Eye closure affects flash VEP latency in dementia

Impaired Visual Inhibition in Amnestic Mild Cognitive Impairment

Objective.The pathophysiology of amnestic mild cognitive impairment (aMCI) and Alzheimer disease (AD) is still a matter of debate. Visual system might be precociously altered, especially for its cholinergic connections. We thus studied patients with aMCI compared to AD with paired-pulse flash-visual evoked potentials (paired-F-VEPs), a putative marker of cholinergic function. Methods. We enrolled 12 adult patients with aMCI and 12 with AD. 14 normal age- and sex-matched subjects acted as controls (HS). Stimuli were single flashes, with interspersed random flash pairs at critical interstimulus intervals (ISIs, 16.5 to 125 ms) with closed eyes. The "single" (unconditioned) F-VEP was split into a "main complex" (50 to 200 ms after the flash) and a "late response" (200 to 400 ms). As for paired stimulation, the "test" F-VEP emerged from electronic subtraction of the "single" F-VEP from the "paired"-F-VEP. Results. In the single F-VEP, P2 latency was prolonged in patients (aMCI and AD) compared to HS (p < .05). As to the paired F-VEPs, in aMCI the "late response" normal inhibition was abolished at ISIs 50-62.5 ms (p ≤ .016), compared to AD and controls. No changes were detected for the "main complex". Conclusions. Paired-F-VEPs demonstrate a defective neural inhibition in the visual system of patients with aMCI at critical intervals. It may represent a compensatory mechanism against neuronal loss, the failure of which may be involved in AD development. Paired-F-VEPs may warrant inclusion in future preclinical/clinical studies, to evaluate its potential role in the pathophysiology and management of aMCI.

Flash-evoked high-frequency EEG oscillations in photosensitive epilepsies

OBJECTIVE

To determine the feasibility of measuring scalp-recorded, flash-evoked, high-frequency EEG oscillations (F-HFOs) using a relatively simple technique. Furthermore, to assess whether F-HFOs are enhanced in photosensitive epileptic patients and if they might be proposed as a putative non-provocative biomarker of photosensitivity.

METHODS

We studied 19 photosensitive patients with idiopathic generalized epilepsy, and 22 controls matched for demographic features. We extracted F-HFOs from the broadband scalp flash-visual evoked potential (b F-VEP) through appropriate filtering. We measured F-HFO amplitude, number and latency. Also, we carried out a time-frequency domain spectral F-HFO analysis. Inter-group statistics was performed. Within-groups, F-HFO features were correlated to the b F-VEP.

RESULTS

The N3-N3^I wave of the b F-VEP was significantly (p = 0.01) larger in patients compared to controls. The same was true for the inter-group F-HFO amplitude (p = 0.01). F-HFOs showed two main spectral peaks (∼88 and ∼125 Hz), whose power was greater (p = 0.001) in patients than in controls. The ∼88 Hz peak power exceeded the upper normal range in 15/19 patients. Patients showed a significant (p = 0.04) correlation between the ∼88 Hz peak power and the size of the N3-N3^I wave.

SIGNIFICANCE

A simplified F-HFO measurement proved feasible. In patients, F-HFOs were enhanced in terms of both size and spectral power, suggesting a role in the generation of the photoparoxysmal response. Some spectral features of the F-HFOs may be proposed as a putative non-provocative marker of epileptic photosensitivity.

The flash visual evoked potential-P2 and the detection of amnestic mild cognitive impairment: A review of empirical literature

Amnestic Mild Cognitive Impairment (aMCI) is now recognized as an early risk state for the development of Alzheimer's dementia (AD). Biomarkers, including those that are cerebrospinal fluid or brain imaging based, have yet to provide the ultimate marker variable. A need currently exists for a non-invasive, easy to administer biomarker that contains aMCI/AD specific pathognomic information.

OBJECTIVE

The objective of the present investigation was to provide an updated review of the Flash Visual Evoked Potential-P2 (FVEP-P2) as a biomarker for aMCI and AD. The FVEP-P2 has been shown to possess AD specific pathognomic information.

METHOD

A review was conducted of all articles published between the years 1976 and 2019 that examined the clinical utility of the FVEP-P2 in the diagnosis of aMCI or AD. Only 17 published investigations met the criteria of the review.

RESULT

The weighted average effect size, as measured by Cohen's d, was 1.07, with patients diagnosed with either aMCI or AD exhibiting a significant delay in the FVEP-P2 latency. The weighted mean latency for the controls was 143.92 ms (SD = 17.13). The weighted mean latency for the aMCI/AD was 164.02 ms (SD = 21.33). Estimates of sensitivity, specificity, and accuracy were based on the weighted means and standard deviations and were equal to 0.73. The area under the curve was equal to 0.78.

CONCLUSION

The results of the current review suggest that the FVEP-P2 latency possesses AD specific pathognomic information and that it should be included as part of a much larger assessment process that includes neuropsychological, cerebrospinal fluid, and brain imaging findings.

Flash visual evoked potentials (FVEP) in various stimulation conditions

Aim

To compare flash visual evoked potentials (FVEP) elicited using a Ganzfeld bowl (G), Mini Ganzfeld (MG) and Flash Goggles (GG) with eyes open and closed.

Patients and method

The study group comprised 17 volunteers with mean age of 30 years; all of them were examined with the Roland Consult electrophysiological diagnostic system. Active electrodes were placed at O₁ and O₂. With the G and MG stimulators, the flash generated by white-light-emitting diodes (LEDs) presented standard flash of 3 cd s m⁻². The GG used red LED flash of 3 cd s m⁻². Stimulus frequency of 1.0 Hz, low-pass filter of 1.0 Hz and high-pass filters of 100 Hz (G); 50 Hz (MG); 30 Hz (GG) were used. P2 amplitude and latency were compared by the means of the Wilcoxon matched-pairs signed-rank test.

