PubMed Health⌕ Search

SEARCH · PubMed Health

Results for “Visual Perception”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,333 records · Page 74Linked to original sources
PubMed2001

Tickling the brain: studying visual sensation, perception and cognition by transcranial magnetic stimulation.

Transcranial magnetic stimulation (TMS) is a means of stimulating the brain from outside the skull with little, and occasionally no discomfort for the subject. A single TMS pulse, lasting less than 1 ms, can briefly disrupt the normal activity of a targeted region of the brain for tens of milliseconds, allowing the effects of disruption on specific perceptual and cognitive tasks to be measured behaviorally. Rapid, repeated pulses can disrupt activity for correspondingly longer periods. The reversibility of the effects make it possible to create 'virtual patients' who can be tested in the same way as actual patients with real brain damage in order to explore regional functional specialization. Although several aspects of TMS continue to be evaluated, such as its safety, the extent and localization of the effective region of induced electrical current, the importance of the waveform of the pulse, the configuration and positioning of the coil, its productivity has been firmly established in little more than 10 years of systematic use. Examples of the latter are given from investigations of the nature of visual phosphenes produced by TMS applied to different regions of the visual cortex in normal subjects and subjects with occipital or ocular damage in an attempt to reveal the role of visual cortex in visual awareness.

Brain↗
PubMed1980

Role of foveal and peripheral visual information in maintenance of postural equilibrium in man.

In a previous report (Amblard & Crémieux, 1976) we demonstrated that the maintenance of postural equilibrium, measured with the subject in Mann's stance on a foam rubber support, was significantly more difficult under stroboscopic rather than normal lighting conditions. The most plausible cause of the difficulty is the subject's loss of visual perception of movement as a result of the stroboscopic lighting. The present study was designed to look at this factor under normal lighting conditions. Also, the relative contributions of foveal and peripheral vision were assessed. During stance, the subjects (5 women and 6 men, aged from 25 to 55 yr.) viewed either a horizontal or a vertical rectangular grating. With horizontal lines, the visual perception of lateral movement is minimized. Lateral acceleration was measured at three anatomical levels: ankles, hips, and head. The horizontal stripe condition was significantly less effective than the vertical stripe one for maintenance of balance, both for measurements at the head level only and for values averaged from all three levels. Balance was significantly impaired with foveal vision alone compared to full vision or to peripheral vision alone, for measurements from each of the three levels. We conclude that the visual perception of movement is a very important factor in the maintenance of the equilibrium, peripheral vision playing the major role, and foveal vision only a supplementary one.

Adult↗
PubMed2003

Memory function in early hepatic encephalopathy.

BACKGROUND: Early hepatic encephalopathy (HE) is characterized by deficits in motor performance, visual perception, visuo-constructive abilities and attention. Whether defective memory is a feature of early HE is controversial. AIMS: To analyze memory function in patients with early HE. METHODS: Memory tests were applied to cirrhotic patients with grade 0 HE, minimal HE and grade I HE (n=45) and controls (n=52). The battery included short and long term memory tests requiring free recall or recognition. Minimal HE was diagnosed by assessing the psychometric hepatic encephalopathy score using the PSE-Syndrom-Test and by carrying out a neurological examination. Group differences of the test results were analyzed using analysis of covariance. RESULTS: HE 0 patients achieved test results similar to the controls in all but two tests. Patients with early HE (minimal and grade I HE) scored lower than the controls in all tests applied. A detailed analysis of test performance showed that the patients' deficits were in attention and visual perception, rather than memory. CONCLUSIONS: Patients with early HE score lower than controls in memory tasks predominantly because of deficits in attention and visual perception.

Adolescent↗
PubMed2004

Training-induced recovery of visual motion perception after extrastriate cortical damage in the adult cat.

