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PubMed1982

[Changes in pattern evoked visual potentials in multiple sclerosis in respect to development of illness].

In 43 patients (28 female, 15 male) with a definitive diagnosis of disseminated sclerosis visual evoked potentials were tested several times with pattern stimulation. The average duration of the disease was 7 years before the first pattern evoked potential. The latency of the evoked potential, estimated from the first positive peak, was in the pathological range in 30 patients in the first investigation, whereas in 38 patients there was a delayed latency in the last measurement. There was an average period of 1.5 years between the first and last investigation. In 20 cases the evoked potentials showed a continuous delay in the latency. Two patients maintained the original pathological values and in 7 cases the control results were below that of the first investigation. In 9 cases it was not possible to show a definite result because of the marked fluctuation in the values. 5 times in the entire investigation was a pathological result absent. There was a clear correlation between both the duration of the disease and the presence of a delayed latency, and the frequency of episodes and a pathological latency.

Adolescent↗
PubMed2005

The effect of interpolation and perceptual difficulty on the visual potentials evoked by illusory figures.

Completion is the process by which the brain unifies and segregates the parts of an incomplete form. It is qualified as amodal when the form is placed behind an obstacle and modal when the form is at the foreground and closed by illusory contours. The N1, and sometimes the N2, deflections of the visual evoked potentials are known to be larger for modal figures, such as the Kanizsa triangle, than for control figures. This result is generally linked to completion or illusory contours, but it could also be related to a third process: the interpolation of the form by connecting its separate parts. To test the influence of interpolation, a modal triangle, an amodal triangle, a figure with outlined inducers, and a no-triangle figure were randomly presented to 26 subjects. The N1 evoked by the three triangle figures were all larger than the N1 to the no-triangle figure. These results suggest that the N1 amplitude is largely determined by the possibility of interpolating a form in the figure. The greatest N1 to the modal figure further suggests that interpolation may be increased by modal completion and decreased by the features that diminish the saliency of triangle in the amodal figure and the figure with outlined inducers. On the other hand, the largest N2 was evoked by the amodal figure. This effect may index processes activated in response to the great difficulty in perceiving the triangle in the amodal figure, a difficulty that is initially caused by a conflict of perceptions characterizing this figure.

Adult↗
PubMed1995

Isolation and characteristics of a steady-state visually-evoked potential in humans related to the motion of a stimulus.

We have examined the visual potential evoked by two motion stimuli. In the first stimulus (termed coherent motion) a random-dot pattern oscillated between phases of coherent and incoherent ("snowstorm") motion, and in the second a random-dot pattern alternated in direction of motion (termed direction change). We found that the response to the coherent motion stimulus is low-pass with respect to speed, has low contrast sensitivity and increases steadily with the contrast of the stimuli. The direction change visually-evoked potential (VEP) is band-pass with respect to speed, has high contrast sensitivity but then saturates and even reduces as the stimulus contrast is raised above 0.1. The behaviour of the direction change VEP is similar in nature to results from psychophysical experiments of motion perception and to the known properties of directionally selective cells of the cortex. On the other hand the behaviour of the coherent motion VEP suggests this may not be mediated by a mechanism specific to motion.

Contrast Sensitivity↗
PubMed1989

Hemispheric asymmetry of the harmonic components ot the visual potentials evoked by the onset-offset of spatially periodic stimuli.

The distribution on the scalp of the first (H1) and second (H2) harmonic components of the steady-state visual evoked potential was studied. The stimulus was the onset-offset of a sinusoidal grating with spatial frequency of 4 c/deg, temporal frequency of 7.1 or 16.7 Hz and contrast of 0.2. The visual evoked potentials were recorded monopolarly from symmetric points at distances of 3, 6 and 9 cm laterally from Oz. It was found that while in the averaged data for all investigated subjects there were no differences in the distributions of H1 and H2 to the right and to the left of Oz, the different individuals had a characteristic asymmetry in the amplitude of the response. The statistical processing of the data revealed a significant prevalence of the second harmonic in the left hemisphere upon stimulation at 16.7 Hz for right-handed subjects. The hypothesis about the dominating role of the left cerebral hemisphere in the processing of signals with high temporal frequency was supported for this group of subjects.

Brain↗
PubMed1991

A PC data-base with analytical applications for evoked potentials.

Visual Evoked Potentials (VEPs) are gaining ground in the research for diagnosis of neurological disorders and visual defects, as a non-invasive diagnostic tool. Yet, the methods used towards these goals are not universal and far from able to provide a common ground among researchers in collecting, analyzing and comparing their results. This paper is an attempt to close the gap. We have developed a PC data-base and a set of analysis programs with graphic capabilities, frequency analysis, as well as an objective way of describing the signals obtained during VEP experiments.

