Visual evoked potentials in school children: a comparative study of transient and steady-state methods with pattern reversal and flash stimulation

Electrophysiological biomarkers of brain function in CDKL5 deficiency disorder

CDKL5 deficiency disorder is a debilitating developmental and epileptic encephalopathy for which no targeted treatment exists. A number of promising therapeutics are under development for CDKL5 deficiency disorder but a lack of validated biomarkers of brain function and clinical severity may limit the ability to objectively assess the efficacy of new treatments as they become available. To address this need, the current study quantified electrophysiological measures in individuals with CDKL5 deficiency disorder and the association between these parameters and clinical severity. Visual and auditory evoked potentials, as well as resting EEG, were acquired across 5 clinical sites from 26 individuals with CDKL5 deficiency disorder. Evoked potential and quantitative EEG features were calculated and compared with typically developing individuals in an age- and sex-matched cohort. Baseline and Year 1 data, when available, were analysed and the repeatability of the results was tested. Two clinician-completed severity scales were used for evaluating the clinical relevance of the electrophysiological parameters. Group-level comparisons revealed reduced visual evoked potential amplitude in CDKL5 deficiency disorder individuals versus typically developing individuals. There were no group differences in the latency of the visual evoked potentials or in the latency or amplitude of the auditory evoked potentials. Within the CDKL5 deficiency disorder group, auditory evoked potential amplitude correlated with disease severity at baseline as well as Year 1. Multiple quantitative EEG features differed between CDKL5 deficiency disorder and typically developing participants, including amplitude standard deviation, 1/f slope and global delta, theta, alpha and beta power. Several quantitative EEG features correlated with clinical severity, including amplitude skewness, theta/delta ratio and alpha/delta ratio. The theta/delta ratio was the overall strongest predictor of severity and also among the most repeatable qEEG measures from baseline to Year 1. Together, the present findings point to the utility of evoked potentials and quantitative EEG parameters as objective measures of brain function and disease severity in future clinical trials for CDKL5 deficiency disorder. The results also underscore the utility of the current methods, which could be similarly applied to the identification and validation of electrophysiological biomarkers of brain function for other developmental encephalopathies.

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.

A survey on stimuli for visual cortical function assessment in infants

Visual processing, as a significant and complex functionality of the human brain, changes during the life span with the most developmental changes in the infancy. Different types of visual stimuli are needed for evaluating different functionalities of the infants' visual system. Selecting appropriate visual stimuli is an important issue in evaluating visual cortical functions in infants. Properties of stimulation influence responses of visual system and must be adjusted according to the age and specific function which is going to be investigated. In this review, the most commonly used stimuli to elicit visual evoked potentials (VEPs) are evaluated and characteristics of VEPs extracted by these stimulations are studied. Furthermore, various studies investigating different functionalities such as selectivity for orientation and directional motion are presented. Valuable results regarding emerging and maturation times of different functions and normative data for clinical diagnosis are provided by these studies.

The Development and Aging of the Magnocellular and Parvocellular Visual Pathways as Indicated by VEP Recordings between 5 and 84 Years of Age

It is well known that pattern reversal visual evoked potentials (VEPs) are age-sensitive. Through the use of this technique, it is possible to assess both of the major visual pathways (i.e., the magnocellular and parvocellular ones) in terms of function and development. What developmental path these pathways follow, and if they develop/age in parallel across the human lifespan is a matter of ongoing debate, yet, only a few VEP studies have dealt with this issue. This cross-sectional study examined a sample of 115 healthy volunteers aged 5 to 84 years. Beyond the standard checkerboard pattern reversal stimulation at 97% contrast, we recorded pattern-reversal VEPs at 6% contrast to selectively stimulate the M pathway and isoluminant red and green checkerboard stimulation was also used to selectively stimulate the P pathway. Our results do not support the developmental advantage of any of the pathways. The development of both pathways appear to take a remarkably long time (well into the 30s), and the signs of aging become marked over 50 years of age, especially in the case of the magnocellular pathway.

Orientation-reversal VEP: comparison of phase and peak latencies in adults and infants

