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Results for “VISUAL PERCEPTION”

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At least 19 recordsLinked to original sources
PubMed2002

The relationship between visual perception and visual mental imagery: a reappraisal of the neuropsychological evidence.

Visual perception and visual mental imagery, the faculty whereby we can revisualise a visual item from memory, have often been regarded as cognitive functions subserved by common mechanisms. Thus, the leading cognitive model of visual mental imagery holds that visual perception and visual imagery share a number of mental operations, and rely upon common neural structures, including early visual cortices. In particular, a single visual buffer would be used "bottom-up" to display visual percepts and "top-down" to display internally generated images. The proposed neural substrate for this buffer consists of some cortical visual areas organised retinotopically, that is, the striate and extrastriate occipital areas. Empirical support for this model came from the report of brain-damaged patients showing an imagery deficit which parallels a perceptual impairment in the same cognitive domain. However, recent reports of patients showing double dissociations between perception and imagery abilities challenged the perception-imagery equivalence hypothesis from the functional point of view. From the anatomical point of view, the available evidence suggests that occipital damage is neither necessary nor sufficient to produce imagery deficits. On the other hand, extensive left temporal damage often accompanies imagery deficits for object form or colour. Thus, visual mental imagery abilities might require the integrity of brain areas related to vision, but at an higher level of integration than previously proposed.

Brain Damage, Chronic↗
PubMed2003

Hemispatial neglect: its effects on visual perception and visually guided grasping.

Hemispatial neglect is a neurological disorder characterized by a failure to represent information appearing in the hemispace contralateral to a brain lesion. In addition to the perceptual consequences of hemispatial neglect, several authors have reported that hemispatial neglect impairs visually guided movements. Others have reported that the extent of the impairment depends on the type of visually guided task. Finally, in some cases, neglect has been shown to impair visual perception without affecting visuomotor control in relation to the very same stimuli. While neglect patients may be able to successfully pick up an object they have difficulty perceiving in its entirety, it does not mean that they are picking up the object in the same way that a neurologically intact individual would. In the current study, patients with hemispatial neglect were presented with irregularly shaped objects, directly in front of them, that lacked clear symmetry and required an analysis of their entire contour in order to calculate stable grasp points. In a perceptual discrimination task, the neglect patients had difficulty distinguishing one object from another on the basis of their shape. In a grasping task, the neglect patients showed more variance in the position of their grasp on the target objects than their control subjects, with an overall shift to the relative right side of the presented objects. The perceptual and visuomotor deficits seen in patients with hemispatial neglect deficits may be the result of an inability to form good structural representations of the entire object for use in visual perception and visuomotor control.

Adult↗
PubMed1999

Covariation of activity in visual and prefrontal cortex associated with subjective visual perception.

Visual areas of the occipitotemporal pathway are thought to be essential for the conscious perception of objects, but the contribution of other cortical regions and the neural mechanisms leading to the awareness of a visual stimulus remain unclear. By using functional MRI in humans exposed to bistable viewing conditions, subjective visual perception was related to covariation of activity in multiple extrastriate ventral, parietal, and prefrontal cortical areas. The coordination of activity among these regions was not linked to external sensory or motor events; rather, it reflected internal changes in perception and varied in strength with the frequency of perceptual events, suggesting that functional interactions between visual and prefrontal cortex may contribute to conscious vision. Because similar cortical systems have been implicated in short-term memory and motor planning, the results also imply that related neural processes may underlie visual awareness and the organization of voluntary behavior contingent on sensory cues.

Adult↗
PubMed1991

Impairment of visual perception and visual short term memory scanning by transcranial magnetic stimulation of occipital cortex.

Transcranial magnetic stimulation (TMS) of occipital cortex was performed using a magneto-electric stimulator with a maximum output of 2 Tesla in 24 normal volunteers. The identification of trigrams, presented for 14 ms in horizontal or vertical arrays was significantly impaired when the visual stimulus preceded the occipital magnetic shock by 40 to 120 ms. The extent of impairment was related to TMS intensity. The latency of perceptual impairment was shorter for more intense TMS. No perceptual impairment was obtained by "sham" stimulation when TMS shocks were applied to the upper cervical region rather than the occipital region to rule out unspecific startle reactions affecting attention possibly responsible for the observed reduction in performance. Occipital TMS did not evoke systematic eye movements except for blink responses at latencies beyond 40 ms which were too late to interfere with visual input. Depending on the required serial order of readout of the trigram perceptual impairment was more marked for the second and third part of the trigram. This demonstrates that TMS interferes with the internal serial processing of visual input. To elucidate this further, TMS was used in a Sternberg short term visual memory scanning task. TMS caused a marked decrease in memory scanning rates whereas visual stimulus encoding and storage remained unaffected when tested at various TMS delays. TMS appears to be a useful method to study processes of visual perception and short term memory handling in the occipital cortex. Advantages over classical visual masking techniques especially regarding topical localisation are discussed.

Adult↗
PubMed1998

Relation of visual perception and visual-motor integration for clumsy children.

The purpose of this study was to examine the relationship between visual perception and visual-motor integration in 30 normal children compared to 30 clumsy children. Difficulty in visual perception, as assessed by the Test of Visual-Perceptual Skills, accounts for about half the variance in the clumsy children's performance in visual-motor integration, as assessed by the Developmental Test of Visual Motor Integration. In contrast, the correlation between scores on these tests for normal control children was low and not significant. These results suggest that visual perception and visual-motor integration may be separate functions in normally developing children. When considering clumsy children, however, these functions cannot be considered as two independent skills.

