Stimulus rate-induced VEP attenuation in preterm infants.

The effect of the stimulus rate on the single flash VEP amplitude and latency (N1) was studied in 26 mechanically ventilated, preterm infants during the first days of life. Significant decreases in VEP amplitude and increases in latency were observed within a series of 3 single flash VEPs recorded with stimulus rates of 1/4 Hz. After a recovery period of 30 sec between series, the amplitude had attained baseline level, whereas the latency delayed progressively. The underlying cause of this rate effect is speculative, as individual degrees of VEP attenuation were not statistically related to gestational age, clinical state or cerebral oxygen delivery.     

Dopaminergic deficiency causes delayed visual evoked potentials in rats

Flash and pattern visual evoked potentials (VEPs) to two temporal frequencies of stimulation were studied in nineteen rats. The effect of a tyrosine hydroxylase inhibitor and of a dopamine receptor blocker on the VEP was explored in ten animals. Significant latency change occurred following injection of either drug, while only the hydroxylase inhibitor reduced the VEP amplitude. These changes were not caused by the anesthesia used in these experiments, although the same anesthetics in higher doses did depress VEP amplitudes. When dopamine blockade was followed by administration of apomorphine, a dopamine agonist, VEP delays could be partially reversed. Besides conduction defects of myelinated axons, synaptic malfunction may also cause delays in sensory evoked potentials.     

Visual evoked potentials in preterm infants during the first hours of life

Flash evoked cortical potentials (VEPs) were studied in 33 preterm infants with gestational ages from 27 to 33 weeks. During the first 12 h after birth a visual response was evoked in all infants. Better estimates of the VEP latency and amplitude were obtained by using the values of 3 VEPs recorded at 30 sec intervals. The VEP latency decreased during the first hours of life, which was accounted for by an increase in core temperature; the latency decreased 6 msec/degrees C increase. Changes in amplitude were less influenced by changes in temperature. Both VEP latency and amplitude were inversely related to gestational age, but there was no association between head circumference and latency. The flash intensity could be reduced from 155 cd to 39 cd without any effect on VEP latency or amplitude. Similarly, a variation of background illumination below 200 lux did not cause VEP changes. The VEP was not affected by development of minor subependymal haemorrhages but it was severely attenuated during a short episode of hypoxia. It is suggested that when taking core temperature into account the VEP can be used to determine changes in the cerebral function in preterm infants.     

Visual evoked potentials in monkeys.

Visual evoked potentials (VEPs) were recorded from 2 cortical sites in stump-tailed macaques. VEPs recorded from striate cortex were basically consistent between animals (especially at low light intensity), remained remarkably stable over time, and compared favorably to VEPs reported by other investigators. We concluded that the VEP recorded from the striate cortex of day-active monkeys consists of 5 major peaks within the first 250 msec. The potentials recorded from post-central gyrus were simpler and more individualized and did not show intensity-related latency changes or increases in inter-subject variability. However, amplitudes of potentials recorded from both electrode placements increased with light intensity apparently reflecting the amplitude of individual potentials rather than the variability of these potentials from which the average VEPs were derived.     

Vector analysis of visual evoked potentials in migraineurs with visual aura.

OBJECTIVE: To assess the capability of vector analysis of visual evoked potentials (VEPs) for revealing alterations in posterior visual pathways in migraineurs with visual aura. METHODS: VEPs to pattern reversal (PR) and LED goggle stimulation were obtained in 23 patients suffering from migraine with visual aura, in an orthogonal Fpz-Oz and T3-T4 montage and displayed as a two-channel Lissajous' trajectory. VEP latency and amplitude at Fpz-Oz, bc segment amplitude (V) and bc vector orientation angle (theta) were compared with a previously collected normative database for individual assessment, and group comparisons with the previously collected normal sample were made. Electrophysiological measures were also correlated with time from onset of disease and from the last crisis, and with the side of symptoms. RESULTS: No individual alterations in VEP latency or amplitude were observed. However, 36.4% of patients showed alterations in vector orientation to PR and 78% to LED goggles. Group differences with respect to normal subjects were found not only in vector orientation but also in midline VEP and V, only for PR stimulation. A significant relationship was found between the laterality of vector deviation and the laterality of symptoms. CONCLUSIONS: Vector analysis of VEP revealed alterations possibly corresponding to asymmetrical visual cortex activation in migraineurs with visual aura, mainly to diffuse light stimulation. SIGNIFICANCE: An electrophysiological parameter of individual value for revealing asymmetric activation of visual cortex in migraineurs is proposed.     

Visual evoked cortical potentials and pattern electroretinograms in Parkinson''s disease and control subjects.

