The recovery cycle of the pattern visual evoked potential in normal subjects and patients with multiple sclerosis

The electrophysiological assessment of visual function in Multiple Sclerosis

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VEPs have largely been replaced by MRI in modern MS diagnosis and management.

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Multifocal VEPs are superior to traditional VEPs in evaluating the integrity of the visual system.

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Physiological asymmetry limits interpretation of small VEP differences.

The assessment of vision is integral to the diagnosis and monitoring of patients with multiple sclerosis (MS). Visual electrophysiology, previously a critical investigation in patients with suspected MS, has in large part been supplanted by magnetic resonance imaging in clinical routine. However, the development of multi-focal visual evoked potentials and the advent of putative re-myelinating therapies that can be monitored with these techniques has led to a resurgence of interest in the field. Here, we review the clinical applications, technical considerations and limitations of visual evoked potentials in the management of patients with MS.

Robust sensory gating in the cortical visual evoked potential using two spatially separated stimuli

OBJECTIVE

Sensory gating has been reported to be either absent or weak in the cortical visual evoked potential (VEP) response to diffuse or spatially overlapping stimuli. In this study, the authors evaluated sensory gating to two spatially separated visual stimuli.

METHODS

Spatially separated stimuli were presented either singly or in combination at the same or different onset times and the VEP recorded at either Oz, or O1 and O2, referenced to Cz.

RESULTS

When one visual stimulus is flashed on, the VEP response to another non-overlapping stimulus is almost completely suppressed.

CONCLUSIONS

The VEP does not reflect the bulk activation of retinotopically organized visual cortex, but rather it primarily reflects a distributed mode of visual cortical activity that only indicates that at least one visual stimulus was presented, and not how many or in what order.

SIGNIFICANCE

Other studies performing intracortical recordings of the local field potential (LFP) in visual cortex have identified a slow-distributed component that exhibits the same nonlinearity found here in the VEP, suggesting that these two phenomena are related.

Auditory and visual refractory period effects in children and adults: An ERP study

OBJECTIVE

This developmental study was designed to investigate event-related potential (ERP) refractory period effects in the auditory and visual modalities in children and adults and to correlate these electrophysiological measures with standard behavioral measures.

METHODS

ERPs, accuracy, and reaction time were recorded as school-age children and adults monitored a stream of repetitive standard stimuli and detected occasional targets. Standards were presented at various interstimulus intervals (ISIs) in order to measure refractory period effects on early sensory components.

RESULTS

As has been reported previously in adults, larger components for standards with longer ISIs were observed for an auditory N1 and the visual occipital P1 and P2 in adults. Remarkably similar effects were observed in children. However, only children showed refractory effects on the amplitude of the visual N1 and P2 measured at anterior sites. Across groups, behavioral accuracy and reaction time were correlated with latencies of auditory N1 and visual P2 across ISI conditions.

CONCLUSIONS

The results establish a normal course of development for auditory and visual ERP refractory period effects across the 6- to 8-year-old age range and indicate similar refractoriness in the neural systems indexed by ERPs in these paradigms in typically developing children and adults. Further, the results suggest that electrophysiological measures and standard behavioral measures may at least in part index similar processing in the present paradigms.

SIGNIFICANCE

These findings provide a foundation for further investigation into atypical development, particularly in those populations for which processing time deficits have been implicated such as children with specific language impairment or dyslexia.