Unilateral ibotenic acid lesions of the lateral suprasylvian (LS) cortex severely impair the ability of cats to integrate local motion signals (measured as direction range thresholds) and to extract motion signals from noise (measured as motion signal thresholds) in their contra-lesional visual hemifields. These deficits were found up to several months after the lesions and were limited to thresholds measured with random-dot stimuli, while contrast sensitivity for discriminating the direction of motion of sine-wave gratings remained unaffected. Our goal was to determine whether deficits of complex motion perception could recover and whether the recovery was spontaneous or required retraining. In each cat, a single location in the impaired visual hemifield was selected for visual retraining, which required the animals to discriminate motion direction using random-dot stimuli in which the range of dot directions was varied. Fifteen to 40 days of intensive retraining led to a gradual, complete recovery of motion integration. The recovery was stimulus specific since it did not transfer from direction range to motion signal thresholds, and it was largely restricted to the visual field locations retrained. Delaying the onset of retraining by several days to several months had no significant impact on the extent or rate of recovery. Once recovery was achieved, performance remained stable over a period of several months. These results suggest that recovery of complex visual motion perception after lesions of extrastriate visual cortex is an active process that requires extensive, stimulus- and retinotopically-specific visual retraining.

Animals↗
PubMedDate not supplied

Spectroscopic analysis of polymer-ceramic dental composites after accelerated aging.

PURPOSE: The aim of the study was to determine the color and surface chemistry changes of two fiber-reinforced composites and one "advanced composite" as a function of an accelerated aging process by light exposure and water spray. MATERIALS AND METHODS: Three composites (Artglass, Targis, and Conquest Sculpture) were assessed for color evaluation in three different shades and with XPS analysis. Four specimens of each shade were analyzed with the UV/VIS/NIR Spectrophotometer in reflectance to determine the baseline color for the CIE L*a*b* system. XPS spectra were obtained from three specimens for each material group. All sample disks were artificially aged in a weathering machine and exposed to water spray and a controlled irradiance xenon arc of 0.55 W/m2/nm measured at 340 nm for a total exposure time of 122 hours. L*a*b* and XPS analyses were repeated after the aging process for all specimens. RESULTS: Artglass had "just perceptible," changes, with deltaE* between 1.8 and 2.7, while Conquest Sculpture showed, in all shades, "visually perceptible" changes from deltaE* 5 to 9. Targis had just perceptible as well as visually perceptible changes after aging. All materials showed decreased Ba and Si on the aged samples, while carbon increased. CONCLUSION: All materials tested underwent a surface change after the aging process. Artglass and Targis changed minimally into whitish/reddish and yellowish appearances, respectively. An organic-rich surface was produced by a combination of washout of filler particles and rearrangement of polymer molecules through diffusion.

Barium↗
PubMed1981

The z-model -- a proposal for spatial and temporal modeling of visual threshold perception.

By considering only the modulation transfer functions of stationary, uniformly moved, and time modulated sinusoidal gratings it is possible to derive a simple model, the "z-model", for the spatio-temporal frequency behaviour of one-dimensional patterns. The transmission function of this model is a band pass function of a single coordinate z, which is a quadratic form of the spatial and temporal frequencies (rotational symmetry with respect to space and time). The model is determined by only three constants. Optionally a time phase which accounts for delay and phase distortion can be added. This model can also be derived from reaction time measurements for switched on sinusoidal gratings. With this model the response of a wide variety of spaito-temporal patterns have been calculated and compared with measured threshold data. For two-dimensional patterns orientational filtering has to be added to the model leading to a further parameter. This model predicts satisfactorily the threshold modulation for a great variety of arbitrary spaito-temporal patterns. However the absolute threshold value for aperiodic transient patterns differs slightly in direction of smaller sensitivity as compared with periodic stationary patterns. This suggests that the peak detection scheme usually used in threshold detection modeling should be replaced by an integrative mechanism.

Mathematics↗
PubMed2002

Neural correlates of visual-motion perception as object- or self-motion.

Both self-motion and objects moving in our visual field generate visual motion by displacing images on the retina. Resolving this ambiguity may seem effortless but large-field visual-motion stimuli can yield perceptual rivalry between the real percept of object-motion and the illusory percept of self-motion (vection). We used functional magnetic resonance imaging to record brain activity in human observers exposed to constant-velocity roll-motion. This stimulus induced responses in areas reaching from calcarine to parieto-occipital and to ventral and lateral temporo-occipital cortex and the anterior insula. During vection, early motion-sensitive visual areas and vestibular parieto-insular cortex deactivated, whereas higher-order parieto- and temporo-occipital areas known to respond to optical flow retained identical activity levels. Within this sustained response, these latter areas displayed transient activations in response to each perceptual switch as identified in event-related analyses. Our results thus show that these areas are responsive to the type of visual motion stimulus and highly sensitive to its perceptual bistability. The only region to be more active during perceived self-motion was in, or close to, the cerebellar nodulus. This activation may correspond to the gain increase of torsional optokinetic nystagmus during vection and/or to changes in sensory processing related to the rotational percept. In conclusion, we identified neural correlates of perceiving self-motion from vision alone, i.e., in the absence of confirmatory vestibular or proprioceptive input. These functional properties preserve the organism's ability to move accurately in its environment by relying on visual cues under conditions when the other spatial senses fail to provide such information.

