Extraction and modeling of the Oscillatory Potential: signal conditioning to obtain minimally corrupted Oscillatory Potentials

Oscillatory potential findings in patients with acute ischaemic central retinal vein occlusion

AIMS

To explore the sensitive components of full-field electroretinography (ERG) as indicators of retina function at the onset of acute ischaemic central retinal vein occlusion (CRVO).

METHODS

11 patients (11 eyes) with ischaemic CRVO and 32 patients (32 eyes) with non-ischaemic CRVO who presented with first-episode unilateral CRVO within 1 month of symptom onset and with no previous intervention were examined by the International Society for Clinical Electrophysiology of Vision standard ERG.

RESULTS

A significant amplitude decline and peak time delay in light-adapted (LA) 3 ERG and LA 30 Hz flicker ERG (p<0.05 for all) was found in the ischaemic CRVO eyes, compared with the non-ischaemic CRVO eyes. The b/a amplitude ratio of dark-adapted (DA) 3 ERG, DA 10 ERG and LA 3 ERG was significantly different between the ischaemic and non-ischaemic groups (p<0.05 for all). Regarding oscillatory potentials (OPs), the amplitudes of OP1, OP2 and OP3 as well as the sum of DA 3 OP1-4 amplitudes (∑OPs) showed significant changes (p<0.01 for all) between two groups. No peak time delay of OPs was found between the ischaemic and non-ischaemic CRVO eyes.

CONCLUSION

The amplitude of DA 0.01 ERG, components of LA 3 ERG and LA 30 Hz flicker ERG, and the b/a amplitude ratio could be among the most sensitive indicators in patients with acute ischaemic CRVO. The amplitudes of OP1, OP2, OP3 and ∑OPs in the CRVO eyes were reduced to 40% of the control values, showing that this quantitative method is reliable for detecting ischaemic retinal diseases, even in early stage.

Full-field and multifocal electroretinogram in non-diabetic controls and diabetics with and without retinopathy

OBJECTIVE

To compare retinal function assessed by full-field electroretinography (ffERG) and multifocal electroretinography (mfERG) in diabetes without retinopathy, diabetes with moderate non-proliferative diabetic retinopathy (NPDR) and in the absence of diabetes.

METHODS

Scotopic and photopic ffERG and mfERG was made in non-fasting volunteers, including 26 diabetic participants without retinopathy, 22 diabetic participants with moderate NPDR and 22 participants without diabetes using full International Society for Clinical Electrophysiology of Vision protocols.

RESULTS

Of the ffERG responses, significant deviation (p ≤ 0.05, corrected for multiple sampling and other relevant confounders) from the non-diabetic participants was seen in the diabetic participants only for the OP1-OP3 oscillatory amplitudes and the OP2 implicit time. This finding was independent of whether retinopathy was present or not. For the mfERG, minor amplitude or implicit time deviations were found for a small number of rings (R2, R4 and R5). Receiver of operating characteristic analysis showed that the single most prominent abnormality of the ffERG in diabetes, regardless of whether retinopathy was present or not, was the OP2 implicit time (area under the curve ≥ 0.80).

CONCLUSION

This bi-modal study of electroretinographic characteristics found that the most prominent anomaly associated with diabetes was a prolongation of the implicit time of the OP2 of the scotopic ffERG, while the most prominent added effect of non-proliferative diabetic retinopathy was a further prolongation of the OP2 implicit time. Although the variation in ERG characteristics is far too large for diagnostic purposes, the close association of the oscillatory potentials with the amacrine cells of the retina indicate that their function is particularly sensitive to diabetes.

Which Comes First in Sports Vision Training: The Software or the Hardware Update? Utility of Electrophysiological Measures in Monitoring Specialized Visual Training in Youth Athletes

In the present study we combined popular methods of sports vision training (SVT) with traditional oculomotor protocols of Optometric Vision Therapy (OVT) and electrophysiological indexes of EEG and VEP activity to monitor training progress and changes in performance of youth ice hockey players without the history of concussion. We hypothesized that administration of OVT protocols before SVT training may result in larger performance improvements compared to the reverse order due to the initial strengthening of visual hardware capable of handling greater demands during training of visuomotor integration and information processing skills (visual software). In a cross-over design 53 youth ice hockey players (ages 13-18) were randomly assigned to one of the two training groups. Group one (hardware-software group) completed 5 weeks of oculomotor training first followed by 5 weeks of software training. For group 2 (software-hardware) the order of procedures were reversed. After 10 weeks of training both groups significantly improved their performance on all but one measure of the Nike/Senaptec Sensory station measures. Additionally, the software-hardware training order resulted in significantly lower frontal theta-to-gamma amplitude ratios on the Nike/Senaptec test of Near-Far Quickness as well as in faster P100 latencies. Both training orders also resulted in significant decreases in post-treatment P100 amplitude to transient VEP stimuli as well as decreased theta-gamma ratios for perception span, Go/No-Go and Hand Reaction time. The observed changes in the electrophysiological indexes in the present study are thought to reflect greater efficiency in visual information processing and cognitive resource allocation following 10 weeks of visual training. There is also some evidence of the greater effectiveness of the software-hardware training order possibly due to the improved preparedness of the oculomotor system in the youth athletes for administration of targeted protocols of the Optometric Vision Therapy.

