Visually evoked potentials: theory, techniques and clinical applications

Reconciliation of weak pairwise spike-train correlations and highly coherent local field potentials across space

Multi-electrode arrays covering several square millimeters of neural tissue provide simultaneous access to population signals such as extracellular potentials and spiking activity of one hundred or more individual neurons. The interpretation of the recorded data calls for multiscale computational models with corresponding spatial dimensions and signal predictions. Multi-layer spiking neuron network models of local cortical circuits covering about $1\,{\text{mm}^{2}}$ have been developed, integrating experimentally obtained neuron-type-specific connectivity data and reproducing features of observed in-vivo spiking statistics. Local field potentials can be computed from the simulated spiking activity. We here extend a local network and local field potential model to an area of $4\times 4\,{\text{mm}^{2}}$, preserving the neuron density and introducing distance-dependent connection probabilities and conduction delays. We find that the upscaling procedure preserves the overall spiking statistics of the original model and reproduces asynchronous irregular spiking across populations and weak pairwise spike-train correlations in agreement with experimental recordings from sensory cortex. Also compatible with experimental observations, the correlation of local field potential signals is strong and decays over a distance of several hundred micrometers. Enhanced spatial coherence in the low-gamma band around $50\,\text{Hz}$ may explain the recent report of an apparent band-pass filter effect in the spatial reach of the local field potential.

Investigating causal effects of pupil size on visual discrimination and visually evoked potentials in an optotype discrimination task

Pupil size primarily changes to regulate the amount of light entering the retina, optimizing the balance between visual acuity and sensitivity for effective visual processing. However, research directly examining the relationship between pupil size and visual processing has been limited. While a few studies have recorded pupil size and EEG signals to investigate the role of pupil size in visual processing, these studies have predominantly focused on the domain of visual sensitivity. Causal effects of pupil size on visual acuity, therefore, remain poorly understood. By manipulating peripheral background luminance levels and target stimulus contrast while simultaneously recording pupillometry and EEG signals, we examined how absolute pupil size affects visual discrimination and visually evoked potentials (VEP) in a task using optotype mimicking the Snellen eye chart, the most common assessment of visual acuity. Our findings indicate that both higher background luminance levels and higher target contrast were associated with improved target discrimination and faster correct reaction times. Moreover, while higher contrast visual stimuli evoked larger VEPs, the effects of pupil size on VEPs were not significant. Additionally, we did not observe inter-individual correlations between absolute pupil size and discrimination performance or VEP amplitude. Together, our results demonstrate that absolute pupil size, regulated by global luminance level, played a functional role in enhancing visual discrimination performance in an optotype discrimination task. The differential VEP effects of pupil size compared to those of stimulus contrast further suggested distinct neural mechanisms involved in facilitating visual acuity under small pupils.

Effects of environmental enrichment on GLUT expression in the visual cortex of amblyopic rats

OBJECTIVE

To investigate the potential changes of glucose metabolism and glucose transporter protein (GLUT) in the visual cortex of formally deprived amblyopic rats, as well as the effects of enriched environments on the levels of nerve conduction and glucose metabolism in the visual cortex of amblyopic rats.

METHODS

36 rats were randomly divided into three groups: CON + SE (n = 12), MD + SE (n = 12) and MD + EE (n = 12). The right eyelids of both MD + SE and MD + EE groups were sutured. After successful modelling, the MD + EE group was maintained in an enriched environment, and the other two groups were kept in the same environment. Pattern visual evoked potentials (PVEP) was used to confirm models' effect, glucose metabolism was analyzed by Micro-PET/CT (18F-FDG), and the protein as well as mRNA expression levels of GLUT were detected by Western Blot and quantitative RT-PCR (quantitative Reverse Transcription-Polymerase Chain Reaction) analyses, site of GLUT expression by immunofluorescence (IF).

RESULTS

After suture modelling, both the MD + EE and MD + SE groups objective visual nerve conduction function decreased, the glucose metabolism in the visual cortex was markedly lower. After the enriched environment intervention, it recovered in the MD + EE group. The expression levels of GLUT1 and GLUT3 were increased in the MD + EE group in comparison with the MD + SE group. GLUT1 was primarily expressed on astrocytes and endothelial cells, but GLUT3 was mainly expressed on neurons.

CONCLUSION

Enrichment of the environment exhibited a therapeutic effect on amblyopia, which could be related to the enhancement of glucose metabolism and GLUT expression in the visual cortex.

Increases in pre-stimulus theta and alpha oscillations precede successful encoding of crossmodal associations

A central aspect of episodic memory is the formation of associations between stimuli from different modalities. Current theoretical approaches assume a functional role of ongoing oscillatory power and phase in the theta band (3-7 Hz) for the encoding of crossmodal associations. Furthermore, ongoing activity in the theta range as well as alpha (8-12 Hz) and low beta activity (13-20 Hz) before the presentation of a stimulus is thought to modulate subsequent cognitive processing, including processes that are related to memory. In this study, we tested the hypothesis that pre-stimulus characteristics of low frequency activity are relevant for the successful formation of crossmodal memory. The experimental design that was used specifically allowed for the investigation of associative memory independent from individual item memory. Participants (n = 51) were required to memorize associations between audiovisual stimulus pairs and distinguish them from newly arranged ones consisting of the same single stimuli in the subsequent recognition task. Our results show significant differences in the state of pre-stimulus theta and alpha power between remembered and not remembered crossmodal associations, clearly relating increased power to successful recognition. These differences were positively correlated with memory performance, suggesting functional relevance for behavioral measures of associative memory. Further analysis revealed similar effects in the low beta frequency ranges, indicating the involvement of different pre-stimulus-related cognitive processes. Phase-based connectivity measures in the theta band did not differ between remembered and not remembered stimulus pairs. The findings support the assumed functional relevance of theta band oscillations for the formation of associative memory and demonstrate that an increase of theta as well as alpha band oscillations in the pre-stimulus period is beneficial for the establishment of crossmodal memory.

Assessment of amblyopic children undergoing occlusion therapy by pattern visual evoked potentials and contrast sensitivity tests

Background: Amblyopia is a case where one or less commonly, both eyes have impaired visual performance, even with the best optical correction and no visible disease of the visual system. Objectives: To assess contrast sensitivity tests (CST) and pattern visual evoked potentials (PVEP) results in amblyopic children who have already started occlusion therapy for durations ranging from 6 to 12 months. Methods: This cross-sectional study was conducted on 200 eyes of 50 patients with monocular amblyopia and 50 age and sex matched controls. Both patients and controls underwent ophthalmological assessment, PVEP, and CST. Results: There was no statistically significant difference in the results of P100 latencies of qualitative PVEP in amblyopic eyes compared to non-amblyopic eyes and control eyes, while the qualitative CST showed a highly statistically significant difference, being affected in 98% of amblyopic eyes compared to unaffected eyes (4%) and control eyes (4%). The maximum contrast level and minimal contrast level of quantitative CST were significantly lower in amblyopic eyes compared to non-amblyopic and control eyes. The cutoff value of maximal contrast level at mean frequencies of 2.5 ± 0.9 Hz, and a range of (1.1-4.1) for amblyopic eyes is ≤21 dB, while the cutoff value of minimal contrast level at mean frequencies of 13.4 ± 2.6 Hz, and a range of (6.7-18) for amblyopic eyes is ≤12 dB. Conclusion: Detection of amblyopia by CST is a noninvasive and easy procedure, which represents a promising tool to support the diagnosis of amblyopia.

