Visual Evoked Cortical Potentials Research Articles

A new procedure for the registration of the visual-evoked cortical potential by multichannel recording of the gradient distribution.

A new bipolar multichannel visual-evoked cortical potential (VECP) procedure is presented, which can supply objective information on visual field losses. The electrical potential is recorded from 11 electrodes applied to the back of the head in the form of an equidistant rectangular grid. Each neighbouring pair of electrodes feeds one of 14 bipolar channels. The adjacent horizontal and vertical channels are used to calculate an approximation to the direction and amplitude of the electrical field gradients. The gradient distribution is represented by a map of arrows (gradient map) for every instant of the sweep, so that the whole sweep can be plotted as a time series of gradient maps. The maps are easy to scale and are well suited for visual evaluation. Twenty normal subjects were stimulated using checkerboard reversal, partial field patterns to simulate visual field defects. The stimulated area varied between full-, half- and quarter-field, and the particular area stimulated could be clearly seen in the resulting gradient maps. Additionally, we developed a computerized classification procedure that detected 86% of the disturbed visual fields from the gradient recordings.

The measurement of steady-state visual evoked cortical potentials using an adaptive noise canceller

A new software method for measuring steady-state visual evoked cortical potentials (VECPs) has been developed using the principle of adaptive noise cancelling. The steady-state VECP is composed of narrow-band frequency components at harmonics of the stimulus frequency. In clinical recordings, these signal components are masked by wide-band noise, predominantly electroencephalographic activity and muscle noise. The stimulus frequency is exactly known and by using a reference sinusoid at the stimulus frequency (or its harmonics) the adaptive noise canceller (ANC) is able to cancel uncorrelated noise components from the recording. In effect, the ANC functions as an adaptive narrow-band-pass filter at the reference frequency. The performance of the ANC has been evaluated using both simulated and physiological signals. The output of the ANC provides temporal information on the signal amplitude and phase, and can be used to calculate the reliability of signal detection. For this application, the ANC has a number of advantages over the fast Fourier transform: it is a more sensitive detector, it requires fewer calculations, it is less computationally intensive, it requires less memory and it can be implemented in real time.

Visual evoked potentials in three-dimensional color space: Correlates of spatio-chromatic processing

Visual evoked potentials (VEPs) were measured for sinusoidal gratings with spatio-chromatic modulation defined in a three-dimensional color space. The spatio-chromatic modulation of the gratings can be decomposed into contributions from an achromatic luminance varying component, an isoluminant component which modulates only the activities of L cones and M cones, and an isoluminant component corresponding to modulation of only S-cone activity. The emphasis of this report is the nature of VEPs resulting from isoluminant spatio-chromatic modulation. The VEP response was characterized along a number of spatial, temporal, and chromatic stimulus dimensions: contrast, spatial frequency, chromaticity in the isoluminant plane, chrominance/luminance ratio, orientation, and temporal frequency. Isoluminant VEPs resulting from stimuli modulating L and M cones are compared with those from S-cone modulation. When appropriate spatiotemporal conditions are employed, both types produce robust VEPs; however, the S-pathway VEPs show considerably longer latencies than do those from LM-pathway activation. The VEP results are compared to psychophysical and single unit electrophysiological observations. VEP latencies exhibit the lowpass character of psychophysical chromatic contrast sensitivity functions but VEP amplitudes show bandpass tuning along both the S and LM axes. An oblique effect, i.e. shorter latencies for horizontal and vertical gratings than for diagonal, is observed in the isoluminant VEP. S-pathway VEPs are used to demonstrate an electophysiological correlate of transient tritanopia. Normative amplitude and latency data for VEPs from selectivity stimulated chromatic mechanisms provide a baseline for clinical electrodiagnostic applications.