Parkinson's disease patients have been shown to have abnormal visually evoked cortical potentials (VEPs) to pattern stimulation. Whereas dopamine is not an important neurotransmitter in the central visual pathways, the retina is rich in dopamine and, together with previous animal and human studies, this suggests that the abnormal VEPs in Parkinson's disease patients may be due to a biochemical and electrophysiological disorder in the retina. This hypothesis has been examined by studying the VEPs and pattern electroretinogram (PERG) of Parkinson's disease patients and matched control subjects. The amplitudes of the cortical and retinal evoked potentials were significantly reduced in Parkinson's disease patients compared with the control subjects and this could not be attributed to any particular feature of the disease or its treatment. There was a significant relationship between the VEP P100 latency and the PERG amplitude. Moreover for those subjects in whom there was an interocular difference in both cortical and retinal evoked potentials, the abnormality was more commonly found in the potentials from the same eye. These findings suggest that the abnormality of the VEP in Parkinson's disease patients is, at least in part, secondary to an abnormality of the retina itself.     

Visual evoked potentials in cerebral white matter hyperintensity on MRI

OBJECTIVES: To investigate the correlation of the cerebral white matter hyperintensities (WMH) on MRI, and latency and amplitude of visual evoked potentials (VEPs) in elderly subjects. PATIENTS AND METHODS: Pattern VEP (PVEP) and flash VEP (FVEP) were recorded in 25 patients with WMH consisting of 12 patients with frontal dominant WMH (FMH) and 13 patients with occipital dominant WMH (OMH) and 25 patients with basal ganglionic hyperintensities (BGH). RESULTS: In WMHs, there were significantly larger P100 and P2 amplitudes than in BGHs and controls. Regarding the distribution of WMH, OMH showed significantly larger P100 amplitudes than FMH. In OMH in males, there was significantly prolonged P100 latency compared with females, and in females, there were significantly larger P100 and P2 amplitudes compared with males. CONCLUSION: Appropriate clinical values in VEP should take into consideration WMH in addition to gender and age-related changes.     

Visual paired-pulse stimulation reveals enhanced visual cortex excitability in migraineurs

Migraine is a common ictal disorder with an interindividual heterogeneous characteristic, whose underlying mechanisms remain elusive. On the one hand migraine is associated with abnormal cortical hyperexcitability. On the other hand, studies reported lower amplitudes of visual-evoked potentials (VEPs) and concluded that low preactivation levels imply decreased excitability. Here we measured visual cortex excitability and paired-pulse suppression in subjects suffering from migraine without aura and in a group of aged- and gender-matched healthy subjects to address the relation between activation levels and excitability. To that aim, we analysed amplitudes of VEPs and paired-pulse suppression evoked by a paired-pulse stimulation paradigm using stimulus onset asynchronies (SOAs) between 80 and 133 ms. We found that in migraineurs in the interictal state the amplitudes of the first VEP were reduced as compared with healthy subjects by approximately 20%. In the case of paired-pulse suppression comparable to healthy controls, the second response amplitude should be reduced as well, which was not the case. Instead, the ratio between the first and second VEP was higher than in healthy controls and did not depend on SOA in the range tested, which demonstrates reduced paired-pulse suppression and therefore implicates increased cortical excitability. Our data show that in migraineurs VEPs were reduced presumably due to reduced activation levels. However, paired-pulse suppression using short SOAs in the range of 100 ms or less was even higher than in normal subjects. Thus, our data show that signatures of both hyper- and hypoexcitability can be found depending on stimulation condition.     

Visual evoked potentials in Leber's hereditary optic neuropathy

Pattern-reversal visual evoked potentials (VEPs) were measured serially in two brothers with Leber's hereditary optic neuropathy during the active phase of the disease. VEP latency and configuration were normal prior to the onset of visual symptoms. The earliest abnormalities consisted of either prolongation of VEP latency or unusual VEP morphology characterized by a double positive peak. As visual acuity declined progressively over a period of months, there was a parallel progressive prolongation of VEP latency accompanied by less consistent diminution of VEP amplitude. In two eyes, VEP ultimately could no longer be measured. VEP latency was normal in most of the asymptomatic family members, including the maternal presumptive carrier. These findings suggest that VEP measurements do not distinguish the presymptomatic or carrier states but do permit quantitative assessment of the activity and progression of the disease.     