The Effects of Directional and Non-Directional Stimuli during a Visuomotor Task and Their Correlation with Reaction Time: An ERP Study

Different visual stimuli can capture and shift attention into different directions. Few studies have explored differences in brain response due to directional (DS) and non-directional visual stimuli (nDS). To explore the latter, event-related potentials (ERP) and contingent negative variation (CNV) during a visuomotor task were evaluated in 19 adults. To examine the relation between task performance and ERPs, the participants were divided into faster (F) and slower (S) groups based on their reaction times (RTs). Moreover, to reveal ERP modulation within the same subject, each recording from the single participants was subdivided into F and S trials based on the specific RT. ERP latencies were analysed between conditions ((DS, nDS); (F, S subjects); (F, S trials)). Correlation was analysed between CNV and RTs. Our results reveal that the ERPs' late components are modulated differently by DS and nDS conditions in terms of amplitude and location. Differences in ERP amplitude, location and latency, were also found according to subjects' performance, i.e., between F and S subjects and trials. In addition, results show that the CNV slope is modulated by the directionality of the stimulus and contributes to motor performance. A better understanding of brain dynamics through ERPs could be useful to explain brain states in healthy subjects and to support diagnoses and personalized rehabilitation in patients with neurological diseases.

VEP examination with new portable device

INTRODUCTION

We developed a new portable device called "VEPpeak" for the examination of visual evoked potentials (VEPs) to extend VEP examination beyond specialized electrophysiological laboratories and to simplify the use of this objective, noninvasive, and low-cost method for diagnostics of visual and central nervous system dysfunctions.

METHODS

VEPpeak consists of a plastic headset with a total weight of 390 g containing four EEG amplifiers, an A/D converter, a control unit, and a visual LED stimulator built in the front, vertically adjustable peak. The device is powered and controlled via USB connection from a standard PC/notebook using custom software for visual stimuli generation and for VEP recording and processing. Up to four electrodes can be placed at any scalp location or in combination with two dry electrodes incorporated into the headset. External visual stimulators, such as a tablet, can be used with synchronization. Feasibility and validation studies were conducted with 86 healthy subjects and 76 neuro-ophthalmological patients including 67 who were during the same session also tested with a conventional VEP system.

RESULTS

VEPpeak recordings to standard (pattern-reversal) and non-standard (motion-onset, red-green alternation) were robust and repeatable and obtained also in immobilized patients. Good comparability of results was achieved between VEPpeak and standard examination. Some systematic differences in peak latencies and amplitudes are consistent with differences in stimulus characteristics of the two compared systems.

DISCUSSION

VEPpeak provides an inexpensive system for clinical use requiring portability. In addition to ISCEV standard VEP protocols, free choice of stimuli and bio-signal recordings make the device universal for many electrophysiological purposes.

Changes in neurovascular coupling with cerebral perfusion pressure indicate a link to cerebral autoregulation

Cerebral autoregulation ensures a stable average blood supply to brain tissue across steady state cerebral perfusion pressure (CPP) levels. Neurovascular coupling, in turn, relies on sufficient blood flow to meet neuronal demands during activation. These mechanisms break down in pathologies where extreme levels of CPP can cause dysregulation in cerebral blood flow. Here, we experimentally tested the influence of changes in CPP on neurovascular coupling in a hydrocephalus-type non-human primate model (n = 3). We recorded local neural and vascular evoked responses to a checkerboard visual stimulus, non-invasively, using electroencephalography and near-infrared spectroscopy respectively. The evoked signals showed changes in various waveform features in the visual evoked potentials and the hemodynamic responses, with CPP. We further used these signals to fit for a hemodynamic response function (HRF) to describe neurovascular coupling. We estimated n = 26 distinct HRFs at a subset of CPP values ranging from 40-120 mmHg across all subjects. The HRFs, when compared to a subject dependent healthy baseline (CPP 70-90 mmHg) HRF, showed significant changes in shape with increasing CPP (ρ_CPP = -0.55, p-value_CPP = 0.0049). Our study provides preliminary experimental evidence on the relationship between neurovascular coupling and CPP changes, especially when beyond the limits of static autoregulation.

Normative Data of Axial Length, Retinal Thickness Measurements, Visual Evoked Potentials, and Full-Field Electroretinography in Female, Wild-Type Minipigs

Purpose

The purpose of this study was to present normative data of optical coherence tomography (OCT), electrophysiological, and ocular biometry parameters and their correlation in minipigs.

Methods

Eighty-eight eyes of 44 minipigs underwent full-field electroretinogram (ERG) recording and ocular biometry. However, 10 eyes of 6 minipigs were excluded because of poor OCT image quality. The thickness of the retinal sublayers was measured on a vertical line at 5 locations with a 1 mm interval from the disc margin to the dorsal periphery and at 10 locations on the visual streak. Visual evoked potentials (VEPs) were measured in 15 eyes of 8 minipigs.

Results

All minipigs were female with a mean age and axial length of 13.83 ± 10.56 months and 20.33 ± 0.88 mm, respectively. The implicit time of the a-wave and b-wave in scotopic 3.0 ERGs was longer than that in photopic 3.0 ERG. The implicit time of the n2-wave and p2-wave in VEP was 25.67 ± 7.41 ms and 52.96 ± 10.38 ms, respectively. The total retinal layer (TRL) and nerve fiber layer (NFL) became thinner near the periphery. The inner retinal sublayers near the visual streak were thicker than those at other locations. Central TRL and NFL thickness on visual streak was 223.06 ± 23.19 µm and 74.03 ± 13.93 µm, respectively. The temporal TRL and NFL on the visual streak were thicker than those on the nasal side.

Conclusions

The normative electrophysiological and OCT parameters used in our study can be used as reference data in further pig studies.

Translational Relevance

This study presents normative data of minipigs, which are adequate animal models for preclinical studies.

