Functional Activity of the Retina and Visual Evoked Cortical Potentials in Simulation the Factors of Space Flight in Conditions of Four Month Isolation in a Hermetic Object with Artificial Habitat

Background. The artificial environment of confined space causes a decrease in the functional reserve of the central nervous system and can affect human health and the success of space missions. In solving this problem, the urgent task is to study adaptation mechanisms that adapt the functioning of the visual sensory system to the conditions of the extreme environment. Purpose to obtain new objective data on the alterations in the functional activity of the visual system during prolonged stay of a person in extreme environmental conditions. Methods. Before and after a 4-month isolation experiment simulating a flight to the moon, an electrophysiological study was conducted of six practically healthy crew members with registration of a set of electroretinograms (ERG) and pattern-reversal visual evoked cortical potentials (VEP) according to the ISCEV standards. In dynamics, corrected monocular visual acuity (MVA) was assessed on board. Results. After the end of the experiment, on average for the group, there were no statistically significant changes in the MVA and functional activity of the retina and visual cortex compared with the initial data. However, individual changes on the part of the flicker ERG and reduction of VEP to small patterns stimulating the parvocellular channel of the visual system were revealed in three testers. These changes were associated with higher visually intense work and physical activity of these crew members, and with an individual reaction to sleep deprivation of pilots with increased responsibility. Conclusion. Four-month isolation with imitation of a space mission did not cause significant changes in the functional activity of the retina and visual pathways in healthy crew members. Individual differences of VEP-responses of the parvocellular visual system were revealed, which can reflect a high level of psychophysiological adaptation and stress resistance in physically active crew members.

Improved spatio-temporal measurements of visually evoked fields using optically-pumped magnetometers

Recent developments in performance and practicality of optically-pumped magnetometers (OPMs) have enabled new capabilities in non-invasive brain function mapping through magnetoencephalography. In particular, the lack of cryogenic operating conditions allows for more flexible placement of sensor heads closer to the brain, leading to improved spatial resolution and source localisation capabilities. Through recording visually evoked brain fields (VEFs), we demonstrate that the closer sensor proximity can be exploited to improve temporal resolution. We use OPMs, and superconducting quantum interference devices (SQUIDs) for reference, to measure brain responses to flash and pattern reversal stimuli. We find highly reproducible signals with consistency across multiple participants, stimulus paradigms and sensor modalities. The temporal resolution advantage of OPMs is manifest in a twofold improvement, compared to SQUIDs. The capability for improved spatio-temporal signal tracing is illustrated by simultaneous vector recordings of VEFs in the primary and associative visual cortex, where a time lag on the order of 10–20 ms is consistently found. This paves the way for further spatio-temporal studies of neurophysiological signal tracking in visual stimulus processing, and other brain responses, with potentially far-reaching consequences for time-critical mapping of functionality in healthy and pathological brains.

Distinct Patterns of P1 and C2 VEP Potentiation and Attenuation in Visual Snow: A Case Report

Visual snow syndrome, characterized by persistent flickering dots throughout the visual field, has been hypothesized to arise from abnormal neuronal responsiveness in visual processing regions. Previous research has reported a lack of typical VEP habituation to repeated stimulus presentation in patients with visual snow. Yet these studies generally used pattern-reversal paradigms, which are suboptimal for measuring cortical responses to the onset of foveal stimulation. Instead, these responses are better indexed by the C2, a pattern-onset VEP peaking 100–120 ms after stimulus onset. In this case study, we analyzed the C2 and its adaptation profile in data previously collected from a single patient with visual snow using a “double-pulse” presentation paradigm. In controls, shorter intervals between stimulus pairs were associated with greater attenuation of the C2 VEP, with recovery from adaptation at longer stimulus onset asynchronies (SOAs). However, the visual snow patient showed the opposite pattern, with reduced C2 amplitude at longer SOAs despite distinct C2 peaks at the shortest SOAs. These results stand in contrast not only to the pattern of C2 VEP attenuation in controls, but also to a lack of adaptation previously reported for the pattern-onset P1 VEP in this patient. Exploratory source localization using equivalent current dipole fitting further suggested that P1 and C2 VEPs in the visual snow patient arose from distinct sources in extrastriate visual cortex. While preliminary, these results support differential patterns of VEP attenuation and potentiation within the same individual, potentially pointing toward multiple mechanisms of abnormal neuronal responsiveness in visual snow syndrome.

