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PubMedDate not supplied

Image quality and visual performance in the peripheral visual field following photorefractive keratectomy.

PURPOSE: A theoretical and experimental study was performed to assess the possible effects of photorefractive keratectomy (PRK) on retinal image quality and thresholds in the peripheral visual field. METHODS: Simple optical calculations suggest that although the quality of the retinal image at the fovea of the postoperative PRK eye may be comparable to that in an emmetropic eye, images in the peripheral field may be markedly worse, since peripheral ray bundles may pass partly through ablated and partly through unablated cornea, giving a simultaneous-vision bifocal effect. This would be expected to create an annular zone of confusion, so that light from two different directions in object space arrives at the same point on the retina. The position of this zone and its width are a function of the ablation zone geometry, the attempted correction, and the pupil diameter, but the major effects typically occur at field angles between about 40 degrees and 60 degrees. To explore this effect, Goldmann static quantitative perimetry was carried out along the 0 degree to 180 degrees meridian in six patients who had undergone myopic PRK and eight emmetropes. RESULTS: The results showed that thresholds for the PRK group at field angles from 40 degrees to 60 degrees were significantly higher than those of the emmetropic group (P < .01). These findings arise because the retinal images in the peripheral field of originally myopic, postoperative PRK patients are, in general, significantly degraded in comparison with those of emmetropes. CONCLUSION: Although under photopic conditions, retinal image quality close to the visual axis in patients who have been corrected by PRK is similar to that in emmetropes, it may be markedly worse in the peripheral visual field.

Adult↗

PubMed1990

Binocular summation in visually evoked responses and visual acuity.

Monocular and binocular transient visually evoked responses (VER) were recorded on 50 adult subjects using a pattern reversal stimulus (check size 5.5 min arc). The peak-to-trough amplitude of the VER wave was measured and compared with the subjective visual acuity (Landolt C). The binocular amplitude enhancement over the monocular amplitude was around 26% across the acuity range studied, whereas the mean binocular subjective visual acuity improvement was around 11.3%, but this varied from 1% at the high acuity end to 17% at the low acuity end of the range.

Adolescent↗

PubMedDate not supplied

[Amplitude-temporal parameters of the evoked potentials of the visual and motor areas in the visual perception and mental representation of an image].

Amplitude-temporal analysis was carried out of the EP components of the visual and motor areas elicited by neutral (diffuse light) and structural (checker board pattern) stimuli in different situations, defined by instruction. Interserial comparisons showed that at any instruction, the latency of the first EP component of the motor areas is reduced; as a result it can appear here simultaneously with the EP of the visual areas. At the instruction involving the subject in the process of active change of perception, activation of the right hemisphere, including the motor area, is manifest by EP parameters, while the right occipital area is activated in response to the structural stimulus, and the left one--in response to the neutral stimulus. At complication of the stimulus or instruction, the period is prolonged when the latency of EP components of the motor area is shorter than the latency of the isopolar components of the visual area--from 120 to 150 ms in response to the neutral stimuli and the neutral with their counting; from 90 to 150 ms in response to the structural stimuli; from 80 to 210 ms in response to the neutral stimuli with mental representation of the structural one.

Adolescent↗

PubMed1988

[Evaluation of the central visual field by the Friedmann Mark I analyzer and color vision in 85 patients with multiple sclerosis. Correlation with visual evoked potentials in 50 cases].

Analysis of the visual field using Friedmann's analyser Mark I and color study in 85 multiple sclerosis patients. Static perimetry of the central visual field and test batteries (Ishihara plates, 15 Hue Standard, 15 Hue of Lanthony) for acquired color vision defects were performed in 85 multiple sclerosis patients (61 definite, 12 probable, 12 possible cases). Results in patients were compared to data obtained in 53 control subjects matched for age. 64% of the 85 patients and 52% of 48 patients with no history of optic nevritis showed visual field abnormalities and/or color vision defects. Comparison with VEP was available in 50 patients. While 10 patients had abnormal VEP and normal static perimetry and coloration tests, 5 patients had the reverse findings.

Adolescent↗

PubMed1978

Visual discrimination in cats: physical parameters of stimuli and effects of visual cortex ablation.