Database Management Systems↗
PubMed1985

Evoked potentials in patients with Huntington's disease and their offspring. II. Visual evoked potentials.

Visual evoked potentials (VEPs) to full-field and half-field stimulation were recorded in a large sample of patients with Huntington's disease (n = 36) and subjects at risk (first-order offspring, n = 55) using a checkerboard pattern reversal stimulus. Whereas a clear diminution of the amplitudes found confirmed earlier findings in patients with Huntington's disease, further alterations of VEP wave forms consisted in the absence of asymmetric hemisphere lateralization to half-field stimulation (i) and severe distortion with amplification of the early NPN/PNP complexes (ii). Considering at least two different abnormal parameters necessary to represent a reliable indicator of the disease, 24 patients (67%) and 14 (25%) of the persons at risk exhibited pathological results. In contrast, only 2 among 36 patients (5%) but 30 (55%) among the offspring exhibited normal results. The diagnostic and predictive value of the investigation of VEPs in Huntington's disease is discussed.

Adolescent↗
PubMed2006

Contrast-response functions for multifocal visual evoked potentials: a test of a model relating V1 activity to multifocal visual evoked potentials activity.

The multifocal visual evoked potential (mfVEP) is largely generated in V1. To relate the electrical activity recorded from humans to recordings from single cells in nonhuman primate (V1) cortex, contrast-response functions for the human mfVEP were compared to predictions from a model of V1 activity (D. J. Heeger, A. C. Huk, W. S. Geisler, & D. G. Albrecht, 2000) based upon single-cell recordings from monkey V1 (e.g., D. G. Albrecht, 1995; D. G. Albrecht, W. S. Geisler, R. A. Frazor, & A. M. Crane, 2002; D. G. Albrecht & D. B. Hamilton, 1982; W. S. Geisler & D. G. Albrecht, 1997). A second purpose was to fully articulate the assumptions of this model to better understand the implications of this comparison. Finally, as the third purpose, one of these assumptions was tested. Monocular mfVEPs were obtained from normal subjects with a contrast-reversing dartboard pattern. The display contained 16 sectors each with a checkerboard. Both the sectors and the checks were scaled approximately for cortical magnification. In Experiment 1, there were 64 checks per sector. The contrast-response functions were fitted well up to 40% contrast by the theoretical population curve for V1 neurons; there was a systematic deviation for higher contrasts. The model, as articulated here, predicts that the contrast-response function should be the same and independent of the size of the elements in the display. Varying the size of the elements by varying the viewing distance in Experiment 2 produced similar results to those in Experiment 1. In Experiment 3, the viewing distance and sector size were held constant, but the size of the elements (and therefore the number of checks per sector) was varied. Changing check size by a factor of 16 had relatively little effect on the contrast-response function. In general, the mfVEP results were consistent with the model based upon the V1 neuron population. However, two aspects of the results require further exploration. First, there was a systematic deviation from the model's contrast-response function for higher contrasts. This deviation suggests that one or more of the model's assumptions may be violated. Second, the latency of the mfVEP changed far less than expected based upon single-cell data.

Adult↗
PubMed1985

[Use of visual evoked potentials in neurology--a review. II].

Visual potentials evoked by whole-field pattern reversal (PRVEP) are most frequently used for the diagnosis of damage of the optic nerve. Already in monocular whole-field stimulation typical PRVEP findings could also be observed in case of a damage of the visual pathway in its further course (chiasm or lesion of the optic tract). The application of a hemi-field stimulation led to a distinct improvement of the information value with respect to the presence of a damage of the optic chiasm. A reduction of the amplitudes at the beginning of the disease, the subsequent extension of the P100 latency and the disappearance of a response to the stimulus are considered to be typical. In recent time, the combined application of whole- and hemi-field stimulation also in the diagnosis of lesions of the posterior visual pathway (especially in the region of the occipital lobe) has brought about a certain advance. The results still are contradictory in some respects and require further clarification. The introduction of the VEP into clinical practice as a non-invasive neurophysiological method involving hardly any stress has led to a considerable enrichment of clinical diagnostics and one can still expect a strengthening of this position in conjunction with the increasing perfection of the examination technique.

Dominance, Cerebral↗
PubMed1997

Variation of topographic visually evoked potentials across the visual field.

