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PubMed1992

Visual hallucinations after acute visual loss in a young child.

A four-year-old boy developed complex visual hallucinations consisting of moving figures, animals and other familiar objects one week after becoming totally blind. This followed a six-month period of gradually deteriorating vision and was precipitated by the debulking of a large optic nerve glioma. Although visual hallucinations are well recognised in adults with visual loss, secondary to both ophthalmological and neurological causes, they have not been reported in young children.

Blindness↗

PubMed1986

Evaluation of visual pathways in multiple sclerosis. I. After-images compared to visual evoked potentials.

The value of flight of colours (FOC), i.e. the succession of coloured afterimages following light stimulation, in diagnosing and following visual impairment was evaluated in 65 patients with multiple sclerosis (MS). Previous optimistic reports of the use of a pocket flashlight (PFL) method could not be confirmed, and are ascribed to inadequate methods. An electronic flashlight (EFL) method developed by us, proved a significantly better diagnostic tool (61% abnormal versus 18% abnormal). The EFL method was diagnostically less sensitive than pattern reversal visual evoked potential (PR-VEP) (80% abnormal). It did, however, add to PR-VEP by diagnosing 16 eyes with normal PR-VEP latency. The EFL method reflected the degree of degenerative changes in the central visual pathways and may be of value in following the disease activity in MS. The potential of the EFL method may be further improved by proposed modifications.

Afterimage↗

PubMed1998

The role of visual experience in the development of columns in cat visual cortex.

The role of experience in the development of the cerebral cortex has long been controversial. Patterned visual experience in the cat begins when the eyes open about a week after birth. Cortical maps for orientation and ocular dominance in the primary visual cortex of cats were found to be present by 2 weeks. Early pattern vision appeared unimportant because these cortical maps developed identically until nearly 3 weeks of age, whether or not the eyes were open. The naïve maps were powerfully dominated by the contralateral eye, and experience was needed for responses to the other eye to become strong, a process unlikely to be strictly Hebbian. With continued visual deprivation, responses to both eyes deteriorated, with a time course parallel to the well-known critical period of cortical plasticity. The basic structure of cortical maps is therefore innate, but experience is essential for specific features of these maps, as well as for maintaining the responsiveness and selectivity of cortical neurons.

Animals↗

PubMed2005

Does visual restitution training change absolute homonymous visual field defects? A fundus controlled study.

AIM: To examine whether visual restitution training (VRT) is able to change absolute homonymous field defect, assessed with fundus controlled microperimetry, in patients with hemianopia. METHODS: 17 patients with stable homonymous visual field defects before and after a 6 month VRT period were investigated with a specialised microperimetric method using a scanning laser ophthalmoscope (SLO). Fixation was controlled by SLO fundus monitoring. The size of the field defect was quantified by calculating the ratio of the number of absolute defects and the number of test points; the training effect E was defined as the difference between these two ratios before and after training. A shift of the entire vertical visual field border by 1 degrees would result in an E value of 0.14. RESULTS: The mean training effect of all right eyes was E = 0.025 (SD 0.052) and all left eyes E = 0.008 (SD 0.034). In one eye, a slight non-homonymous improvement along the horizontal meridian occurred. CONCLUSIONS: In one patient, a slight improvement along the horizontal meridian was found in one eye. In none of the patients was an explicit homonymous change of the absolute field defect border observed after training.

Adult↗

PubMed1989

Pattern electroretinography in patients with delayed pattern visual evoked potentials due to distal anterior visual pathway dysfunction.

Between March 1983 and January 1988 delayed pattern visual evoked potentials (PVEP) were observed in 67 patients with distal visual pathway dysfunction. Many of these patients had been referred for neurophysiological examination because of possible optic nerve dysfunction. These patients also had pattern electroretinography (PERG) performed which in all cases showed an abnormality of the main positive P50 component. None of these patients had an abnormality confined to the negative N95 component, the type of abnormality usually found if the PERG is abnormal in optic nerve disease. It is suggested that PERG recording should now be a routine adjunct to the PVEP in the assessment of anterior visual pathway dysfunction.

Adolescent↗

PubMed2006

On-line attentional selection from competing stimuli in opposite visual fields: effects on human visual cortex and control processes.

