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PubMed2001

The electroretinogram and visual evoked potential of freely moving rats.

The vascularised rat retina could be one of the most useful experimental objects in visual neuroscience to understand human visual physiological and pathological processes. We report here on a new method of implantation for studying the visual system of freely moving rats that provides a rat model for simultaneous recording at corneal and cortical level and is stable enough to record for months. We implanted light emitting diodes onto the skull behind the eyeball to stimulate the eye with flashes and to light adapt the retina with constant light levels. A multistrand, stainless steel, flexible fine wire electrode placed on the eyeball was used for electroretinogram recording and screw electrodes (left/right visual and parietal cortical) were used to record the visual evoked potential and the electroencephalogram. In the present report we focus on the new method of implantation for recording the corneal flash electroretinogram of normal, freely moving rats simultaneously with the visual evoked cortical potential showing examples in various visual experiments. We also introduce a program for retinogram and visual evoked potential analysis, which defines various measures (latencies, areas, amplitudes, and durations) and draw attention to the benefits of this method for those involved in visual, functional genomic, pharmacological, and human ophthalmologic research.

Animals↗
PubMed1995

Visual evoked potentials generator model derived from different spatial frequency stimuli of visual field regions and magnetic resonance imaging coordinates of V1, V2, V3 areas in man.

Visual evoked potentials (VEPs) to pattern reversal vertical bar stimuli were recorded from 24 scalp derivations (including zygomatic and inion) referenced to digitally linked earlobes in 50 controls. 1, 2 and 4 cpd patterns were presented as full field (FF) stimuli, on Upper Hemifields (UHF) and Lower Hemifields (LHF), upper and lower quadrants and with the occlusion of central and peripheral UHF and LHF. VEPs to octant stimuli were also recorded with 2 cpd patterns. N1, P1 and N2 components were recorded from posterior and inion derivations with FF stimuli, from posterior derivations with LHF stimuli, only from inion leads with UHF stimuli, from derivations ipsilateral to stimuli with quadrants and octants, and consistently from midline derivations only with lower quadrants. Polarity inverted sequences (iN1-iP1-iN2) were recorded from the other scalp derivations, with similar latency and spatial frequency sensitivity as N1-P1-N2. Single Equivalent Dipole (ED) calculations were performed on N1 and P1 recorded in the different stimulus conditions. Our findings contradict previous hypotheses on VEP generators and contradict the predictions of VEPs polarity and distribution based on the "cruciform model" of VEPs generators. In order to explain the distribution of VEPs to upper and lower half fields and to quadrant and octants, we propose a model based on the position of the medial and occipito-polar surface of visual cortex in man.

Adult↗
PubMed1990

Results with multichannel topographical bipolar recordings of the visual evoked potential in normals and amblyopes.

The method currently used to record topographical voltage distributions of visual evoked potentials is that of monopolar recording with respect to some arbitrary reference electrode. As an alternative we investigated the method of bipolar topographical recording of the checkerboard reversal evoked potential in four amblyopic patients and ten normal persons. The recordings were performed with a system of 14 bipolar channels, arranged in a rectangular grid with an interelectrode distance of 15% of the inion-nasion distance and centered 3 cm above the inion. The gradient distributions of the visual evoked potential obtained from normal persons showed at the time of component P2 a maximum about 3 cm above the inion. The healthy eyes of the amblyopic patients showed similar distributions and latencies. The amblyopic eyes showed normal or reduced amplitudes and normal or increased latencies with large checks and low amplitudes with smaller checks. The topographical distributions recorded with the bipolar recording system showed shifts of the maxima with the larger patterns in three of four amblyopic eyes. A technical comparison showed that these shifts can be recorded by the bipolar recording method with higher accuracy than with the monopolar method, promising a better understanding and more accurate diagnosis of amblyopia.

Adult↗
PubMed1992

Cortical visual impairment following bacterial meningitis: magnetic resonance imaging and visual evoked potentials findings in two cases.

Cortical visual impairment (CVI) following bacterial meningitis is a very uncommon complication. Two children with CVI following bacterial meningitis are reported. Bacterial agents were Haemophilus influenzae type B in one and meningococci in the other child. Both children showed only insufficient recovery from CVI, mental retardation and residual neurological symptoms. Flash visual evoked potentials (VEP) showed preserved cortical response at onset of CVI. Re-evaluations several months later showed significantly reduced amplitudes, but normal latencies for P100. Thus, flash VEP does not allow prediction of visual outcome. MRI results have not been reported before. MRI at onset of diagnosis showed occipital parenchymal irregularities with enlarged sulci and subarachnoid spaces. Follow up MRI 15 months after onset of CVI in one patient showed marked atrophy of the occipital cortex, hyperintensities of the cortical white matter and no visible optic radiation. The MRI findings indicate hypoxic-ischaemic lesions in the border zone between the distribution of the great cerebral arteries.