Effects of fixed cutoff filtering on dark- and light-adapted ERG components and the application of variable cutoff filter

Purpose

Human oscillatory potentials (OPs) are derived from dark-adapted (DA) electroretinograms (ERGs) with fixed frequency cutoff filters while light-adapted (LA) OPs are generally not isolated from ERGs. Our purpose was to analyze the effect of cutoff frequencies on DA and LA ERG components using a series of fixed and variable filters.

Methods

DA and LA ERGs were recorded from 10 healthy eyes of 10 subjects (mean age = 20.5 ± 6.7 years) following ISCEV standards. Each signal was filtered in the Fourier domain to acquire slow (a- and b-waves; below cutoff frequency) and fast (OPs; above cutoff frequency) components. Fixed cutoff frequencies ranged from 60 to 105 Hz and a variable cutoff frequency was calculated. Results were analyzed with statistical tests and specific models.

Results

DA ERG components were slightly influenced by the filter cutoff frequency. In contrast, fixed and variable filters significantly changed LA components: the lower the cutoff frequency the smaller the b-wave and OP3 and the higher the OP2/OP4 amplitudes. Analyzing the filter frequency limits a transition range between 68.9 Hz and 83.9 Hz was observed where amplitudes vary.

Conclusions

The present report shows that DA OPs may be isolated from ERGs using filtering procedures with high-pass cutoff frequency at about 75 Hz as recommended by ISCEV. On the other hand, the spectral distribution of low-frequency and high-frequency LA ERG components may overlap. Accordingly, filtering the signal using different cutoff frequencies is not necessarily separating b-wave and OPs.

Visually Evoked Potential Markers of Concussion History in Patients with Convergence Insufficiency

Purpose

We investigated whether differences in the pattern visual evoked potentials exist between patients with convergence insufficiency and those with convergence insufficiency and a history of concussion using stimuli designed to differentiate between magnocellular (transient) and parvocellular (sustained) neural pathways.

Methods

Sustained stimuli included 2-rev/s, 85% contrast checkerboard patterns of 1- and 2-degree check sizes, whereas transient stimuli comprised 4-rev/s, 10% contrast vertical sinusoidal gratings with column width of 0.25 and 0.50 cycles/degree. We tested two models: an a priori clinical model based on an assumption of at least a minimal (beyond instrumentation’s margin of error) 2-millisecond lag of transient response latencies behind sustained response latencies in concussed patients and a statistical model derived from the sample data.

Results

Both models discriminated between concussed and nonconcussed groups significantly above chance (with 76% and 86% accuracy, respectively). In the statistical model, patients with mean vertical sinusoidal grating response latencies greater than 119 milliseconds to 0.25-cycle/degree stimuli (or mean vertical sinusoidal latencies >113 milliseconds to 0.50-cycle/degree stimuli) and mean vertical sinusoidal grating amplitudes of less than 14.75 mV to 0.50-cycle/degree stimuli were classified as having had a history of concussion. The resultant receiver operating characteristic curve for this model had excellent discrimination between the concussed and nonconcussed (area under the curve = 0.857; P < .01) groups with sensitivity of 0.92 and specificity of 0.80.

Conclusions

The results suggest a promising electrophysiological approach to identifying individuals with convergence insufficiency and a history of concussion.

Contribution of Nav1.8 sodium channels to retinal function

We examined the contribution of the sodium channel isoform Na_v1.8 to retinal function using the specific blocker A803467. We found that A803467 has little influence on the electroretinogram (ERG) a- and b-waves, but significantly reduces the oscillatory potentials (OPs) to 40-60% of their original amplitude, with significant changes in implicit time in the rod-driven range. To date, only two cell types were found in mouse to express Na_v1.8; the starburst amacrine cells (SBACs), and a subtype of retinal ganglion cells (RGCs). When we recorded light responses from ganglion cells using a multielectrode array we found significant and opposing changes in two physiological groups of RGCs. ON-sustained cells showed significant decreases while transient ON-OFF cells showed significant increases. The effects on ON-OFF transient cells but not ON-sustained cells disappeared in the presence of an inhibitory cocktail. We have previously shown that RGCs have only a minor contribution to the OPs (Smith et al., 2014), therefore suggesting that SBACs might be a significant contributor to this ERG component. Targeting SBACs with the cholinergic neurotoxin ethylcholine mustard aziridinium (AF64A) caused a reduction in the amplitude of the OPs similar to A803467. Our results, both using the ERG and MEA recordings from RGCs, suggest that Na_v1.8 plays a role in modulating specific aspects of the retinal physiology and that SBACs are a fundamental cellular contributor to the OPs in mice, a clear demonstration of the dichotomy between ERG b-wave and OPs.

