The effect of background illumination on pattern onset visual evoked potentials

The early part (first 200 msec) of pattern onset VEPs elicited by a dartboard pattern was studied in conditions of varying level of background illumination. The effect of pattern adaptation and pattern blurr was also studied. The observed complex behaviour of the main negativity within this part of the VEP can be best described in terms of a composite of two independent negative peaks labelled N100 and N130. In high luminance conditions peak N100 was dominant and the presence of N130 was indicated only by a 'notch' on the rising slope of the negativity. As luminance decreased the situation was reversed and N130 became a dominant feature of the negative wave. This finding did not depend on the particular choice of reference site. For checkerboard stimulation the same features were present, but variability of the VEP wave form was greater than in the case of dartboard stimulation. Present results relate the well-known pattern specific properties of the negativity in onset VEPs to N100 only, whereas N130 is not pattern specific. Lower and upper half-field stimulation produced peaks of opposite polarity at 100 msec but no change was observed in polarity of N130. These findings support the suggestion that these two parts of the negativity in pattern onset VEPs may have different cortical sources.     

Pattern-reversal visual evoked potentials and retinal eccentricity

The effect of stimulation of discrete areas of the retina on visual evoked potentials (VEP) was studied in 16 normal volunteers. The stimulus consisted of a constant luminance 2 degrees 18' field containing checks of 34'30' reversing at a frequency of 500 msec. The amplitude of the VEP was highest at the fixation point and inside the 2 degrees isopter. Rapid amplitude decrement was noted with stimuli located within the 2-4 degrees isopters. No identifiable response was obtained outside the 4 degrees or 6 degrees isopter with a 2 degrees 18' stimulus. VEP could, however, be elicited by increasing the size of the stimulus. The smallest size of a field required to evoke a detectable response also varied in relations to retinal eccentricity. Stimulation at 0 degree, 8 degrees and 14 degrees horizontal eccentricities with fields of a size estimated to activate an equivalent amount visual cortex produced VEPs of similar amplitude.     

Developmental changes of pattern reversal visual evoked potentials

Analysis of developmental changes in pattern reversal visual evoked potentials (PVEPs) was performed on 141 normal children ranging in age from 1 month to 19 years 4 months. The stimulus was a black and white checkerboard pattern on a television screen, and the check edge subtended 50 min of retinal arc. The major positive peak (P2) was observed in all subjects, and the incidence of other peaks tended to increase with age. The P2 latency decreased rapidly during the first six months of life and reached a constant value after the age of 2 years. The P2 latency with monocular stimulation was significantly longer than that with binocular stimulation in most age groups. Developmental changes of the N1-P2 or P2-N2 amplitudes were unclear, and the standard deviation of the amplitude in each age was too large for clinical application. The P2 latency with binocular stimulation was shorter for females than for males only at the age of 8-11 years.     

The effect of different hypertension models on visual evoked potentials

Even though there is an abundance of information about the complications of hypertension, studies of its influence on visual evoked potentials (VEPs) are rare. In previous studies, it was pointed out that hypertension induces changes on VEPs. However, it has not yet been clarified which models of hypertension are more effective on VEPs. The aim of this study was to investigate this subject in rats. Animals were divided equally into six groups: control group (C), sham operated (Sham), two kidney-one clip (2K-1C), one kidney-one clip (1K-1C), deoxycorticosterone-salt (DOCA), and N-omega-nitro-L-arginine-methyl ester (L-NAME) groups. Mean arterial pressure was significantly higher in four hypertensive groups compared with control and sham groups, but there were no significant differences either among hypertensive groups or between sham and control groups. At the end of the experimental period, flash visual evoked potentials were recorded. The mean latencies of P1, N1, P2, N2, and P3 components were significantly prolonged in all hypertensive groups compared with the control and sham groups. The mean latencies of all VEPs components in the L-NAME group were longer than in the other hypertensive groups. Thiobarbituric acid-reactive substances (TBARS) were determined as an indicator of lipid peroxidation. Our data showed that hypertension caused a significant increase of lipid peroxidation in brain and retinal tissues. Additionally, plasma renin activity (PRA) was highest in the 2K-1C group and lowest in the DOCA group.     

Effect of gamma-vinyl GABA on human pattern evoked visual potentials

We studied the effects on human visual evoked potentials of gamma-vinyl GABA, an anticonvulsant drug that increases cerebral levels of gamma-aminobutyric acid. The subjects were 15 epileptic patients undergoing a clinical trial of this drug. Serial recordings were made from each patient periodically for 1 year. The stimulus was a reversing checkerboard pattern with a check size of 50 minutes. Plasma levels of other antiepileptic medications remained constant throughout the study. No changes, other than a normal variation of +/- 5 msec, were observed in peak latency throughout the study. These results indicate that there are no GABA-ergic effects on the P100 component evoked by large checkerboard pattern-reversal.     

