfrich illusion of elliptical motion of a target moving linearly when viewing the motion with one eye as opposed to the well recognised binocular manifestation of the phenomenon. The perception of the illusion was independent of the wave form or velocity characteristics of target motion or of retinal image position. We suggest that the occurrence of the phenomenon does not simply reflect delay in the visual system but is a function of an abnormality of perceptual interpretation of visual stimuli occurring at a high integrative level.

Adult↗
PubMed2004

Visual perception and social foraging in birds.

Birds gather information about their environment mainly through vision by scanning their surroundings. Many prevalent models of social foraging assume that foraging and scanning are mutually exclusive. Although this assumption is valid for birds with narrow visual fields, these models have also been applied to species with wide fields. In fact, available models do not make precise predictions for birds with large visual fields, in which the head-up, head-down dichotomy is not accurate and, moreover, do not consider the effects of detection distance and limited attention. Studies of how different types of visual information are acquired as a function of body posture and of how information flows within flocks offer new insights into the costs and benefits of living in groups.

Journal Article↗
PubMed1980

[Visual perception under the influence of a tranquilizer (author's transl)].

In a double blind study, the influence of the tranquilizer Camazepam on different ophthalmological parameters was examined. The 12 test persons were between 20 and 30 years of age and had entirely normal ocular findings. Visual acuity was uninfluenced by a single dose of 40 mg of Camazepam, reflecting the very slightly hypnotic effect of the drug. Roughly half of the patients showed a considerable widening of the visual field, thought to be related to the stimulation of attention caused by Camazepam. Finally the lack of muscle relaxing properties of Camazepam was confirmed as regards the extraocular muscles, since their balance remained uninfluenced as well.

Accommodation, Ocular↗
PubMed2004

The visual perception of length along intrinsically curved surfaces.

The ability of observers to perceive three-dimensional (3-D) distances or lengths along intrinsically curved surfaces was investigated in three experiments. Three physically curved surfaces were used: convex and/or concave hemispheres (Experiments 1 and 3) and a hyperbolic paraboloid (Experiment 2). The first two experiments employed a visual length-matching task, but in the final experiment the observers estimated the surface lengths motorically by varying the separation between their two index fingers. In general, the observers' judgments of surface length in both tasks (perceptual vs. motoric matching) were very precise but were not necessarily accurate. Large individual differences (overestimation, underestimation, etc.) in the perception of length occurred. There were also significant effects of viewing distance, type of surface, and orientation of the spatial intervals on the observers' judgments of surface length. The individual differences and failures of perceptual constancy that were obtained indicate that there is no single relationship between physical and perceived distances on 3-D surfaces that is consistent across observers.

Depth Perception↗
PubMed1989

Visual perception of surface curvature: psychophysics of curvature detection induced by motion parallax.

The continuous approach to optic-flow processing shows that the curvature of a moving surface is related to a second spatial derivative of the velocity field, the spin variation (Droulez & Cornilleau-Pérès, 1989). With this approach as a theoretical framework, visual sensitivity to the curvature of a cylinder in motion was measured using a task of discrimination between cylindrical and planar patches. The results confirm the predictions suggested by the theory: (1) Sensitivity to curvature was always greater when the cylinder axis and the frontal translation were parallel than when they were orthogonal. The ratio of curvature detection thresholds in the two cases was between 1.3 and 2.5; the value predicted from the spin variation theory is about 2. (2) Sensitivity to curvature increased strongly with the velocity of the motion but was only weakly affected by its amplitude and the duration of viewing for the range of values used in our experiments.

Adult↗
PubMed2002

Reversed hemispheric asymmetry during simple visual perception in schizophrenia.

Processing of sensory information in the human brain progresses from primary areas, dedicated to a single sensory feature, to multimodal areas, which integrate many features across sensory modalities. For some of these processes hemispheric dominance has developed. Here we report the results of a passive viewing task using positron emission tomography. Subjects were scanned twice while staring at a stationary visual noise pattern. Normal subjects showed a significant reduction of regional cerebral blood flow (rCBF) in a distributed right hemisphere network of brain regions during the second visual task. Schizophrenic subjects, however, showed significant increases of right hemisphere rCBF during the second visual task and showed significant decreases only in the left hemisphere. These results are consistent with the notion of reversed hemispheric asymmetry during the processing of sensory information in schizophrenia.

