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Results for “Evoked Potentials, Visual”

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PubMed1993

Fractal analysis of steady-state-flicker visual evoked potentials: feasibility.

Electrophysiological models of visual evoked potential recording have assumed that response variability is caused predominantly by random noise added to a true steady signal. Since neuronal geometry has a fractal structure, neural activity may demonstrate deterministic nonlinear dynamics, i.e., chaos. We recorded several-minute time-series traces of the visual evoked potential magnitude in response to full-field flicker from three glaucoma patients and one normal subject. When plotted in phase space, the steady-state response derived from a lock-in amplifier shows an apparent so-called strange attractor (extended nonrepeating loops) rather than the pattern expected from a signal-plus-noise model (a fuzzy dot). The fractal dimension of this attractor may be a more sensitive indicator of early optic-nerve damage than are visual evoked potential latency or amplitude measures.

Electrophysiology↗

PubMed1995

Delayed visual evoked potentials in adults after monocular visual deprivation by a dense cataract.

PURPOSE: To study the effect of monocular visual deprivation caused by dense unilateral cataracts in adults. METHODS: Visual evoked potentials have been recorded in 11 patients after removal of a dense unilateral cataract acquired in adulthood. These were compared with those from 8 control patients after removal of a mild lens opacity. RESULTS: Visual evoked potentials recorded on the first day after removal of a longstanding, dense, unilateral cataract showed a marked delay to stimulation of the operated eye compared to the unoperated eye. No delay was found in the operated or unoperated eye of control patients. Delays in the visual evoked potential returned to normal within approximately 3 months after surgery, with the exception of the two patients with the most longstanding cataracts in whom the delays persisted much longer. CONCLUSIONS: The adult central visual system is sensitive to visual deprivation caused by a longstanding, dense, unilateral cataract. The changes found may be important in understanding the causes of intractable diplopia, which sometimes occurs after the removal of such cataracts.

Adult↗

PubMed1983

Visual evoked potentials in neurosyphilis.

The visual evoked potential (VEP) to pattern reversal was recorded in 79 patients with neurosyphilis. Sixteen patients (20%) had abnormal VEP latencies with a predominance of pathological VEP values in the group of tabes dorsalis (50%) as compared to general paresis (18%) or meningovascular forms (13%). A comparison of the frequency of abnormal VEPs with that of other ophthalmological tests (visual acuity, visual field, central campimetry, pupillary reactions, dark adaptation, optic fundus) yielded no diagnostic superiority of VEP.

Adult↗

PubMed1990

Handedness and hemispheric asymmetry of pattern reversal visual-evoked potentials.

Pattern reversal visual-evoked potentials (EP) from temporal leads in the two hemispheres of 26 right-handed (14 right-eye-dominant and 12 left-eye-dominant) and 10 left-handed (left-eye-dominant) adults were recorded. Checkerboard patterns (check sizes: 5.7 and 17 min of arc) at 1 and 8 Hz were reversed. Stimuli (a) subtended 6 degrees of visual field, (b) subtended 1 degree of visual field (foveal condition), and (c) were restricted to the annular portion of the visual field around the fovea (peripheral condition). Larger EP amplitudes in right or left hemisphere in relation to handedness, temporal frequency, and visual field condition were recorded. Eye dominance of dextrals appeared to play a role in determining the hemispheric asymmetry. Previous literature data and present results in relation to the hypothesis of different hemispheric specialization for basic visual information are discussed.

Adult↗

PubMed1989

[Decrease of relative amplitude of evoked visual potentials caused by the use of a prism].

In search for an objective method of defining the quality of the binocular vision the author used the visual evoked potentials. The point was before all to estimate the added up records from both eyes checking up whether the amplitude of this record increases in dependence on the fact whether we apply the stimulus to one or to both eyes. A straight dependence between a binocular record from the whole retina and a record from the maculae cannot be discovered. By macular stimulation after placing a prism occurs a lowering of the relative amplitude connected with a disturbance of binocular vision and this enables the differentiation between the presence or lack of the stereoscopic vision in dependence on the fact wheather diplopia occurs or it does not.

Humans↗

PubMed1986

Binocular summation in normal, monocularly deprived, and strabismic cats: visual evoked potentials.