Variability, repeatability and test-retest reliability of equine flash visual evoked potentials (FVEPs)

Background

Visual evoked potentials (VEPs) are electrical potentials generated by neurons in the central nervous system in response to visual stimuli. A series of positive and negative wavelets in response to flash-stimuli (flash-VEP; FVEP) or reversing, iso-luminant patterns (pattern-VEP; PVEP) are recorded. Pathological conditions affecting the post-retinal pathways can alter overall waveform morphology, and also affect wavelet peak times and amplitudes. FVEPs have recently been described in horses, but more data on the variability within and between subjects is required, to adequately interpret results from clinical equine patients. Therefore, the purpose of this study was to describe the variability, repeatability and test-retest reliability of equine FVEPs in normal, adult horses.

Results

Equine FVEPs were recorded from one randomly selected eye in 17 horses, from both eyes in eight of these horses, and also at two separate recording sessions in six horses. N1, P2, N2 and P4 wavelets were present in 100% of the recordings in all horses, while P1, N2a, P3 and P5 were only present in some recordings. Coefficients of variation (CVs) were low for P2, N2 and P4 peak times, but higher for all amplitudes. There were no statistically significant differences comparing peak times and amplitudes between eyes or between sessions. Coefficients of repeatability (CRs) are reported for P2, N2 and P4 peak times between eyes (P2; 5 ms, N2; 18 ms, P4; 18 ms) and also between sessions (P2; 5 ms, N2; 16 ms, P4; 39 ms). Intraclass correlation coefficients (ICCs), as an estimate of test-retest reliability, was assessed to be fair to excellent for most parameters.

Conclusions

This study provides important data on variability, repeatability and test-retest reliability of FVEPs in normal, adult horses. We conclude that P2, N2 and P4 peak times should be included in the evaluation of equine FVEPs. The large inherent variability of FVEP amplitudes is likely to make them less suitable and useful for establishing a diagnosis on their own in most clinical patients, but they may occasionally provide support to a clinical diagnosis.

Diagnosis and Evaluation of Visual Dysfunction in Premature Infants with Low Birth Weight

Visual impairment of many kinds is common in premature infants with low birth weight. The infant's visual system is quite plastic in the early stages, and early screening may help to discover treatable pathology. Clinical techniques are available that are not used in the routine examination given by the pediatrician. These include optokinetic nystagmus, preferential looking, and visually evoked potential. If such techniques were more widely employed where visual impairment is suspected, nonsurgical remedial intervention might be more successful.

An Overview of Preschool Vision Screening

Although several vision screening techniques are available for infants, including Optokinetic Nystagmus, Preferential Looking and Visual Evoked Potential, none is routinely utilized. Visual deficits are often first detected through required preschool vision screenings. These screenings vary in their comprehensiveness and administration. No standardization for testing or administration exists. Referrals for further examination appear to be higher for those children receiving a thorough battery, suggesting that some of the testing procedures used have inappropriately high rates of false negatives.

Vision Screening: A Study of 297 Head Start Children

Vision assessments were provided to 297 Head Start children in nine Westchester County, New York programs. An optometrist and an orthoptist administered a protocol which included retinoscopy, visual acuity, fly, cover, motility and convergence tests. Sixty-three children (21.2%) were referred for further evaluation. The visual deficits detected by this screening included decreased acuity, strabismus combined with decreased acuity, astigmatism, and nonspecific visual problems. The protocol used provided a thorough evaluation of the visual system and required only seven minutes per patient.

Special Visual Assessment Techniques for Multiply Handicapped Persons

Techniques to assess vision capabilities of multihandicapped persons are discussed as used in a specialty Optometric clinic. The major emphasis of the assessment is to obtain information about visual abilities and to make recommendations that promote optimum vision functioning. Techniques for working with children from special populations are discussed. The assessment includes a modified ophthalmic examination to identify and correct refractive error and to assess oculomotor functioning and ocular health. Visual acuity is measured using preferential looking and visual evoked potential techniques. Confrontation visual fields are measured. Several aspects of visual function are assessed with a parent interview.

Active Exploration of the Environment by Young Children with Low Vision: A Review of the Literature

Young children with low vision experience delays in cognitive development, motor development, and social skills because of the lack of motivation or insufficient opportunities to explore their environments actively. As a result, they have difficulty acquiring practical knowledge and spatial and environmental concepts. This review of the literature explores these problems, discusses current approaches to addressing them, and suggests areas for future research, including the use of optical aids.

Electrodiagnostic Testing of Visually Impaired Children under Sedation

The use of chloral hydrate as a sedative to enable electrodiagnostic testing of hyperkinetic children is discussed, and procedures for the administration of chloral hydrate are described. Case examples are presented to illustrate that assessment during the sleep-induced state enables further counseling regarding spasmus nutans, optic nerve hyoplasia, and Leber's congenital amaurosis. Management based on comparative electroretinogram and visual-evoked-response recordings is emphasized.

A BCI Gaze Sensing Method Using Low Jitter Code Modulated VEP

Visual evoked potentials (VEPs) are used in clinical applications in ophthalmology, neurology, and extensively in brain-computer interface (BCI) research. Many BCI implementations utilize steady-state VEP (SSVEP) and/or code modulated VEP (c-VEP) as inputs, in tandem with sophisticated methods to improve information transfer rates (ITR). There is a gap in knowledge regarding the adaptation dynamics and physiological generation mechanisms of the VEP response, and the relation of these factors with BCI performance. A simple, dual pattern display setup was used to evoke VEPs and to test signatures elicited by non-isochronic, non-singular, low jitter stimuli at the rates of 10, 32, 50, and 70 reversals per second (rps). Non-isochronic, low-jitter stimulation elicits quasi-steady-state VEPs (QSS-VEPs) that are utilized for the simultaneous generation of transient VEP and QSS-VEP. QSS-VEP is a special case of c-VEPs, and it is assumed that it shares similar generators of the SSVEPs. Eight subjects were recorded, and the performance of the overall system was analyzed using receiver operating characteristic (ROC) curves, accuracy plots, and ITRs. In summary, QSS-VEPs performed better than transient VEPs (TR-VEP). It was found that in general, 32 rps stimulation had the highest ROC area, accuracy, and ITRs. Moreover, QSS-VEPs were found to lead to higher accuracy by template matching compared to SSVEPs at 32 rps. To investigate the reasons behind this, adaptation dynamics of transient VEPs and QSS-VEPs at all four rates were analyzed and speculated.

32-channel mouse EEG: Visual evoked potentials

BACKGROUND

Measuring visual evoked potentials (VEP) by means of EEG allows the quasi non-invasive assessment of visual function in mice. Such sensory phenotyping is important to screen for genetic or aging effects on vision in preclinical mouse models. Thus, a standardized EEG-like approach for the assessment of sensory evoked potentials in mice is desirable.

NEW METHOD

We describe a method to obtain the topographical distribution of flash evoked VEPs with 32-channel thin-film EEG electrode arrays in anesthetized mice. Further, we provide suggestions for the optimal choice of adequate digital filtering, referencing, and stimulus parameters for fast and reliable assessment of VEP parameters and distribution.