We examined the variation of monocular pattern reversal topographic visually evoked potentials across a 13.6 degrees visual field and measured the parameters of the VEP waveforms in 37 locations across the field. Eighteen right and 20 left normal eyes were tested from 23 subjects. The mean response densities of the VEP decreased with increasing eccentricity; response densities are higher in the lower hemifield than in the corresponding mirror symmetric locations in the upper hemifield. The incidence of polarity reversal is higher in the upper hemifield, especially in more superior locations. The latency in the temporal field is shorter than that in the other locations. The coefficients of variations (CVs) of latencies are less than those of amplitudes in corresponding locations and the CVs of latencies of P1 and N2 components are less than those of the N1 component in corresponding locations. The CVs of amplitudes of the waveforms in mid-peripheral locations are larger than those of the other central areas. The parameters of the human topographic visually evoked potential are distributed regularly across the visual field and appear to reflect the underlying anatomy of the retina and visual cortex, and the placement of the recording electrode.

Adolescent↗
PubMed2001

[Usefulness of visual evoked potentials in visual acuity examination].

Determination of visual acuity by means of VEP is carried out usually by the calculation of the amplitude function response from the value of pattern single element. That purpose is achieved by carrying out several tests of transient VEP performed each time with stimulation, using the pattern of different size of individual elements or changing sweep the following sizes of the pattern single element. Concentration of a patient is necessary during any examination and therefore in clinical practise the second of the listed methods is preferred because of its short duration. Correlation factor of visual acuity, determined by means of VEP and Snellen tests, differs depending on the examined group from about 0.4 to 0.9 and it is the lowest in case of ophthalmological disorders connected with optic nerve. Fixing of VEP optimal parameters when evaluating the visual acuity remains an open matter.

Evoked Potentials, Visual↗
PubMed2002

Pattern visual evoked potential (PVEP) evaluation in hypothyroidism.

Dysfunction of the central nervous system (CNS) is an important consequence of thyroid hormone deficiency. Evoked potentials like visual evoked potentials (VEP) provide a reliable and objective measure of function in related sensory system and tracts. In this study pattern-shift VEP (PVEP) recordings were performed on 48 newly diagnosed hypothyroid patients. Twenty-four had sub-clinical and 24 had overt hypothyroidism. None of the patients had clinical symptoms or signs referable to CNS dysfunction. Their mean age was 44+/-12 yr. The response to pattern stimulation on the normal control subjects was a triphasic response with a prominent positive wave (P100) with a peak latency of 84-105 (mean: 96+/-4) milliseconds (ms). In patients with hypothyroidism mean P100 latency was (mean: 97+/-6) ms and the difference between the 2 groups was not statistically significant. (p>0.05) Delays above the average latency +/-2.5 SD of the mean of the control subjects was defined as a criteria for an abnormality. According to defined criteria 6 (12.5%) patients demonstrated abnormal PVEP at least on one tested side. Previous studies conducted on small patient populations stated there is high percentage of VEP abnormalities in hypothyroid patients. However, this fact was not confirmed by our study. We believe abnormalities of PVEP will be more prominent in untreated patients in the advanced stage of the disease, or in patients who have a neurological involvement; such as apathy, impaired memory or cerebellar dysfunction. Consecutive studies, in a more clearly defined and selected patient population, are needed to confirm and settle this issue.

Adult↗
PubMed1989

Central effects of drugs used in migraine prophylaxis evaluated by visual evoked potentials.

The present study used recordings of visual potentials evoked by pattern reversal (VEPs) to investigate the central effects of three drugs used in migraine prophylaxis: the calcium channel blocker nifedipine, the beta-1-selective blocker metoprolol, and the nonselective beta adrenoreceptor blocker propranolol. The study involved 58 patients with common or classical migraine who were treated in a double-blind randomized study over a period of 7 months, while the effectiveness of prophylactic treatment was recorded in headache diaries that were subjected to time series analysis. VEPs were recorded at the beginning of a 2-month baseline period without treatment, after 4 months of treatment, and at the end of a 3-month washout period. At baseline, migraine patients had significantly higher VEP amplitudes and longer latencies than did a group of 87 healthy control subjects. Patients were separated by statistical analysis into responders and nonresponders to each prophylactic treatment. Nifedipine had no effects on the frequency, intensity, and duration of migraine attacks, nor on amplitude and latency of the VEPs. In contrast, the use of beta blockers resulted in a significant decrease in VEP amplitude, both in responders and nonresponders, whereas VEP latency remained unchanged. VEP amplitudes returned to the initial values at follow-up in the nonresponders, but stayed at lower levels in responders. Beta blockers thus appear to have a significant effect on the increased excitability of the visual system in patients with migraine, although their action is not directly related to their reduction of migraine frequency.