Macular Morpho-Functional and Visual Pathways Functional Assessment in Patients with Spinocerebellar Type 1 Ataxia with or without Neurological Signs

Spinocerebellar ataxia type 1 (SCA-ATXN1) is an autosomal dominant, neurodegenerative disease, caused by CAG repeat expansion in the ataxin-1 gene (ATXN1). In isolated reports of patients with neurological signs [symptomatic patients (SP)], macular abnormalities have been described. However, no reports exist about macular anomalies in SCA1 subjects carrying the ATXN1 mutation without neurological signs [not symptomatic carriers (NSC)]. Therefore, the main aim of our work was to evaluate whether the macular functional and morphological abnormalities could be detectable in SP, genetically confirmed and with neurological signs, as well as in SCA-ATXN1-NSC, harboring pathogenic CAG expansion in ATXN1. In addition, we investigated whether the macular involvement could be associated or not to an impairment of RGCs and of their fibers and of the neural conduction along the visual pathways. Herein, nine SCA-ATXN1 subjects (6 SP and 3 NSC) underwent the following examinations: visual acuity and chromatic test assessments, fundus oculi (FO) examination, macular and peripapillary retinal nerve fiber layer thickness (RNFL-T) analysis by Spectral domain-Optical Coherence Tomography (Sd-OCT) acquisition, multifocal electroretinogram (mfERG), pattern reversal electroretinogram (PERG) and visual evoked potentials (VEP) recordings. In four eyes of two SP, visual acuity reduction and chromatic abnormalities were observed; in three of them FO changes associated with macular thinning and outer retinal defects were also detected. In three NSC eyes, slight FO abnormalities were associated with qualitative macular morphological changes. By contrast, abnormal mfERG responses (exclusively from foveal and parafoveal areas) were detected in all SP and NSC (18 eyes). No abnormalities of PERG values, RNFL-T, and VEP responses were found, but in one SP, presenting abnormal papillo-macular bundle neural conduction. Results from our SCA-ATXN1 cohort suggest that a macular dysfunction, detectable by mfERG recordings, may occur in the overt disorder, and unexpectedly in the stage of the disease in which there is still an absence of neurological signs. In NSC, an exclusive dysfunction of preganglionic macular elements can be observed, and this is associated with both normal RGCs function and neural conduction along the visual pathways.

Long-term visual pathway alterations after elemental mercury poisoning: report of a series of 29 cases

Background There are few clinical data on retinal involvement after acute exposure to high concentrations mercury and the available reports are based on a small number of patients suffering chronic exposure. The purpose of this paper is to report findings in workers acutely exposed to very high concentrations of mercury vapor with the aim of providing data on a possible direct retinal involvement. Methods Twenty-nine patients and 16 controls were evaluated in a comparative case series. Mercury levels in blood and urine samples, visual acuity (VA), contrast sensitivity (CS), visual field (VF), color discrimination and optical coherence tomography (OCT) were recorded. The pattern reversal visual-evoked potentials (PRVEP), full-field and multifocal electroretinography (ffERG/mfERG), pattern electroretinography (PERG), systemic symptoms, presence of erethism, and electromyography (EMG) were also gathered. A descriptive analysis was performed. The correlations between variables also were studied. In addition, electrophysiological data from those patients with deeper VF defects (group 1) were compared with a normal control group. Results Twenty-six workers exhibited symptoms of erethism. The EMG showed sensorimotor polyneuropathy and multiple mononeuropathy. The VA was slightly affected in 48.27% (n = 14) of subjects. Loss of CS in at least one of four spatial frequencies and color vision alterations occurred in 96.5% (n = 28) and 44.8% (n = 13), respectively. VF alterations were identified in 72.4% (n = 21) patients. No morphologic changes were seen in the OCT scans. Latencies over 100 milliseconds and reduced amplitudes of P100 were found in the PRVEP (p < 0.05). The reduced amplitude of the b wave at the ffERG, of the P50 at the PERG and of the P1 wave at the mfERG results (p < 0.05) suggested that the outer retina was involved. Significant negative correlations among blood mercury levels, VA, and ffERG were observed. Conclusions In this case series, showed that acute exposure to mercury vapor had a hazardous effect on the visual system. Although neurologic and visual pathway involvement was clearly demonstrated, the differences found compared to control support the existence of a direct functional retinal damage and participation in impaired vision in mercury poisoning.