Serial pattern shift visual evoked potentials in multiple sclerosis

Forty patients with MS initially tested in our laboratory were recalled for repeat PSVEP testing approximately two years later. Twelve normal controls were tested in a similar manner approximately two years apart. The PSVEP positive peak latency changed little in the 24 control eyes (mean 1.4 msec, range 0-6) over the study interval. Most MS patient eyes also showed little change in PSVEP latency over the two year study interval. Fifty-eight eyes changed 8 msec or less. Eighteen eyes showed a PSVEP latency increase of 10 msec or more. Six of these eighteen eyes were symptomatic (attack of clinical optic neuritis), twelve asymptomatic during the study interval. Symptomatic eyes tended to have greater latency increases during the study interval than asymptomatic eyes. Significant latency increases occurred with equal frequency in previously normal eyes (normal PSVEP on first test) and abnormal eyes (abnormal PSVEP on first test or previous clinical optic neuritis). Significant latency increases occurred with greater frequency in patients with a mixed or progressive course than in patients with a remitting-relapsing course, and in patients with greater disability rating (Kurtzke 3-7) than in patients with lower disability ratings (Kurtzke 0-2). Bilateral latency increases occurred during the study interval more frequently than expected by chance. Patient age and disease duration did not significantly influence the number of PSVEP latency increases seen during the study interval. Four eyes decreased in latency by 10 msec or more during the study interval. All these eyes had had an episode of acute optic neuritis which began in the 5 weeks immediately preceding the 1st PSVEP test.(ABSTRACT TRUNCATED AT 250 WORDS)     

The evaluation of sellar region tumours with pattern visual evoked potentials

Pattern visual evoked potentials (PVEPs) were performed on 20 patients with sellar tumours. The aim was to assess the validity of full-field PVEPs combined with a topographic recording in chiasmatic compression. They were abnormal in 95% of the patients, with variable patterns (absent or distorted responses). Our results show that PVEPs are very sensitive in revealing compression on visual pathways and that they correlate with the progression of the tumour. However, they are not reliable in detecting the chiasmatic or retrochiasmatic site of the lesion and relative visual field defect, because of the multidirectional extension of the tumour.     

The effect of adaptation to the stimulating pattern on the latency and wave form of visual evoked potentials

Visual evoked potentials (VEPs) elicited with a checkerboard stimulus after adaptation to an unpatterned grey field were compared to those obtained after adaptation to the stimulating pattern for an equal interval of time. When 3--4 sec elapsed between the adapting interval and recording of the VEP, the latency of the principal positive peak increased by 4 msec with pattern-reversal, but not with pattern-onset stimulation, while the amplitude was unchanged. However, when the pattern-onset VEP was recorded immediately after adaptation, the principal negative peak of the response was lost and the latency of its principal positive peak increased by 14 msec, causing it to mimic the conventional pattern-reversal VEP in both wave form and latency, and suggesting that adaptation may cause the wave form differences in the VEPs obtained by these two methods.     

Effects of age and sex on pattern electroretinograms and visual evoked potentials

Pattern-electroretinograms (P-ERGs) and visual evoked potentials (VEPs) were simultaneously recorded in 112 normal individuals aged 20-75. Two-sized checks subtending 15' and 31' were used as stimuli. A weighted regression analysis was used to determine which of the variables, sex or age, was significant. The latency of the a and b wave of the P-ERGs showed a progressive increase with age but no difference between sexes. The effect was statistically significant for both 15' and 31' checks. There was no statistically significant aging effect for VEPs elicited by 31' checks. Aging, however, affected N70, P100, and the interpeak interval between b wave to N70 and b wave to P100 for responses to 15' checks. Shorter VEP latencies were noted in females for both 15' and 31' checks. The simultaneous recording of P-ERGs and VEPs has demonstrated that aging is a major variable at the retinal level. The effects on the a and b waves are mostly due to optic changes with aging and only partially to aging changes in the neuronal retinal circuitry. The effect of aging on VEPs is different for different size stimuli. The cause is a random neuronal cell loss in the visual pathways from the optic nerve to the visual cortex as the individual ages. The difference in VEP data between sexes may be related to anatomical size and hormonal influences.     