Adult↗
PubMed1988

A selective impairment of motion perception following lesions of the middle temporal visual area (MT).

Physiological experiments indicate that the middle temporal visual area (MT) of primates plays a prominent role in the cortical analysis of visual motion. We investigated the role of MT in visual perception by examining the effect of chemical lesions of MT on psychophysical thresholds. We trained rhesus monkeys on psychophysical tasks that enabled us to assess their sensitivity to motion and to contrast. For motion psychophysics, we employed a dynamic random dot display that permitted us to vary the intensity of a motion signal in the midst of masking motion noise. We measured the threshold intensity for which the monkey could successfully complete a direction discrimination. In the contrast task, we measured the threshold contrast for which the monkeys could successfully discriminate the orientation of stationary gratings. Injections of ibotenic acid into MT caused striking elevations in motion thresholds, but had little or no effect on contrast thresholds. The results indicate that neural activity in MT contributes selectively to the perception of motion.

Animals↗
PubMed1977

ENG analysis of 150 cases of posterior fossa disease.

Analysis of the ENG records of 150 patients with well substantiated neurologic disease involving the structure of the posterior fossa revealed a high incidence of diminished optic fixation inhibition (DOFI) of caloric induced nystagmus. DOFI is defined as less than 50 percent decrease of speed of the slow phase when eyes are open and fixated. The whole group was subdivided into seven subgroups according to type and location of disease and the incidence of DOFI was found to vary significantly in the different subgroups. The highest incidence was found in the group of cerebellar mass lesions and in the group of degenerative diseases, most of which had cerebellar involvement. A review of the literature reveals work done by others showing that suppression of vestibular reflexes induced by high speed turning and also habituation of caloric induced nystagmus is dependent on optic fixation inhibition (opto-vestibular reflex.). It has been shown that the flocculonodular lobe of the cerebellum has an inhibitory effect on the effects of vestibular activity by both destructive and stimulating techniques. Afferent and efferent pathways between the cerebellum and primary vestibular reflex arcs have been extensively described. It has been shown that cerebellar nodulectomy severely interferes with acquisition of habituation and retention of previously acquired habituation. Since habituation and suprression of vestibular nystamus depend on optic fixation inhibition and equally on the cerebellum, it is reasonable to infer that the opto-vesitubular inhibitory reflex is mediated by the crebellum. The clinical data of the 150 cases were analyzed for correlation of DOFI and other cerebellar signs. This analysis gives a high incidence of correlation: 66 percent of the cases having DOFI also had other cerebellar signs, whereas only 19 percent of cases that did not have DOFI had other cerebellar signs. These data lend further support to the inference that the cerebellum is involved in mediation of the opto-vestibular inhibitory reflex. Under most circumstances visual perception of the environment during movement depends on an inter-reaction between the vestibulo-oculomotor and the opto-vestibular reflexes. Eye movements during head movements are entirely different with eyes closed than with eyes open and fixating. In fact, it can be shown that ocular fixation is far more efficient during head movements than it is during movement of an object with head stationary. It is probable that fixation in the former case is the result of vestibulo-oculomotor slow phase activity modified by the opto-vestibular inhibitory reflex and elimination of the fast phase. A defect in the latter should result in a sense of spatial insecurity during movement by virtue of altered visual perception. The opto-vestibular inhibitory reflex is a central descending reflex probably mediated through the cerebellum. It is easy to test during and ENG. It is essential to normal visual perception during head movement...

Brain Diseases↗
PubMed1996

Binocular visual surface perception.

Binocular disparity, the differential angular separation between pairs of image points in the two eyes, is the well-recognized basis for binocular distance perception. Without denying disparity's role in perceiving depth, we describe two perceptual phenomena, which indicate that a wider view of binocular vision is warranted. First, we show that disparity can play a critical role in two-dimensional perception by determining whether separate image fragments should be grouped as part of a single surface or segregated as parts of separate surfaces. Second, we show that stereoscopic vision is not limited to the registration and interpretation of binocular disparity but that it relies on half-occluded points, visible to one eye and not the other, to determine the layout and transparency of surfaces. Because these half-visible points are coded by neurons carrying eye-of-origin information, we suggest that the perception of these surface properties depends on neural activity available at visual cortical area V1.