Binocularity in the little owl, Athene noctua. II. Properties of visually evoked potentials from the Wulst in response to monocular and binocular stimulation with sine wave gratings

Visually evoked potentials (VEPs) have been recorded from the Wulst surface of the little owl, Athene noctua, in response to counterphase-reversal of sinusoidal gratings with different contrast, spatial frequency and mean luminance, presented either monocularly or binocularly. Monocular full-field stimuli presented to either eye evoked VEPs of similar amplitude, waveform and latency. Under binocular viewing, VEPs approximately doubled in amplitude without waveform changes. VEPs with similar characteristics could be obtained in response to stimulation of the contralateral, but not ipsilateral, hemifield. These results suggest that a 50% recrossing occurs in thalamic efferents and that different ipsilateral and contralateral regions converge onto the same Wulst sites. The VEP amplitude progressively decreased with increase of the spatial frequency beyond 2 cycles/degree, and the high spatial frequency cut-off (VEP acuity) was under binocular viewing (8 cycles/degree) higher than under monocular (5 cycles/degree) viewing (200 cd/m2, 45% contrast). The VEP acuity increased with increase in the contrast and decreased with reduction of the mean luminance. The binocular gain in both VEP amplitude and VEP acuity was largest at the lowest luminance levels. Binocular VEP summation occurred in the medium-high contrast range. With decreased contrast, both monocular and binocular VEPs progressively decreased in amplitude and tended to the same contrast threshold. The VEP contrast threshold depended on the spatial frequency (0.6-1.8% in the range 0.12-2 cycles/degree). Binocular VEPs often showed facilitatory interaction (binocular/monocular amplitude ratio greater than 2), but the binocular VEP amplitude did not change either by changing the stimulus orientation (horizontal vs. vertical gratings) or by inducing different retinal disparities.(ABSTRACT TRUNCATED AT 250 WORDS)     

Visual evoked potential in patients with cerebral asthenopia

Cerebral asthenopia is often overlooked as a symptom in diffuse brain lesion. An objective correlate of this symptom has so far never been demonstrated. Averaged visual evoked potential (VEP) in 10 patients with asthenopia was compared with 20 normal subjects. Both eyes and each eye alone were stimulated using bipolar recording in the midline and over each of the occipital lobes. There was no difference of latency of the VEPs in the two groups, but the amplitude of the most prominent component was significantly reduced in the patients. There was also a difference in the two groups regarding habituation and lateralisation. No amplitude difference could be found in the somatosensory evoked potential. The amplitude difference in VEP, as an objective correlate of asthenopia, is probably modal specific and suggests involvement of the visual cortex. VEP is unsuitable as a diagnostic tool due to the great overlap between amplitudes in asthenopic patients and control subjects.     

Acutely Administered Haloperidol Has Little Effect on Steady-State Visual Evoked Potentials from Pattern-Reversal Stimulations in Treated Schizophrenics

Abstract: The amplitude changes in steady-state visual evoked potentials (VEPs) from pattern-reversal stimulations at 30 min after an intramuscular injection of haloperidol were examined in 10 treated schizophrenics. The VEP amplitudes to hemi-field or full-field pattern-reversal stimulations with a standard check size were almost unchanged after the haloperidol injection as compared with those before the injection. However, while the VEP amplitudes to the full-field stimulations were significantly higher in the midline occipital portion than in the right occipital portion before the injection, no significant difference was observed after the injection. Further, the VEPs similarly responded to the change in the check sizes used in the full-field stimulations before and after the injection, both showing significantly lower amplitudes only at the large check size of 4.1° in a visual angle than that of 1.0°. These results indicate that acutely administered haloperidol has little effect on steady-state VEPs from pattern-reversal stimulations in treated schizophrenics.     

Visual evoked potential changes in migraine. Influence of migraine attack and aura

OBJECTIVE: To assess the visual evoked potential (VEP) changes in migraines with and without aura. STUDY DESIGN: A clinical study in which the VEP results of 45 migraineurs (study group) and 22 healthy volunteers (control group) were compared. Of 45 migraineurs, 29 had migraine with aura (MA) and 16 had migraine without aura (MOA), and they were examined both during and between the migraine attacks. METHODS: The patients and healthy controls underwent VEP assessment. On VEP recording, mono-ocular stimulation was performed by means of the pattern reversal check board. The latencies of N1, P1 and N2, and the N1--P1 amplitude were noted. The following comparisons were made between NI, P1 and N2 latencies and N1--P1 amplitudes of the migraine and control groups; during and between attack the VEP results of the patients with MA and MOA. RESULTS: The VEP results of the migraineurs and healthy controls were similar (P>0.05). The during attack results of MA, during and between attack results of MOA, and the results of the control group were also similar (P>0.05). N2 latency significantly elongated in patients with MA in the attack free period than it was during the attack (P=0.01), and was also longer than it was in the control group (P=0.01). CONCLUSIONS: There is involvement of the visual pathway in MA rather than MOA, and differentiation between these subtypes of the migraine disease may be performed on the basis of VEP findings manifesting by the prolongation of the N2 wave latency. This contention should be confirmed by further studies.     

