Developments in non-invasive visual electrophysiology

Silicone oil insulation effects on flash electroretinogram and visual evoked potential in patients with retinal detachment

BACKGROUND

Silicone oil is used as endotamponade following vitreoretinal surgery to maintain the retina reattached when indicated. This study investigates the hypothesis that silicone oil causes insulation effects on the retina by affecting its response to light.

METHODS

Electrophysiological responses to a flash stimulus were recorded using full-field electroretinography (ERG) and visual evoked potentials (VEP). Recordings were performed in 9 patients who underwent surgery for retinal detachment, before (1-2 days) and after (2-3 weeks) silicone oil removal (SOR) in both the study and the control eye. Flash ERG and VEP recordings were performed according to the ISCEV standard protocol.

RESULTS

Statistically significant differences were found in the study eye in the amplitudes of the ERG responses and their corresponding ratios, i.e. the amplitude after SOR over the amplitude before SOR, in all conditions tested. No differences were observed in the control eye. The mean ratio of photopic ERG response was 3.4 ± 2.4 for the study and 1.0 ± 0.3 for the control eye (p<0.001). The mean ratio of ERG flicker response was 3.1 ± 2.4 and 1.0 ± 0.3, respectively (p = 0.003). Scotopic flash ERG ratio was 5.0 ± 4.4 for the study and 1.3 ± 0.6 for the control eye (p = 0.012). No differences were observed for the amplitude and latency of flash VEP response after SOR.

CONCLUSIONS

Silicone oil causes a reduction in flash ERG responses; no effect was found on flash VEP responses. ERGs in eyes filled with silicone oil should not be considered representative of retinal functionality, in contrast to VEPs, which are not affected by silicone oil presence.

Retina Oculomics in Neurodegenerative Disease

Ophthalmic biomarkers have long played a critical role in diagnosing and managing ocular diseases. Oculomics has emerged as a field that utilizes ocular imaging biomarkers to provide insights into systemic diseases. Advances in diagnostic and imaging technologies including electroretinography, optical coherence tomography (OCT), confocal scanning laser ophthalmoscopy, fluorescence lifetime imaging ophthalmoscopy, and OCT angiography have revolutionized the ability to understand systemic diseases and even detect them earlier than clinical manifestations for earlier intervention. With the advent of increasingly large ophthalmic imaging datasets, machine learning models can be integrated into these ocular imaging biomarkers to provide further insights and prognostic predictions of neurodegenerative disease. In this manuscript, we review the use of ophthalmic imaging to provide insights into neurodegenerative diseases including Alzheimer Disease, Parkinson Disease, Amyotrophic Lateral Sclerosis, and Huntington Disease. We discuss recent advances in ophthalmic technology including eye-tracking technology and integration of artificial intelligence techniques to further provide insights into these neurodegenerative diseases. Ultimately, oculomics opens the opportunity to detect and monitor systemic diseases at a higher acuity. Thus, earlier detection of systemic diseases may allow for timely intervention for improving the quality of life in patients with neurodegenerative disease.

Subsequent and simultaneous electrophysiological investigation of the retina and the visual cortex in neurodegenerative and psychiatric diseases: what are the forecasts for the medicine of tomorrow?

Visual electrophysiological deficits have been reported in neurodegenerative disorders as well as in mental disorders. Such alterations have been mentioned in both the retina and the cortex, notably affecting the photoreceptors, retinal ganglion cells (RGCs) and the primary visual cortex. Interestingly, such impairments emphasize the functional role of the visual system. For this purpose, the present study reviews the existing literature with the aim of identifying key alterations in electroretinograms (ERGs) and visual evoked potentials electroencephalograms (VEP-EEGs) of subjects with neurodegenerative and psychiatric disorders. We focused on psychiatric and neurodegenerative diseases due to similarities in their neuropathophysiological mechanisms. Our research focuses on decoupled and coupled ERG/VEP-EEG results obtained with black-and-white checkerboards or low-level visual stimuli. A decoupled approach means recording first the ERG, then the VEP-EEG in the same subject with the same visual stimuli. The second method means recording both ERG and VEP-EEG simultaneously in the same participant with the same visual stimuli. Both coupled and decoupled results were found, indicating deficits mainly in the N95 ERG wave and the P100 VEP-EEG wave in Parkinson’s, Alzheimer’s, and major depressive disorder. Such results reinforce the link between the retina and the visual cortex for the diagnosis of psychiatric and neurodegenerative diseases. With that in mind, medical devices using coupled ERG/VEP-EEG measurements are being developed in order to further investigate the relationship between the retina and the visual cortex. These new techniques outline future challenges in mental health and the use of machine learning for the diagnosis of mental disorders, which would be a crucial step toward precision psychiatry.