We have studied visual evoked potentials (VEP) in the cat using dichoptically presented sinusoidal gratings. Our goals were to determine if binocular disparity causes differential responses in the VEP, and to examine the effects of monocular deprivation and convergent or divergent strabismus on the degree of binocular summation. Binocular disparity in stimuli causes no regular alterations of visual evoked responses, except at very low spatial frequencies. However, this apparent selectivity is probably due to luminance modulation in the central retina at low frequencies. The insensitivity to binocular disparity establishes that binocular summation in the VEP may be estimated without regard to the relative phase of gratings presented to the two eyes. Binocular summation of the VEP was examined in normal animals. We found that the ratio of the binocularly evoked response to the largest monocular response (averaged across spatial frequency) ranged from 1.27 to 2.12 (4 animals) and had a mean of 1.48. These values fall within the range which has been reported for human subjects. The degree of summation might be expected to be greatly reduced in strabismic and monocularly deprived animals, in which the majority of the cells are functionally monocular. While summation was found to be reduced in 5 esotropic (convergent) animals (range = 1.13-1.24; mean = 1.18) it was approximately normal in three exotropic (divergent) animals (range = 1.29-2.12; mean = 1.61). However, single unit recordings carried out on the same animals show similar reductions of cells that can be driven through either eye for both groups of animals. Recordings from three monocularly deprived animals, on the other hand, show evidence of binocular interaction in the form of suppression. In this case, response amplitudes obtained using binocular stimulation were consistently and substantially smaller than those obtained from the normal eye alone (range = 0.76-0.85; mean = 0.80). We conclude that convergent and divergent strabismus differ substantially in the degree to which binocular summation is exhibited in the VEP, which in the latter condition, is indistinguishable from the normal cat. Monocular deprivation causes an effect which is markedly different from either form of strabismus in that the deprived eye suppresses the response of the normal eye.

Animals↗

PubMed1993

Real-time monitoring of visual evoked potentials.

Steady-state visual evoked potentials (SSVEP) were recorded in response to flashes from eyepatch-mounted light-emitting diodes during neurological surgery and in the intensive care unit. SSVEP were detected and measured with a microprocessor-based waveform correlator. The system enabled a continuous display, in real time, of SSVEP amplitude and latency. SSVEP amplitude and latency changes were monitored throughout a variety of surgical procedures. Some of these procedures directly affect the central visual pathway, some affect the central nervous system as a whole, while others do not affect central nervous system function. In addition, intensive care unit patients with a variety of intracranial pressures were monitored. The results of this study indicate that SSVEP recorded with this method showed changes in SSVEP within seconds of surgical and/or medical decompression of intracranial pressure and were sensitive to specific changes in the visual pathway. Surgical procedures that directly affected the visual system, or elevation of intracranial pressure, resulted in changes in SSVEP. In contrast, procedures that did not affect the functional integrity of the visual system did not affect the recordings.

Adenoma↗

PubMed1981

Life-span alterations in visually evoked potentials and inhibitory function.

Visually evoked potentials (VEPs) elicited by patterned and unpatterned flashes were recorded from 211 healthy males aged 4-90 years. A measure of similarity between the two kinds of VEPs was obtained by correlating the digital values comprising the two waveforms. Across the life-span, correlations followed a U-shaped curve; patterned and unpatterned flash VEPs were most alike for the youngest and oldest subjects. This age effect, localized to scalp areas overlying visual cortex, is compatible with a concept of reduced inhibitory functioning within the visual systems of the young and the old. At central scalp, patterned and unpatterned flash VEP waveforms were more effectively differentiated by the right hemisphere. This observation agrees that the right hemisphere specializes in analyses of spatial material.

Adolescent↗

PubMed1985

Repetitively pulsed small spot light flashes. Depression of steady-state pattern visual evoked potentials.

Steady-state visual evoked potentials (VEP's) were elicited from human volunteers by contrast reversal of a 43% contrast checkerboard pattern resolvable with 6/24 (20/80) Snellen acuity. Superimposed on the fixation point was a Xenon flash lamp subtending 0.358 degrees visual angle, which produced a 0.05 J/cm2 sr flash in 2 microseconds (full width, half maximum). Single pulses of light had no significant effect on the VEP. Increasingly significant decreases in VEP signal magnitude were noted with 2 and 5 pulses of 20 Hz white light. No disruption of VEP phase entrainment was noted with any of the flash exposures. Partial reciprocity was demonstrated between number of flashes at this frequency and response decrement.

Adolescent↗

PubMedDate not supplied

[Effect of phenytoin and carbamazepine on evoked potentials in patients with newly diagnosed epilepsy. Part I. Visual evoked potentials].