Blindness↗
PubMed1996

Fetal brain sparing is associated with accelerated shortening of visual evoked potential latencies during early infancy.

OBJECTIVE: Our purpose was to assess the effects that fetal growth restriction exerts on the myelination of the developing brain. STUDY DESIGN: Fetal haemodynamic centralization, an adaptive strategy to growth restriction caused by placental insufficiency, was determined by Doppler ultrasonography. Infants with a raised ratio between umbilical artery pulsatility index and cerebral artery pulsatility index are severely growth restricted. Visual evoked potentials give information on the degree of brain myelination. Shortening of visual evoked potential latencies is a normal feature of myelination. In a consecutive series of 105 Neonates, visual evoked potentials were recorded at the corrected ages of 6 months and 1 years. Correction for possible confounders, such as cranial ultrasonographic findings, gestational age, and head circumference, was performed. RESULTS: At 6 months, infants with a raised umbilical artery/cerebral artery pulsatility index ratio have shorter visual evoked potential latencies. Opposite of neonates with a normal umbilical artery/cerebral artery ratio, they show no postnatal maturational shortening of visual evoked potential latencies. CONCLUSION: Accelerated neurophysiologic maturation, found in infants with a high umbilical artery/cerebral artery ratio, might be the result of a beneficial adaptive process to severe fetal growth restriction.

Brain↗
PubMed1993

Visual evoked potentials after optic neuritis. Effect of time interval, age and disease dissemination.

Visual evoked potentials (VEPs) were compared among six groups of patients tested at various times after an episode of acute unilateral optic neuritis (ON). The incidence of abnormalities ranged from above 90% in patients tested during the first 6 months to about 70% when more than 2 years had elapsed. Compared with the acute stage (1-8 weeks), latency prolongation was almost 50% less in patients tested after 2-19 years. In the latter group, latencies were significantly correlated with the patients' age at the time of the attack. The findings confirm and extend the evidence of serial studies which shows that the shortening of VEP latency is a general phenomenon which proceeds for up to 2 years and possibly for longer in younger patients. When the patients with clinically isolated ON were compared with those who had a history of additional neurological episodes suggestive of multiple sclerosis (MS), up to 4 weeks after ON latencies were more prolonged in the MS group but between 4 and 8 weeks amplitudes were larger and between 8 weeks and 2 years latencies were significantly shorter. No significant differences were found in patients tested after more than 2 years. It is suggested that the electrophysiological deficit may initially be more severe in patients with disseminated disease, but that recovery may occur faster.

Adolescent↗
PubMed2005

Recovery from parkinson syndrome and prolonged visually evoked potentials in hepatic encephalopathy.

The combination of Parkinson syndrome (PS) and prolonged visually evoked potentials (VEPs) in a single patient with hepatic encephalopathy (HE) has not been reported. A 52-year-old male with chronic HE developed PS in early 2001. Treatment with L-DOPA was only of minimal effect. In August 2001 he was admitted because of worsening PS and HE. There was anemia, hyperlipidaemia, markedly elevated liver-function-parameters, hyperammonemia, elevated resting-lactate, steatosis hepatis and hepatomegalia. VEPs showed markedly prolonged P100-latencies. Under L-DOPA, pramipexol, L-ornithin-L-aspartate, paromomycin-sulfate, and lactulose liver-function-parameters normalized and PS markedly improved. In February 2003, VEPs were normal. L-DOPA was discontinued by the patient in April 2003 and pramipexol in December 2003. Since then PS did not recur. This case shows that HE may go along with reversible PS and prolonged VEPs. Under adequate therapy liver-function-parameters and VEPs normalize and PS disappears.

Anti-Bacterial Agents↗
PubMed1986

Dissociation of frontal N100 from occipital P100 in pattern reversal visual evoked potentials.