Disruption of early visual processing in amyloid-positive healthy individuals and mild cognitive impairment

BACKGROUND

Amyloid deposition is a primary predictor of Alzheimer's disease (AD) and related neurodegenerative disorders. Retinal changes involving the structure and function of the ganglion cell layer are increasingly documented in both established and prodromal AD. Visual event-related potentials (vERP) are sensitive to dysfunction in the magno- and parvocellular visual systems, which originate within the retinal ganglion cell layer. The present study evaluates vERP as a function of amyloid deposition in aging, and in mild cognitive impairment (MCI).

METHODS

vERP to stimulus-onset, motion-onset, and alpha-frequency steady-state (ssVEP) stimuli were obtained from 16 amyloid-positive and 41 amyloid-negative healthy elders and 15 MCI individuals and analyzed using time-frequency approaches. Social cognition was assessed in a subset of individuals using The Awareness of Social Inference Test (TASIT).

RESULTS

Neurocognitively intact but amyloid-positive participants and MCI individuals showed significant deficits in stimulus-onset (theta) and motion-onset (delta) vERP generation relative to amyloid-negative participants (all p < .01). Across healthy elders, a composite index of these measures correlated highly (r =  - .52, p < .001) with amyloid standardized uptake value ratios (SUVR) and TASIT performance. A composite index composed of vERP measures significant differentiated amyloid-positive and amyloid-negative groups with an overall classification accuracy of > 70%.

DISCUSSION

vERP may assist in the early detection of amyloid deposition among older individuals without observable neurocognitive impairments and in linking previously documented retinal deficits in both prodromal AD and MCI to behavioral impairments in social cognition.

Neural Research on Depth Perception and Stereoscopic Visual Fatigue in Virtual Reality

Virtual reality (VR) technology provides highly immersive depth perception experiences; nevertheless, stereoscopic visual fatigue (SVF) has become an important factor currently hindering the development of VR applications. However, there is scant research on the underlying neural mechanism of SVF, especially those induced by VR displays, which need further research. In this paper, a Go/NoGo paradigm based on disparity variations is proposed to induce SVF associated with depth perception, and the underlying neural mechanism of SVF in a VR environment was investigated. The effects of disparity variations as well as SVF on the temporal characteristics of visual evoked potentials (VEPs) were explored. Point-by-point permutation statistical with repeated measures ANOVA results revealed that the amplitudes and latencies of the posterior VEP component P2 were modulated by disparities, and posterior P2 amplitudes were modulated differently by SVF in different depth perception situations. Cortical source localization analysis was performed to explore the original cortex areas related to certain fatigue levels and disparities, and the results showed that posterior P2 generated from the precuneus could represent depth perception in binocular vision, and therefore could be performed to distinguish SVF induced by disparity variations. Our findings could help to extend an understanding of the neural mechanisms underlying depth perception and SVF as well as providing beneficial information for improving the visual experience in VR applications.