Pattern-reversal visual evoked potentials (VEPs) were recorded in 64 patients with newly diagnosed epilepsy. Before starting medication the patients with partial and primary generalized epilepsy, had prolonged latencies of the VEPs component P100, as compared with controls. VEPs were repeated after 3 months in 43 patients with focal epilepsy, during carbamazepine (22 cases) or phenytoin (21 cases) treatment. The plasma concentration of the drugs were within therapeutic levels. Carbamazepine but not phenytoin, was associated with prolongation of the P100 peak latency and induced increase of its amplitude, as compared with the baseline condition. The VEPs abnormality was most pronounced in patients whose seizures were poorly controlled. We conclude, that administration of carbamazepine or phenytoin, at therapeutic serum level, have minimal effect on the VEPs.

Adolescent↗

PubMedDate not supplied

Brainstem auditory and visual evoked potentials in infants with myelomeningocele.

Brainstem auditory evoked potentials (BAEPs) and visual evoked potentials (VEPs) were recorded in 47 infants with myelomeningocele to determine if the evoked potentials reflected the early neurological status, and if they had prognostic value as to the children's neurological outcome. The infants were tested between 1 day and 3 months of age (mean 24 days), while still in hospital after the myelomeningocele repair. Outcome was assessed at a mean of 2 years of age. Normal BAEPs were found in 41% and normal VEPs in 62% of the patients. BAEPs were abnormal in all infants studied who had symptomatic Arnold-Chiari (AC) malformation (n = 9); VEPs were abnormal in only 55% of symptomatic infants. Of the infants who did not have symptomatic AC malformation, 53% had normal BAEPs, 69% had normal VEPs. Of the patients with normal BAEPs, 81% had normal cerebral function on follow-up. Of the patients with abnormal BAEPs, 87% had central neurological abnormalities on follow-up. Of the patients with normal VEPs, 63% were normal on follow-up; of the patients with abnormal VEPs, 71% were abnormal on follow-up. Thus, the VEPs studied early in the neonatal course do not appear to be sufficiently sensitive to be valuable prognostically in these infants. However, the BAEPs were consistently abnormal in symptomatic AC malformation and showed a positive predictive value of 88% and an accuracy in predicting central neurological sequelae of 84%.

Arnold-Chiari Malformation↗

PubMed1988

[Topography and reactivity of the visual evoked potential].

Augmenting/reducing (A/R) of visual evoked potentials (VEP) has been repeatedly observed in central derivations, with some subjects increasing, and others decreasing, VEP amplitude with increasing intensity of stimulation. Central derivations also exhibit hemispheric lateralization regarding A/R. This paper explores central and occipital VEP in the same population of 16 healthy, right-handed male subjects, stimulated with binocular light flashes 10 usec in duration delivered by à Grass PS2 stimulator at a rate of 1/sec and at the intensities of 0.36, 0.72 and 1.44 joules at source. Amplitudes and latencies of components P1, N1 and P2 agreed with those reported in the literature. On the basis of amplitude/intensity slope functions of "peak to peak" amplitudes (P1N1 and NIP2) at Cz, 10 augmenters (slope greater than 0) and 6 reducers (slope smaller than 0) were found. Compared to central leads (C3 and C4) occipital ones (O1 and O2) did not exhibit significant interhemispheric differences. Vertex augmenters for N1P2 were occipital reducers and vice-versa. The different characteristics of A/R at occipital and central leads are interpreted in terms of stages of visual information processing in primary and association areas and functional significance of VEP components.

Adolescent↗

PubMed1981

The effect of stimulus orientation on the visual evoked potential in multiple sclerosis.

Visual evoked potential (VEP) latency was measured in 89 patients with multiple sclerosis (MS) using both a checkerboard stimulus and an alternating vertical grating. Thirty-seven patients had abnormal VEP latencies to the check stimulus, but 63 were classified as abnormal when the grating stimulus was employed. In an additional 22 MS patients, each eye was tested with a checkerboard stimulus, a vertical grating, and a horizontal grating to determine if edge orientation of the stimuli was responsible for the difference between grating and check diagnostic yields. The resulting diagnostic differences could be explained by taking into account the sinusoidal components of these stimuli. After equating gratings and checks for the fundamental Fourier component, each eye of 15 of the MS patients was retested with gratings in three orientations: vertical, horizontal, and oblique. Nine of the 15 patients had an orientation-dependent abnormality of VEP latency. The results suggest that MS causes an orientational imbalance in the human visual system and that VEP testing with stimuli in more than one orientation can increase the diagnostic yield in MS patients.

Adolescent↗

PubMedDate not supplied

[Informative value of forms of visual evoked potentials in assessing the location of a visual tract block].