We used fMRI to investigate competition and on-line attentional selection between targets and distractors in opposite visual hemifields. Displays comprised a high-contrast square-wave grating, defined as target by its orientation, presented alone (unilateral) or with a similar distractor of orthogonal orientation in the opposite hemifield (bilateral displays). The target appeared unpredictably on the left or right, precluding anticipatory attention to one side. We found greater activation in target-contralateral superior occipital gyrus for unilateral than for bilateral displays, indicating suppression of the target's visual representation by distractor presence despite the competing distractor projecting to a different occipital hemisphere. Several frontal and parietal regions showed greater activation for bilateral than unilateral trials, suggesting involvement in on-line attentional selection. This was particularly pronounced for regions in bilateral intraparietal sulcus (IPS), which also showed greater functional coupling with occipital cortex specifically on bilateral trials that required selection plus some repetition-suppression effects when target side was repeated, but again only on bilateral trials requiring selection. Our results indicate that competition between visual stimuli in opposite hemifields can influence occipital cortex, and implicate IPS in resolution of this competition by selection.

Adult↗

PubMed1997

Dynamic model of visual recognition predicts neural response properties in the visual cortex.

The responses of visual cortical neurons during fixation tasks can be significantly modulated by stimuli from beyond the classical receptive field. Modulatory effects in neural responses have also been recently reported in a task where a monkey freely views a natural scene. In this article, we describe a hierarchical network model of visual recognition that explains these experimental observations by using a form of the extended Kalman filter as given by the minimum description length (MDL) principle. The model dynamically combines input-driven bottom-up signals with expectation-driven top-down signals to predict current recognition state. Synaptic weights in the model are adapted in a Hebbian manner according to a learning rule also derived from the MDL principle. The resulting prediction-learning scheme can be viewed as implementing a form of expectation-maximization (EM) algorithm. The architecture of the model posits an active computational role of the reciprocal connections between adjoining visual cortical areas in determining neural response properties. In particular, the model demonstrates the possible role of feedback from higher cortical areas in mediating neurophysiological effects due to stimuli from beyond the classical receptive field. Simulations of the model are provided that help explain the experimental observations regarding neural responses in both free viewing and fixation conditions.

Animals↗

PubMed2006

Sweep visual evoked potential grating acuity thresholds paradoxically improve in low-luminance conditions in children with cortical visual impairment.

PURPOSE: Children with cortical or cerebral visual impairment (CVI) often experience photophobia. In a study conducted to test whether this clinical phenomenon affects visual function, the sweep visual evoked potential (VEP) was used to evaluate cortical responses to grating stimuli in two luminance conditions: low and normal. METHODS: Twenty children (age range, 7 months to 4 years 10 months) with CVI and 17 age-matched control subjects were examined. Testing conditions consisted of a swept grating stimulus shown against a normal background luminance (109 cd/m2) and against a low-luminance background (20 cd/m2). Thresholds in these two luminance conditions were compared. Response amplitudes across the spatial frequency domain were also compared. RESULTS: Children with CVI paradoxically have improved grating acuity thresholds when the stimulus is shown using a low-luminance background (P=0.006). Response amplitudes are also increased in low luminance. In control children, luminance had no significant effect on response amplitudes or thresholds. CONCLUSIONS: Increased luminance causes a worsening of acuity thresholds in children with CVI. Response amplitudes are also diminished in normal luminance. This finding has implications for optimal viewing and learning conditions for children with CVI.

Blindness, Cortical↗

PubMedDate not supplied

Visual field indices for the nasal step: different calculation procedures and their correlation with the clinical classification of visual field defects.

We calculated normal values for the normal population of the Octopus G1 program (n = 836) and values for defective fields due to glaucoma and other diseases (n = 147) to determine indices for a nasal step in the interpretation of glaucomatous visual fields. We used different calculation procedures and correlated the results with the clinical evaluation of the visual field. All indices tested were able to detect and quantify nasal steps in defective fields and to separate them from normal findings (area under curve of Receiver Operating Characteristic curves 0.963 to 0.999). Indices calculated excluding sensitivity values of positions with the greatest distance from the horizontal meridian or weighting of sensitivity values according to their closeness to the horizontal meridian showed the best results (auc = 0.999). We can thus conclude that the calculation of an index for nasal step is a simple and effective adjunct to any interpretation system for visual fields.