High-frequency changes in single-trial visual evoked potentials for unattended stimuli in chronic schizophrenia

Patients with schizophrenia reveal changes in information processing associated with external stimuli, which is reflected in the measurements of brain evoked potentials. We discuss actual knowledge on electro- (EEG) and magnetoencephalographic (MEG) changes in schizophrenia. The commonly used averaging technique entails the loss of information regarding the generation of evoked responses. We propose a methodology to describe single-trial (non-averaged) visual evoked potentials (VEP) using spectral and statistical analyses. We analysed EEG data registered in the O1-Cz and O2-Cz leads during unattended pattern-reversal stimulation, collected from a group of adult patients with chronic schizophrenia, and compared them to those of healthy individuals. Short-time single-trial VEP were transformed to the frequency domain using the FFT algorithm. Changes of the spectral power were visualized using spectrograms which were created by stacking single-trial spectra across all trials. Measures of the absolute and the relative spectral power were calculated and compared statistically. In schizophrenia, the energy density of VEP oscillations is shifted towards higher (gamma) frequencies, compared to healthy individuals. These differences are statistically significant in all analysed frequency bands for the relative power. This indicates distorted early processing of visual stimuli in schizophrenia. The presented observations complement the knowledge on gamma oscillations acquired from computationally more complex methods of time--frequency analysis.

Replication of Reduced Pattern Electroretinogram Amplitudes in Depression With Improved Recording Parameters

Background: The retina has gained increasing attention in non-ophthalmological research in recent years. The pattern electroretinogram (PERG), a method to evaluate retinal ganglion cell function, has been used to identify objective correlates of the essentially subjective state of depression. A reduction in the PERG contrast gain was demonstrated in patients with depression compared to healthy controls with normalization after remission. PERG responses are not only modulated by stimulus contrast, but also by check size and stimulation frequency. Therefore, the rationale was to evaluate potentially more feasible procedures for PERG recordings in daily diagnostics in psychiatry.Methods: Twenty-four participants (12 patients with major depression (MDD) and 12 age- and sex-matched healthy controls) were examined in this pilot study. We investigated PERG amplitudes for two steady-state pattern reversal frequencies (12.5/18.75 rps) and four sizes of a checkerboard stimulus (0.8°, 1.6°, 3.2°, and 16°) to optimize the PERG recordings in MDD patients.Results: Smaller PERG amplitudes in MDD patients were observed for all parameters, whereby the extent of the reduction appeared to be stimulus-specific. The most pronounced decline in the PERG of MDD patients was observed at the higher stimulation frequency and the finest pattern, whilst responses for the largest check size were less affected. Following the PERG ratio protocol for early glaucoma, where similar stimulus dependent modulations have been reported, we calculated PERG ratios (0.8°/16°) for all participants. At the higher frequency (18.75 rps), significantly reduced ratios were observed in MDD patients.Conclusion: The “normalization” of the PERG responses—via building a ratio—appears to be a very promising approach with regard to the development of an objective biomarker of the depressive state, facilitating inter-individual assessments of PERG recordings in patients with psychiatric disorders.

Visual Processing During the Interictal Period Between Migraines: A Meta-Analysis

Migraine is a poorly understood neurological disorder and a leading cause of disability in young adults. Migraines are characterized by severe pulsating unilateral headache and visual symptoms. Whether visual function is also impaired in the interictal period between migraines remains controversial. Thus, this meta-analysis investigated the evidence for altered visual function as measured electrophysiologically via pattern-reversal visual evoked potential (VEP) amplitudes and habituation in adult migraineurs with or without visual aura and control in the interictal period. Twenty-three studies were selected for random effects meta-analysis, demonstrating slightly diminished VEP amplitudes and substantially reduced habituation in the early P100 component in migraineurs without aura and with aura compared to controls. No differences were found between migraineurs with and without aura. Although heterogeneity between studies and insufficient published data for VEP latencies and the earlier N75 VEP component data was observed and require further testing, P100 anomalies may indicate abnormal functioning of the fast-conducting magnocellular visual pathway, in episodic migraineurs.