Adult↗
PubMed1975

Visual perception of line direction in patients with unilateral brain disease.

The accuracy of identifying the slope of briefly exposed lines was assessed in patients with lesions of the left or right hemisphere and in a group of control patients without history or evidence of brain disease. The frequency of impaired performance was remarkably high in the patients with right hemisphere lesions. In contrast, the patients with left hemisphere lesions did not perform differently from the control group. Visual field defect, aphasic disorder, and age were not related to performance level. The striking interhemispheric difference in performance on this visuospatial task suggests its further development for clinicodiagnostic purposes.

Adult↗
PubMed2001

The stationarity hypothesis: an allocentric criterion in visual perception.

Having long considered that extraretinal information plays little or no role in spatial vision, the study of structure from motion (SfM) has confounded a moving observer perceiving a stationary object with a non-moving observer perceiving a rigid object undergoing equal and opposite motion. However, recently it has been shown that extraretinal information does play an important role in the extraction of structure from motion by enhancing motion cues for objects that are stationary in an allocentric, world-fixed reference frame (Nature 409 (2001) 85). Here, we test whether stationarity per se is a criterion in SfM by pitting it against rigidity. We have created stimuli that, for a moving observer, offer two interpretations: one that is rigid but non-stationary, another that is more stationary or less rigid. In two experiments, with subjects reporting either structure or motion, we show that stationary, non-rigid solutions are preferred over rigid, non-stationary solutions; and that when no perfectly stationary solutions is available, the visual system prefers the solution that is most stationary. These results demonstrate that allocentric criteria, derived from extra-retinal information, participate in reconstructing the visual scene.

Adult↗
PubMed1978

[Impaired visual perception in progressive multifocal leukoencephalopathy; a clinical diagnosis based on sequential computerized axial tomography (author's transl)].

Progressive multifocal leukoencephalopathy (PML) is a viral illness affecting principally cerebral white matter of patients, who have diminished immunologic resistance. A patient is presented, whose initial complaint was blurred vision. Relentless progression over a period of seven months to left hemianopsia, prosopagnosia, alexia with agraphia and eventually cortical blindness was observed. Accompanying the visual impairment was a steady deterioration of mental function. Sequential computerized axial tomography defined low density lesions in the occipital white matter with sparing of the cortical ribbon. As hemianopsia progressed to cortical blindness, the scans confirmed the evolution of bilaterial occipital lesions. The diagnosis was made clinically and established on histological and cultural grounds. Computerized tomography seems to be valuable in diagnosis and monitoring progress of PML.

Aged↗
PubMed2003

Auditory influences on visual temporal rate perception.

Visual stimuli are known to influence the perception of auditory stimuli in spatial tasks, giving rise to the ventriloquism effect. These influences can persist in the absence of visual input following a period of exposure to spatially disparate auditory and visual stimuli, a phenomenon termed the ventriloquism aftereffect. It has been speculated that the visual dominance over audition in spatial tasks is due to the superior spatial acuity of vision compared with audition. If that is the case, then the auditory system should dominate visual perception in a manner analogous to the ventriloquism effect and aftereffect if one uses a task in which the auditory system has superior acuity. To test this prediction, the interactions of visual and auditory stimuli were measured in a temporally based task in normal human subjects. The results show that the auditory system has a pronounced influence on visual temporal rate perception. This influence was independent of the spatial location, spectral bandwidth, and intensity of the auditory stimulus. The influence was, however, strongly dependent on the disparity in temporal rate between the two stimulus modalities. Further, aftereffects were observed following approximately 20 min of exposure to temporally disparate auditory and visual stimuli. These results show that the auditory system can strongly influence visual perception and are consistent with the idea that bimodal sensory conflicts are dominated by the sensory system with the greater acuity for the stimulus parameter being discriminated.