The peak latency of pattern-reversal (PR)-VEP has been found to develop rapidly, reaching the adult level around 15 weeks of age. However, the development of orientation-reversal (OR)-VEP, reflecting the specific spatial organization of cortical receptive fields, still remains unknown. OR-VEP was tested in 81 adults at 1-12 reversals/sec (r/s) and 94 infants (age 4-79 weeks) at 2-8r/s. OR data at 4r/s from an additional 123 infants (age 4.0-20.3 weeks) studied previously were also analyzed. In addition to peak transient latencies at 1-4r/s, latency values derived from the gradient of phase against temporal frequency in steady-state recording were also calculated. For both adults and infants, no significant latency differences in the initial positive peaks were found among the low reversal rates. The calculated latency was statistically longer than the transient latency in both groups. While the transient latency asymptoted to adult value of 102 ms at around 50 weeks of age, the calculated latency, unlike that for PR-VEP, showed little variation across the age span. The data suggest a dominant effect of transmission delay on the initial peak in infancy, which reduces with age. However, the overall timing of the cortical response to orientation change remains slower than for pattern reversal in the fully developed visual cortex. Upon reaching maturity, the latencies of the initial positive peak in both pattern and orientation VEPs may arise from the same level of cortical processing in V1, but the overall time course reflected in the steady-state phase continues to show a much more prolonged response to orientation change than the transmission delay seen in the transient VEPs.

Aging effect in pattern, motion and cognitive visual evoked potentials

An electrophysiological study on the effect of aging on the visual pathway and various levels of visual information processing (primary cortex, associate visual motion processing cortex and cognitive cortical areas) was performed. We examined visual evoked potentials (VEPs) to pattern-reversal, motion-onset (translation and radial motion) and visual stimuli with a cognitive task (cognitive VEPs - P300 wave) at luminance of 17 cd/m(2). The most significant age-related change in a group of 150 healthy volunteers (15-85 years of age) was the increase in the P300 wave latency (2 ms per 1 year of age). Delays of the motion-onset VEPs (0.47 ms/year in translation and 0.46 ms/year in radial motion) and the pattern-reversal VEPs (0.26 ms/year) and the reductions of their amplitudes with increasing subject age (primarily in P300) were also found to be significant. The amplitude of the motion-onset VEPs to radial motion remained the most constant parameter with increasing age. Age-related changes were stronger in males. Our results indicate that cognitive VEPs, despite larger variability of their parameters, could be a useful criterion for an objective evaluation of the aging processes within the CNS. Possible differences in aging between the motion-processing system and the form-processing system within the visual pathway might be indicated by the more pronounced delay in the motion-onset VEPs and by their preserved size for radial motion (a biologically significant variant of motion) compared to the changes in pattern-reversal VEPs.

Early signs of visual perception and evoked potentials in radiologically asymptomatic boys with X-linked adrenoleukodystrophy

The aim was to identify the electrophysiological and psychological signs at a very early stage in asymptomatic boys with childhood cerebral X-linked adrenoleukodystrophy. Flash visual evoked potentials, pattern reversal, and visual event-related potentials were recorded in 6 radiologically asymptomatic boys with adrenoleukodystrophy and 22 control boys. The latency and amplitude of P100 of visual evoked potentials and P1 of event-related potentials were evaluated. Though all patients had normal intelligence quotient, performance intelligence quotient was significantly lower than verbal intelligence quotient in 2 patients. Both P100 and P1 amplitudes were significantly greater in adrenoleukodystrophy than in controls. The difference between performance intelligence quotient and verbal intelligence quotient exhibited significant correlation with P100 amplitude. Enlargement of visual evoked potentials might be a sign of cerebral involvement preceding the appearance of abnormalities on magnetic resonance imaging. Follow-up of asymptomatic boys with both electrophysiological and neuropsychological tests may serve as an aid for deciding the timing of therapeutic intervention.

Frequency-Following and Connectivity of Different Visual Areas in Response to Contrast-Reversal Stimulation

The sensitivity of visual areas to different temporal frequencies, as well as the functional connections between these areas, was examined using magnetoencephalography (MEG). Alternating circular sinusoids (0, 3.1, 8.7 and 14 Hz) were presented to foveal and peripheral locations in the visual field to target ventral and dorsal stream structures, respectively. It was hypothesized that higher temporal frequencies would preferentially activate dorsal stream structures. To determine the effect of frequency on the cortical response we analyzed the late time interval (220-770 ms) using a multi-dipole spatio-temporal analysis approach to provide source locations and timecourses for each condition. As an exploratory aspect, we performed cross-correlation analysis on the source timecourses to determine which sources responded similarly within conditions. Contrary to predictions, dorsal stream areas were not activated more frequently during high temporal frequency stimulation. However, across cortical sources the frequency-following response showed a difference, with significantly higher power at the second harmonic for the 3.1 and 8.7 Hz stimulation and at the first and second harmonics for the 14 Hz stimulation with this pattern seen robustly in area V1. Cross-correlations of the source timecourses showed that both low- and high-order visual areas, including dorsal and ventral stream areas, were significantly correlated in the late time interval. The results imply that frequency information is transferred to higher-order visual areas without translation. Despite the less complex waveforms seen in the late interval of time, the cross-correlation results show that visual, temporal and parietal cortical areas are intricately involved in late-interval visual processing.

