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Results for “Evoked Potentials, Visual”

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At least 793 records · Page 44Linked to original sources
PubMed1982

Visual evoked potential diagnosis of field defects in patients with chiasmatic and retrochiasmatic lesions.

Visual evoked potentials were studied in 18 patients with visual field defects assessed by perimetry. Stimuli were reversing checkerboard and grating patterns of 13', full field and hemifield. Two reversal rates were studied: 1 Hz (transient VEPs) and 8 Hz (steady-state VEPs). Using monocular hemifield stimulation the "paradoxical lateralisation" of transient but not of steady-state VEPs was confirmed. Diagnostic yield was 90% using transient, but less than 20% using steady-state stimuli.

Brain↗
PubMed1981

Comparison of two types of visual evoked potentials: pattern reversal and eye movement (lambda).

The averaged visual evoked potentials of 10 normal subjects were studied either with eyes fixated while a chequerboard reversed or when they moved their eyes across the stationary chequerboard. The resulting occipital potentials averaged using a computer of averaged transients were discrepant for the 2 conditions. The latency of the P100 component was similar allowing for the difference in latency in the computer triggering system but the resulting wave forms were different. There was a prominent negative component preceding the P100 in the lambda response and a prominent negative component following the P100 for the pattern shift. The possible reasons for the findings are discussed.

Adult↗
PubMed1998

Investigation of multifocal visual evoked potential in anisometropic and esotropic amblyopes.

PURPOSE: To investigate the variation of visual evoked potential (VEP) function at different eccentricities of the visual field in esotropic amblyopes and anisometropic amblyopes. METHODS: Data from 5 esotropic amblyopic eyes, 6 anisometropic amblyopic eyes, and 45 control eyes were analyzed. A VERIS system was used to generate a stimulus matrix containing 61 hexagons on a computer monitor. Each hexagon of the display contained a number of small black and white hexagonal patches that reversed in polarity during stimulation according to a pseudorandom binary m-sequence. The VERIS system extracted the local responses by cross-correlating the input and output signals. The latencies and amplitudes of the responses from the central 8.6 degrees of arc in the visual field were analyzed. RESULTS: In esotropic amblyopia, the multifocal VEP latency is prolonged, and the amplitude is reduced in the central region of the visual field. The mean amplitude is significantly smaller, and the mean latency is significantly longer in the temporal visual field than in the nasal visual field. In anisometropic amblyopia, latencies are markedly prolonged, and the amplitudes of multifocal VEP are attenuated in the central region of the visual field, and these effects are lessened in the periphery. CONCLUSIONS: The results are in agreement with psychophysical studies reporting a greater foveal deficit in amblyopia and a greater visual loss in the temporal field than in the nasal field in esotropic amblyopia.

Adolescent↗
PubMed1999

Visual evoked potential (VEP) measured by simultaneous 64-channel EEG and 3T fMRI.

We present the first simultaneous measurements of evoked potentials (EPs) and fMRI hemodynamic responses to visual stimulation. Visual evoked potentials (VEPs) were recorded both inside and outside the static 3T magnetic field, and during fMRI examination. We designed, constructed, and tested a non-magnetic 64-channel EEG recording cap. By using a large number of EEG channels it is possible to design a spatial filter capable of removing the artifact noise present when recording EEG/EPs within a strong magnetic field. We show that the designed spatial filter is capable of recovering the ballistocardiogram-contaminated original EEG signal. Isopotential plots of the electrode array recordings at the peak of the VEP response (approximately 100ms) correspond well with simultaneous fMRI observed activated areas of primary and secondary visual cortices.

Artifacts↗
PubMed2000

Functional bitemporal quadrantopia and the multifocal visual evoked potential.

Cases of functional bitemporal hemianopia rarely have been reported. The authors describe the case of a patient with dense bilateral inferotemporal quadrantic field defects on a functional basis that was confirmed by multifocal visual evoked potential. The normality of the multifocal visual evoked potential provides evidence of the functional basis of the field defects.

Adult↗
PubMedDate not supplied

Pattern visual evoked potentials represent an early index for the evolution of optic chiasma syndrome of tumoral etiology.

