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Results for “Evoked Potentials, Visual”

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PubMed1985

Changes in visual evoked potentials in children on chronic dialysis treatment.

Visual evoked potentials (VEP) were recorded in 20 children undergoing dialysis for chronic renal failure. VEP before treatment (72 h after last dialysis) were pathological in 17 patients (85%); responses obtained 3 h after treatment were abnormal in only 6 cases (30%). Furthermore, all patients improved after treatment, except two who were unchanged. However, VEP recorded immediately after dialysis were worse in 4 of 7 patients than before treatment, probably as an effect of the dysequilibrium syndrome; they improved spontaneously afterwards. The acute changes caused by dialysis seem to be more evident in children than in adults. No correlations have been found between blood chemistry indexes and VEP modifications. Finally, VEP have proved to be more sensitive than EEG in identifying a central nervous system (CNS) dysfunction in these uremic patients.

Adolescent↗

PubMed2006

Detection of motion onset and offset: reaction time and visual evoked potential analysis.

Manual reaction time (RT) and visual evoked potentials (VEP) were measured in motion onset and offset detection tasks. A considerable homology was observed between the temporal structure of RTs and VEP intervals, provided that the change in motion was detected as soon as the VEP signal has reached critical threshold amplitude. Both manual reactions and VEP rise in latency as the velocity of the onset or offset motion decreases and were well approximated by the same negative power function with the exponent close to -2/3. This indicates that motion processing is normalised by subtracting the initial motion vector from ongoing motion. A comparison of the motion onset VEP signals in two different conditions, in one of which the observer was instructed to abstain from the reaction and in the other to indicate as fast as possible the beginning of the motion, contained accurate information about the manual response.

Adult↗

PubMed1983

Pattern-reversal visual evoked potentials in patients with chronic renal insufficiency.

Visual evoked potentials (VEPs) were recorded from 88 patients with chronic renal insufficiency twice at a 6 month interval. In dialysed patients statistically significant decreases in amplitudes and prolongations of VEP peak latencies were found. Azotaemic non-dialysed patients showed related, but smaller, changes in their VEPs. In patients after kidney transplantation the VEP amplitudes were normal but peak latencies were prolonged. No systematic significant relationship between the VEPs and creatinine and urea levels in the blood could be shown.

Brain↗

PubMed1984

Auditory and visual evoked potentials in Huntington's disease.

Auditory and visual evoked potentials elicited by acoustic click (AEP) and checkerboard pattern flash (VEP) were recorded from 15 locations in 21 patients with Huntington's disease (HD) and 21 controls matched for age and sex. Peak latencies were approximately the same for both groups, except that AEP peak N100 appeared earlier in HDs. The main deviation in HDs consisted of generally reduced amplitude of VEP and AEP components; these deviations can be considered non-specific. Early VEP and AEP peaks contained no specific abnormalities comparable to those found in early somatosensory evoked potentials, as previously reported for the same subjects. VEP and AEP amplitudes were lower in medicated than in unmedicated HD patients, but amplitudes for both medicated and unmedicated HDs deviated from normal. Drugs may act to further diminish the already lower than normal amplitudes in HDs.

Adult↗

PubMed1993

Interactions of spectral, spatial, and temporal mechanisms in the human pattern visual evoked potential.

The human pattern onset-offset visual evoked potential (VEP) was studied with different colours and spatial frequencies presented on a steady homogeneous intensive yellow background. Under this condition a broad, late negative wave (N2) dominated by the blue-sensitive mechanism and a sharper earlier positive wave (P1) dominated by the red-green-sensitive mechanism can be observed. With a 460 nm pattern N2 shows a strong amplitude tuning at low-medium spatial frequencies. With 550 nm an early negative component (N1) is added showing an amplitude tuning at high spatial frequencies. Different spatial resolutions of the two colour mechanisms are thus indicated. With pattern-reversal stimuli the 550 nm stimulus shows a higher temporal resolution than the 460 nm stimulus.

Color Perception↗

PubMed1982

The effect of attentional effort on visual evoked potential N1 latency.

The latency of the visual evoked potential N1 component evoked by nontarget stimuli increases with an increased attention to nontarget stimuli. The latency increase seems related to a general effort at processing, rather than any early filtering. This phenomenon is illustrated in one study of hyperactive children and another of normal young adults. The literature of this phenomenon is reviewed, and various explanations are considered. It does not appear to be a result of a slow negative wave, but rather a genuine effect of one aspect of attention on N1.

Adolescent↗

PubMedDate not supplied

Sequence-dependent deterioration in the visual evoked potential in the absence of drowsiness.