We studied the relationship between occipital P100 and frontal N100 in visual evoked potentials produced by pattern reversal in normal subjects and two groups of patients. Recording derivation was critical for interpretation since both Fz and Oz electrode sites are active. In 9 patients, but no normal subjects, P100 was absent. In these patients, use of a standard Oz-Fz montage resulted in the erroneous impression of a 'normal' P100 since a downward deflection was produced by the inverting effect of the amplifier on an intact N100 at Fz. When both P100 and N100 were present (at Oz and Fz respectively), their latencies were usually similar but not identical which contributed to apparent latency shifts or W-shaped wave forms in the Oz-Fz derivation. We conclude that use of a non-cephalic or relatively inactive scalp position (such as the mastoid) should be used as a reference site in addition to Fz to reduce interpretive errors.

Adolescent↗
PubMedDate not supplied

Multifocal visual evoked potential is dependent on electrode position.

PURPOSE: To investigate whether the multifocal visual evoked potential (mVEP) is dependent on the electrode position, and to confirm the reproducibility of the mVEP. METHODS: The mVEPs were recorded using the Veris III system with two different bipolar electrode settings. In Position 1, electrodes were placed at equal distances in vertical alignment 2 cm above and below the inion. In Position 2, the electrodes were placed in horizontal alignment at equal distances 2 cm to the left and right of the inion. Dartboard pattern stimulation was conducted. The mVEPs were repeatedly recorded from 4 volunteers, and mainly the second-order kernel response components were analyzed. RESULTS: Although the reproducibility of mVEP was good in both Position 1 and Position 2, each waveform of the mVEP was drastically different between the two positions. This difference in the waveforms was clearly shown in the center and at the horizontal meridian. We also investigated the first-order kernel response components of the mVEPs. Several traces of the first-order kernel response components did not reveal flat traces. This point is also worthy of consideration. CONCLUSION: Responses from mVEPs are clearly dependent on the electrode position.

Adult↗
PubMed2001

A first comparison of the human multifocal visual evoked magnetic field and visual evoked potential.

Our objectives were to determine the feasibility of recording reliable multifocal visual evoked magnetic fields (mfVEFs), to investigate the maximum stimulus eccentricity for which the mfVEF responses can be obtained, and to study how this changes with checksize (spatial frequency tuning). Using a checksize of 30', we recorded 8-channel pattern-onset mfVEFs three times to obtain responses from 19 channels located around the inion. Multifocal visual evoked potentials (mfVEPs) were recorded under the same conditions. Eccentricity changes with spatial frequency were studied using checksizes from 7.5' to 60'. We obtained, for the first time, reliable mfVEFs, and found they could be elicited from more peripheral stimulus elements than could mfVEPs. The larger the checksize, the greater the eccentricity reached.

Adult↗
PubMed1999

Prognostic value of flash visual evoked potentials in preterm infants.

This paper investigates the prognostic value of flash visual evoked potentials (VEPs) recorded in preterm infants at birth and at term age with respect to severe neurological outcome. Flash VEPs were recorded in 81 preterm infants at birth (i.e. <35 weeks' gestation) and repeated in 56 of these infants at term age. The preterm infants were assigned to either a healthy or at-risk subgroup based on clinical birth factors. Normal ranges of flash-VEP latencies, amplitudes, and number of components present were obtained from the subgroup of healthy preterm infants and from 68 term infants tested postnatally. The flash-VEP results of the entire preterm group were compared with the normal ranges and any abnormalities noted. Seven preterm infants in the at-risk group died, six of whom had abnormal flash VEPs before term age. Of the five infants from the at-risk group diagnosed with cerebral palsy (CP), three had abnormal flash VEPs before term age. Thus the sensitivity and specificity of the flash VEP with respect to survival was 86% and 89% respectively, and with respect to the development of CP was 60% and 92%. The abnormal features of the flash VEP associated with adverse outcomes comprised a delayed N3 component before term age and the absence of a positive component (P2) at term age.

Cerebral Palsy↗
PubMed1996

Visual evoked potentials are affected by trunk rotation in neglect patients.

Steady-state visual evoked potentials (VEPs) were recorded in four patients with unilateral visuo-spatial neglect, stimulating either the left or the right hemifield. In the standard condition (head and body oriented straight ahead towards the stimulus) the left hemifield VEP was delayed. When the body was turned to the left, however, the two hemifield latencies were comparable. These results were confirmed with the transient VEP technique. No effect of trunk rotation was observed in a group of patients with left brain damage and without neglect. The results imply that the sensory afferents from neck muscles might restore the altered occipital activity and suggest that the same conditions which modulate neglect modulate VEPs latencies.