Influence of Long-Term Stimulation on Pattern-Reversal Visual Evoked Cortical Potentials

In 20 subjects, we recorded the pattern-reversal visual evoked cortical potential (VECP) over a period of 30 min. We could not find any remarkable alterations of the peak times, they were unchanged over the time. During the first 10 min, the amplitudes showed a clear tendency to decrease if one evaluated the averaging value of the whole group. After this time, there were no significant changes. But we learned a quite different behavior of amplitudes evaluating the record of each subject. We found 5 types of different behavioral modulated responses. The problems of long-term investigation are sophisticated and need further investigation to learn which different factors have an impact on VECP. From a clinical point of view, a long-term investigation shows no relevant potential alterations.

Chapter 34 Colour vision: isolating mechanisms in overlapping streams

Publisher Summary This chapter discusses electrophysiologically the basis of color vision, and show that cortical visual evoked potentials (VEPs) correlate with the known spatio-temporal limits of color vision. First, the extent of overlap of the spatio-temporal properties of achromatic and chromatic-opponent cells is discussed. Second, an assessment of the relationship between local and global electrophysiological measures of color vision is made, and field potential recordings are compared with single-unit and multi-unit recordings made in the same cortical site. Third, it is shown that the use of low-contrast, low-spatial frequency gratings presented at a low temporal rate dichotomize the global responses measured as VEPs: achromatic gratings elicit transient on-off responses, consistent with the responses of most transient M-cells, which are dependent on a step change in contrast, whereas chromatic gratings generate sustained responses that are different for onset and offset presentations, and thus similar to the responses of chromatic opponent cells. Fourth, chromatic VEPs are shown to be selectively diminished by administration of N-Methyl-D-aspartate (NMDA) antagonists, consistent with the notion that NMDA receptors facilitate sustained responses. Finally, chromatic VEPs recorded from subjects with lesions of cortical visual area V4 are seen to be unchanged in shape, and little different in magnitude, relative to normal. This suggests that color detection and categorization are mediated by mechanisms operating not later than V2 in the chromatic processing stream.

Visual thresholds in albino and pigmented rats.

Albino rats have recently been reported to have increment thresholds against dim backgrounds that are two log units higher than those of pigmented rats. We, on the other hand, have failed to confirm these differences using electroretinogram b-waves and pupillary light reflexes. This paper reports on experiments using evoked potentials from cortex and colliculus and single-unit recordings from colliculus. We recorded visual-evoked potentials from cortex and superior colliculus in the strains of albino (CD) and pigmented (Long-Evans) rats used in the earlier studies. Thresholds were determined on eight fully dark-adapted animals by extrapolating intensity-response curves to the point at which there was zero evoked potential. The average dark-adapted threshold for the visual-evoked cortical potential was -5.6 log cd/m2 in pigmented and -5.80 log cd/m2 in albino animals. The average dark-adapted threshold for the superior colliculus evoked response was -5.54 log cd/m2 in pigmented and -5.84 log cd/m2 in albinos. The differences were not statistically significant. On the same apparatus, the average absolute threshold for three human observers was -5.3 log cd/m2, a value close to the rat dark-adapted thresholds. Thus, visual-evoked cortical potentials and superior collicular evoked potentials failed to confirm the report of higher dark-adapted thresholds for albinos. In addition, we find that single units in superior colliculus in the albino rat respond to very dim flashes.

Assessment of central visual function after successful retinal detachment surgery by pattern visual evoked cortical potentials.

The pattern of visual recovery after successful surgery by pattern visual evoked cortical potentials (VECP), visual acuity, colour vision, and critical fusion frequency was investigated in 14 eyes with retinal detachment involving the macula. The temporal tuning characteristics in the evoked potentials were measured as based on the P100 amplitude and the frequency necessary for evoking 0 mu V amplitude, which was defined as an objective critical fusion frequency by extrapolating the curve. Significant improvement in visual acuity and colour vision was observed within 2 months postoperatively. A significantly increased P100 peak latency became shorter as the postoperative days increased. In general, a good correlation was noted between the P100 peak latency and subjectively measured visual acuity, colour vision, and critical fusion frequency. The objective critical fusion frequency measured by VECP recovered gradually during the 6 months after surgery. Functional recovery was not related to the length of time the macula was detached before surgery.