Form Perception↗
PubMed1993

Vibrotactile support: initial effects on visual speech perception.

The study investigated the initial effects of the implementation of vibrotactile support on the individual's speech perception ability. Thirty-two subjects participated in the study; 16 with an acquired deafness and 16 with normal hearing. At a general level, the results indicated no immediate and direct improvement as a function of the implementation across all speech perception tests. However, when the subjects were divided into Skilled and Less Skilled groups, based on their performance in the visual condition of each test, it was found that the performance of the Skilled subjects deteriorated while that of the Less Skilled subjects improved when tactile information was provided in two conditions (word-discrimination and word-decoding conditions). It was concluded that tactile information interferes with Skilled subjects' automaticity of these functions. Furthermore, intercorrelations between discrimination and decoding tasks suggest that there are similarities between visually and tactilely supported speechreading in how they relate to sentence-based speechreading. Clinical implications of the results were discussed.

Adult↗
PubMed1981

Eye oscillations in strobe reared cats.

Cats reared from birth in stroboscopic illumination develop abnormal spontaneous eye oscillations of low amplitude. The present experiments were undertaken to define these eye movements as recorded in the dark, in stroboscopic light of various frequencies, after exposure to normal light and after attenuation of the vestibulo-ocular reflex (VOR) gain by optical reversal of vision. The interaction of spontaneous eye oscillations with voluntary saccadic eye movements, and optokinetic tracking (OKN), were also studied. Two cats, reared from birth to 18 months in 8 Hz strobe light, and one normally reared control animal, were used. Horizontal movement of the right eye was measured by the scleral eye coil method. The frequency content of eye movement records was determined by power spectral analysis. VOR gain was estimated in the dark, by rotating the animals sinusoidally at 1/8 Hz and 5 degrees/sec velocity amplitude. In the dark, both strobe reared cats had abnormal spontaneous eye oscillations at a frequency close to 8 Hz, with peak-to-peak amplitudes of 0.5--1.0 degrees. These abnormal eye movements did not interfere with, nor were they abolished by, normal oculomotor activity. The introduction of strobe light modified the spontaneous eye movements by entraining the oscillations at a given 'forcing' frequency, and by producing a number of harmonics or sub-harmonics. In one of the strobe reared animals, the effect of normal light was to reduce the characteristic 'dark' value of 9 Hz, to a new maintained 'light' value of 2.7 Hz. Adaptive attenuation of the VOR gain caused the abolition of regular spontaneous eye oscillations in the dark; nevertheless, transient oscillations to single strobe flashes could still be elicited in the VOR adapted condition. The results are interpreted as representing an organised attempt by the developing oculomotor system to attain the goal of stable visual perception in a new visual environment.

Animals↗
PubMed2002

The function of bursts of spikes during visual fixation in the awake primate lateral geniculate nucleus and primary visual cortex.

When images are stabilized on the retina, visual perception fades. During voluntary visual fixation, however, constantly occurring small eye movements, including microsaccades, prevent this fading. We previously showed that microsaccades generated bursty firing in the primary visual cortex (area V-1) in the presence of stationary stimuli. Here we examine the neural activity generated by microsaccades in the lateral geniculate nucleus (LGN), and in the area V-1 of the awake monkey, for various functionally relevant stimulus parameters. During visual fixation, microsaccades drove LGN neurons by moving their receptive fields across a stationary stimulus, offering a likely explanation of how microsaccades block fading during normal fixation. Bursts of spikes in the LGN and area V-1 were associated more closely than lone spikes with preceding microsaccades, suggesting that bursts are more reliable than are lone spikes as neural signals for visibility. In area V-1, microsaccade-generated activity, and the number of spikes per burst, was maximal when the bar stimulus centered over a receptive field matched the cell's optimal orientation. This suggested burst size as a neural code for stimuli optimality (and not solely stimuli visibility). As expected, burst size did not vary with stimulus orientation in the LGN. To address the effectiveness of microsaccades in generating neural activity, we compared activity correlated with microsaccades to activity correlated with flashing bars. Onset responses to flashes were about 7 times larger than the responses to the same stimulus moved across the cells' receptive fields by microsaccades, perhaps because of the relative abruptness of flashes.