Aged↗
PubMed1985

Flicker electroretinograms and visual evoked potentials in the evaluation of laser flash effects.

Electroretinograms (ERG's) and visual evoked potentials (VEP's) were recorded from four cynomolgus monkeys in response to a sinusoidally flickering argon laser beam (514 nm) producing a 50-micron spot on the fovea. Super-position of a 20-Hz train of six pulses of Q-switched (120 ns) frequency-doubled neodymium laser light (532 nm) at "safe" exposure energies in 250 ms had no significant effects on the ERG (p greater than 0.05). The VEP was disrupted significantly (p less than 0.001) but demonstrated recovery within 500 ms of the initial pulse. Therefore, flash effects of pulsed visible lasers at these doses on suprathreshold luminance processing are probably limited only to the exposure period.

Animals↗
PubMed2000

Subspace averaging of steady-state visual evoked potentials.

A new algorithm for doing signal averaging of steady-state visual evoked potentials (VEP's) is described. The subspace average is obtained by finding the orthogonal projection of the VEP measurement vector onto the signal subspace, which is based on a sinusoidal VEP signal model. The subspace average is seen to out-perform the conventional average using a new signal-to-noise-ratio-based performance measure on simulated and actual VEP data.

Algorithms↗
PubMedDate not supplied

Recording of visual-evoked potentials in dogs with scalp electrodes.

Following unsuccessful attempts to record visual-evoked potentials (VEPs) in dogs with scalp electrodes, adoption of a new stimulation technique seems to be beneficial. Previously, flashes of white light administered after dark adaptation induced relatively high amplitude electroretinograms (ERGs) covering any VEP activity over the surface of the skull. ERG amplitude, however, can be significantly reduced using flashes of red light after light adaptation (mostly cone stimulation). Simultaneous ERG and VEP recording allows identification of VEPs composed of three significantly different negative peaks (N1, N2, and N3) measured in dogs anesthetized with chloralose and halothane. No more than two of the three peaks were seen in one recording. Only the N1 and N3 waves were consistently recorded in dogs anesthetized with thiopental and thiopental combined with halothane. In 50% of all recordings, N1 was seen alone. The other VEPs consisted of N1 and N2, or N1 and N3 occurring concurrently. The simultaneous occurrence of N2 and N3 waves, however, was never seen. Among all recordings, N1 was most frequently recorded (85% of measurements), followed by N3 and N2 (38% and 31% of measurements, respectively). Peaks of less than 90 ms are highly reproducible. Anesthesia is necessary to eliminate frequent artifacts obtained in conscious and sedated dogs. Thiopental and/or halothane had no effect on measured latencies compared with chloralose.

Anesthesia↗
PubMed1985

Pattern-reversal visual evoked potential in a case of nitrous oxide abuse and recovery.

Pattern reversal visual evoked potentials were recorded in a dentist who had abused nitrous oxide while he showed neurologic impairment and during recovery. The latency of the major positive peak (P100) remained constant during both phases. However, the amplitude of this peak was markedly reduced during the stage of neurologic involvement, and increased during recovery after withdrawal. The clinical significance of this is discussed.

Adult↗
PubMedDate not supplied

[Visual evoked potentials in children with tension headaches and migraine].

OBJECTIVE: To determine whether visual evoked potentials (VEP) change, and to what degree, in different types of headache (migraine with or without aura, or tension headache). PATIENTS AND METHODS: We made a transversal study of 78 children (aged 3-14 years) studied between March 1997 and August 1998, classified into three groups according to HIS diagnostic criteria of 1988 and HIS-R 1997. A VEP of geometric pattern was done using the recording technique recommended by the International Society in their standards for VEP and the reference values were used for an amplitude of less than 5 microV and a latency of P100 +/- 15 ms. The qualitative variable was frequency, and the quantitative variables were the mean and the standard deviation. We studied the association between qualitative variables using the chi-squared test and the differences in means between the groups with ANOVA. All differences were considered to be statistically significant when p < 0.05. RESULTS: Girls made up 55%, with an average age of 8.84 years and a standard deviation of 3 years. There were no statistically significant differences in the mean of the VEP findings between the different types of headache with regard to amplitude (p = 0.975) and latency (p = 0.941). Neither were there any significant differences in the response to VEP in the different types of headache as far as sex and age were concerned, with p = 0.268 and p = 0.147 respectively. CONCLUSION: Our results show no statistically significant differences and do not support the idea of using VEP as a neurophysiological method for studying headaches and differentiating the various types.

Adolescent↗