Diagnosis, Computer-Assisted↗

PubMed2001

The influence of binocular visual deprivation on the development of visual-spatial attention.

This article examines the effects of visual input on the development of attention by comparing normal children to children, all more than 8 years old, who had been treated for bilateral congenital cataracts during infancy. In Experiment 1, patients pushed a button as soon as they detected a target that appeared 100, 400, or 800 msec after a central cue. The cue either validly cued the upcoming location or invalidly cued the wrong location. Patients (n = 16) performed normally at the 100 msec and 400 msec stimulus onset asynchrony (SOA). However, when the cue preceded the target by the 800 msec SOA, patients' reaction times were not affected by the validity of the cue, especially when deprivation had extended past 4 months of age. In Experiment 2, patients indicated which of two shapes appeared in the periphery 400 msec after a central cue, with those shapes surrounded by compatible or incompatible distractors. Patients (n = 15) differed from age-matched controls in (a) being slowed more by incompatible distractors on invalid trials, and (b) tending to show a larger than normal effect of the validity of the cue preceding targets in the upper visual field. Together, these findings suggest that the normal development of attention is influenced by early visual experience.

Adolescent↗

PubMed1995

Scrolled and rapid serial visual presentation texts are read at similar rates by the visually impaired.

Visually impaired observers read dynamically displayed text faster than text displayed in a normal page view. The goal of this study was to compare reading rates from two dynamic-presentation methods that have been proposed to facilitate reading from computer-based displays. Prior research has shown that both normally sighted and low-vision observers read text displayed to the same location, one word at a time [known as rapid serial visual presentation (RSVP)], faster than a page of text. A similar comparison with text scrolled continuously across the screen also shows faster reading for low-vision patients, but the relative change from a standard page view is substantially less (15% faster for the scroll display versus 80% faster for RSVP). In this study we directly compared these techniques. For those with normal vision, reading from the RSVP display was 1.3 times faster than reading from the scroll display [t(9) = 3.32, P = 0.009]. Although the difference in reading rates for the visually impaired group did not reach statistical significance, as a group they read 13% slower from the RSVP than from the scroll display.

Adult↗

PubMed1980

Role of structured visual field and visual reafference in accuracy of pointing movements.

In a visuo-motor pointing task, the accuracy of response was measured in situations which differ in the visual information available and the speed of execution of movement. Results confirm the role of visual cues received from self-produced movement in the control of the trajectory of a limb. Furthermore, it can be shown that a structured visual background is important in the treatment of sensory data concerned with the movement and in target localization. This effect is absent in rapidly executed pointing response.

Cues↗

PubMed1990

The effects of visual field size on hemispheric asymmetry of pattern reversal visual evoked potentials.

Visual evoked potentials (VEPs) were recorded in 17 females (10 right-handers and 7 left-handers) and 17 males (10 right-handers and 7 left-handers). The stimulus was a checkerboard pattern phase-reversed at a rate of 1 Hz, binocularly viewed by the subject. Two experimental conditions were tested: field size (the stimulus was enlarged from 1 to 30 degrees of visual angle) and occlusion (the central part of the stimulus was occluded by experimental scotomata of various size--from 1 to 16 degrees). Responses recorded at occipital and temporal leads of the two hemispheres were compared. The P100-N145 amplitude was larger at occipital than at temporal leads, it augmented increasing the size of the stimulus and diminished occluding progressively larger portions of the central field. Hemispheric asymmetries emerged with large field sizes and were not abolished by the presence of experimental scotomata. The hemispheric asymmetry was related to handedness (left-handers had larger amplitudes in the right hemisphere, right-handers in the left hemisphere), but not to sex. Hemispheric asymmetries in VEPs latency were not present. In comparison to males, females showed larger amplitudes and shorter latencies. These results point out that hemispheric asymmetries are found also for the processing of elementary visual stimuli like checkerboards, and they depend both on stimulus parameters (field size) and subject variables (handedness).

Adult↗

PubMed1998

Why are there eccentricity effects in visual search? Visual and attentional hypotheses.