Electroretinographic responses to luminance and cone-isolating white noise stimuli in macaques

Electroretinograms (ERGs) are mass potentials with a retinal origin that can be measured non-invasively. They can provide information about the physiology of the retina. Often, ERGs are measured to flashes that are highly unnatural stimuli. To obtain more information about the physiology of the retina, we measured ERGs with temporal white noise (TWN) stimuli that are more natural and keep the retina in a normal range of operation. The stimuli can be combined with the silent substitution stimulation technique with which the responses of single photoreceptor types can be isolated. We characterized electroretinogram (ERG) responses driven by luminance activity or by the L- or the M-cones. The ERGs were measured from five anesthetized macaques (two females) to luminance, to L-cone isolating and to M-cone isolating stimuli in which luminance or cone excitation were modulated with a TWN profile. The responses from different recordings were correlated with each other to study reproducibility and inter-individual variability. Impulse response functions (IRFs) were derived by cross-correlating the response with the stimulus. Modulation transfer functions (MTFs) were the IRFs in the frequency domain. The responses to luminance and L-cone isolating stimuli showed the largest reproducibility. The M-cone driven responses showed the smallest inter-individual variability. The IRFs and MTFs showed early (high frequency) components that were dominated by L-cone driven signals. A late component was equally driven by L- and M-cone activity. The IRFs showed characteristic similarities and differences relative to flash ERGs. The responses to TWN stimuli can be used to characterize the involvement of retinal cells and pathways to the ERG response. It can also be used to identify linear and non-linear processes.

Investigate the Neuro Mechanisms of Stereoscopic Visual Fatigue

Stereoscopic visual fatigue (SVF) due to prolonged immersion in the virtual environment can lead to negative user experience, thus hindering the development of virtual reality (VR) industry. Previous studies have focused on investigating the evaluation indicators associated with SVF, while few studies have been conducted to reveal the underlying neural mechanism, especially in VR applications. In this paper, a modified Go/NoGo paradigm was adopted to induce SVF in VR environment with Go trials for maintaining participants' attention to experimental viewing tasks and NoGo trials for investigating the neural effects under SVF. Random dot stereograms (RDSs) with 11 disparities and 2 types of shapes (arrow and rectangle) were presented to evoke the depth-related visual evoked potentials (DVEPs) during 64-channel EEG recordings. EEG datasets collected from 15 participants in NoGo trials were selected to conduct individual processing and group analysis, in which the characteristics of the DVEPs components for various fatigue degrees were compared with one-way repeated-measurement ANOVA and independent components were clustered to explore the original cortex areas related to SVF. Point-by-point permutation statistics revealed that DVEPs sample points from 230ms to 280ms in most brain areas changed significantly with SVF. More specifically, we found that amplitudes of component P2 changed significantly when SVF increased. Additionally, independent component analysis (ICA) identified that component P2 which originated from posterior cingulate cortex and precuneus, was associated statistically with SVF. We believe that SVF is rather a conscious status concerning the changes of self-awareness or self-location awareness than the performance reduction of retinal image processing. Moreover, we suggest that indicators representing higher conscious state may be a better indicator for SVF evaluation in VR environments.

In the Eye of the Storm: Bi-Directional Electrophysiological Investigation of the Intact Retina

Electrophysiological investigations reveal a great deal about the organization and function of the retina. In particular, investigations of explanted retinas with multi electrode arrays are widely used for basic and applied research purposes, offering high-resolution and detailed information about connectivity and structure. Low-resolution, non-invasive approaches are also widely used. Owing to its delicate nature, high-resolution electrophysiological investigations of the intact retina until now are sparse. In this Mini Review, we discuss progress, challenges and opportunities for electrode arrays suitable for high-resolution, multisite electrophysiological interfacing with the intact retina. In particular, existing gaps in achieving bi-directional electrophysiological investigation of the intact retina are discussed.

Blue-Yellow VEP with Projector-Stimulation in Glaucoma

Background and aim

In the past, increased latencies of the blue-on-yellow pattern visually evoked potentials (BY-VEP), which predominantly originate in the koniocellular pathway, have proven to be a sensitive biomarker for early glaucoma. However, a complex experimental setup based on an optical bench was necessary to obtain these measurements because computer screens lack sufficient temporal, spatial, spectral, and luminance resolution. Here, we evaluated the diagnostic value of a novel setup based on a commercially available video projector.