Correct reaction probability and minimum exposition time in visual form discrimination task were studied under the conditions of varying pattern parameters. The minimal exposition time depended on pattern contrast and pattern background ratio and on the number of screen position variations. However, it remained unchanged when the background luminance and pattern size varied within the suprathreshold range. The deficits of discrimination in the minimal exposition time conditions in animals with damages to the striate and parastriate cortical regions suggest that the visual cortex participates in the analysis of shortterm stimuli and is responsible for prolongation of the visual trace within the time necessary for object discrimination.

Animals↗

PubMed1998

Visual attention in cirrhotic patients: a study on covert visual attention orienting.

Attentional dysfunction, which influences overall cognitive productivity, is not well characterized in cirrhotic patients. The aim of this study was to clarify the features of covert visual attention orienting in cirrhotics without overt hepatic encephalopathy. One hundred consecutive cirrhotic patients and 40 controls were enrolled. Visual covert attention orienting was assessed by the Posner test, which evaluates the effect of a cue on visual reaction times. Patients were characterized by the number connection test (NCT) and electroencephalographic (EEG) spectral analysis. The severity of liver disease was graded using standard laboratory parameters and the Child-Pugh's classification. Fifty-five psychometric and EEG evaluations were performed in the follow-up of 17 patients to assess the relationship between the variations of psychometric and neurophysiological findings. NCT and quantified-EEG parameters (altered in 19% and 40% of cirrhotic patients, respectively) were linked to each other and to the severity of liver disease. The Posner test showed a delay of visual reaction times in class B-C cirrhotic patients. Reaction times were correlated with ammonia and EEG parameters. The effect of the cue was higher in cirrhotic patients than in controls, particularly in the invalid position. This study suggests that cirrhotic patients have a reduced activity rate and reduced capacity to disengage attention previously focused on a cue. Such alterations are linked to NCT and EEG findings.

Adult↗

PubMed1976

[The alteration of visual evoked potentials in light- and dark-adapted states as a measure for the visual field sensibility at elevated intraocular pressure (author's transl)].

On 12 voluntary subjects with normal visual function the intraocular pressure was increased dependent on the ophthalmic blood pressure Pm = (Ps + Pd)/2. The visual field was tested under this pressure load by means of a subjective method and the visual evoked potential was derived under light and dark adaptation. Comparison of subjective and objective investigations demonstrated that the different visual evoked potentials which are dependent on the state of adaptation are a measure for the pressure sensibility of prelaminar optic nerve fibers.

Adaptation, Ocular↗

PubMed1992

Effects of cholinergic and non-cholinergic drugs on visual discrimination and delayed visual discrimination performance in rats.

The effects of several centrally active drugs were investigated using two visual discrimination tasks: a two-lever food-rewarded conditional brightness discrimination, and a similar conditional brightness discrimination where a delay was introduced between the disappearance of the stimulus and the opportunity to respond on the levers for food. The substances tested (amphetamine, scopolamine, methylscopolamine, physostigmine, diazepam and beta-carboline benzodiazepine receptor antagonist, ZK 93426), all produced differing profiles of action on the performance parameters recorded. In the simple conditional visual discrimination, amphetamine increased omissions without significant effects on accuracy or response latency. Physostigmine enhanced response latencies and failures to respond without significant effects on accuracy. ZK 93426 had no consistent effects on accuracy although at higher doses, some increase in response latency was seen in the delayed responding version of the visual discrimination task. Diazepam had negative effects on all parameters in both discrimination procedures. Scopolamine disrupted responding, but not accuracy in the simple discrimination, whereas accuracy was reduced in a dose, but not delay dependent manner in the delayed discrimination. A similar effect to that observed with scopolamine was observed following methylscopolamine in the delayed discrimination procedure. In the simple visual discrimination small increases in accuracy were recorded, accompanied by increased response latencies.

Animals↗

PubMed1994

Effects of flunitrazepam on responses to lateralized visual stimuli: evidence for cerebral asymmetry of execution of manual movements to targets in contralateral and ipsilateral visual space.

