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At least 1,063 records · Page 59Linked to original sources
PubMed2005

Change perception using visual transients: object substitution and deletion.

In three experiments we studied change detection and identification when no extraneous transients were present in the image at the time of change. Each image consisted of 12 different objects, sorted by color into three different levels of probability of change. In Experiment 1, change of one object was detected and identified frequently in objects having the highest probability of change (central interest), which we hypothesize were mainly visited by attention. Changes in other objects with a lower probability of change (marginal interest), however, although detected efficiently were unlikely to be identified. Identification improved for less attended objects if the changed stimulus simply disappeared, allowing visual persistence to hold information about the object until attention could be shifted to it (Experiment 2). Contrary to previous findings showing that response times (RTs) for luminance change detection in a multi-element display are not altered by attention, we found changes in objects of central interest to be detected faster than in objects of marginal interest when objects' identity was to be held in working memory. However, no differences in RTs emerged in the same change detection task when objects' identity was not stored in working memory (Experiment 3).

Adult↗
PubMed1995

Object perception by visually impaired people at different light levels.

We investigated the relationship between the illumination level and the ability of visually impaired subjects to detect and recognize objects in a realistic visual environment. Subjects often continued to show substantial improvement at light levels where normal subjects have reached maximum performance. Integrated contrast sensitivity, a summary measure for the contrast sensitivity function, was better at predicting performance than either visual acuity or peak contrast sensitivity. However, when combined, the latter two predicted performance as well as the former. We conclude that when we try to find the best illumination for orientation and day-to-day activities we should optimize it for both visual acuity and contrast sensitivity.

Contrast Sensitivity↗
PubMed1992

Preserved figure-ground segregation and symmetry perception in visual neglect.

A central controversy in current research on visual attention is whether figures are segregated from their background preattentively, or whether attention is first directed to unstructured regions of the image. Here we present neurological evidence for the former view from studies of a brain-injured patient with visual neglect. His attentional impairment arises after normal segmentation of the image into figures and background has taken place. Our results indicate that information which is neglected and unavailable to higher levels of visual processing can nevertheless be processed by earlier stages in the visual system concerned with segmentation.

Aged↗
PubMed2006

Mechanisms of top-down facilitation in perception of visual objects studied by FMRI.

Prior knowledge regarding the possible identity of an object facilitates its recognition from a degraded visual input, though the underlying mechanisms are unclear. Previous work implicated ventral visual cortex but did not disambiguate whether activity-changes in these regions are causal to or merely reflect an effect of facilitated recognition. We used functional magnetic resonance imaging to study top-down influences on processing of gradually revealed objects, by preceding each object with a name that was congruent or incongruent with the object. Congruently primed objects were recognized earlier than incongruently primed, and this was paralleled by shifts in activation profiles for ventral visual, parietal, and prefrontal cortices. Prior to recognition, defined on a trial-by-trial basis, activity in ventral visual cortex rose gradually but equivalently for congruently and incongruently primed objects. In contrast, prerecognition activity was greater with congruent priming in lateral parietal, retrosplenial, and lateral prefrontal cortices, whereas functional coupling between parietal and ventral visual (and also left lateral prefrontal and parietal) cortices was enhanced in the same context. Thus, when controlling for recognition point and stimulus information, activity in ventral visual cortex mirrors recognition success, independent of condition. Facilitation by top-down cues involves lateral parietal cortex interacting with ventral visual areas, potentially explaining why parietal lesions can lead to deficits in recognizing degraded objects even in the context of top-down knowledge.

Adult↗
PubMed1988

The perception of visual ambiguous figures in schizophrenia and Parkinson's disease.

The frequency of reversal and dominance of aspect of three versions of an ambiguous Schröder staircase were studied in patients with a diagnosis of either schizophrenia or Parkinson's Disease (PD), and in a mixed group of psychiatric patients before and after prolonged neuroleptic therapy. It was found that schizophrenics perceived the staircases from above for significantly less time, and had a (nonsignificant) tendency to have more reversals than controls. PD patients saw the staircases from above for significantly more time, and also had (nonsignificantly) more reversals than controls. In the psychiatric patients, long term neuroleptic therapy had no significant effect on either reversal rate or dominance of aspect. When the contrast and the luminance of the stimulus were manipulated, normal subjects reported significantly less reversals and significantly increased dominance of the superior aspect when the contrast of the stimulus was reduced. Changing the luminance had no significant effect. The implications of these results for visual abnormalities in schizophrenia and Parkinson's Disease are discussed.