The use of visual evoked potentials in the detection of optic chiasma syndrome of tumoral etiology has been controversial in the literature. In our study the pattern visual evoked potentials (PVEP) were recorded in 22 healthy free volunteers and in 32 patients with optic chiasma syndrome (OCS) produced by tumors of the hypothalamic-pituitary area, mainly pituitary adenomas with suprasellar extension, proved by CT scan. The PVEP were recorded bilateral after monocular photic stimulation of each atropinized eye, in parallel with a complete ophthalmologic exam consisting in visual field, visual acuity and optic fundus. The main results showed that the latencies P100 recorded bilateral were correlated (p < 0.01) with the types of visual field deficiency, in each hemifield. The changes in P100 latency are more sensitive than the evolution of visual field deficiency by campimetry. The same correlation was observed between the "W" form of P wave with the visual field defect. A significant decrease (p < 0.05) of the amplitude of P100 was observed only if the visual acuity was less than 1/2. It is suggested that the PVEP is a reliable index of diagnosis and evolution for the optic chiasma syndrome aside the usual ophthalmologic exam.

Adolescent↗
PubMed1992

[Pattern visual evoked potential and pattern electroretinogram in patients with retinitis pigmentosa].

The pattern visual evoked potential (PVEP) was analyzed in 22 patients (42 affected eyes) with retinitis pigmentosa, and in 9 cases the pattern electroretinogram (PERG) was simultaneously examined to find that (1) PVEP could be recorded in 31 (73.8%) and was normal in 17 (40.5%) of the affected eyes; (2) PVEP was related to visual acuity, being abnormal in all the eyes with visual acuity below 0.2, but normal in most eyes with visual acuity over 0.2; (3) the changes in PERG and PVEP were mostly consistent; and (4) PVEP and PERG might well be utilized for the morbid evaluation of central retina in patients with retinitis pigmentosa.

Adolescent↗
PubMed1989

Visual field versus visual evoked potentials in maculopathies and optic neuropathies.

Both kinetic and static visual fields and visual evoked potential (VEP) were tested in 52 patients with maculopathies and in 29 patients with axial optic neuropathies. The results of central static quantitative perimetry and pattern reversal VEP were compared. The amplitude and latency of P100 were correlated with the total loss of central visual threshold in maculopathies. While the P100 latency was not correlated with the total loss of the central visual threshold in neuropathies, their latency delay was more obvious than that in maculopathies. We arrived at the conclusion that both VEP and visual field are necessary for assessing function loss and reflecting different pathological mechanisms in macular and optic nerve diseases.

Adolescent↗
PubMedDate not supplied

Age-related changes in pattern visual evoked potentials: differential effects of luminance, contrast and check size.

We recorded visual evoked potentials (VEPs) to checkerboard pattern-reversal stimulation in 109 normal subjects (51 males and 59 females; aged 19-84 years) in order to study the aging effect on the multiple channels of the visual system in humans. Transient VEPs to 3 check sizes (15', 30' and 50') were obtained by monocular stimulation. Two test conditions were employed: (1) a high luminance (180 cd/m2) and a low luminance (11 cd/m2) both with a fixed contrast (90%), and (2) a high contrast (85%) and a low contrast (10%) both at a fixed luminance (57 cd/m2). The major features of our results included: (1) the presence of a curvilinear relationship between P100 latency and age for all conditions, while the P100 amplitude did not show any such aging effect, (2) the age-latency function was similar between the two luminance conditions, while it was different between the two contrast conditions, and (3) the differential age effect on the P100 latency caused by changes in contrast depended on the check size. These results suggest that age-related changes in the human visual system are not uniform, but rather are different in the specific functional subdivisions. It is thus hypothesized that aging may differentially influence the separate channels of the human visual system.

Adult↗
PubMed2002

The effect of disparity change on binocular visual evoked potential parameters elicited by convergent dynamic random-dot stereogram stimuli in humans.

Binocular visual evoked potentials (VEP) were recorded from the left and right occipital cortices of right-handed subjects in response to six different levels of convergent disparity using the stimulus of dynamic random-dot stereogram (DRDS). The VEP obtained consisted of a negative (N1) and a positive peak (P1) within intervals of 200-400 ms and 400-600 ms, respectively. The latency of the N1 wave decreased with disparity and the amplitude of the same wave displayed a concave-down curve. The tuning-curve of N1 showed that the cortical focus, which produced this wave, was sensitive to mid-disparities. The fact that the tuning-curve of the right hemisphere was sharper than that of the left implied that the right hemisphere was more disparity-selective. In contrast to N1, the change of the latency and the amplitude of the P1 with disparity consisted of concave-up curves. The inverse correlation between the amplitudes of N1 and P1 with disparity made me think that the activation of the neuron population, which gave rise to N1 wave, changed the synchronization level of the cortical focus responsible for the P1 wave. If we keep in mind that the perception of depth is the result of the disparity detection, then we may conclude that N1 is related to disparity detection and P1 is the electrophysiological correlate of a depth percept.