Steady-state visual evoked potentials during migraine prophylaxis by propranolol and femoxetine

ABSTRACT— The amplitude of visual evoked potentials (VEPs) for flickering light has been reported to be increased in migraine. In the present study, we have examined whether the VEPs are attenuated when the clinical state of the patient improves during a double-blind experiment with propranolol and femoxetine. VEPs for sinusoidally-modulated light were measured by spectral analysis, and an index depicting the visual reaction type was calculated. The group mean VEP index closely followed the group mean attack frequency, but individual variance was considerable. The changes were most evident in VEPs elicited by stimuli of about 20 Hz. During the treatments, the VEP and headache were also significantly correlated among subjects. The results suggest a close relationship between the enlarged VEPs and the headache mechanisms.     

Spatial attention has different effects on the magno- and parvocellular pathways.

Attention was directed to the left or to the right of the fixation point by the lateral presentation of a target on which the subject had to perform an attention demanding task. A (task-irrelevant) grating displayed in the left visual field was the visual evoked potential (VEP) stimulus. Gratings modulated either in luminance or colour contrast at various temporal frequencies were used in order to maximise the activation of magno- or parvocellular pathways. VEPs recorded in attended and unattended conditions were compared. For luminance stimuli, both latency and amplitude of VEPs were modified by attention. For chromatic stimuli, attention affected the amplitude but not the latency of VEPs. Spatial attention uses different mechanisms when magno- or parvocellular systems are involved.     

Early visual processing in neglect patients: A study with steady-state VEPs

Reliable steady-state visual evoked potentials (VEPs) were recorded in a group of 19 right brain-damaged patients with visuospatial hemineglect (Neglect), and two control groups: 15 left brain-damaged (LBD) patients and 12 right brain-damaged (RBD) patients without neglect. Moreover, VEPs were recorded in two rare cases of left brain damage and right visuospatial hemineglect. Stimuli were gratings phase-reversed at various temporal frequencies presented in the left and right visual field. In the Neglect group, VEPs to stimuli displayed in the left visual field (contralesional stimuli) had longer latencies. The delay was not present for the two control groups. As regards the VEP amplitudes, the Neglect group data showed a less distinctive pattern than in the case of latency. VEPs to stimuli contralateral to the lesion were smaller than those recorded for stimuli ipsilateral to the lesion in both Neglect and RBD groups. On the contrary, the VEP amplitudes for the two hemifields were comparable in the LBD group. In the case of left brain damage and neglect, VEPs to right visual field stimuli had longer latencies and lower amplitudes compared to the ipsilesional responses in both patients. Overall, the data support the view that, in most cases, early visual processing is not intact in the neglected hemifield.     

Visual evoked potentials: a diagnostic test for migraine headache in children

Visual evoked potentials (VEPs) were performed in a blind fashion in 114 children aged 3 to 17 years with different types of headaches, in order to investigate whether this could be helpful in the diagnosis of migraine versus other types of headaches in children. The study showed that the amplitude between P100 and N2 was significantly larger in children with migraine headaches (mean 19.8 nvol, SD 7.75) compared with other types of headaches (mean 13.1 nvol, SD 7.45). These results suggest that the sensitivity of VEP P100 amplitude to distinguish migraine headaches in children is 67%, the specificity of VEP is 83%, and the predictive positive value is 83%. VEP study might be helpful in workup of a child with headache, particularly a young child, when signs and symptoms may not be characteristic.     

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.     

Visual evoked potentials in the detection of subclinical optic toxic effects secondary to ethambutol

In 14 patients with tuberculosis treated with ethambutol hydrochloride, pattern-reversal visual evoked potentials (VEPs) were recorded to monocular, whole-field stimulation before the commencement of treatment and one month and three months subsequently. In six subjects, the VEPs showed changes in the latency and amplitude of the P100 component at the one- or three-month interval. In three cases, the VEP changes reversed after cessation of treatment. In five of the six cases, changes were not associated with a change in visual function, as measured by clinical neuro-ophthalmologic examination. Our findings confirm the usefulness of VEPs in the detection of subclinical optic nerve disease and suggest their use in routine monitoring of ocular function in patients treated with ethambutol.     

Cortical visual evoked potentials during the sleep wakefulness cycle of the freely moving cat. Characterization and statistical comparisons

Visual evoked potentials (VEPs) were obtained during the stages of wakefulness (W), slow sleep (SS) and paradoxical sleep (PS) by means of a light-emitting diode chronically implanted in the frontal sinus of the freely moving cat. Statistical analysis of the variables: latencies, latency intervals and amplitudes, between each of the mentioned stages shows that, for the first components, variations occurred only in the first interval of latency during SS vs. W. Lengthening of VEP latencies and increase of VEP amplitudes were observed for all secondary components in the comparisons between both SS and W, and SS and PS. PS-VEPs vs. W-VEPs showed shortening of latencies and decrease of amplitudes of all secondary components of the former case. The results confirm that in the freely moving cat, the secondary VEP response is more intensely affected by sleep than the primary VEP response, but indicate that there are different mechanisms in the generation of the VEP during SS and PS.