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Results for “Evoked Potentials, Visual”

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At least 19 recordsLinked to original sources

PubMed1992

Colour, contrast and the visual evoked potential.

Visual evoked potentials exhibit interesting morphological changes when they are elicited by checkerboards of different spatial and chromatic contrast, counterphasing in the foveal and lower macula field. The characteristic, positive wave of the phase-reversal visual evoked potential, for example, is preceded by an increasingly prominent negative peak as luminance contrast progressively increases above 10% and, at isoluminance, the response to red and green checkerboards becomes a predominantly monophasic negative wave. To study the nature of the morphological change we synthesized these waveforms with a computer simulation consisting of Gaussian components. The amplitudes of positive and negative components were altered until the synthesized response was closely similar to the recorded data. These Gaussian components have response characteristics which are identified with those of magnocellular and parvocellular neurones.

Color Perception↗

PubMedDate not supplied

White noise effects on pattern-reversal visual evoked potentials.

Visual evoked potentials (VEPs) were recorded in 9 subjects from occipital and temporal leads. The stimulus was a checkerboard phase-reversed at the frequency of 1 Hz, binocularly viewed by the subject. VEPs were recorded during white noise stimulation (9 different levels of intensity) and without noise stimulation. P100 latency was not affected by the white noise stimulation, whereas the N75-P100 amplitude turned out to be affected by the simultaneous auditory stimulation with different patterns in relation to the site of the recording. The results are discussed in terms of general activation aroused by the white noise on visual information processing.

Acoustic Stimulation↗

PubMed1979

Contrast sensitivity of the human neonate measured by the visual evoked potential.

Visual evoked potentials (VEPs) were recorded from a total of 97 1- to 10-day-old infants, with phase-reversing sinusoidal grating stimuli. The grating contrast or spatial frequency for which 50% of infants gave a statistically significant VEP was taken as a measure of threshold. This procedure yielded an estimate of neonatal acuity of 0.85 cycles/degree and an optimal contrast threshold of 50%. VEPs from an older infant showed good agreement with behavioral measures of sensitivity on the same individual. Comparison of the neonatal VEP results with behavioral data from 5-week-old infants, suggests little change in visual performance over the first month of life.

Evoked Potentials↗

PubMed1991

Visual evoked potentials and visual prognosis following perinatal asphyxia.

Twenty-five children born at term with perinatal asphyxia were studied at age 2.5 to 4.5 years to evaluate visual function and to determine the prognostic value of postnatal assessments of visual outcome. Postnatal assessments included several visual evoked potentials and electroretinograms in the first week of life. Follow-up assessments included flash and pattern visual evoked potentials, visual evoked potential threshold measurements, and clinical eye examinations. Nineteen children had normal visual function, three were visually impaired, and three remained blind. A strong association was found between normal, abnormal, or absent visual evoked potentials in the early postnatal period and long-term visual outcome (P less than .0001). Other perinatal indicators of asphyxia, including neurologic status, Apgar scores, and arterial pH values, were poor predictors of visual outcome. The risk of visual impairment was limited to those survivors with neurodevelopmental deficits.

Asphyxia Neonatorum↗

PubMedDate not supplied

[Cortical and spinal evoked potentials in parkinsonism. Part I. Visual potentials evoked with a checkerboard pattern].

In 21 patients with parkinsonism and 20 healthy controls visual potentials evoked with checker pattern used as an alternating stimulus were studied. The left and right eye were examined separately recording visual cortical responses in the central occipital area. The analysed elements included the latency of the first highest positive wave P100 and the amplitude of the P100/N120 complex. Prolongation of the mean latency of P100 was found in patients with parkinsonism, but it was not significant statistically. No differences were found of the P100/N120 amplitude in the group with parkinsonism in relation to healthy controls. In the patients with parkinsonian syndrome (mainly of atherosclerotic origin) the mean latency of the visual potentials was longer and the amplitude was lower than in cases of Parkinson's disease. Attention is called to the high variability of the visual evoked potentials related to the clinical state and origin of the disease (parkinsonian syndrome and Parkinson's disease).

Aged↗

PubMed1991

Visual evoked potentials and visual acuity after transurethral resection of the prostate.

Changes in visual evoked potentials, visual acuity, blood ammonia levels and serum electrolytes (Na+ and K+) after transurethral resection of the prostate using glycine as an irrigating fluid performed under subarachnoid block were studied in 12 patients, in the pre-operative and immediate postoperative periods. Visual evoked potentials (p100 latency), recorded by shift of a checkerboard pattern, increased significantly from a pre-operative value of mean (SEM) 101.18 (1.63) msec in the right eye, and 102.5 (1.47) msec in the left eye to 108.91 (1.8) msec (p less than 0.01) and 108.08 (2.53) msec (p less than 0.01) respectively in the postoperative phase. There were no changes in visual acuity as assessed by a Snellen's chart, blood ammonia levels and serum electrolyte concentrations. The amount of glycine used intra-operatively for irrigation ranged from 3 to 31 litres.