Temporal recovery of auditory evoked potentials in individuals at risk for alcoholism

The present investigation examined auditory evoked potential (AEP) recovery functions in both high-risk (HR, N = 23, mean = 22.3) and low-risk (LR, N = 27, mean = 23.0) males. A series of binaural auditory stimuli, with randomly interposed interstimulus intervals (ISIs) of 0.5, 1.0, and 10.0 s, were used to elicit the N1 and P2 components of the AEP. Scalp potentials were recorded from the 19 electrodes comprising the 10/20 International System. For purposes of statistical analysis, five electrode groups were created: frontal (F), central (C), parietal (P), occipital (O), and temporal (T). The results of within-group MANOVA demonstrated that in both LR and HR individuals, increases in the ISI produced significant N1 and P2 amplitude increases without significant latency differences. These amplitude increases occurred primarily in the F and C regions. However, the results of between-group MANOVA indicated that there were no differences in the recovery functions of the two risk groups. Our results indicate that in both LR and HR individuals, recovery functions are responsive to changes in increasing ISI. However, they did not effectively discriminate between risk groups. It is speculated that risk group differences may be apparent with the use of an ethanol challenge.

Cortical and retinal refractory periods in the human visual system

Refractory periods of the visual system were investigated in 12 healthy subjects by simultaneously recording retinal (ERG) and cortical (VEP) evoked electrical activity. Double-flash stimuli were presented at different interstimulus intervals, and response components evoked by the second flash were analyzed in detail, and related to psychophysical detection thresholds. With short interstimulus intervals ERG b-wave peak latencies were increased and b-wave amplitudes were significantly reduced, while P100 component latencies of the VEP were significantly influenced only at long interstimulus intervals. Regression analysis of the individual data as well as analysis of the retinocortical transmission times showed that the cortical latency changes were not simply caused by changes on the level of the retina. Additional influences of the interstimulus interval on nonretinal structures of the human visual system must be assumed. The subjective psychophysical detection thresholds were significantly higher than the threshold values at which reliable electrical or cortical response components could be elicited.

First-order Wiener kernel visually evoked potentials obtained from multiple sclerosis patients

Visually evoked potentials (VEP) were measured in multiple sclerosis patients with five sizes of reversing check stimuli. The VEPs were obtained using random binary sequence triggered check reversals. The random binary sequence was cross-correlated with scalp potential responses to obtain the estimates of the linear response of the system. For each evaluation a series of five VEPs, one for each of five check sizes, was done. A correlation coefficient was calculated to evaluate check size versus amplitude of the major negative wave. A tabulation was made to compare Snellen chart visual acuity with these correlation coefficients. For the eyes with a Snellen acuity of 20/20, 63% of the VEP amplitudes increased as the check size was decreased with correlation coefficients of -0.6 to -1. For the eyes with a Snellen acuity of 20/70 or less the amplitude decreased with the check size decrease, showing a correlation coefficient of +0.6 to +1 in 45% of the evaluations. The check size giving the largest amplitude was also tabulated with respect to the visual acuity, but did not appear to have as strong a relationship to visual acuity.

The refractory period of the visual evoked potential produced by pattern reversal in multiple sclerosis

Refractoriness of the visual evoked potential to pattern-reversal stimulation has been studied using a light-emitting diode stimulator, the effect on a test P100 of a conditioning pattern reversal being determined for conditioning-test intervals of 20 ms, 50 ms and 100 ms. In 20 normal subjects, the latency of the test P100 was increased by a mean of 9.8% (SD 5.0%) with the 20-ms interval, 5.8% (SD 6.6%) with the 50-ms interval, and 2.8% (SD 4.7%) with the 100-ms interval. Patients with clinically definite multiple sclerosis were divided into 3 groups based on the latency of P100 in the conventional VEP: within 2 SD of the normal mean (7 patients), plus 3-6 SD (7 patients), and greater than 10 SD (7 patients), representing normal, mildly abnormal and grossly abnormal latencies, respectively. Only 4 patients had "abnormal" results in the conditioning-test paradigm, 2 of whom were in the "grossly abnormal" group and 1 in each of the other two groups. The prolongation in latency tended to be greater the longer the control P100. The conditioning-test paradigm did not reveal evidence of a significantly prolonged refractory period of demyelinated axons in the optic nerve, and did not assist in the identification of changes "typical of demyelination" in patients with normal or mildly abnormal control P100s. It is concluded that testing the refractoriness of the VEP may not enhance the diagnostic yield of the VEP in multiple sclerosis.