RESULTS

32-channel thin-film electrodes provided clear information on the VEP topography across the skull. Re-referencing, such as bipolar, common average, and local average montages could be used to further refine the information on VEP topography. A balanced choice of digital high-pass filter, signal averaging and stimulus rate allowed to minimize measurement duration and at the same time assured good VEP signal-to-noise ratio.

COMPARISON WITH EXISTING METHODS

Subdermal electrodes or single skull screws provide only limited topographical information of the VEP. Assessment of VEPs with 32-channel thin-film electrodes can provide comparable signal quality with superior spatial resolution and standardized topographical and hemispheric information of VEP distribution.

CONCLUSIONS

EEG-like thin-film electrodes are an efficient tool for fast, comprehensive sensory phenotyping with topographical information in mice. This is a step towards the use of standardized mouse EEG to characterize EEG biomarkers in mouse models of human diseases.

Evaluation of Neurotoxicity of Multiple Anesthesia in Children Using Visual Evoked Potentials

Objectives:

Anesthetic applications may cause increased neuronal damage in infants and children. Commonly cognitive or learning disability tests were used to investigate the neurological progress in children. Visual Evoked Potential is a gross electrical signal generated by the occipital regions of the cerebral cortex in response to visual stimulation and an objective assessment of brain function. In this study, to acquire more objective results, Visual Evoked Potential responses of children who had multiple exposures to anesthesia during the treatment of corrosive esophagitis were compared to children who have never received anesthesia before.

Methods:

In this prospective, single-blinded, randomized, controlled study, 25 children, who were admitted to our pediatric surgery clinic because of corrosive esophagitis and who received general anesthesia more than 15 times composed Group-P; 25 children, who admitted to our well-child-clinic and who had never received anesthesia before consisted Group-C. The flash and pattern VEP responses of both groups were measured at the electrophysiology laboratory without any anesthetic drug application. The VEP responses of children in Group-P were recorded at least three days after the last exposure to anesthesia.

Results:

Latencies and amplitudes of the N2 and P2 components of the pattern and flash VEP responses were statistically significantly different between the two groups (p=0.000).

Conclusion:

This study shows that in children who had repeated anesthetic applications VEP parameters are significantly altered. We believe that VEP responses may be a reliable objective criterion for the evaluation of anesthesia neurotoxicity.

Characteristics of Pattern Visual Evoked Potential in Two Eyes with Varying Visual Acuity in One Eye and Forensic Application

In this study, we aimed to study the pattern visual evoked potentials (P-VEPs) in two eyes with varying visual acuity in one eye and to provide an objective estimation of visual acuity by comparing P-VEPs in one and two eyes. Thirty subjects were chosen, who had one eye with an acuity of 5.0, 4.85, 4.6, 4.0, or scieropia and obstructed vision and the other eye with an acuity of 5.0, respectively. P-VEPs were detected under the large grating stimuli at 3×4 spatial frequency, moderate grating stimuli (12×16 spatial frequency) and small grating stimuli (48×64 spatial frequency). Under large grating stimuli, there was no significant difference in P100 peak latency between the groups, nor was there a significant difference between the amplitude of two eyes and the amplitude of one normal-vision eye. Under moderate and small grating stimuli, there was a significant difference in P100 peak latency between the group with both eyes having an acuity of 5.0 and the group with visual acuity below 4.0 in one eye. There was a significant difference in P100 amplitude between the group with visual acuity of 5.0 in both eyes and the group with one normal-vision eye. There was no significant difference in the amplitude of two eyes and the amplitude of one normal-vision eye between any other two groups. In forensic identification, characteristics and variability of P-VEPs in one and two eyes can be used to identify malingering or decline in visual acuity.

Association of Diopsys® Short-duration Transient Visual Evoked Potential Latency with Visual Field Progression in Chronic Glaucoma

Aim

To determine the association of Diopsys® NOVA-LX amplitude and latency abnormality scores with perimetric staging of chronic glaucoma, and to explore potential single-visit short-duration transient visual evoked potential (SD-tVEP) trend detection ability utilizing Humphrey 30-2 field progression data.

Materials and methods

Setting: Glaucoma subspecialty clinic. Participants: Treated adult chronic glaucoma patients undergoing SD-tVEP evaluation. Main outcome measures: (1) Proportion of eyes designated as suspect or abnormal by the NOVA-LX multifactorial algorithm were determined as a function of glaucoma severity using the most recent Humphrey visual field analyzer (HVFA) 30-2 field. (2) Association between long-term HVFA-guided progression analysis (GPA) annual slopes and SD-tVEP abnormality was assessed to determine whether a single VEP test might help to identify eyes more prone to progressive visual field (VF) loss.

Results

One hundred and thirty-three eyes of 84 patients (mean age 68 years) were analyzed. The SD-tVEP abnormality increased proportionately with severity of VF loss under high-contrast (Hc) test conditions for both latency (p = 0.001) and amplitude (p < 0.01). The HVFA progression analysis printouts existed for 91 eyes (mean 12.3 fields per eye/range 5-18). Nearly three-quarters (72.5%) of eyes with mean annual HVFA progression >0.7 dB/year (n = 29) had single-visit VEP latency abnormalities. Fewer than half (46.7%) of the remainder (n = 62) showed latency abnormality. Mean progression for eyes with abnormal vs normal VEP latency was -0.87 ± 0.3 dB/year vs -0.32 ± 0.4 dB/year.

Conclusion

Diopsys NOVA-LX Hc latency abnormality shows strong association with VF loss among a diverse population of clinical patients undergoing active treatment for chronic glaucoma, and appears likely to afford clinically useful trend-detecting test.

Clinical significance

The SD-tVEP has the potential to serve as a single-visit clinical indicator to identify glaucoma patients at high risk for VF progression.

How to cite this article: Trevino R, Sponsel WE, Majcher CE, Allen J, Rabin J. Association of Diopsys® Short-duration Transient Visual Evoked Potential Latency with Visual Field Progression in Chronic Glaucoma. J Curr Glaucoma Pract 2018;12(1):29-35.

Visual system pathology in humans and animal models of blast injury

Injury from blast exposure is becoming a more prevalent cause of death and disability worldwide. The devastating neurological impairments that result from blasts are significant and lifelong. Progress in the development of effective therapies to treat injury has been slowed by its heterogeneous pathology and the dearth of information regarding the cellular mechanisms involved. Within the last decade, a number of studies have documented visual dysfunction following injury. This brief review examines damage to the visual system in both humans and animal models of blast injury. The in vivo use of the retina as a surrogate to evaluate brain injury following exposure to blast is also highlighted.

P100 Wave Latency in Anisometropic and Esotropic Amblyopia versus Normal Eyes

Purpose:

To evaluate cortical activity using pattern visual evoked potentials (PVEPs) in patients with mild and moderate amblyopia (esotropic and anisometropic).