In order to examine the effects of benzodiazepines on response execution by the left and right hemisphere, flunitrazepam (1 mg) or placebo was administered to healthy, right handed volunteers in two separate experiments. In experiment 1, drug was administered daily during a treatment period of 8 days, and subjects were instructed to fixate vision centrally and to execute laterally directed manual responses corresponding to the position of visual stimuli presented in either the right or left hemifield. Experiment 2 was performed with a single dose and cross-over design, and subjects responded to the laterally presented visual stimuli by key press of a centrally positioned response device, i. e. neither detection of position of the stimulus in space nor response selection was required before initiation of the response. In experiment 1, intake of flunitrazepam generally increased reaction time more during response execution by the left as compared to the right hemisphere, and the most pronounced effect was observed on responses with the right hand, directed across the body axis, to visual stimuli presented in the left visual field. In contrast to these observations, in experiment 2, flunitrazepam impaired responses with the right and left hand practically to the same extent. Together, the results indicate that benzodiazepines may affect manual responses executed by left and right hemisphere differently, and that this asymmetry may be related to a stimulus-response compatibility effect in tasks that require response selection.

Adult↗

PubMed2003

Cross-modal congruency and visual capture in a visual elevation-discrimination task.

Participants in this experiment were required to discriminate the elevation of visual target stimuli (upper versus lower) presented to either side of fixation, while simultaneously trying to ignore vibrotactile distracters presented independently from the index finger (up) or thumb (lower) of either hand. The participants' hands were occluded from view under an opaque screen while the visual targets were situated directly over the hands above the screen. Participants responded more slowly on this elevation-discrimination task when the vibrotactile distracters were incongruent with the elevation of the visual targets (i.e. a "lower" vibration to the thumb during the presentation of an upper visual target) than when they were congruent (presumably due to the competing responses primed by the target and distracter stimuli on incongruent trials). The vibrotactile distracters had less of an effect on performance when a pair of rubber hands was placed on top of the screen, "holding" the target lights, in a posture consistent with that of the participant. Those participants who agreed most strongly with a statement about feeling the vibrations at the location of the rubber hands in a post-congruency task questionnaire evidenced the greatest reduction in the magnitude of the cross-modal congruency effect.

Adult↗

PubMed1974

Vision at work visual defect and the visual demand of tasks.

Several related studies of visual tasks in a large communications undertaking have revealed much defect of vision and many sources of visual fatigue and inefficiency. The nature of visual fatigue is described briefly and a programme to facilitate visual tasks is outlined.

Journal Article↗

PubMed2006

Audio-visual biofeedback for respiratory-gated radiotherapy: impact of audio instruction and audio-visual biofeedback on respiratory-gated radiotherapy.

PURPOSE: Respiratory gating is a commercially available technology for reducing the deleterious effects of motion during imaging and treatment. The efficacy of gating is dependent on the reproducibility within and between respiratory cycles during imaging and treatment. The aim of this study was to determine whether audio-visual biofeedback can improve respiratory reproducibility by decreasing residual motion and therefore increasing the accuracy of gated radiotherapy. METHODS AND MATERIALS: A total of 331 respiratory traces were collected from 24 lung cancer patients. The protocol consisted of five breathing training sessions spaced about a week apart. Within each session the patients initially breathed without any instruction (free breathing), with audio instructions and with audio-visual biofeedback. Residual motion was quantified by the standard deviation of the respiratory signal within the gating window. RESULTS: Audio-visual biofeedback significantly reduced residual motion compared with free breathing and audio instruction. Displacement-based gating has lower residual motion than phase-based gating. Little reduction in residual motion was found for duty cycles less than 30%; for duty cycles above 50% there was a sharp increase in residual motion. CONCLUSIONS: The efficiency and reproducibility of gating can be improved by: incorporating audio-visual biofeedback, using a 30-50% duty cycle, gating during exhalation, and using displacement-based gating.

Adult↗

PubMed1999

Modification of saccade-contingent visual mislocalization by the presence of a visual frame of reference.

The effect of three kinds of background visual field on saccade-contingent mislocalization was compared so as to explore the critical factor responsible for visual stability during saccadic eye movements. A large mislocalization observed in the dark (DARK condition) was substantially decreased when a faint frame pattern was projected against the dark background (FRAME condition) as well as when the background screen being illuminated (LIGHT condition). Large mislocalization in the DARK condition occurred both when targets were presented just before the beginning of a saccade and when they were presented at the end of the saccade, irrespective of the position at which the target was presented. In contrast, in the FRAME and LIGHT conditions, mislocalization appeared to depend on target position. Mislocalization of targets presented immediately before the saccade onset diminished when they were presented near the saccade goal, whereas mislocalization of targets presented at the end of the saccade diminished when they were presented near the original fixation point. It was concluded from these results that the subject made use of the frame and the illuminated background as a visual cue for exocentric localization of the target, and that a saccade-contingent shift of visual attention was involved in producing the selective diminution of mislocalization observed in FRAME and LIGHT conditions.