In standard visual search experiments, observers search for a target item among distracting items. The locations of target items are generally random within the display and ignored as a factor in data analysis. Previous work has shown that targets presented near fixation are, in fact, found more efficiently than are targets presented at more peripheral locations. This paper proposes that the primary cause of this "eccentricity effect" (Carrasco, Evert, Chang, & Katz, 1995) is an attentional bias that allocates attention preferentially to central items. The first four experiments dealt with the possibility that visual, and not attentional, factors underlie the eccentricity effect. They showed that the eccentricity effect cannot be accounted for by the peripheral reduction in visual sensitivity, peripheral crowding, or cortical magnification. Experiment 5 tested the attention allocation model and also showed that RT x set size effects can be independent of eccentricity effects. Experiment 6 showed that the effective set size in a search task depends, in part, on the eccentricity of the target because observers search from fixation outward.

Adolescent↗

PubMedDate not supplied

[The evaluation of central visual fatigue in computer terminal users by visual evoked potentials].

Visual evoked potentials (VEPs) were recorded in a group of 20 operators aged 25-45 years during two sessions, before and after a 5-hour work at computer terminals, in order to assess visual system fatigue. The outcomes were referred to the norms established for the age-matched population of 30 subjects not exposed to professional contact with computers. Most of the examined operators suffered various complaints concerning not only the organ of vision but the central nervous system as well. VEPs were elicited by means of monocular stimulation using the reversal checkerboard pattern and recorded simultaneously from the scalp electrodes placed over the right and the left cerebral hemisphere. Only recording acquired after finishing of a working day demonstrated significant statistically abnormalities of the VEPs waveforms, which suggests their transient character. The changes mentioned above included prolongation of peaks P100 and N2 latency with concomitant attenuation of their amplitudes and a decrease of the correlation coefficient values for the recordings from the hemispheres. The preliminary findings presented in this paper seem to justify a conclusion that the non-invasive VEPs method can be considered as the objective indicator of the central visual fatigue induced by the workload at computer terminals.

Accommodation, Ocular↗

PubMed2001

[Usefulness of visual evoked potentials in visual acuity examination].

Determination of visual acuity by means of VEP is carried out usually by the calculation of the amplitude function response from the value of pattern single element. That purpose is achieved by carrying out several tests of transient VEP performed each time with stimulation, using the pattern of different size of individual elements or changing sweep the following sizes of the pattern single element. Concentration of a patient is necessary during any examination and therefore in clinical practise the second of the listed methods is preferred because of its short duration. Correlation factor of visual acuity, determined by means of VEP and Snellen tests, differs depending on the examined group from about 0.4 to 0.9 and it is the lowest in case of ophthalmological disorders connected with optic nerve. Fixing of VEP optimal parameters when evaluating the visual acuity remains an open matter.

Evoked Potentials, Visual↗

PubMed1992

Age and visual field differences in computing visual-spatial relations.

Age and brain hemispheric differences in visual-spatial performance were investigated using 2 versions of categorical and coordinate (metric) spatial relations tasks. Thirty-two young adults (M = 19.2 years) and 32 older adults (M = 68.8 years) participated. An overall age-related decrement in computing visual-spatial relations was obtained for lateralized presentations and when items were presented centrally. In contrast to some previous findings, there was no evidence to suggest differential aging of the right hemisphere in computing visual-spatial relations.

Adult↗

PubMedDate not supplied

[Asymmetry of the amplitude-temporal properties of visually-guided saccades in monkeys depending on the complexity of the spatial scheme of the visual stimulation].

The properties of visually-guided saccades were studied in three monkeys (Macaca rhesus). Traditional single-step GAP-OVERLAP temporal paradigms were used. Target stimuli were presented in accordance with two spatial stimulation schemes: (i) along the horizontal meridian (one-dimensional scheme) and (ii) within rectangle part of the visual field (two-dimensional scheme). We revealed asymmetric focuses of short- and long-latency saccades in the two-dimensional scheme of stimulation. The MANOVA revealed that the factor of dimensionality influenced saccadic latency to a greater extent than the factor of lateralization of stimuli (presenting in the right or left visual hemifields). Accuracy of saccades decreased with increasing in their eccentricity in both spatial schemes of stimulation.

Animals↗