Effect of diazepam on visual potentials evoked by reversible checkerboard pattern

Visual potentials evoked by means of reversible checkerboard pattern were studied in 12 healthy subjects, 5 patients with probable multiple sclerosis and 3 patients with photosensitive epilepsy before and after intravenous injection of 10 mg of Valium. The most characteristic feature was lowering of the amplitude of the evoked visual potentials (in 75%) which appeared as a rule between 30 seconds and 4 minutes after Valium injection. Prolongation of Pmax latency was observed less frequently (in 65%) and this change was of low grade. In nearly half the cases sings of synchronization appeared with development of late multiphasic components of the visual evoked potential. No significant difference was noted in the effect of Valium on the latency and amplitude of the visual evoked potentials between healthy subjects and patients. The possible mechanisms of diazepam action on the conduction in visual pathways are discussed.     

The effect of levodopa treatment on the visual evoked potentials in Parkinsonian patients

There is extensive literature on the effects of levodopa treatment on the visual evoked potentials (VEP) in laboratory animals and in depressed patients. The effects of levodopa on the VEP of parkinsonian patients were overlooked to a certain extent. In this work we searched for levodopa effects on the VEP of 42 parkinsonian patients. The VEP were data-reduced and analyzed by several techniques: (a) 256 data values (256 msec) were reduced to 70 variables by averaging; (b) time-domain parametric extraction: latencies to peak-and-trough points, amplitudes, etc., resulted in 14 variables; (c) frequency transformation into power spectral density bands resulted in 14 variables. Each one of the above variables was univariate, paired t-tested for levodopa effects. The overall effects of levodopa on the power variables and the time-domain parameters were evaluated by Hotelling paired T2. The 70 time variables were further reduced by the tolerance function of the discriminant procedure and analyzed both by direct and by stepwise discriminant analyses and by SAS-MANOVA in a pairwise design. Only few sporadic univariate t values reached significance levels. No overall multivariate significant effects of levodopa were found. In view of known dopaminergic involvement in parts of the visual system it is postulated that levodopa might have antagonizing effects at different levels of the visual pathways. This hypothesis and better understanding of levodopa effects on the visual system should probably be achieved by recording VEP from the visual subsystems.     

Relationship between amplitude of pattern displacement and visual evoked potentials.

Thirteen subjects looked at a 7 mm2 black and white checkerboard which was displaced through 0.25, 0.50, 0.75 and full pattern displacement. One hundred and twenty eight averages were taken for each displacement and the peak to peak amplitude of the visual evoked potential was measured. This showed a linear relationship between pattern displacement and size of the evoked potential. Latency was unaffected for 0.25, 0.50 and 0.75 displacement but was significantly longer for the full pattern stimulation.     

The effect of levo-acetyl-carnitine on visual cognitive evoked potentials in the behaving monkey

We studied acute and chronic effects of levo-acetyl-carnitine (LAC) on event-related potentials (ERPs) in 3 monkeys trained in a “go”/“no-go” visual “oddball” discrimination task. The stimuli were 2.5 cpd sinusoidal gratings differing in their respective orientation only (0° or 45°). Each monkey was trained to release a lever during a prespecified time window. Target stimulus presentation probabilities were between 0.25 and 0.5 ERPs had comparable mean latencies and amplitudes in all monkeys. Primary evoked potentials recorded to either the target or non-target stimulus did not change significantly as a result of LAC treatment. On the other hand, P300 latency decreased following LAC administration, with a maximum occurring in 15–20 min. The major effects of LAC were consistent within each animal and for all three of them.     

The relationship between visual seizures and visual evoked potentials

We have studied a patient (pt) in status epilepticus with visual seizures (szs) arising focally from the right occipital area and have recorded the visual evoked potential (VEP) to three different stimuli under three different conditions (during, between and with no szs). The pt experienced "sparkling" in the contralateral visual field as the sz and the intensity of the "sparkling" was directly related to the frequency of the ictal activity recorded on the EEG. During the szs the VEPs could still be recorded, but the amplitude of the P100 was higher on the contralateral side with pattern reversal (PR) stimuli, and with flashes (FL) the positive peak after the P100 was less evident on the ipsilateral side. Latencies to these latter two positivities were generally shorter than in normals, with much greater standard deviations ipsilaterally with FL and contralaterally with PR, especially during the actual szs. The relationship between VEP generation and visual sz phenomena is discussed.     

Effects of spatially structured stimulus fields on pattern reversal visual evoked potentials

Pattern-reversal visual evoked potentials (PRVEPs) were tested in 11 healthy individuals. A photic stimulator for pattern production on a TV monitor, an electroencephalograph for EEG signal amplification and a computer for averaging the potentials and displaying the traces were used in the experiments. The peak latencies and both trough-to-peak and baseline-to-peak amplitudes of the main PRVEP components were found to be closely related to both the check size of checkerboard pattern and the bar width of barred pattern. The latencies decreased exponentially and amplitudes varied non-monotonically with check size or bar width. Most of the amplitudes of these components were greatest when checkerboard patterns with check sizes subtending visual angles of 60--70 min of arc, or barred patterns with bar widths subtending angles of 7--15 min of arc, were presented.     