Methods:

PVEP was recorded in 43 unilateral amblyopic patients, including 15 esotropic (ET) and 28 anisometropic (AM) patients, selected from three different medical centers in the city of Shiraz, Iran and compared to that obtained from 15 age and sex matched normal subjects who served as controls. Visual acuity (VA) in amblyopic eyes was equal to or less than 0.7 LogMAR. The latency of P100 was recorded monocularly using two check sizes of 15 and 60 min of arcs at two different levels of contrasts (30% and 100%).

Results:

P100 latency in amblyopic eyes was significantly increased compared to the normal group (P < 0.001). There was a significant difference (P < 0.001) in P100 latency in anisometropic and esotropic amblyopic eyes as compared to normal subjects, using high spatial frequency and with both levels of contrast. A significant difference was observed with large check sizes and high contrast between anisometropic amblyopic and normal eyes (P = 0.03). However, there was no significant difference between these two groups and the control group with other stimuli.

Conclusion:

The neural response based on p100 latency in PVEP was different between amblyopic groups and normal subjects. PVEP may be valuable for diagnosis, prognosis and treatment of amblyopia.

Effects of refractive errors on visual evoked magnetic fields

Background

The latency and amplitude of visual evoked cortical responses are known to be affected by refractive states, suggesting that they may be used as an objective index of refractive errors. In order to establish an easy and reliable method for this purpose, we herein examined the effects of refractive errors on visual evoked magnetic fields (VEFs).

Methods

Binocular VEFs following the presentation of a simple grating of 0.16 cd/m² in the lower visual field were recorded in 12 healthy volunteers and compared among four refractive states: 0D, +1D, +2D, and +4D, by using plus lenses.

Results

The low-luminance visual stimulus evoked a main MEG response at approximately 120 ms (M100) that reversed its polarity between the upper and lower visual field stimulations and originated from the occipital midline area. When refractive errors were induced by plus lenses, the latency of M100 increased, while its amplitude decreased with an increase in power of the lens. Differences from the control condition (+0D) were significant for all three lenses examined. The results of dipole analyses showed that evoked fields for the control (+0D) condition were explainable by one dipole in the primary visual cortex (V1), while other sources, presumably in V3 or V6, slightly contributed to shape M100 for the +2D or +4D condition.

Conclusions

The present results showed that the latency and amplitude of M100 are both useful indicators for assessing refractive states. The contribution of neural sources other than V1 to M100 was modest under the 0D and +1D conditions. By considering the nature of the activity of M100 including its high sensitivity to a spatial frequency and lower visual field dominance, a simple low-luminance grating stimulus at an optimal spatial frequency in the lower visual field appears appropriate for obtaining data on high S/N ratios and reducing the load on subjects.

Auditory evoked potential response and hearing loss: a review

Hypoacusis is the most prevalent sensory disability in the world and consequently, it can lead to impede speech in human beings. One best approach to tackle this issue is to conduct early and effective hearing screening test using Electroencephalogram (EEG). EEG based hearing threshold level determination is most suitable for persons who lack verbal communication and behavioral response to sound stimulation. Auditory evoked potential (AEP) is a type of EEG signal emanated from the brain scalp by an acoustical stimulus. The goal of this review is to assess the current state of knowledge in estimating the hearing threshold levels based on AEP response. AEP response reflects the auditory ability level of an individual. An intelligent hearing perception level system enables to examine and determine the functional integrity of the auditory system. Systematic evaluation of EEG based hearing perception level system predicting the hearing loss in newborns, infants and multiple handicaps will be a priority of interest for future research.

Pattern visual evoked potentials for identifying malingering

PURPOSE

To investigate the efficacy of pattern visual evoked potentials (VEPs) in evaluating objective visual acuity (VA) and discriminating malingerers.

METHODS

Two hundred and forty-nine eyes of 249 patients aged 20-65 years were included. There were 147 eyes with macular diseases (group 1) and 102 eyes with optic nerve diseases (group 2). Amplitudes and latencies were analyzed and correlated with best-corrected visual acuity by a regression analysis. We found the best-correlated mode of pattern VEP, determined the relations, and then calculated the pattern VEP-estimated VA (PVEP-VA) of all 249 eyes, another 30 malingering eyes, 13 eyes with macular diseases, and 17 eyes with optic nerve diseases, and used a receiver operating characteristic (ROC) curve to determine a cutoff for acceptable variance between PVEP-VA and subjective VA to discriminate malingerers.

RESULTS

The best correlation was between the amplitude of 50' checkerboard size (Amp50') and VA in every group. Significant correlation was between Amp50' and VA, where p < 0.0001 in group 1 and p = 0.020 in group 2. A logarithmic curve best fitted the correlation in the regression analysis, where y = 1.731 - 1.569x (R(2) = 0.611, p < 0.0001) in group 1 and y = 2.413 - 2.169x (R(2) = 0.531, p < 0.0001) in group 2 [x: log(Amp50'), y: PVEP-VA (logMAR)]. By using the relations and ROC curve, we determined a variance value of 0.4041 (logMAR) with 100% sensitivity and 94.0% specificity in group 1 and 0.3658 with 70.6% sensitivity and 50.5% specificity in group 2 to discriminate malingerers.

CONCLUSIONS

The pattern VEP amplitude of 50' checkerboard size was useful to assess VA and can be helpful in discriminating malingering from real disability.

A supplementary system for a brain-machine interface based on jaw artifacts for the bidimensional control of a robotic arm

Non-invasive Brain-Machine Interfaces (BMIs) are being used more and more these days to design systems focused on helping people with motor disabilities. Spontaneous BMIs translate user's brain signals into commands to control devices. On these systems, by and large, 2 different mental tasks can be detected with enough accuracy. However, a large training time is required and the system needs to be adjusted on each session. This paper presents a supplementary system that employs BMI sensors, allowing the use of 2 systems (the BMI system and the supplementary system) with the same data acquisition device. This supplementary system is designed to control a robotic arm in two dimensions using electromyographical (EMG) signals extracted from the electroencephalographical (EEG) recordings. These signals are voluntarily produced by users clenching their jaws. EEG signals (with EMG contributions) were registered and analyzed to obtain the electrodes and the range of frequencies which provide the best classification results for 5 different clenching tasks. A training stage, based on the 2-dimensional control of a cursor, was designed and used by the volunteers to get used to this control. Afterwards, the control was extrapolated to a robotic arm in a 2-dimensional workspace. Although the training performed by volunteers requires 70 minutes, the final results suggest that in a shorter period of time (45 min), users should be able to control the robotic arm in 2 dimensions with their jaws. The designed system is compared with a similar 2-dimensional system based on spontaneous BMIs, and our system shows faster and more accurate performance. This is due to the nature of the control signals. Brain potentials are much more difficult to control than the electromyographical signals produced by jaw clenches. Additionally, the presented system also shows an improvement in the results compared with an electrooculographic system in a similar environment.