Fixation, Ocular↗

PubMed2002

Visual motion stimulation, but not visually induced perception of self-motion, biases the perceived direction of verticality.

Large-field torsional optokinetic stimulation is known to affect the perceived direction of gravity with verticality judgements deviating towards the direction of visual stimulus rotation. The present study aimed to replicate this effect and to examine it further by subjecting participants to optokinetic stimulation in roll, resulting in spontaneous alternations between the perception of object-motion and that of contradirectional self-motion (vection), as reported by the subjects. Simultaneously, subjects were oscillated laterally in a flight simulator and indicated their perception of postural verticality. Results confirmed that rotation of the visual environment in the frontal plane biases the perceived orientation of gravity towards the direction of visual stimulus motion. However, no differential effect of perceptual state on postural verticality was obtained when contrasting verticality judgements made during the perception of object-motion with those obtained during reported self-motion perception. This finding is likely to reflect a functional segregation of central nervous visual-vestibular subsystems that process the perception of self-tilt and that of self-rotation to some degree independently.

Adult↗

PubMed2003

Visual marking: using time in visual selection.

Given human capacity limitations, to behave adaptively we need to prioritise the order of visual processing to ensure that the most relevant information is available to control action. One way to do this is to prioritise processing at a particular location in space. However, there are many situations where this strategy is not possible and recent studies have shown that, in such circumstances, observers can use time as well as space to prioritise selection. We propose that selection by time can be influenced by a process of visual marking, involving an active bias applied in parallel against old items in the field. Here we describe the properties of visual marking in relation to other mechanisms of visual selection.

Journal Article↗

PubMed2003

Olfactory and/or visual cues for spatial navigation through ontogeny: olfactory cues enable the use of visual cues.

This study analyzed the spatial memory capacities of rats in darkness with visual and/or olfactory cues through ontogeny. Tests were conducted with the homing board, where rats had to find the correct escape hole. Four age groups (24 days, 48 days, 3-6 months, and 12 months) were trained in 3 conditions: (a) 3 identical light cues; (b) 5 different olfactory cues; and (c) both types of cues, followed by removal of the olfactory cues. Results indicate that immature rats first take into account olfactory information but are unable to orient with only the help of discrete visual cues. Olfaction enables the use of visual information by 48-day-old rats. Visual information predominantly supports spatial cognition in adult and 12-month-old rats. Results point out cooperation between vision and olfaction for place navigation during ontogeny in rats.

Age Factors↗

PubMed1996

Meningeal hyperperfusion visualized by MRI in a patient with visual hallucinations and migraine.

A 41-year-old patient with a history of migraine but with no history of seizures had intermittent prolonged and variable complex visual hallucinations and illusions lasting 9 days, accompanied by unilateral headache. Electroencephalography during these visual symptoms revealed occipital epileptic discharges. Distinction between focal migrainous attacks and ictal phenomena was difficult. Magnetic resonance imaging showed a lesion in the right visual cortex probably related to low perfusion and hyperemia of meningeal vessels, representing the rarely described transient MRI changes associated with migraine. Continued treatment with antiepileptic drugs and calcium-channel blocking agents completely resolved the headache and visual symptoms, while minor EEG changes persisted. After discontinuation of treatment, a second attack occurred with a similar and reversible pattern on EEG.

Adult↗

PubMed1995

Visual Displays and Visual Perception in Minimal Access Surgery.

Visual perceptual processing underlies safe execution of endoscopic surgery. This review deals with the limitations of the present visual display technology used in endoscopic surgery with reference to the normal direct stereoscopic vision and pints to the research and development needed in this important technological and psychomotor aspect of endoscopic surgery. Eyeball movements (saccadic and smooth pursuit), visual cues (stereoscopic and monoscopic), accommodation, individual visual attributes, and the display technology itself are all important. Monocular depth cues are degraded by the cureent display systems, and technological advances in this are will improve perceptual processing and reduce both fatigue and human error during endoscopic interventions. Depth perception can be improved by alternative techniques to three-dimensional imaging such as the VISTRAL system and the Suspended Image System based on projection of image by parabolic mirrors and advanced beam spitter technology.

Journal Article↗