It is proposed the explanation of the form of visual evoke potential (VEP) on the base of physical imagination of the electrical field of double electrical layers that can approximate the wave of excitement of the visual tract. We mast take into consideration the electrical potential as active as indifferent electrode simultaneously and estimate visual evoke potential as diversity. It is shown on mathematical model the more smooth form of visual evoke potential under block and unimportant to arrangement of the electrode over the place of block.

Evoked Potentials, Visual↗

PubMed1985

Laser flash effects on laser speckle shift visual evoked potential.

Steady-state visual evoked potentials (VEP's) were recorded from four cynomolgus monkeys in response to a sinusoidally oscillating 10 degrees helium-neon laser speckle field (632.8 nm), moving vertically 2.5 degrees at 8 shifts per second. A 5-pulse flash train at the maximum permissible exposure (MPE) dose from a collimated Q-switched frequency-doubled neodymium laser (532 nm) was superimposed on the foveal stimulus and the subsequent disruption and recovery of the VEP measured. Minimal disruption of the response signal magnitude was demonstrated (0.1 greater than p greater than 0.05) which recovered within 300 ms of the termination of the pulse train. A small but significant (p less than 0.01) disruption of phase entrainment was also noted that recovered within the same period. This is contrasted with a second experiment with three monkeys in which an argon (514 nm) laser served both as the speckle stimulus source and as the shuttered flash. Exposure to collimated MPE argon radiation for 250 ms immediately depressed the VEP (97%, p less than 0.01) and showed recovery to 70% of the pre-flash baseline only after 3 s. Phase lock was also severely degraded for several seconds. These results imply that visual processing of nonacuity-limited medium contrast stimuli with broad spatial frequency content will probably not be materially affected by ultra-short pulsed laser exposure at these energy levels and frequencies. However, even safe levels of collimated continuous laser light may have severe effects on vision that could parallel flash effects seen with Xenon discharge flash lamps.

Animals↗

PubMed1986

[Lissajous figures of the visual evoked potentials].

The value of the visual evoked potential (VEP) technique for diagnosis of epilepsy is limited. Except for photosensitive epilepsies, the visual evoked potentials (VEPs) do not reveal more than a routine EEG. As one of the reasons of that, it is pointed out that a review of VEP literature reveals different "normal values" for peak latencies and amplitudes. Thus in this study, lissajous figures of VEPs (VEP-L) were attempted instead of the measurement of peak latencies and amplitudes of VEPs. Thirty six epileptic subjects (14 have partial seizures, 22 have generalized seizures) and 22 control subjects were investigated. Several EEGs to storoboscopic flashes were recorded from scalp electrodes placed at the frontal (Fz) and occipital (Oz) regions according to the 10-20 electrode system, using a right ear as nonreference with a EEG amplifier (Nihon Kohden ME-175 E) and a data recorder (TEAC R-60). Then the phasic relationships between the VEPs from the two areas were analyzed with a medical computer (Nohon Kohden ATAC-2300) and printed out as VEP-L. The results are summerized as follows: First, as time passes from 100 msec to 200 msec in the VEP-L, the epileptic patient group showed more right-roted types than that of the control group (p less than 0.01). Second, the VEP-L at the periods from 50 msec after photic stimulus, were classified into 5 types inspectively. The 5 types are the right-ascending type, the left-ascending type, the vertical type, the horizontal type, and the circular type.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1999

The effects of caffeine withdrawal on cognitive P300 auditory and visual evoked potentials.

Auditory and visual P300 cognitive evoked potentials were obtained in 13 individuals who regularly consumed caffeine prior to and during a 4 day period of abstinence from caffeine. During the period of caffeine abstinence auditory P300 evoked potentials showed significant decreases in amplitude measures but no significant changes in latency, while the visual P300 evoked potentials showed significant latency decreases but no significant changes in amplitude. The reason for these opposite effects is unclear and further research in this area is needed.

Adolescent↗

PubMed1979

Effects of the inter-stimulus interval on visual evoked potentials to patterned stimuli.

Visual evoked potentials to patterned stimulation of the upper and lower half of the visual field were investigated in nine adult Ss. The inter-stimulus interval varied between 350 and 2,600 msec. With patterned stimulation, no inter-stimulus interval effects were obtained for occipital components. In contrast for the vertex lead, on the other hand, as inter-stimulus interval increased, the N140-P200 amplitude increased. Because of the heterogeneity subjects' responses evoked by upper half-field stimulation, this effect was more pronounced in lower half-field stimulation.

Adult↗