Abnormalities of the electroretinogram and visual-evoked potential in vitamin E deficient rats

Flash electroretinograms, retinal oscillatory and cortical visual-evoked potentials were recorded in 10-month vitamin E deficient rats and in age-matched controls. A significant increase in the latency ( P < 0·0001) and decrease in amplitude ( P < 0·0001) of the electroretinogram a- and b-waves were observed in the vitamin E deficient rats compared with controls. The vitamin E deficient rats also showed a significant delay ( P < 0·002) in early oscillatory potentials. No significant group differences were obtained in the P1 latency or P1-N1 amplitude of the visual-evoked potential; however, the N1 peak was significantly delayed ( P = 0·01) in the vitamin E deficient rats. This study shows, for the first time, that the electroretinogram and visual-evoked potential to flash stimulation provide a sensitive index for monitoring the visual effects of vitamin E deficiency in the rat.

Visual function before and after ACTH therapy for vision loss in multiple sclerosis

The visual function of a patient with multiple sclerosis (MS) was evaluated by visually evoked cortical potentials (VEPs) and the contrast sensitivity function (CSF) before and after adrenal corticotrophic hormone (ACTH) therapy for an acute exacerbation of the condition. Although Snellen visual acuity returned to normal in each eye, steady-state pattern reversal VEP waveforms remained degraded long after therapy and monocular CSFs were reduced unequally 18 days after therapy, and even three years later. On the other hand, flash VEPs, recorded within three weeks of ACTH therapy, showed an improvement that parallelled the patient's overall improvement which appeared to be associated with the ACTH therapy. These observations indicate that MS can affect spatial and temporal channels differentially, reinforce the previously reported clinical utility of VEPs for the diagnosis of MS, and suggest that they may be simple, clinically useful, objective indices of the effectiveness of any interventive medical management of the disease. Language: en

Electrophysiology and colour perimetry in dominant infantile optic atrophy.

A typical finding in dominant infantile optic atrophy (DIOA) is the variation of the phenotypic expression of the DIOA gene even within one family. It is of special interest for genetic consultation to evaluate an examination method for detecting subclinically involved patients. Seven patients of two families were examined. Three of them had the typical symptoms of DIOA: reduced visual acuity, tritan defect, temporal pallor of both optic discs, and a relative central scotoma for white test spots. In visual evoked cortical potentials (VECP) the amplitudes were reduced, and in one patient the latencies were slightly delayed and two patients considerably so. The amplitude of the negative component of the PERG was markedly reduced, while the positive component was normal. In the remaining four family members normal retinal and cortical responses were recorded under standard conditions and visual fields and colour vision (FM 100 hue) were also normal. However, static perimetry with blue test spots showed in two family members enlarged central scotomas, thus proving that they had subclinical DIOA.

Electroretinograms and pattern visually evoked cortical potentials in central areolar choroidal dystrophy

We describe three patients with central areolar choroidal dystrophy whose electroretinograms (ERGs) and pattern visually evoked cortical potentials (VECPs) confirmed their macular dysfunction. Visual fields measured by Goldmann perimetry showed central relative scotomata corresponding to a dystrophic lesion. Dark adaptation curves were slightly abnormal either in the first curve or in the second curve, depending on the visual acuity. Color vision was disturbed irregularly. Fluorescein angiography revealed a loss of the choriocapillaries, and a hyperfluorescent border outlined the dystrophic lesion. Single-flash dark-adapted ERGs and scotopic ERGs were almost normal, while photopic and flicker ERGs showed slightly attenuated amplitudes. Conversely, VECPs to pattern reversal stimulation were greatly affected for both transient and steady-state stimuli. We conclude that, in central areolar choroidal dystrophy, pattern VECPs can provide more information on macular dysfunction than does the ERG.