Electrophysiological examination in uveitis: a review of the literature

Purpose

Uveitis is the inflammation of the uveal tract, which usually also affects the retina and vitreous humor. The electrophysiological examination is an objective ocular examination that includes the electroretinogram, visual evoked potentials, the electrooculogram, the multifocal electroretinogram, and multifocal visual evoked potentials. Our aim is to review the literature of the use of the electrophysiological examination in cases of uveitis.

Methods

We performed a systematic search of the literature of published papers until October 2012 using the PubMed search engine. The key terms that were used were “uveitis”, “electrophysiological examination”, “electroretinogram”, “visual evoked potentials”, “electrooculogram”, “multifocal electroretinogram”, and “multifocal visual evoked potentials” in multiple combinations. To the best of our knowledge, this is the first review concerning the assessment of electrophysiology in uveitis.

Results

Our search of the literature demonstrated that the electrophysiological examination, mainly by means of electroretinogram, multifocal electroretinogram, and visual evoked potentials, is performed in several cases of uveitis for many purposes, including diagnosis and monitoring of disease progression and treatment efficacy. The electrophysiological examination is more useful in patients with multiple evanescent white dot syndrome, acute posterior multifocal placoid pigment epitheliopathy, birdshot chorioretinopathy, Vogt–Koyanagi–Harada disease, Adamantiades–Behçet disease, ocular syphilis, and Fuchs heterochromic cyclitis.

Conclusion

This review summarizes the use of the electrophysiological examination in uveitic patients and underlines its value as a useful tool in the objective assessment and the monitoring of the disease.

Short duration transient visual evoked potentials in glaucomatous eyes

PURPOSE

To investigate the correlation between structural and functional damage in patients with asymmetric glaucoma using a newly developed short duration transient visual evoked potential (SD-tVEP) device.

METHODS

Twenty-five patients with visual acuity ≥20/30 and asymmetric visual field (VF) loss [inter-eye difference in mean deviation index (MD) of at least 3 dB] were enrolled. Patients underwent optical coherence tomography (OCT) for macular thickness measurement, scanning laser polarimetry with variable corneal compensation for retinal nerve fiber layer measurement, and SD-tVEP (10% and 85% Michelson contrast, acquisition time of 20 s) in both eyes within 2 months. We correlated VF MD and structural test results with SD-tVEP P100 latency and Delta Amplitude (N75-P100).

RESULTS

Using 10% contrast, there was a significant difference in SD-tVEP latency and amplitude between eyes with better and worse VF MD (P<0.001). MD correlated significantly with both SD-tVEP parameters (r>0.33, P≤0.01). When using 85% contrast, SD-tVEP amplitude differed between eyes (P=0.01) and MD values correlated significantly with amplitude results (r=0.32, P=0.01), but not with latency (P=0.46). In eyes with more advanced VF loss, there was a positive and significant correlation between SD-tVEP amplitude (85% contrast) and macular thickness on OCT (r=0.47, P=0.01), but not with retinal nerve fiber layer measured with polarimetry (P=0.26).

CONCLUSIONS

In cases of asymmetric glaucoma, SD-tVEP results correlate significantly with the level of VF damage as measured by MD. In the eyes with more advanced VF loss, reduced SD-tVEP amplitude was associated with decreased macular thickness on OCT. These findings suggest that SD-tVEP may be a fast and objective method to assess or screen for functional damage in glaucomatous eyes.