Adult↗
PubMed1987

Scalp distribution of pattern visual evoked potentials in normal and hemianopic monkeys.

We examined the hemispheric distribution of the pattern visual evoked potential in 3 Cynomolgus monkeys, before and after right optic tractotomy or left occipital lobectomy. The stimuli were vertical gratings of 4 spatial frequencies, presented using counterphase and on-off modulation at 1 and 8 Hz. In the normal monkey, the amplitude and latency of the PVEPs elicited by 1 Hz stimulation were similar across all electrode sites. While no differences were found in the phase of the PVEP elicited by 8 Hz stimulation across the different derivations, PVEP amplitude generally decreased over the lateral electrodes. Once experimental hemianopia was created, PVEPs recorded over the midline and the intact hemisphere were normal. However, partially polarity-inverted PVEPs of smaller amplitude were recorded over the deafferented hemisphere. In addition, the interhemispheric phase difference became more prominent in the PVEP recorded over the deafferented hemisphere as the spatial frequency was increased.

Animals↗
PubMed1982

A comparison of levodopa treatment and task demands on visual evoked potentials in hemi-parkinsonism.

In patients with hemi-Parkinsonism the amplitude of visual evoked potential (EP) components was relatively reduced over the hemisphere contralateral to Parkinsonian symptomatology. Chronic levodopa treatment tended to reverse this lateral asymmetry. Before levodopa treatment, participation in a simple attentional task brought about similar EP changes. After levodopa treatment, only patients with left symptomatology responded to the attention task with an enlargement of EP components, largely in the noninvolved hemisphere. These findings suggest that task demands cause changes in the sensory system which are similar to those contributing to "paradoxical kinesia."

Adult↗
PubMedDate not supplied

Visual evoked potentials in Alzheimer's disease: correlations with age and severity.

Flash and pattern reversal visually evoked potentials were recorded in 25 patients fulfilling NINCDS-ADRDA criteria for 'probable' Alzheimer's disease (AD). Results were compared with those obtained from groups of normal elderly subjects and patients with mild memory impairment not amounting to dementia. The P2 component of the flash VEP was significantly delayed in AD patients of 74 years or less and correlated with severity whereas the P100 component of the pattern reversal VEP remained relatively unchanged. The latency difference between the P2 and the P100 proved to be a valid discriminator between demented and non-demented subjects only in the younger age group. This limits its usefulness as a diagnostic test of dementia in patients over the age of 75.

Age Factors↗
PubMed2006

Pattern electroretinography and visual evoked potentials in optic nerve diseases.

BACKGROUND: To evaluate transient pattern electroretinography (PERG) and pattern visual evoked potential (VEP) for the diagnosis, differential diagnosis and follow-up of optic nerve diseases. METHODS: Twenty-nine consecutive patients (14 female, 15 male) with the diagnosis of ischaemic optic neuropathy (n=14) and optic neuritis (n=15) were included in this study. Mean age of the patients with ischaemic optic neuropathy was 63.3+/-3.3 (60-78) years and the mean age of the patients with optic neuritis was 28.3+/-8.4 (19-43) years. In each patient ophthalmological examination and systemic evaluation were done and VEP and PERG were recorded. As a control group, VEP recordings of 35 healthy subjects were included. RESULTS: In the ischaemic optic neuropathy group (group 1), mean VEP amplitude (+/-SD) (1.96+/-0.95 microV) was found to be decreased significantly in the affected eyes in comparison to the control group and the unaffected eyes. The delay in latency (116.3+/-20.14 msec in the affected eyes compared with 101.31+/-6.19 msec in unaffected eyes) was statistically significant when compared with the healthy subjects. In the optic neuritis group (group 2), VEP amplitude was decreased (4.13+/-4.04 microV vs 6.97+/-3.35 microV and 6.97+/-4.43 microV) and latency was increased (122.59+/-20.09 msec vs 101.31+/-6.19 msec and 108.76+/-13.57 msec) in affected eyes significantly in comparison to the unaffected eyes and control group, respectively. Even though there were no significant differences for P50 latency and N95/P50 ratios between affected and unaffected eyes in both groups, N95 amplitude decreased significantly in the affected eyes of the ischaemic optic neuropathy patients and N95 latency was found to be decreased in optic neuritis patients. There was no correlation between VEP and PERG findings in both groups. CONCLUSION: VEP amplitude decreased significantly in ischaemic optic neuropathies while latency delay was more significant in patients with optic neuritis. PERG findings showed decreased N95 amplitude in ischemic optic neuropathy without associated latency changes.