Haloperidol Delays Visually Evoked Cortical Potentials but Not Electroretinograms in Mice

Visually evoked cortical potentials (VECPs) and electroretinograms (ERG) to flash stimulation were studied in one ICR strain mouse and 23 ICR mice, respectively. The effect of a dopamine receptor blocker, haloperidol, on both responses was investigated. Significant peak latency prolongation was found in VECPs following injection of haloperidol, while the amplitude reduction of the b-wave was found in ERG. No effect of anesthetics only on ERG was found. It has been assumed that haloperidol affects VECPs related function mainly the higher visual pathway. However, the slight decrease of the b-wave amplitude of ERG could not exclude the possibility of the toxic effect on the retina.

Visual Acuity Measurements by Swept Spatial Frequency Visual-Evoked-Cortical Potentials (VECPs): Clinical Application in Children with Various Visual Disorders

Previous studies have indicated that visual acuities of normal infants can be estimated with good accuracy using swept spatial frequency visual-evoked-potentials (VECPs). In this report we describe acuity measurements obtained with this technique from 304 examinations performed on 135 children having various visual disorders. When possible, two or more different stimulation frequencies (8, 12, 15 or 24 contrast reversals/sec) were used in each patient, and three to eight sweep VECPs were obtained from each patient under each simulation and recording condition. High correlation coefficients (0.94 - 0.96) between the acuity estimated on each patient from either the single sweep giving the best visual acuity (BSS) or from vector averages (VeA) of the EEG data obtained from several sweeps confirmed previous findings in normal infants. We also found high correlation coefficients among BSS recorded at different temporal frequencies (0.79-0.97) and among comparisons of BSS or VeA acuity to optotype visual acuity (0.6-0.89). Children with clinically undetectable optokinetic responses showed lower visual acuity estimated by BSS than those who demonstrated optokinetic nystagmus. We conclude that the sweep VECP is a valid method, giving estimates of acuity which correlate well with optotype acuity and correspond well to other clinical findings, and that it can be useful in the clinical management of nonverbal patients.

Preliminary studies of cytokine-induced functional effects on the visual pathways in the rabbit

Epidural visual evoked potentials (VEP) were used to study the role of cytokines in the induction of pathophysiologic changes associated with inflammation in the central nervous system (CNS) of the rabbit. In normal rabbits, intraocular injection of human recombinant interferon-γ (IFN-γ) and tumor necrosis factor (TNF) increased the peak latency of the cortical VEP by more than 2 ms within 3 h of injection; equal volume injections of control subtances had no effect. Alterations in conduction induced by IFN-γ and TNF reversed within 24 h and could be reinduced by reinjection. Intraocular injection of recombinant human interleukin-1β (IL-1) induced a more progressive delay in conduction that peaked 24 h after intraocular challenge and reversed over the ensuing 48 h. Pathologic examination of the tissues indicated that the primary effect of these cytokines is on the vasculature and induces changes associated with inflammation. The results suggest that the acute reversible effects of cytokines on CNS function are associated with vascular events; further they support the sensitivity of the ‘rabbit eye model’ for studies on the pathophysiologic effect of inflammatory mediators on the CNS in vivo.

Retinal and retinocortical times to pattern stimulation in amblyopic children.

In order to determine whether in amblyopes retinal conduction delays contribute to the cortical measureable delays in the visual evoked cortical potential (VECP), peak latencies of the pattern electroretinogram (ERG) are measured in amblyopic children. The results are compared with those of the normal fellow eyes and those of a healthy control group. Simultaneously the latencies in the VECP are recorded and the determination of the retinocortical times is performed. Statistically retinal b-wave (Q) and a-wave (P) of the pattern ERG of amblyopic eyes do not show significant delays of peak latency. In retincortical times, however, there are significant prolongations. During occlusion therapy retinocortical values of normal fellow eyes are also delayed in comparison with the control group. A pathological conduction delay of visual information on the retinal level up to the generators of the pattern ERG can thus be excluded in amblyopia. The total latency delay in the VECP of amblyopes consists solely in a prolongation of retinocortical times.

A model for the determination of VECP latencies.