When visual evoked potentials (VEPs) are tested patients are often expected to focus on a pattern screen for prolonged periods of time. This may lead to fatigue, failure of concentration and drowsiness, and consequently to a deterioration in the recorded VEP. To determine whether there may be time-dependent changes in the VEP of normal subjects independent of the degree of alertness, attention and altertness were controlled using a reaction time (RT) task in which the subjects were required to re-illuminate the fixation point in the middle of the stimulating screen for the VEP. It was first established that the switching of the fixation point produced little contamination of the background VEP to pattern reversal and that the latency and amplitude of P100 to pattern reversal were identical whether or not the subject was engaged in the RT task. A sequence of 16 averages of the VEP to 256 pattern reversals was recorded, alternately with or without the RT task. The measured RTs decreased during the sequence, presumably due to practice. There was a progressive decrease in the amplitude N70-P100, accompanied by an increase in the variability of the latency of P100. These changes cannot be attributed to lack of alertness, given the improvement in RT. Clinicians should be aware of the possibility of a deterioration in the VEP due to physiological mechanisms when the testing protocol involves multiple averages.

Adult↗

PubMed1998

Development of the spatio-chromatic visual evoked potential (VEP): a longitudinal study.

Most prior visual evoked potential (VEP) research on the development of color vision has employed pattern-reversing stimuli that are not optimal for producing chromatic responses. We measured infant VEPs using low spatial frequency, onset-offset stimuli, modulated along the three axes of a cone-based color space (Derrington et al. [J. Physiol 1984;357, 241-265.]). Three color-normal infants were tested in a longitudinal design over the first postnatal year. One red/green color-deficient infant was also tested at 197 days. We found that VEP responses to S-axis (tritan) stimuli have their initial onset later than responses to red/green (L-M) or achromatic stimuli, and that developmental changes in VEP waveforms are more complex and longer lasting for chromatic than for achromatic stimuli. Possible mechanisms underlying these changes are discussed.

Adult↗

PubMed2003

Normalisation of visual evoked potentials after optic neuritis.

Normalisation of the visual evoked potential (VEP) in patients with optic neuritis (ON) appears to be a rare phenomenon. However, although several workers have indicated that it can happen, they have not followed up with subsequent VEP tests to confirm how long the VEP latency of the affected eye remains in the normal range. To resolve this, 18 patients with a clinical diagnosis of acute unilateral ON were followed for 5 years with repeated VEP tests to determine if the latency of the P2 wave from affected eye could return to the normal range. Furthermore, in cases where the latency returned to normal, the length of time that it remained so was also assessed. The normal range for the latency of the P2 wave was determined by measuring VEPs from a group of 18 healthy control subjects with a similar age distribution to the patients. This established an upper limit of 115.9 ms. At presentation the mean P2 latency of the affected eyes of the patients was 140 ms with a standard deviation of 16 ms. In general, the VEP latencies remained constant over the period of the investigation. However two patients demonstrated a return to normal latencies but this was only temporary. Their latencies become prolonged again within 2 years. These results provide evidence that the delayed P2 latency observed in patients with ON can return to the normal range in a small percentage of cases. However, this improvement may spontaneously deteriorateonce more as a result of further episodes of subacute demyelination.

Acute Disease↗

PubMed1990

Effects of enflurane on visual evoked potentials in humans.

Effects of enflurane on visual evoked potentials (VEP) were studied in eight healthy female patients. Control VEP were recorded from the occipital electrode (OZ-CZ), and prominent negative (N70) and positive (P100) peaks were observed following flash light stimulation. After induction of anaesthesia with thiopentone, VEP were recorded at expired concentrations of 1.2, 1.8, 2.4 and 2.7% of enflurane in 100% oxygen. The amplitude of the P100 was decreased and the latency of the N70 was increased significantly from the control with all concentrations of enflurane. No significant difference was found between different concentrations of enflurane. The latency of the P100 was increased significantly only at concentrations above MAC and at 2.7% when it was significantly longer than those at 1.2 and 1.8%. We conclude that VEP are altered significantly with enflurane in clinically used concentrations.

Adult↗

PubMed1997

Visual evoked potentials in rubber factory workers.

Pattern reversal visual evoked potentials (pVEP) were studied in 39 male rubber factory workers in the age range of 18-55 years and 20 control subjects (aged 18-46 years) not exposed to the rubber factory environment. Results revealed that 20 (51%) rubber factory workers had abnormal latencies of wave P1 (dominant component of pVEP) as per accepted criteria of 99% tolerance limit set for the control group (i.e. any value above mean +3 SD of control was considered abnormal). The section-wise per cent distribution of abnormalities was vulcanization (83%), tubing (75%), calendering (60%), loading (38%) and mixing (14%). This study provides electrophysiological evidence that rubber factory environments affect the conduction processes in optical pathways from their origin in the retina to striate cortex. However, this study has its limitations in not identifying the specific chemical(s) causing these changes in VEP.

Adult↗

PubMed1998

Optimal detection of visual evoked potentials.

We consider the problem of detecting visual evoked potentials (VEP's). A matched subspace filter is applied to the detection of the VEP and is demonstrated to perform better than a number of other evoked potential detectors. Unlike single-harmonic detectors, the matched subspace filter (MSF) detector is suitable for detecting multiharmonic VEP's. Moreover, the MSF is optimal in the uniformly most powerful sense for multiharmonic signals with unknown noise variance.