Motion-onset VEPs reflect long maturation and early aging of visual motion-processing system

Pattern-reversal and motion-onset visual evoked potentials (VEPs) were simultaneously tested in a group of 70 healthy subjects between the ages of 6-60 years to verify suspected differences in maturation and aging dynamics of the pattern and motion processing subsystems of the visual pathway. The motion-onset VEPs displayed dramatic configuration development and shortening of latencies up to 18 years of age (correl. coeff. -0.85; p < 0.001) and systematic prolongation from about 20 years of age (correl. coeff. 0.70; p < 0.001). This confirms long-lasting maturation of the magnocellular system and/or motion processing cortex and their early age related changes. Less significant changes of pattern-reversal VEPs in the tested age range can be interpreted as a sign of early maturation of the parvocellular system and its enhanced functional endurance in the elderly.

Interaction between the flash evoked SSVEPs and the spontaneous EEG activity in children and adults

OBJECTIVE

To evaluate the interaction between the steady-state visual evoked potentials (SSVEPs) recorded during the intermittent photic stimulation (IPS) and the spontaneous EEG activities both in children and adults.

METHODS

EEG was recorded during the rest and under 5, 7.5, 10 and 12.5 Hz IPS in 41 children between 3 and 16 years and 10 adults. We distinguished between the spontaneous resting EEG spectra, SSVEPs (1st harmonic) and undriven (ongoing) EEG spectra recorded during the IPS.

RESULTS

We show that IPS influences spontaneous EEG activity by specifically suppressing or desynchronizing individual posterior dominant resting EEG frequencies (DF) in both children and adults. Further, this highly significant and consistent suppressing effect positively correlates with the SSVEPs amplitude.

CONCLUSIONS

Our data suggest that the desynchronization of the spontaneous EEG activity under IPS and the SSVEPs are related to each other.

SIGNIFICANCE

These relationships could be interesting to study in pathological conditions where the neural synchronization and the responses to IPS have been shown to be affected, such as epilepsy and schizophrenia.

Pattern ERG and VEP maturation in schoolchildren

OBJECTIVE

The maturation of the visual system has been studied with pattern electroretinograms (PERG) and pattern visual evoked potentials (PVEP) mostly in children under the age of 6 years. To address the question of maturation of the visual system in childhood and adolescence we investigated age-dependent PERG and PVEP changes in children aged 7-18 years.

METHODS

PERG were recorded with skin electrodes attached to the lower eyelid, and PVEP were recorded with 5 electrodes. Visual stimuli, consisting of pattern-reversal 50' checks to full-field and to half-field stimulation, were applied to obtain macular (N70, P100, N145) and paramacular waves (P80, N105, N135).

RESULTS

We found an age-dependent decrease (linear regression P<0.05) of PERG P50 amplitude and full-field PVEP P100 latency to monocular right and left eye stimulation, indicating central retinal and postretinal changes. In addition, waveform changes were found in responses to half-field stimulation. The paramacular wave N105 was typically enhanced in younger schoolchildren and diminished with age. The age-dependent decrease (linear regression P<0.01) of paramacular N105 amplitude indicated the increasing predominance of the macular structures of the visual system.

CONCLUSIONS

Our findings suggest that central retinal and postretinal electrophysiological maturation persists throughout childhood. Age-dependent PVEP changes seem to correlate with the morphological and metabolic findings that maturation of the visual cortex continues until puberty and even later.

Identification of the stimulated hemiretina in primary school children and adults based on left and right hemifield pattern reversal visual evoked potentials--a comparative study

OBJECTIVES

The analysis of left and right hemifield pattern-reversal visual evoked potentials (PVEPs) in children and the identification of the stimulated hemiretina testing different identification procedures previously applied to adults.

METHODS

Lateral hemifield PVEPs were recorded in 40 children (6-11 years) and 27 adults (25-40 years) from, at least, 19 standard electrodes. Two procedures were tested for the determination of the stimulated hemifield: firstly, the evaluation of the values of instantaneous frequency at the occipital electrodes at P100 latency (determined by the global field power), and secondly, the application of a generalised dynamic neural network (GDNN) using the PVEP time course at selected electrode positions as the external input.

RESULTS

P100 latency as well as P100 amplitude over the contralateral occiput in children were significantly greater than in adults. Contrary to the behaviour in adults, instantaneous frequency is not a robust identifier of left and right hemiretina stimulation in children. The best identification performances were achieved when using group trained GDNNs with the bipolar difference signals of electrodes P3/P4 or T5/T6 as the external input.

CONCLUSIONS

The PVEPs at electrodes P3/P4 and T5/T6 contain essential information for the determination of the stimulated hemifield. This should be further considered during the development of on-line procedures for automatic PVEP detection in future studies.