The time between stimulus event and onset of the visually evoked cortical potential (VECP) reflects the dynamic transmission characteristics of the precortical part of the visual system. A simple mathematical model describing this sub-system with regard to the triggering of luminance VECP is presented. The four functional parameters of the model can be calculated from the measured latency versus stimulus intensity curve of luminance VECP and can provide a quantitative evaluation of the functional state of the precortical visual system for clinical use. Some experiments and first clinical results show the applicability of the model.

Accommodation power determined with transient pattern visual evoked cortical potentials in diabetes.

Changes of accommodation power by aging were measured with pattern reversal visual evoked cortical potentials (VECPs) in 45 normal subjects and 32 patients suffering from diabetes mellitus. Transient VECPs were recorded by increasing minus power lenses in front of the eye at one-diopter steps, up to the point where no VECP was recordable. It was found that the amplitude of the P100 component became attenuated linearly with increased accommodation stimulus. The regression line was calculated from the VECP amplitude vs. accommodation stimulus plots (in diopters) and the objective accommodation was determined by extrapolating the line to 0 microV amplitude. The measured accommodation power with pattern VECPs was attenuated significantly in the normal group over 40 years old and in diabetics, and also was found to be larger by approximately 2.5 diopters than that obtained subjectively by the near-point rule.

Determinants of visual evoked potentials in preterm infants

The latency and amplitude of NI, the first negative peak of single flash visual evoked cortical potentials (VEP) were evaluated in 86 preterm infants (25–34 weeks of gestation), investigated during the first day of life. All infants were regarded stable at the time of investigation and none had experienced severe asphyxia. Both the VEP latency and amplitude were inversely related to gestational age. When latency was corrected for age it was longer in boys than in girls. Age corrected VEP amplitude was inversely related to head circumference, umbilical cord pH and EEG activity. There was no difference between VEP parameters in asymmetrically growth retardated infants compared to those with normal growth. Administration of dexamethasone to the mother before delivery, mode of delivery, presence of hyaline membrane disease, mode of assisted ventilation or imminent development of peri-intraventricular haemorrhage did not affect the VEP.

Vitrectomy for diabetic eye disease. The prognostic value of pre-operative electroretinography and visual evoked cortical potentials.

An investigation was carried out into the prognostic value of standard flash electroretinography (ERG) and visual-evoked cortical potentials (VECP) recorded for 87 diabetic eyes prior to vitrectomy. 95% of the eyes with a recordable ERG b-wave achieved a post-operative visual acuity (VA) of 0.05 or better (positive predictive value), but only 35% of the eyes with a good post-operative VA had a recordable b-wave (sensitivity). The corresponding values for the ERG a-wave were 67 and 72%, and for the VECP 77 and 69%, respectively. A non-recordable b-wave in the ERG was associated with a poor visual outcome (VA less than 0.05) in 45% of cases (negative predictive value), and 97% of the eyes with a poor visual outcome had a nonrecordable b-wave (specificity). The corresponding values for the ERG a-wave were 42 and 37%, and for the VECP 53 and 63%, respectively. Consequently, the odds ratio for an unfavourable visual outcome (VA less than 0.05) was greater for a non-recordable b-wave (15.7) than for a nonrecordable VECP (3.8) or a non-recordable a-wave (1.5). Recordability of the b-wave gave the best prediction for attached macula. Extensive photocoagulation diminished all ERG and VECP amplitudes. Vitreous opacities had no significant effect on the ERG or VECP.

Direct and transcallosal contribution to the cortical visual evoked response in rats

The averaged visually evoked cortical potential (VECP) in response to contrast reversal of a grating was measured on striate cortex over a range of spatial frequencies and contrasts. The response to binocular or monocular stimulation was almost abolished by unilateral section of the optic tract on the side of the recording, indicating that the transcallosal pathway makes little contribution to the VECP. Additional section of the corpus callosum, and application of spreading depression to the normal hemisphere shows that the small response following tract section was transcallosal. It was confined to stimuli of low spatial frequencies and high contrast.