Cellular responses following retinal injuries and therapeutic approaches for neurodegenerative diseases

Retinal neurodegenerative diseases like age-related macular degeneration, glaucoma, diabetic retinopathy and retinitis pigmentosa each have a different etiology and pathogenesis. However, at the cellular and molecular level, the response to retinal injury is similar in all of them, and results in morphological and functional impairment of retinal cells. This retinal degeneration may be triggered by gene defects, increased intraocular pressure, high levels of blood glucose, other types of stress or aging, but they all frequently induce a set of cell signals that lead to well-established and similar morphological and functional changes, including controlled cell death and retinal remodeling. Interestingly, an inflammatory response, oxidative stress and activation of apoptotic pathways are common features in all these diseases. Furthermore, it is important to note the relevant role of glial cells, including astrocytes, Müller cells and microglia, because their response to injury is decisive for maintaining the health of the retina or its degeneration. Several therapeutic approaches have been developed to preserve retinal function or restore eyesight in pathological conditions. In this context, neuroprotective compounds, gene therapy, cell transplantation or artificial devices should be applied at the appropriate stage of retinal degeneration to obtain successful results. This review provides an overview of the common and distinctive features of retinal neurodegenerative diseases, including the molecular, anatomical and functional changes caused by the cellular response to damage, in order to establish appropriate treatments for these pathologies.

Assessment of visual disability using visual evoked potentials

Background

The purpose of this study is to validate the use of visual evoked potential (VEP) to objectively quantify visual acuity in normal and amblyopic patients, and determine if it is possible to predict visual acuity in disability assessment to register visual pathway lesions.

Methods

A retrospective chart review was conducted of patients diagnosed with normal vision, unilateral amblyopia, optic neuritis, and visual disability who visited the university medical center for registration from March 2007 to October 2009. The study included 20 normal subjects (20 right eyes: 10 females, 10 males, ages 9–42 years), 18 unilateral amblyopic patients (18 amblyopic eyes, ages 19–36 years), 19 optic neuritis patients (19 eyes: ages 9–71 years), and 10 patients with visual disability having visual pathway lesions. Amplitude and latencies were analyzed and correlations with visual acuity (logMAR) were derived from 20 normal and 18 amblyopic subjects. Correlation of VEP amplitude and visual acuity (logMAR) of 19 optic neuritis patients confirmed relationships between visual acuity and amplitude. We calculated the objective visual acuity (logMAR) of 16 eyes from 10 patients to diagnose the presence or absence of visual disability using relations derived from 20 normal and 18 amblyopic eyes.

Results

Linear regression analyses between amplitude of pattern visual evoked potentials and visual acuity (logMAR) of 38 eyes from normal (right eyes) and amblyopic (amblyopic eyes) subjects were significant [y = −0.072x + 1.22, x: VEP amplitude, y: visual acuity (logMAR)]. There were no significant differences between visual acuity prediction values, which substituted amplitude values of 19 eyes with optic neuritis into function. We calculated the objective visual acuity of 16 eyes of 10 patients to diagnose the presence or absence of visual disability using relations of y = −0.072x + 1.22 (−0.072). This resulted in a prediction reference of visual acuity associated with malingering vs. real disability in a range >5.77 μV. The results could be useful, especially in cases of no obvious pale disc with trauma.

Conclusions

Visual acuity quantification using absolute value of amplitude in pattern visual evoked potentials was useful in confirming subjective visual acuity for cutoff values >5.77 μV in disability evaluation to discriminate the malingering from real disability.

Assessing optic nerve pathology with diffusion MRI: from mouse to human

The optic nerve is often affected in patients with glaucoma and multiple sclerosis. Conventional MRI can detect nerve damage, but it does not accurately assess the underlying pathologies. Mean diffusivity and diffusion anisotropy indices derived from diffusion tensor imaging have been shown to be sensitive to a variety of central nervous system white matter pathologies. Despite being sensitive, the lack of specificity limits the ability of these measures to differentiate the underlying pathology. Directional (axial and radial) diffusivities, measuring water diffusion parallel and perpendicular to the axonal tracts, have been shown to be specific to axonal and myelin damage in mouse models of optic nerve injury, including retinal ischemia and experimental autoimmune encephalomyelitis. The progression of Wallerian degeneration has also been detected using directional diffusivities after retinal ischemia. However, translating these findings to human optic nerve is technically challenging. The current status of diffusion MRI of human optic nerve, including imaging sequences and protocols, is summarized herein. Despite the lack of a consensus among different groups on the optimal sequence or protocol, increased mean diffusivity and decreased diffusion anisotropy have been observed in injured optic nerve from patients with chronic optic neuritis. From different mouse models of optic nerve injuries to the emerging studies on patients with optic neuritis, directional diffusivities show great potential to be specific biomarkers for axonal and myelin injury.

Multifocal visual evoked potentials in the anesthetized non-human primate

PURPOSE

To evaluate monkey multifocal visual evoked cortical potentials (mfVEPs) recorded from central and peripheral fields for reliability and isolation from electroretinographic (ERG) activity.

METHODS

The mfVEP stimulus consisted of a 7-element hexagonal array that subtended 80 degrees of the central visual field. Recordings were made under intravenous pentobarbital sodium (15 mg/kg) anesthesia. Two monkeys with absent optic nerve and ganglion cell function after combined unilateral optic nerve transection and experimental ocular hypertension (ONT/OHT) were followed longitudinally. In a second study, 16 ophthalmologically normal monkeys were tested once.

RESULTS

Testing of the non-transected eye in two transected animals revealed robust first- and second-order kernel, first slice (K1 and K2.1) mfVEPs. Stimulation of the transected eye revealed no contamination of the mfVEP from the concurrently recorded multifocal ERGs. There was complete separation of the root-mean-square (RMS) mfVEP amplitudes from the transected and the fellow eyes tested repeatedly across a 4- to 17- month period. The largest amplitude mfVEP was generated by the central element; however, mfVEPs were recorded from outside the central 20 degrees element. The 16 normal animals showed waveforms similar to the normal eyes of the ONT/OHT animals both in shape and distribution throughout the visual field. A scalar-product measure showed both K1 and K2.1 mfVEPs from central and some peripheral elements were statistically distinct from noise.

CONCLUSIONS

mfVEPs can be reliably recorded from non-human primates anesthetized with pentobarbital. Under the recording conditions described, mfVEPs are not contaminated by ERG activity. mfVEPs may be useful in animal models of diseases that differentially affect macular and peripheral visual field responsiveness.