Adult↗
PubMed2003

Effect of brain damage and source location on left-right asymmetry of visual evoked potentials in a realistic model of the head.

The left-right asymmetry of visual evoked potentials (VEP) was studied using a realistic three-dimensional model of the head, constructed from CT scans. The integral equation describing the potential distribution due to an inner current-dipole (simulating the VEP source), inside the biological volume conductor (the head) was solved numerically using the finite volume method. The effect of several structural parameters, such as the natural head geometry, the location and orientation of the current source, and conductivity changes (simulating a brain damage) on the VEP asymmetry was examined. The results revealed that the natural anatomical asymmetry presented in a normal head induces some degree of VEP asymmetry (up to 0.42% at the occipital electrodes), yet the major cause of left-right asymmetry is due to asymmetrical location of the source: up to 6.53% in the O(2)-O(1) electrodes for an angular shift of 3 degrees to the left. It was also found that conductivity changes inside the head have a smaller effect on the VEP asymmetry (up to 3.35% in hemorrhaged brain compared to 1.96% in the normal brain at the C(4)-C(3) electrodes). These findings may help in better understanding VEP sources of asymmetry, originating not only due to cerebral functionality, but from structural parameters as well.

Action Potentials↗
PubMedDate not supplied

Visual evoked potentials to red-green stimulation in schoolchildren.

The aim is to study chromatic visual evoked potentials (VEP) to isoluminant red-green (R-G) stimulus in schoolchildren. Sixty children (7-19 years) with normal color vision were examined, 30 binocularly and 30 monocularly. The isoluminant point was determined for each child subjectively by using heterochromatic flicker photometry, and objectively from recordings. The stimulus was a 7 degrees circle composed of horizontal sinusoidal gratings, with spatial frequency 2 cycles/degrees and 90% contrast, presented in onset-offset mode. VEP were recorded from Oz (mid-occipital) position. Age-dependent waveform changes and changes of the positive and negative wave were studied to both binocular and monocular R-G stimulation. Age-dependent waveform changes were observed to binocular and monocular R-G stimulation. In younger children the positive wave was prominent, whereas in older children also the negative wave became more evident. The latency of the positive wave decreased linearly with age to R-G binocular stimulation. To monocular stimulation no significant changes of the latency were observed. The amplitude of the positive wave dropped exponentially with age to binocular and monocular stimulation. The latency of the negative wave increased linearly with age to binocular and monocular stimulation, whereas the amplitude did not show age-dependent changes. These findings suggest that the chromatic VEP response undergoes evident age-dependent changes during the school-age period.

Adolescent↗
PubMed1991

Postnatal development of flash visual evoked potentials in the jaundiced Gunn rat.

In this study, the postnatal development of flash visual evoked potential (VEP) has been monitored in the jaundiced (jj) Gunn rat model of neonatal hyperbilirubinemia to determine whether this evoked response is affected by bilirubin-induced neurotoxicity. VEP could first be recorded at 16 d of age. The jj rats exhibited prolonged wave latencies and lower wave amplitudes during the 3rd wk of postnatal life, when compared with their nonjaundiced littermates. There was no correlation at 21 d of age between VEP parameters and either bilirubin levels or body weight. About one third of the jj animals died between 21 and 28 d of age. The average VEP wave latencies at 21 d of age of the rats who were to die was prolonged compared with those of rats who survived till at least 28 d of age. Thus, the latency of VEP waves at the age of 21 d appears to be related to the further outcome of jj Gunn rats. Although wave amplitudes were lower in jj as compared with nonjaundiced 21-d-old animals, there were no amplitude differences between the jj rats who would die and those who would survive during the 4th wk of life. These findings may contribute to the understanding of the pathogenesis of bilirubin encephalopathy in the neonatal period.

Age Factors↗