Aged↗

PubMed1978

Clinical aspects of the visually evoked potential.

The visually evoked potential (VEP) was studied in normal and abnormal human subjects, and in Rhesus monkeys. A relatively simple protocol for clinical VEP testing is described. The monkeys showed similar but smaller VEP responses compared to those obtained from human subjects. Central, but not paracentral or peripheral photocoagulation retinal lesions were associated with VEP abnormalities. The second, smaller wave of the response complex to 10 Hz flash stimuli corresponds to the primary evoked response, and is closely related to visual acuity. It was possible to recognize visually this VEP waveform and subjectively interpret the record correctly in 85% of eyes with regard to visual acuity. Therefore, the clinician can "read" the VEP record in response to non-patterned flash stimuli. This test was further validated in a series of patients with opacities of the ocular media. VEP promises to become a procedure of diagnostic and prognostic value in ophthalmology.

Adolescent↗

PubMed1990

Comparison of the effects of Alzheimer's disease, normal aging and scopolamine on human transient visual evoked potentials.

Transient visual evoked potentials elicited by the onset of a patterned stimulus were recorded in patients suffering from Alzheimer's disease (AD), in healthy elderly controls and in healthy young individual. The latencies and amplitudes of both the components studied were adversely affected by normal aging and one of the components, CI, but not the other, CII, showed further deterioration in AD. These changes occurred over a range of stimulus contrast levels. The changes found in AD, but not those seen in normal aging, could be mimicked by administration of the cholinergic antagonist scopolamine to young volunteers.

Adult↗

PubMed1991

Refractive state, contrast sensitivity, and resolution in the freshwater turtle, Pseudemys scripta elegans, determined by tectal visual-evoked potentials.

Visual-evoked potentials (VEPs) were recorded from the surface of the optic tectum of the freshwater turtle, Pseudemys scripta elegans, in response to phase reversal of square-wave gratings of different spatial frequency and contrast. The refractive state of a group of 12 turtles in air was assessed from VEPs by placing trial lenses in front of the eye. The group mean refraction did not differ significantly from emmetropia, as compared to 4.8 diopters of hyperopia when refracted retinoscopically. The difference was explained by the retinoscopic reflex originating from the interface between vitreous humor and retina. Peak VEP amplitude was approximately linear with log grating contrast; extrapolation to zero VEP amplitude yielded contrast thresholds as low as 1%. High spatial-frequency cutoffs ranged from 4.4-9.9 cycle/deg in different animals, the highest values corresponding to the intercone spacing in the area centralis and to behavioral measures of acuity in a related species.

Adaptation, Ocular↗

PubMed1977

Clinical aspects of the visually evoked potential.

The visually evoked potential (VEP) was studied in normal and abnormal human subjects, and in Rhesus monkeys with central, paracentral, and peripheral photocoagulation lesions. A relatively simple protocol for clinical VEP testing is described. The monkeys showed similar VEP responses but these were smaller in amplitude than those obtained from human subjects. Central, but not paracentral or peripheral retinal lesions were associated with VEP abnormalities. For both monkey and human subjects, some variability of responses between normal and subjects was noted. Generally, there are differences in VEP responses obtained from the affected eye of abnormal subjects who had one eye which could serve as a control, as compared to responses from the normal eye. In these subjects as well as in subjects with two abnormal eyes, computer analysis of digitized VEP data from 10 Hz stimulus responses was performed. Fourier transformation analyses showed abnormalities which could be detected easily by evaluating the pattern of the amplitudes of the fundamental and first three harmonics. With this technique, it was possible to group correctly normal VEP's with eyes with normal visual acuity (greater than or equal to 20/30 or 0.67), and abnormal VEP's with eyes with poor visual acuity (less than 20/30 or 0.67) in 72% of cases. Analysis of the data obtained with 1 Hz and 10 Hz stimulation suggests that the components of the VEP related to visual acuity occur within the first 60-100 msec of the response, corresponding to the primary evoked response of Chiganek. The second, smaller wave of the response complex to 10 Hz flash stimuli corresponds to the primary evoked response, and is closely related to visual acuity. This was further supported in another series in which the digitized data was filtered around the stimulating frequency. It was possible to recognize visually this VEP waveform and subjectively interpret the record correctly in 85% of eyes with regard to visual acuity. Therefore, the clinician can "read" the VEP record in response to nonpatterned flash stimuli. This test was further validated in a series of patients with opacities of the ocular media, such as cataract, corneal scarring, and vitreous hemorrhage. VEP promises to become a procedure of diagnostic and prognostic value in ophthalmology.

Adolescent↗

PubMed1978

The effects of chiasmal compression on the pattern visual evoked potential.