Animals↗
PubMed2003

Environmental tilt illusion as the only symptom of a thalamic astrocytoma.

A 14-year-old girl experienced two episodes of environmental tilt illusion. During both episodes, which lasted less than 1 minute, she perceived all objects within view as rotated 45 degrees clockwise. There were no auras, accompanying symptoms, or sequelae. Neuro-ophthalmic examination findings were normal except for a right relative afferent pupil defect (RAPD). Imaging disclosed a cystic mass in the left posterior thalamus with compression of the brachium of the left superior colliculus. Stereotactic biopsy revealed a pilocytic astrocytoma. This is the first case documenting environmental tilt illusion as an isolated symptom of a thalamic lesion. Disruption of vestibular connections between the posterior thalamus and the posterior parietal cortex may be the cause of this visual perceptive disorder.

Adolescent↗
PubMed1996

Neuropsychological aspects of Marfan syndrome.

We evaluated the neuropsychological status of 13 adults patients with Marfan syndrome. All subjects were administered the same neuropsychological test battery that included nine measures covering a broad range of cognitive abilities such as attention and concentration, learning and memory, and verbal and non-verbal abilities. Compared to a control group of 13 healthy subjects matched for sex, age and verbal intelligence, Marfan patients only performed significantly worse on tests measuring sustained visual attention and visuoconstruction. Although these tests use visual material and depend on visual perception and processing, the visual acuity problems associated with the syndrome could not explain these differences, nor could the use of beta-blocking medication or the presence of joint hypermobility. The findings suggest that problems with sustained visual attention and visuoconstruction may be present in Marfan syndrome over and above visual acuity problems and other phenomena associated with the disease. Further research on the neuropsychological aspects of Marfan syndrome is needed, using larger patient groups and more adequate control groups such as non-affected siblings and matched controls with similar visual impairment.

Adolescent↗
PubMed2000

The effect of learned perceptual associations on visuomotor programming varies with kinematic demands.

The present set of experiments investigated the possibility that learned perceptual information can, under certain circumstances, be utilized by visuomotor programming. In Experiment 1 (N = 28), an association was established between the color and size of square wooden blocks (e.g., red = large; yellow = small, or vice-versa). In Experiment 2 (N = 28), an association was established between the shape and size of plastic objects (e.g., hexagon = large; circle = small, or vice-versa). It was expected that the learned associations would change the perceived size of two probe objects halfway in size between the large and small objects (the probe object matched by color or shape to the large group of objects would appear smaller than the probe object matched to the small group of objects as a result of within-group relative size comparisons). In both experiments, half of the participants grasped the target objects, and the other half estimated the size of the objects by opening their thumb and finger a matching amount. For Experiment 1, it was predicted that an influence of the learned association on the treatment of the probe objects would be seen in manual estimations and in grip scaling because the kinematics of the grasping movement were very similar across trials. As predicted, the learned association between size and color was as easily incorporated into visually guided grasping as it was into visual perceptions. In Experiment 2, it was predicted that an influence of the learned perceptual association would be seen only in manual estimations, and not in grip scaling, because the variability in target object shape from trial to trial would demand changes in precontact finger posture across trials. Despite the significant effect of the size-shape association on size estimations, no influence was seen in preparatory grip scaling, probably because varying shape increased the metrical demands on visuomotor programming from those in Experiment 1. Together, the results suggest that visuomotor programming can make use of learned size information under some, but not all, conditions.

Association Learning↗
PubMed2001

Visual marking and the perception of salience in visual search.

In the present study, the gap paradigm originally developed by Watson and Humphreys (1997) was used to investigate whether the process of visual marking can influence the perceptual salience of a target in visual search. Consistent with previous studies (Watson & Humphreys, 1997), the results showed that search was not affected by the presence of the preceding distractors when the target was relatively low in salience. This finding suggests that visual marking can increase the efficiency of visual search by decreasing the size of the search set. However, more important, the results also showed that search was affected by the presence of the preceding distractors when the target was relatively high in salience. This finding suggests that visual marking may be limited in its ability to increase the perceptual salience of the target. Together, the results of the present study suggest that the effectiveness of visual marking may vary as a function of search context.

Fixation, Ocular↗