To act or not to act. Neural correlates of executive control of learned motor behavior

Successful behavior requires contextual modulation of learned "programs", that is, the retrieval or nonretrieval (inhibition) of behavioral elements depending on situative context. Here we report neural correlates of these elementary aspects of behavior as identified with functional magnetic resonance imaging (fMRI). Inhibition of a "ready-to-go" behavioral program was represented in the brain by reduction of net synaptic activity in the cerebro-cerebellar pathway. The metabolic correlate of inhibition was a multifocal (premotor, primary sensorimotor, superior parietal, cingulate cortex, and cerebellum) decrease of the blood oxygenation level-dependent (BOLD) signal to below the resting state (negative BOLD) with a concomitant decrease of motor cortical excitability. The reverse was true for retrieval. We propose that contextual modulation of learned behavioral programs depends on an interplay of focal increases and decreases of neural activity and that the inhibitory changes are reflected by negative BOLD responses in an extended cerebro-cerebellar network of sensorimotor structures.

Processing Deficits in Primary Visual Cortex of Amblyopic Cats

Early esotropic squint frequently results in permanent visual deficits in one eye, referred to as strabismic amblyopia. The neurophysiological substrate corresponding to these deficits is still a matter of investigation. Electrophysiological evidence is available for disturbed neuronal interactions in both V1 and higher cortical areas. In this study, we investigated the modulation of responses in cat V1 to gratings at different orientations and spatial frequencies (SFs; 0.1–2.0 cycles/°) with optical imaging of intrinsic signals. Maps evoked by both eyes were well modulated at most spatial frequencies. The layout of the maps resembled that of normal cats, and iso-orientation domains tended to cross adjacent ocular dominance borders preferentially at right angles. Visually evoked potentials (VEPs) were recorded at SFs ranging from 0.1 to 3.5 cycles/° and revealed a consistently weaker eye for the majority of squinting cats. At each SF, interocular differences in VEP amplitudes corresponded well with differences in orientation response and selectivity in the maps. At 0.7–1.3 cycles/°, population orientation selectivity was significantly lower for the weaker eye in cats with VEP differences compared with those with no VEP amplitude differences. In addition, the cutoff SF, above which gratings no longer induced orientation maps, was lower for the weaker eye (≥1.0 cycles/°). These data reveal a close correlation between the loss of visual acuity in amblyopia as assessed by VEPs and the modulation of neuronal activation as seen by optical imaging of intrinsic signals. Furthermore, the results indicate that amblyopia is associated with altered intracortical processing already in V1.

Detecting glaucomatous damage with multifocal visual evoked potentials: how can a monocular test work?

PURPOSE

To understand and improve the detection of glaucomatous damage with multifocal visual evoked potentials (mfVEP) obtained from single eyes.

PATIENTS AND METHODS

Monocular mfVEP recordings were obtained from both eyes of 30 individuals with no known visual abnormalities. The 44.5 degrees -diameter display contained 60 sectors. Probability plots, analogous to the total deviation probability plot of the Humphrey Visual Field Analyzer, were created based on tests that compared each eye (monocular test), or the ratio of the responses from the 2 eyes (interocular test), to group norms.

RESULTS

For the monocular test, the number of points exceeding the 5% confidence level was not distributed randomly among individuals or among field locations within an individual. Individuals with small signals (i.e., low signal-to-noise ratios [SNR]) showed too many "abnormal" points, while those with relatively large SNR values showed too few. Reasonably good specificity was obtained by defining an abnormality in terms of a cluster of significant points in the mfVEP probability plot. For the interocular test, the results were close to those expected by chance.

CONCLUSIONS

Both monocular and interocular tests will be of value when testing glaucoma patients with the mfVEP technique. The interocular test is a more sensitive indicator of glaucomatous damage when a region when healthy has a large signal (SNR) and damage is largely unilateral, whereas the monocular test will be a more sensitive test when a region when healthy has a small signal (SNR). However, the statistics underlying monocular comparisons of mfVEPs are not simple. To obtain high specificity, criteria based on clusters of points need to be used and norms obtained for every laboratory.

Inhibitory control of acquired motor programmes in the human brain

An important basis of skilled human behaviour is the appropriate retrieval of acquired and memorized motor programmes ('motor memory traces'). Appropriate retrieval is warranted if motor programmes are only activated if necessary and are, probably more often, inhibited if required by the context of a given situation. It is unknown how this type of inhibition is accomplished in the brain. We studied context-dependent modulation of motor memory traces in 18 volunteers and six patients with focal dystonia. Cortical function was assessed with transcranial magnetic stimulation over the primary motor cortex (M1) and with task-related analysis of oscillatory EEG activity. An activation (ACT) and inhibition (INH) condition were compared. In both, visual cues were presented at 1/s. In ACT, subjects had to respond to these cues with individual finger movements as learned in a preceding training session. In INH, subjects had to observe the cues without retrieval of motor responses. During INH, inhibitory control of the motor memory trace was confirmed by significant amplitude reduction of motor evoked potentials (MEPs) compared with baseline. This was accompanied by a significant increase of 11-13 Hz oscillatory activity over the sensorimotor areas during INH. During active retrieval of the motor memory traces, the reverse was true (increased MEP amplitudes, decreased oscillatory 11-13 Hz activity). In a small sample of dystonic patients (n = 6), the increase of 11-13 Hz oscillatory activity during INH was consistently absent. The present data demonstrate for the first time cortical correlates of appropriate, context-dependent inhibition of motor memory traces. We propose that focal increases of oscillatory activity are instrumental for inhibitory control at the cortical level. This concept is supported by the preliminary observations in dystonic patients who are known to have deficits of inhibitory motor control and in whom these context-dependent focal increases of oscillatory activity were absent.

Dynamic stretch correlates to both morphological abnormalities and electrophysiological impairment in a model of traumatic axonal injury

In this investigation, the relationships between stretch and both morphological and electrophysiological signs of axonal injury were examined in the guinea pig optic nerve stretch model. Additionally, the relationship between axonal morphology and electrophysiological impairment was assessed. Axonal injury was produced in vivo by elongating the guinea pig optic nerve between 0 and 8 mm (Ntotal = 70). Morphological damage was detected using neurofilament immunohistochemistry (SMI 32). Electrophysiological impairment was determined using changes in visual evoked potentials (VEPs) measured prior to injury, every 5 min for 40 min following injury, and at sacrifice (72 h). All nerves subjected to ocular displacements greater than 6 mm demonstrated axonal swellings and retraction bulbs, while nerves subjected to displacements below 4 mm did not show any signs of morphological injury. Planned comparisons of latency shifts of the N35 peak in the VEPs showed that ocular displacements greater than 5 mm produced electrophysiological impairment that was significantly different from sham animals. Logit analysis demonstrated that less stretch was required to elicit electrophysiological changes (5.5 mm) than morphological signs of damage (6.8 mm). Moreover, Student t tests indicated that the mean latency shift measured in animals exhibiting morphological injury was significantly greater than that calculated from animals lacking morphological injury (p < 0.01). These data show that distinct mechanical thresholds exist for both morphological and electrophysiological damage to the white matter. In a larger context, the distinct injury thresholds presented in the report will aid in the biomechanical assessment of animate models of head injury, as well as assist in extending these findings to predict the conditions that cause white matter injury in humans.