Adult↗
PubMed1990

Pattern-reversal visual evoked potentials recorded in children with generalized epilepsy.

Visual potentials evoked by pattern reversal (PRVEPs) were studied in 64 normal subjects (age range 7 to 15 years) and in 15 patients with primary generalized epilepsy (age range 8 to 13 years), 10 of whom were without anticonvulsant medication. Most of them were studied during sodium valproate (VPR) therapy and some during carbamazepine (CBZ) medication. A Quadristim set (Alvar) was used to present checkerboard patterns on a TV monitor, to amplify the EEG signals and to average and plot the evoked potentials. The potentials were elicited by binocular full-field 2/s checkerboard reversals, recorded from an electrode 4 cm above the inion, and analyzed for latency, amplitude and waveform. Our PRVEP measurements examined peak latency of positive P2 (or P100) component and trough-to-peak amplitude on N1P2 wave complex. The degree of similarity between pairs of PRVEP plots were determined by Pearson correlation coefficient r for an analysis time of 150 ms. In most of our patients, no pronounced influence of the disease itself on the parameters and waveform of the normal PRVEP pattern was demonstrated if anticonvulsant drugs were not taken. In patients who were under complete seizure control, the anticonvulsant did not change the PRVEP morphology as well. The PRVEP abnormality was most pronounced in patients who were taking anticonvulsant medication, but whose seizures were poorly controlled. This pattern distortion can be revealed by the correlation coefficient, but not by other PRVEP parameters. Therefore, this coefficient may be useful as a sensitive and objective measure both of PRVEP distortion and PRVEP improvement. Our results give further evidence that nondemyelinating disorders, but with synaptic transmission defects, can produce measurable changes in PRVEP morphology.

Adolescent↗
PubMed2005

Topographic analysis of flash visual evoked potentials in dogs.

Visual evoked potentials by flash stimulation (flash VEP) were analyzed in dogs using a topographic method. The flash VEP consisted of 3 positive (P1, P2 and P3) and 2 negative (N1 and N2) components by 150 msec after the flash stimuli. On the topographic mappings, a negative response area was observed in the frontal region of the scalp in the stimulated site followed by the shifting of the area to the contralateral frontal region and occipital region, during the first 100 msec. The negative response area in the frontal region in the stimulated site, contralateral frontal and temporal region, and occipital region were corresponded to N1, P2, and N2 on the flash VEP, respectively, according to their latencies. In the dogs with experimentally impaired the right lateral geniculate body, the latency of P2 was prolonged, and N2 and P3 were disappeared after the left eye stimulation. On the topographic mapping, only the early negative response area was detected on the stimulated site of the frontal region of the brain. Therefore, it is concluded that P1 and N1, P2, and N2 are referred to the retinal potentials, the potentials from the retina to the brainstem included the lateral geniculate body, and those from the brainstem to the visual cortex, respectively.

Animals↗
PubMed1985

Testing of concentric visual field constriction by means of scotopic visually evoked potentials.

Using scotopic visually evoked potentials (VEP), an objective test of concentric absolute field defects is presented. At 0.8 log units above the mean VEP threshold, the full field, the central area of 50 degrees diameter, and the complementary peripheral field were flash stimulated. In 13 normal subjects the peripheral VEP response was larger in amplitude and shorter in latency compared to the central response. In four cases of concentric field restriction due to hysteria and malingering, the same results were found. In three cases of retinitis pigmentosa and advanced glaucoma, the peripheral VEP sensitivity was worse than the central one or no response could be found. The amount of stray light was estimated as the difference of the thresholds for central and peripheral stimulation (1.6 to 1.8 log units) in a patient with a residual central field of 20 degrees.

Adolescent↗
PubMed1992

Activation state of the cortex could be changed by reinforcement of low-amplitude visual evoked potentials in rabbits.

Visually evoked potentials (VEP) were recorded chronically from occipital cortex in awake rabbits. The VEP typically consisted of a triphasic response (positive deflection P60--negative deflection N205--positive deflection P450) that was followed by a late negative shift starting about 750 ms after the eliciting flash. After computation of a discriminating amplitude value (200 or 250 microV) the VEP were divided for comparison in high- and low-amplitude groups. Significant differences between these groups existed in amplitudes and latencies of the early VEP components. High-amplitude VEP were followed by larger late negative shifts and had significantly (P less than 0.01) earlier second positive deflections. In these high-amplitude cases, the EEG-baseline was more negative than in the low-amplitude VEP. In addition, we found a more synchronized background EEG in the low-amplitude VEP group. We conclude that different VEP amplitudes depended on different activation states of the cortex which could be changed by reinforcement.

Animals↗