Results

After right eye stimulation (from O₁; n = 17), the mean amplitudes of P2, elicited with the G, MG and GG, were 13, 7 and 10 µV, respectively. The respective latencies were 129, 114 and 110 ms. Hence, the difference between the results obtained with these stimulators was statistically significant (p < 0.05). The mean P2 amplitudes, acquired by the means of the G, MG and GG, were 13 µV, 7 µV and 10 µV for open eyes, and 11 µV, 8 µV and 8 µV for closed eyes. The respective latencies were 129, 114 and 110 ms for eyes open, and 127, 125 and 121 ms for eyes closed. These results of the MG (latency only) and GG (latency and amplitude) stimulation differed significantly (p < 0.05).

Conclusion

The amplitudes and latencies of the FVEP P2 elicited with different stimulators are not suitable for comparison. Closing the eye during the examination had a significant effect on the components of FVEP waveform elicited with the Flash Goggle and on the latency of P2 elicited with the MG.

Transcranial Direct Current Stimulation Effects on Single and Paired Flash Visual Evoked Potentials

Transcranial direct current stimulation (tDCS) applied over the occipital cortex has a controversial effect on the visual cortex excitability. Paired flash visual evoked potentials (paired F-VEPs) offer a unique method to express neural inhibition within the visual system. However, no studies have explored the effects of tDCS on F-VEPs in humans. The aim of this study was to evaluate the changes of single- and paired-F-VEPs during and after tDCS in healthy humans. Twenty-six healthy volunteers participated. F-VEPs were recorded from occipital electrodes with closed eyes. Stimuli were single flashes, intermingled to flash pairs at the interstimulus interval of 125, 62.5, 50, 33.3, 16.6, and 11.1 ms (internal frequency of 8, 16, 20, 30, 60, and 90 Hz). The single F-VEP was split into a "main complex" and a "late response." As to paired stimuli, the "test" F-VEP emerged from electronic subtraction of the single-F-VEP to the paired-F-VEP. In experiment 1, the return electrode was located on the scalp and we studied changes in F-VEPs after anodal, cathodal (1 mA, 15 min) and sham stimulation. A second experiment was performed in which F-VEPs were recorded before, during and after tDCS stimulation (anodal and cathodal) with the return electrode on the neck. F-VEPs recorded in experiment 1 did not detect any significant change after tDCS. In experiment 2 anodal polarization significantly increased the P2 latency (P = .031) and reduced the amplitude of the "late response" of the single F-VEP (P = .008). As for the paired F-VEPs, no significant changes were detected. In conclusion, low-intensity anodal tDCS has weak inhibitory aftereffects on the single F-VEP and no effects on the paired F-VEPs. Further methodological studies are needed to improve polarization efficacy.

Resting state in Alzheimer's disease: a concurrent analysis of Flash-Visual Evoked Potentials and quantitative EEG

BACKGROUND

To investigate to what extent Alzheimer's Disease (AD) affects Resting State activity, the possible impairment of independent electrophysiological parameters was determined in Eye-open and Eye-closed Conditions. Specifically, Flash-Visual Evoked Potential (F-VEP) and quantitative EEG (q-EEG) were examined to establish whether abnormalities of the former were systematically associated with changes of the latter.

METHODS

Concurrently recorded F-VEP and q-EEG were comparatively analysed under Eye-open and Eye-closed Conditions in 11 Controls and 19 AD patients presenting a normal Pattern-Visual Evoked Potential (P-VEP). Between Condition differences in latencies of P2 component were matched to variations in spectral components of q-EEG.

RESULTS

P2 latency increased in 10 AD patients with Abnormal Latency (AD-AL) under Eye-closed Condition. In these patients reduction of alpha activity joined an increased delta power so that their spectral profile equated that recorded under Eye-open Condition. On the opposite, in Controls as well as in AD patients with Normal P2 Latency (AD-NL) spectral profiles recorded under Eye-open and Eye-closed Conditions significantly differed from each other. At the baseline, under Eye-open Condition, the spectra overlapped each other in the three Groups.

CONCLUSION

Under Eye-closed Condition AD patients may present a significant change in both F-VEP latency and EEG rhythm modulation. The presence of concurrent changes of independent parameters suggests that the neurodegenerative process can impair a control system active in Eye-closed Condition which the electrophysiological parameters depend upon. F-VEP can be viewed as a reliable marker of such impairment.

Visual evoked responses in Alzheimer's disease: a review

This review of the literature has shown that a delayed flash P2, in the presence of a normal flash P1 and pattern-reversal P100, can distinguish groups of Alzheimer's patients from groups who are healthy, psychiatrically ill, or suffering from other types of dementia. The VER's usefulness in the individual patient remains undetermined. With today's techniques, too many patients are misclassified. False negatives may result if Alzheimer's patients with visual symptoms are a distinct subgroup. Those with a normal flash VER may have cortical dysfunction outside the visual association areas. This would most likely be in the earliest stages of the disease before the atrophy becomes widespread. It may be that a VER test would only be valid in established disease. Future research should use adequate numbers of patients who are in a single category of mild, moderate or severe, so that the applicability of a VER test in the early stages of the disease can be determined. Strict diagnostic criteria such as the NINCDS-ADRDA guidelines should be used. Patients should be drug free, or at least not taking medications with anticholinergic properties. Flash VER should be obtained using a strobe light with eyes closed, and the pattern VER using a black and white television with a large pattern and high contrast.