Response characteristics of the normal retino-cortical pathways as determined with simultaneous recordings of pattern visual evoked potentials and simple motor reaction times

PURPOSE

In an attempt to explain the existing discrepancies regarding the relationship between electrophysiological and psychophysical measurements of visual transmission time we compared, in humans, the response characteristics of the normal retino-cortical pathways with simultaneously obtained pattern visual evoked potentials (PVEP) and simple motor reaction times (RT).

METHODS

PVEPs and manual RTs were recorded simultaneously using a reversing checkerboard with different spatial frequency and contrast combinations chosen to elicit responses favoring the magnocellular or parvocellular pathways. The amplitude and peak time of the P1 wave of the PVEP were compared to the mean RT. Other parameters of the RT, such as mode and standard deviation were also considered.

RESULTS

The RT is not modified in the same fashion as the peak time of the P1 wave of the PVEP, the peak time of the PVEP demonstrating a spatial frequency selectivity, while the RT does not. Further comparative analysis of the PVEP and RT shows that the RT is faster for stimuli of lower contrast and spatial frequency, while the PVEP amplitude is larger and its peak time shorter for higher contrast and spatial frequency stimuli.

CONCLUSIONS

Our findings suggest that PVEP and RT measures recruit distinct physiological characteristics and appear to be differently modulated while travelling along the retino-cortical pathway. Our results also show the importance of obtaining electrophysiological and psychophysical measures concomitantly to insure elimination of combined inter-stimulus and inter-session variability.

Pattern visual evoked potential in the diagnosis of functional visual loss

OBJECTIVE

To study the pattern visual evoked potential (P-VEP) in the diagnosis of functional visual loss.

STUDY DESIGN

Retrospective study of observational case series.

PARTICIPANTS

Seventy-two subjects whose best corrected visual acuity (VA) was 20/50 or worse, with or without visual field defect, and whose visual abnormalities could not be explained by the findings of ophthalmologic and neurologic examination were included in this study.

MAIN OUTCOME MEASURES

To compare the P-VEP estimated acuity to the initial subjective VA and to the best-performed VA.

RESULTS

Seventy-two subjects with functional visual loss had normal P-VEPs. The initial subjective VA was 20/50 in 9 subjects and < or =20/200 in 42 subjects. After clinical examination and reassurance, the best-performed VA was > or =20/50 in 53 subjects and < or =20/200 in 8 subjects. The discrepancy between the P-VEP estimated acuity and the best-performed VA was less than 3 lines of Snellen acuity in 63 of 72 (87.5%) subjects and more than 4 lines in 6 subjects. These six subjects were three women with loss of vision of unknown origin and three men with injury-related visual loss.

CONCLUSIONS

P-VEP has the advantage of objectively predicting VA and is a useful test in the diagnosis of functional visual loss.

Visual evoked cortical potential can be used to differentiate between uncorrected refractive error and macular disorders

The visual evoked cortical potential (VECP) is widely used to verify complaints of reduced visual performance and to identify the site of the disorder. In this study, we investigated the correlation between reduced visual acuity and VECP in volunteers with normal corrected visual acuity and in patients suffering from inherited macular degeneration or from age related macular degeneration (ARMD). Flash evoked VECP was not affected by the visual acuity in the cases of refractive error and in ARMD patients but was reduced in amplitude and delayed in implicit time in the patients suffering from inherited macular degeneration. The VECP elicited by pattern reversal checkerboard (PVECP) was not affected by the quality of the visual image in volunteers with uncorrected refractive error when checks of 60' or larger were used but were considerably reduced in size and prolonged in implicit time for checks smaller than 15'. In both groups of patients suffering from macular dysfunction, pattern reversal VECP was very subnormal and was characterized by prolonged implicit time compared to values expected from their visual acuity. These findings indicate that the PVECP does not directly correlate with visual acuity but rather with foveal function. Therefore, we suggest that recordings of PVECP can be used to differentiate between refractive error and macular disorders as causing reduction in visual acuity when other clinical signs are missing or not available.

Tissue-level thresholds for axonal damage in an experimental model of central nervous system white matter injury

In vivo, tissue-level, mechanical thresholds for axonal injury were determined by comparing morphological injury and electrophysiological impairment to estimated tissue strain in an in vivo model of axonal injury. Axonal injury was produced by dynamically stretching the right optic nerve of an adult male guinea pig to one of seven levels of ocular displacement (Nlevel = 10; Ntotal = 70). Morphological injury was detected with neurofilament immunohistochemical staining (NF68, SM132). Simultaneously, functional injury was determined by the magnitude of the latency shift of the N35 peak of the visual evoked potentials (VEPs) recorded before and after stretch. A companion set of in situ experiments (Nlevel = 5) was used to determine the empirical relationship between the applied ocular displacement and the magnitude of optic nerve stretch. Logistic regression analysis, combined with sensitivity and specificity measures and receiver operating characteristic (ROC) curves were used to predict strain thresholds for axonal injury. From this analysis, we determined three Lagrangian strain-based thresholds for morphological damage to white matter. The liberal threshold, intended to minimize the detection of false positives, was a strain of 0.34, and the conservative threshold strain that minimized the false negative rate was 0.14. The optimal threshold strain criterion that balanced the specificity and sensitivity measures was 0.21. Similar comparisons for electrophysiological impairment produced liberal, conservative, and optimal strain thresholds of 0.28, 0.13, and 0.18, respectively. With these threshold data, it is now possible to predict more accurately the conditions that cause axonal injury in human white matter.

The effect of check size on VEPs in healthy humans: a parabolic relationship between check size and N145 wave

The effect of check size on pattern reversal VEPs was examined in healthy adult volunteers. The subjects were exposed to full-field binocular stimulation. Five checkerboard patterns with check sizes 13', 25', 50', 75' and 100' were used as the visual stimuli. The patterns reversed at a rate of 1 reversal/sec. Well-defined pattern reversal VEPs were obtained for all check sizes in all subjects. As the check size increased a transition from a triphasic to an almost monophasic wave form was observed. N75 and P100 amplitudes, together with N75 latency, revealed linear relationships with check size; the N145 amplitude displayed a parabolic relationship with check size. No significant relationships were found for P100 and N145 latencies. It is concluded that the N75 wave originated mainly from the activity of the foveola, whereas the more eccentric regions contribute more to the formation of P100, and the interaction of both regions elicited the N145 wave.