Child↗
PubMedDate not supplied

[Visual suppression during eye movements (a brief review on the problem of the mechanisms and their role in visual perception). II. Their role in visual perception].

A role of both geniculate and tecto-thalamic channels in visual suppression has been considered. Some evolutional aspects of visual suppression are discussed. Both a hypothetical scheme of the multilevel structural and functional organization of visual suppression and a neurophysiological basis of the integration of object's elements to whole image are suggested from the author's papers. The functional role of visual suppression has been not limited the visual stability only but has been considered wider as the important stage of the visual perception by which both the proprioceptive sensation of the object's spatial position arise and the preparation of the visual system for the survey and the processing object's pure visual information (brightness, color, form, etc.) go on. The match of both proprioceptive and visual sensations lead in finally to spatial-visual image of the object.

Animals↗
PubMed1978

Analysis of the Development Test of Visual Perception and the Motor-Free Visual Perception Test.

To identify children with learning problems and to compare test results, 28 kindergarten children were tested with the Developmental Test of Visual Perception and the Motor-free Visual Perception Test. The tests identified some children already noted by the teacher to exhibit characteristics of learning difficulty, failed to confirm the teacher's judment in others, and identified some children with previously undetected difficulties. Basic differences in the two tests are reported.

Child↗
PubMed1976

Delayed visual perception and delayed visual evoked potentials in the spinal form of multiple sclerosis and in retrobulbar neuritis.

(1) We have used both subjective and evoked potential tests to study cases of multiple sclerosis with no history of retrobulbar neuritis (spinal patients) and compared them with patients with multiple sclerosis who had experienced an attack of retrobulbar neuritis (RBN). We measured the delay of steady-state evoked potentials (EPs) elicited by flicker in the medium-frequency (13-25 c/s) range, by flicker in the high-frequency (30-60 c/s) range, and by pattern-reversal. We also measured the delay in seeing (perceiving) both an increase of light intensity and a decrease of light intensity. (2) The difference between perceptual delays for the left and right eyes (D s) was abnormal when retrobulbar neuritis affected only one eye (22/22 patients) even when acuity and discs were normal. It might be supposed that this perceptual test would be ineffective when both eyes were affected by retrobulbar neuritis. However, the value of D was abnormal in cases of bilateral retrobulbar neuritis (5/5 patients). Probably the principal reason is that demyelination was patchy in the patients studied. For this same reason the difference between perceptual delays for two sites in the visual field (T s) may also be abnormal. In principle the perceptual delay test can be effective even when both eyes are similarly delayed: abnormal values of T were recorded in 5 spinal patients for whom D was normal. (3) Perceptual delays were measured for an extended group of 19 patients suffering from spinal multiple sclerosis. Taking both D and T into account, the perceptual delay test alone picked out 12/19 spinal patients. The perceptual delay test has the advantage over EP tests that it can detect islands of demyelination as small as 3 degrees diameter, and the apparatus is cheap and straightforward to use. (4) Thirteen patients with spinal multiple sclerosis, including 6 with no ocular signs or symptoms, were examined with a battery of two evoked potential and one perceptual test. Ten patients had clearly abnormal visual delays. Results for the remaining 3 were equivocal. Delay tests can reveal visual damage in most patients who have not experienced an attack of RBN as well as in practically all patients who have experienced an attack. (5) Correlations between the results of the various tests were different in spinal patients and in multiple sclerosis patients who had experienced an attack of retrobulbar neuritis. Flicker EPs, pattern EPs and visual perception were all delayed in every RBN patient, whereas for spinal patients different tests could pick up different patients. Flicker EPs picked up 5/13 spinal patients, pattern EPs 6/13, perceptual delay (D) picked up 4/13 and perceptual delay T picked up 7/13. (6) Delay tests divided spinal multiple sclerosis patients into two fairly distinct groups. In one group pattern EPs and perception were delayed; in the other group flicker EPs were delayed. This grouping corresponded to a clinical distinction between long-standing patients with visual signs and recent patients without visual signs...

Adult↗
PubMed1994

The generic viewpoint assumption in a framework for visual perception.

A visual system makes assumptions in order to interpret visual data. The assumption of 'generic view' states that the observer is not in a special position relative to the scene. Researchers commonly use a binary decision of generic or accidental view to disqualify scene interpretations that assume accidental viewpoints. Here we show how to use the generic view assumption, and others like it, to quantify the likelihood of a view, adding a new term to the probability of a given image interpretation. The resulting framework better models the visual world and reduces the reliance on other prior assumptions. It may lead to computer vision algorithms of greater power and accuracy, or to better models of human vision. We show applications to the problems of inferring shape, surface reflectance properties, and motion from images.

Humans↗
PubMed1994

Intact visual imagery and impaired visual perception in a patient with visual agnosia.

Although it is now well accepted that visual mental imagery and visual perception share common underlying mechanisms, there are several reports in which they are dissociated. Evidence for the separability of these processes is provided by a patient, C.K., who has a profound visual object recognition deficit attributable to an impairment in grouping or segmenting visual images. Despite this perceptual deficit, C.K. was able to draw objects in considerable detail from memory, and his knowledge of the visual appearance of objects was preserved on a variety of mental imagery tasks. Together with previous cases, these findings confirm the double dissociation between object recognition and perception. Interestingly, C.K. could also recognize newly constructed objects in his internal imagery. To accommodate these results, we propose a model in which imagery and perception are strongly associated but are also functionally specialized.

Adult↗