Acoustic Stimulation↗
PubMed2005

EEG phase synchrony differences across visual perception conditions may depend on recording and analysis methods.

OBJECTIVE: (1) To investigate the neural synchrony hypothesis by examining if there was more synchrony for upright than inverted Mooney faces, replicating a previous study; (2) to investigate whether inverted stimuli evoke neural synchrony by comparing them to a new scrambled control condition, less likely to produce face perception. METHODS: Multichannel EEG was recorded via nose reference while participants viewed upright, inverted, and scrambled Mooney face stimuli. Gamma-range spectral power and inter-electrode phase synchrony were calculated via a wavelet-based method for upright stimuli perceived as faces and inverted/scrambled stimuli perceived as non-faces. RESULTS: When the frequency of interest was selected from the upright condition exhibiting maximal spectral power responses (as in the previous study) greater phase synchrony was found in the upright than inverted/scrambled conditions. However, substantial synchrony was present in all conditions, suggesting that choosing the frequency of interest from the upright condition only may have been biased. In addition, artifacts related to nose reference contamination by micro-saccades were found to be differentially present across experimental conditions in the raw EEG. When frequency of interest was selected instead from each experimental condition and the data were transformed to a laplacian 'reference free' derivation, the between-condition phase synchrony differences disappeared. Spectral power differences were robust to the change in reference, but not the combined changes in reference and frequency selection criteria. CONCLUSIONS: Synchrony differences between face/non-face perceptions depend upon frequency selection and recording reference. Optimal selection of these parameters abolishes differential synchrony between conditions. SIGNIFICANCE: Neural synchrony is present not just for face percepts for upright stimuli, but also for non-face percepts achieved for inverted/scrambled Mooney stimuli.

Adult↗
PubMed1997

[Maculo-ocular reflex and visual perception during vertical acceleration].

BACKGROUND: We investigated the effect of vertical acceleration upon the otolithic-ocular reflex of 22 healthy people. The study was performed to obtain standard values for subsequent investigations at patients. METHODS: People sitting on a chair were accelerated in the vertical axis with an amplitude of 4 cm and the frequencies of 0.5 Hz, 1 Hz, 1.5 Hz, 2 Hz, 2.5 Hz and 2.7 Hz. The movements of the ocular globe were initially recorded during vertical acceleration with the eyes closed. Then visual acuity was tested during linear acceleration with the eyes open. As parameters of evaluation we used coherence and alteration of the visual acuity. RESULTS: When the frequency was increased while the eyes were closed, coherence increased and the number of people with vertical eye movements increased. Amplitude was observed to increase and a phase shift occurred. A significant value of coherence (> 0.8) was observed at a frequency above 2.5 Hz. During the test of visual acuity, coherence also increased but did not reach quantity we observed initially. A significant loss of visual acuity occurred at a frequency above 2.5 Hz. A phase shift was also observed. The reason for the loss of visual acuity was the increment of amplitude and the phase shift, which had a negative influence on fixation. CONCLUSIONS: In summary, reactions with closed eyes are best tested at frequencies of 2.5 and 2.7 Hz. We recommended frequencies of 1.5 and 2 Hz for testing visual acuity.

Acceleration↗
PubMed1997

Phenomenal causality: impressions of pulling in the visual perception of objects in motion.

Subjects observed computer-generated images of the five opaque rectangles arranged in a vertical column and separated by small gaps. In order from top to bottom, the rectangles began to move horizontally at intervals of a fraction of a second, at constant and identical speeds. Subjects reported a strong impression that the top object was pulling the others, despite the fact that the objects never came into contact or approached each other, moved in different planes, and had no visible connection. The impression was not much affected by speed, direction of motion, or length of delay between successive objects beginning to move. The effect was attenuated if there was prior motion in the opposite direction, if each object in turn rapidly decelerated to standstill, and if all objects began to move simultaneously. It is unlikely that this impression could be mediated by an innate visual mechanism, and it may reflect perceptual learning.

Adolescent↗