Methods

BY-VEPs were recorded in 126 participants (42 healthy control participants, 12 patients with ocular hypertension, 17 with “preperimetric” glaucoma, and 55 with perimetric glaucoma). Stimuli were created with a video projector (DLP technology) by rear projection of a blue checkerboard pattern (460 nm) for 200 ms (onset) superimposed on a bright yellow background (574 nm), followed by an offset interval where only the background was active. Thus, predominantly S-cones were stimulated while L- and M-cone responses were suppressed by light adaptation. Times of stimulus onset to VEP onset-trough (N-peak time) and offset-peak (P-peak time) were analyzed after age-correction based on linear regression in the normal participants.

Results

The resulting BY-VEPs were quite similar to those obtained in the past with the optical bench: pattern-onset generated a negative deflection of the VEP, whereas the offset-response was dominated by a positive component. N-peak times were significantly increased in glaucoma patients (preperimetric 136.1 ± 10 ms, p < 0.05; perimetric 153.1 ± 17.8 ms, p < 0.001) compared with normal participants (123.6 ± 7.7 ms). Furthermore, they were significantly correlated with disease severity as determined by visual field losses retinal nerve fiber thinning (Spearman R = –0.7, p < 0.001).

Conclusions

Video projectors can be used to create optical stimuli with high temporal and spatial resolution, thus potentially enabling sophisticated electrophysiological measurements in clinical practice. BY-VEPs based on such a projector had a high diagnostic value for detection of early glaucoma.

Registration of study

Registration site: www.clinicaltrials.gov

Trial registration number: NCT00494923.

A novel system for measuring visual potentials evoked by passive head-mounted display stimulators

PURPOSE

The objective of this work is to evaluate the performances of a novel integrated device, based on passive head-mounted display (HMD), for the pattern reversal visual evoked potential (PR-VEP) clinical test.

METHODS

Google Cardboard® is used as passive HMD to generate the checkerboard pattern stimuli through an Android® application. Electroencephalographic signals are retrieved and processed over 20 subjects, 12 females and 8 males between 20 and 26 years. Morphological PR-VEPs and frequency response were compared with previous literature results, to test the reproducibility and the efficacy of the proposed solution.

RESULTS

PR-VEPs evoked by our novel prototype showed typical triphasic waveforms in moderate agreement with those obtained with other more expensive HMDs and standard commercial devices. Statistical analysis did not highlight strong differences among the systems over the features analyzed except for the P100 amplitude and peak time (**p < 0.005).

CONCLUSION

The proposed solution opens the door for a new generation of non-invasive first-level diagnostic devices of optic nerve pathologies inexpensive and easy to access.

Multimodal imaging and electroretinography highlights the role of VEGF in the laser-induced subretinal fibrosis of monkey

Age-related macular degeneration (AMD) is a leading cause of blindness. Laser-induced nonhuman primate choroidal neovascularization (CNV) is a widely used animal model of neovascular AMD. Subretinal fibrosis (SFb) is the major limiting factor of effective anti-VEGF therapy for neovascular AMD, yet SFb has never been systematically analyzed in the primate CNV model and if VEGF directly affect SFb is unknown. We recruited a large cohort of rhesus macaques to study the occurrence, multimodal imaging and electroretinography (ERG) features, and related cytokines of SFb. Here we show that among 33 rhesus macaques, 88% CNV eyes developed SFb. Spectral domain optical coherence tomography (SD-OCT) identified four types of subretinal hyper-reflective material (SHRM) of SFb in primate. Multimodal imaging is reliable for monitoring SFb and matches the histological results well. Reduced amplitude of oscillatory potentials correlates with the thinning of inner retina layers and is a possible SFb indicator. Iba1⁺ microglia/macrophage cells infiltrated in the fibrotic lesions, and aqueous cytokine analysis identified four fibrosis-related factors (GM-CSF, IL-10, TGFβ2 and VEGF). Unexpectedly, we found sustained expression of VEGF may be an important inducer of SFb, and anti-VEGF therapy actually partially suppresses SFb. Taken together, our data suggest the laser-induced primate SFb model, coupled with multimodal imaging and ERG recording, is a useful system to dissect the pathogenesis and explore the rationale of treatment for SFb; and combined therapy with anti-VEGF and anti-fibrosis agents is necessary for AMD treatment.