Algorithms↗

PubMed1995

Maturation of electroretinograms and visual evoked potentials in preterm infants.

Electroretinograms (ERGs) and visual evoked potentials (VEPs) to flash stimulation were recorded from 51 infants (gestational age 26 to 42 weeks; post-conceptional age (PCA) 31 to 47 weeks) to give cross-sectional data on the maturation of these responses. Sequential recordings were taken from a separate group of 24 preterm infants (gestational age 28 to 33 weeks) to give longitudinal data. There was a significant decrease in ERG a-wave latency and increase in a-b amplitude with increasing PCA in both groups. For the VEPs there was a significant decrease in latency of the early negative component (N1) and the major positive component (P2). Comparison between recordings made on preterm infants with those from term infants at an equivalent PCA suggested faster maturation of VEPs in the extra-uterine environment, but no difference in maturation of the ERG.

Brain↗

PubMed1985

Comparative study of visual evoked potentials in spinocerebellar ataxias and multiple sclerosis.

Visual evoked potentials (VEPs) were delayed in 11 out of 18 patients with Friedreich's ataxia, in 1 out of 8 patients with Strumpell's hereditary spastic ataxia, in 2 out of 5 cases with cerebellar atrophy and in 42 out of 50 patients with multiple sclerosis (MS). Responses were normal in 5 cases with Pierre Marie's disease. Amplitude and temporal dispersion were statistically analyzed in the above-mentioned groups of patients with respect to controls. An abnormal temporal dispersion, also considered as interpeak N1P2, was frequently found in MS but only occasionally in spinocerebellar ataxias. Amplitude was statistically reduced in Friedreich's ataxia group, where an inverse relationship between latency and amplitude was found. No relation was found between VEP delay and duration of the disease, in any group considered.

Adolescent↗

PubMed1995

The influence of age on the flash visual evoked potentials.

The effect of age on flash visual evoked potentials was investigate in 148 normal subjects aged 6 yo 84 years. The occurrence of the P1 wave progressively increased with age, while the P2 wave was recordable in all the subjects. There were significant effects of age on the P2 latency (p < or = 0.01). The latency of the N1 and N2 waves significantly changed with age. In particular, the N2 wave that occurred in the absence of the P1 wave had a latency similar to that of the N1 wave was present. We suggest that the cases apparently without the P1 wave could be explained with a merging of this was with the P2 wave in a major single deflection.

Adolescent↗

PubMed1989

The visual evoked potential in acute primary angle closure glaucoma.

Visual evoked potentials (VEPs) were elicited from 29 patients who had experienced a previous attack of acute primary angle closure glaucoma. The VEPs were shown to be abnormal in at least one of the measures (latency, amplitude, contrast threshold, or slope) in 72.4% of affected eyes, whereas only 41.4% indicated obvious optic nerve damage. It is notable that 48.1% of fellow eyes with no (known) history of acute pressure rise also showed some form of VEP abnormality. The possible pathophysiological mechanisms operating in both affected and fellow eyes are discussed. It is concluded that, despite the presence of possible artefactual influences, the results probably reflect the presence of primary angle closure glaucoma.

Adult↗

PubMed1982

Migraine patients exhibit abnormalities in the visual evoked potential.

An experiment is described which compared visual evoked potential (VEP) amplitudes and latencies in normal subjects and migraine patients. Several VEP abnormalities were found: at vertex and to a lesser extent at temporal sites, P100-N120 amplitudes were larger in patients; at vertex and temporal sites N120 amplitudes were larger in patients; at temporal sites patents had larger N120-P200 amplitudes but smaller P100 amplitudes. Peak latencies of the VEP were also found to be abnormal in patients. At vertex, patients had delayed N120 peak latencies while at temporal sites delays were found in the P200 latencies. Patients were subdivided according to side of headache. Right-sided headache patients showed larger temporal P100 amplitudes and larger left temporal P100-N120 amplitudes than bilateral headache patients.

Adult↗

PubMed1990

Visual evoked potentials in rotogravure printers exposed to toluene.

Visual evoked potentials (VEPs) from stimulation by checkerboard pattern reversal were examined in 54 rotogravure printers exposed to toluene (all men, aged 22-64 years, duration of exposure 1-41 years). A control group consisted of 46 subjects (23 men and 23 women; aged 22-54 years). Compared with controls the exposed group showed more frequent responses with reduced reproducibility or absence of some waves, or both; the mean P1 wave latency was prolonged and mean amplitudes N1P1 and P1N2 were reduced. The VEPs were abnormal in 24% of workers. The frequency of abnormal VEPs correlated positively with the duration of exposure to toluene and also with the degree of alcohol drinking. No association was found between measurements of VEP and electroencephalogram (EEG) or electromyogram (EMG) examinations. A VEP measurement was made in 78% of the exposed workers two years after the first examination. No statistically significant difference between the two results was found. This suggests a marked stability of the observed VEP changes. These changes can be interpreted as a subclinical sign of dysfunction of the central nervous system (CNS) related to exposure to toluene and also to alcohol consumption.

Adult↗