The visual evoked potentials (VEPs) to pattern reversal stimulation (26' individual check subtense, 11 degrees total field) have been examined in 10 patients with proven chiasmal compression. Bipolar occipital-sylvian and occipital-parietal recordings were taken with electrodes placed according to the Modified Maudsley system. In all patients abnormalities, taking the form of latency delays or amplitude reductions, were found in response to monocular stimulation which, without exception, were maximal in the traces corresponding to the hemisphere contralateral to the field defect. In seven patients responses to a 13' pattern were also examined. In six of these the lateralisation of abnormality was enhanced. The discrepancy between these and previous findings is discussed, the importance of stimulus and recording parameters being stressed.

Adult↗

PubMed1991

A PC data-base with analytical applications for evoked potentials.

Visual Evoked Potentials (VEPs) are gaining ground in the research for diagnosis of neurological disorders and visual defects, as a non-invasive diagnostic tool. Yet, the methods used towards these goals are not universal and far from able to provide a common ground among researchers in collecting, analyzing and comparing their results. This paper is an attempt to close the gap. We have developed a PC data-base and a set of analysis programs with graphic capabilities, frequency analysis, as well as an objective way of describing the signals obtained during VEP experiments.

Database Management Systems↗

PubMed1992

Activation state of the cortex could be changed by reinforcement of low-amplitude visual evoked potentials in rabbits.

Visually evoked potentials (VEP) were recorded chronically from occipital cortex in awake rabbits. The VEP typically consisted of a triphasic response (positive deflection P60--negative deflection N205--positive deflection P450) that was followed by a late negative shift starting about 750 ms after the eliciting flash. After computation of a discriminating amplitude value (200 or 250 microV) the VEP were divided for comparison in high- and low-amplitude groups. Significant differences between these groups existed in amplitudes and latencies of the early VEP components. High-amplitude VEP were followed by larger late negative shifts and had significantly (P less than 0.01) earlier second positive deflections. In these high-amplitude cases, the EEG-baseline was more negative than in the low-amplitude VEP. In addition, we found a more synchronized background EEG in the low-amplitude VEP group. We conclude that different VEP amplitudes depended on different activation states of the cortex which could be changed by reinforcement.

Animals↗

PubMed1992

Use of 10-Hz flash visual evoked potentials in prediction of final visual acuity in diabetic eyes with vitreous hemorrhage.

In 44 diabetic eyes with vitreous hemorrhage, monocular steady-state visual evoked potentials were elicited through closed eyes by a 10-Hz flash. Visual evoked potentials were rated as normal or abnormal on the basis of amplitude and waveform. Abnormal visual evoked potentials were subdivided into mildly abnormal, markedly abnormal and nonrecordable categories. Patients with normal potentials were predicted to have visual acuities of 6/15 (20/50) or better. Patients with abnormal potentials were predicted to have visual acuities of 6/18 (20/60) or worse. Final visual acuities were the best visual acuities recorded in the 6 months after vitreous surgery (vitrectomy) or spontaneous clearing of the vitreous hemorrhage. The visual evoked potential categories and final acuities were compared with a 2 x 2 contingency table. The accuracy was 86%. The visual evoked potential categories and final acuities were associated at a statistically significant level.

Adult↗

PubMed1991

Pattern reversal visual evoked potentials in fencers.

Visual evoked potentials were recorded from occipital and temporal leads in the two cerebral hemispheres of eight fencers and eight control subjects. The stimulus was a checkerboard subtending a small (1 degree) or large (30 degrees) visual field. Significant differences in P60, N75 and P100 latency and amplitude were found between the two subject groups, especially during the processing of the large visual field. In fencers and left-handers shorter latencies were found for the large visual field condition, whereas right-handers showed an opposite trend. The results give further evidence of special patterns of visual processing in athletes, like fencers, in agreement with the literature.

Adolescent↗

PubMed1992

Visual evoked potential estimation of visual activity with a Laplacian derivation.

Visual acuity was estimated with visual evoked potentials in 13 healthy subjects. A Laplacian derivation of 5 electrodes was used to improve the signal-to-noise ratio and to enhance striate cortical activity selectively. In 6 subjects, the Laplacian derivation gave a more reliable estimation of visual acuity than did a single midline derivation. In the other 7 subjects, the quality of both estimations was comparable.

Electrodes↗

PubMed1975

[A simple method for the evaluation of differences in averaged evoked potentials as demonstrated of visual evoked potentials changed through hippocampal stimulation].

A simple method is presented which by subtraction of amplitudes of averaged evoked potentials (AEP) within regular time-intervals enables us to estimate differences in the course of the potential. First results demonstrate that all parts of the potential may be influenced and that most striking differences must not necessarily occur in the peak region of the AEP. The late negative complex of the AEP is likely to be a result of summation of some subcomponents, which may be altered differently. The method presented allows exact measurements of each of them.

Animals↗

PubMed1976

Pattern reversal visually evoked potentials in infants.

Visually evoked potentials (VEP) were recorded from infants two to six months of age using a checkerboard pattern reversal stimulus. By six months, infants produced the largest amplitude VEP to checks subtending visual angles of 7.5 or 15 minutes of arc, as do adults with 20/20 acuity. This finding indicates that by six months an infant's sensory capacity for a visual acuity of 20/20 is established.

Adult↗