Adult↗
PubMed1985

Directional preferences in perception of visual stimuli.

The present paper reviews a series of studies regarding the effects of hemispheric asymmetry and reading and writing habits on directional preferences in reproducing horizontally-displayed visual stimuli. Hebrew readers, English readers, and Arabic readers were presented with arrays of horizontally-displayed directional and nondirectional stimuli, as well as with single stimuli. They were asked to reproduce the stimuli, and the direction of their reproduction, left-right or right-left, was recorded for analysis. Generally, in reproducing arrays of stimuli, English readers showed left-right directionality, whereas Hebrew readers showed right-left directionality. But in reproducing arrays of English and Hebrew letters, subjects of both groups showed left-right and right-left preferences, respectively. However, the right-left directional preferences shown by Hebrew readers were weaker than the left-right preferences shown by English readers. It was hypothesized that these differences are due to differential reading and writing habits acquired in school by English- and Hebrew-readers. In support of the reading and writing habit hypothesis, it was subsequently found that: (a) Arabic readers, who have stronger right-left reading and writing habits than Hebrew readers, show relatively stronger right-left directional preferences, and (b) with the introduction of English as a foreign language in the fifth grade, children show an increase in left-right directionality. Further investigation showed that, depending on the experimental conditions, directional preferences may be a function of either reading and writing habits, or hemispheric asymmetry, or both. Finally, the bearing of these findings on the "nature-nurture" controversy regarding the development of perceptual exploration in children is discussed.

Dominance, Cerebral↗
PubMedDate not supplied

[Hemispheric interaction in man during perception of visual stimuli].

Averaged evoked potentials (AEP) to verbal (letters) and nonverbal (random shapes) stimuli exposed in the left and right visual fields were registered in healthy subjects with normal vision. Analysis of the later AEP latencies pointed to asymmetry in the temporal parameters of the interhemispheric interaction. The late AEP latency is shorter in the right hemisphere than in the left hemisphere. The difference is more pronounced in responses to nonverbal stimuli. The earlier development of the evoked potential in the right hemisphere (or the later one in the left hemisphere) accounts for the interhemispheric difference in the temporal parameters of the late AEP components. Comparison of the latency of the component P300 to verbal and nonverbal stimuli presented in the ipsilateral or the contralateral visual fields reveals a transfer of the results of the cortical processing of visual information in the course of interhemispheric interaction.

Adult↗
PubMed2003

New insights into the subjective perception of visual field defects.

Most glaucoma patients are unaware of their visual problem until a late stage of the disease, probably because their visual field defects are being filled-in by the brain. According to new insights into how our brain works, plasticity of the visual cortex allows the brain to reorganize after damage to the retina or the visual pathways. This plasticity involves activation of existing but normally ineffective synapses, in a highly organized system of long-range horizontal connections in the visual cortex. Normal cells in the cortex surrounding the lesion thus take control of the deprived ones. As a result, glaucomatous field defects are probably concealed in the colors and patterns of the surround, up to the point where visual input has decreased to such an extent that the brain is not able anymore to compose a plausible image.

Adult↗
PubMed1992

Effects of gravitoinertial force variations on optokinetic nystagmus and on perception of visual stimulus orientation.

Recordings of horizontal and vertical eye movement were obtained with subjects exposed to vertical, horizontal, and oblique optokinetic stimulation during parabolic flight. When the optokinetic stimulation was vertical, the upward slow phase eye velocity increased during transition from high force level to free-fall, and decreased during transition from free-fall to high force level. During optokinetic stimulation in the horizontal and oblique plane, the gravitoinertial forces of parabolic flight induced changes in the velocity of the vertical component of the eye movements, and, therefore, changes in the plane of the eye movements. Some subjects also perceived modifications in the apparent orientation of the visual motion. These findings are in agreement with previous observations on the presence of a vertical nystagmus induced by changes in plane vertical acceleration. They also suggest a close interaction of reflexive eye movements induced by graviceptor inputs and visual inputs for visual stabilization during variations of gravitoinertial force level.

Adult↗