Effects of oestrogen replacement therapy on pattern reversal visual evoked potentials.

As a result of a regression in the ovarian functions, oestrogen level in circulation during the menopause drops to 1/50 of its value in the normal reproductive cycle. Excitatory oestrogen increases the sensitivity of the central nervous system to catecholamines by changing the opening frequency of voltage-related L-type calcium channels and augmenting the effect of glutamate; in addition it inhibits the formation of gamma-amino butyric acid (GABA) by the inhibition of glutamate decarboxylase enzyme. It is argued that oestrogen increases transmission in the optic pathways and that oestrogen is responsible for the shorter latency values and higher amplitudes of visual evoked potentials in women. We recorded the monocular pattern reversal visual evoked potentials (PRVEP) of both eyes of 54 post-menopausal women before treatment and of 30 of them after replacement therapy with Tibolon, and of 24 women receiving placebo treatment. The explicit values of P100 latency of right and left eyes before treatment were 98.8 +/- 3.5 and 99.0 +/- 3.3 ms, respectively. The explicit values of P100 latency of right and left eyes after placebo treatment were 98.6 +/- 3.7 and 98.8 +/- 4.0, respectively. The explicit values of P100 latency of right and left eyes after replacement treatment were 94.6 +/- 3.7 and 94.8 +/- 4.0, respectively. We found a statistically significant decrease in the mean PRVEP latencies and a statistically significant increase in mean amplitudes after replacement treatment (P < 0.001) compared with those before treatment and those after placebo treatment. We attributed the changes in PRVEP values after replacement treatment to the action of Tibolon, which acted as a natural sex steroid and speeded the visual transmission time via the widespread receptors in the central nervous system. It is concluded that PRVEP is an objective electrophysiological assessment method in evaluating the efficiency of hormone replacement therapy in post-menopausal women.     

Brain computer interface using flash onset and offset visual evoked potentials.

OBJECTIVE: This paper presents a brain computer interface (BCI) actuated by flash onset and offset visual evoked potentials (FVEPs). Flashing stimuli, such as digits or letters, are displayed on a LCD screen for inducing onset and offset FVEPs when one stares at one of them. Subjects can shift their gaze at target flashing digits or letters to generate a string for communication purposes. METHODS: By designing the flickering sequences with mutually independent flash onsets (or offsets) and employing the inherent property that onset (or offset) FVEPs are time-locked and phase-locked to flash onsets (or offsets) of gazed stimuli, segmented epochs based on the flash onsets (or offsets) of gazed stimuli will be enhanced after averaging whereas those based on the onsets (or offsets) of non-gazed stimuli will be suppressed after averaging. The amplitude difference between the N2 and P2 peaks of averaged onset FVEPs, denoted by Amp(onset), and that between the N1 and P1 peaks of averaged offset FVEPs, denoted by Amp(offset), are detected during experiments. The stimulus inducing the largest value of the sum Amp(onset)+Amp(offset) is identified as the gazed target and the representative digit or letter is sent out. RESULTS: Five subjects participated in two experiments. In the first experiment, subjects were asked to gaze at 25 flickering stimuli one by one with each for a duration of 1min. The mean accuracy with 10-epoch averages was 97.4%. In the second task, subjects were instructed to generate a string '0287513694E' by staring at stimuli on a pseudo keypad comprising ten digits '0-9' and two letters 'B' and 'E'. The mean accuracy and information transfer rates were 92.18% and 33.65bits/min, respectively. CONCLUSIONS: The onset and offset FVEP-based BCI has shown that high information transfer rate has been achieved. SIGNIFICANCE: A novel FVEP-based BCI system is proposed as an efficient and reliable tool for disabled people to communicate with external environments.     

Computed tomography and pattern reversal visual evoked potentials in chronic schizophrenic patients

Eighteen chronic schizophrenic subjects treated with a uniform dosage (4-6 mg/day p.o.) of haloperidol were submitted to computed tomography (CT) and to pattern reversal visual evoked potentials (VEPs). Compared to age-matched controls, schizophrenic patients showed lateral and third ventricular enlargement, greatly delayed VEP latencies and reduced amplitudes. These abnormalities were not related to diagnostic subgroups. Schizophrenic patients with a positive family history for major psychiatric disorders showed normal CT scan measures and greatly abnormal VEP measures, whereas patients with a negative family history showed CT scan signs of atrophy and less pronounced VEP abnormalities.