Repeatability of short-duration transient visual evoked potentials in normal subjects

To evaluate the within-session and inter-session repeatability of a new, short-duration transient visual evoked potential (SD-tVEP) device on normal individuals, we tested 30 normal subjects (20/20 visual acuity, normal 24-2 SITA Standard VF) with SD-tVEP. Ten of these subjects had their tests repeated within 1–2 months from the initial visit. Synchronized single-channel EEG was recorded using a modified Diopsys Enfant™ System (Diopsys, Inc., Pine Brook, New Jersey, USA). A checkerboard stimulus was modulated at two reversals per second. Two different contrasts of checkerboard reversal patterns were used: 85% Michelson contrast with a mean luminance of 66.25 cd/m² and 10% Michelson contrast with a mean luminance of 112 cd/m². Each test lasted 20 s. Both eyes, independently and together, were tested 10 times (5 times at each contrast level). The following information was identified from the filtered N75-P100-N135 complex: N75 amplitude, N75 latency, P100 amplitude, P100 latency, and Delta Amplitude (N75-P100). The median values for each eye’s five SD-tVEP parameters were calculated and grouped into two data sets based on contrast level. Mean age was 27.3 ± 5.2 years. For OD only, the median (95% confidence intervals) of Delta Amplitude (N75-P100) amplitudes at 10% and 85% contrast were 4.6 uV (4.1–5.9) and 7.1 uV (5.15–9.31). The median P100 latencies were 115.2 ms (112.0–117.7) and 104.0 ms (99.9–106.0). There was little within-session variability for any of these parameters. Intraclass correlation coefficients ranged between 0.64 and 0.98, and within subject coefficients of variation were 3–5% (P100 latency) and 15–30% (Delta Amplitude (N75-P100) amplitude). Bland–Altman plots showed good agreement between the first and fifth test sessions (85% contrast Delta Amplitude (N75-P100) delta amplitude, mean difference, 0.48 mV, 95% CI, −0.18–1.12; 85% contrast P100 latency delay, −0.82 ms, 95% CI, −3.12–1.46; 10% contrast Delta Amplitude (N75-P100) amplitude, 0.58 mV, 95% CI, −0.27–1.45; 10% contrast P100 latency delay, −2.05 mV, 95% CI, −5.12–1.01). The inter-eye correlation and agreement were significant for both SD-tVEP amplitude and P100 latency measurements. For the subset of eyes in which the inter-session repeatability was tested, the intraclass correlation coefficients ranged between 0.71 and 0.86 with good agreement shown on Bland–Altman plots. Short-duration transient VEP technology showed good within-session, inter-session repeatability, and good inter-eye correlation and agreement.

The oscillatory potentials of the dark-adapted electroretinogram in retinopathy of prematurity

PURPOSE

To study the development of the electroretinographic (ERG) oscillatory potentials (OPs) in two rat models of ROP and in human subjects with a history of ROP.

METHODS

Sprague-Dawley rats (n = 36) were studied longitudinally. Rat models of ROP were induced, either by exposure to alternating 50%/10% oxygen (50/10 model) from postnatal day (P) 0 to P14 or by exposure to 75% oxygen (75 model) from P7 to P14. Control rats were reared in room air. Infant and adult human subjects with and without a history of ROP (n = 91) were also studied. Dark-adapted ERGs were recorded and filtered to demonstrate the OPs. Discreet Fourier transform (DFT) allowed evaluation of the OP power spectrum. OP energy (E), dominant frequency (F(peak)), and sensitivity (log i(1/2)) were evaluated.

RESULTS

In 50/10 model rats, E was low compared with that in the 75 model rats and control animals. F(peak) (approximately 95 Hz) did not vary with age or group. Intriguingly, log i(1/2) in 75 model rats was greater than that in controls or 50/10 model rats. Human adults with a history of ROP had lower-energy OPs than did the control adults, but infants with a history of ROP had higher-energy OPs than did the control infants. F(peak) was lower (approximately 120 Hz) in infants than in adults (approximately 130 Hz). ROP did not affect log i(1/2) in humans.

CONCLUSIONS

Differences between OPs in healthy rats and healthy humans were substantial, suggesting that OPs in rat models of ROP are unlikely to provide insight into the effects of ROP on human OPs. Indeed, neither ROP model studied showed a pattern of effects similar to that in human ROP.

Wavelet analysis reveals dynamics of rat oscillatory potentials

We characterised the dynamics in the oscillatory potentials (OPs) of the rat electroretinogram (ERG) using a continuous complex Morlet wavelet transform. Dark-adapted (>12h) full field ERG responses were recorded from five anaesthetized (ketamine:xylazine, 60:5mg/kg) adult Long-Evans rats (10-12weeks). Five responses were obtained for brief LED flashes (1-4ms) in a ganzfeld at exposures ranging from -4.2 to 1.58logcdsm(-2). Signals were recorded across a bandwidth of 0.3-1kHz and digitized at 10kHz. Morlet wavelets with frequencies between 50 and 250Hz were correlated with raw ERG signals at 1ms intervals. The amplitude of the correlation at each time and frequency was given by the modulus of the complex wavelet response. Candidate OPs were identified as local peaks within 10% of the maximum amplitude. As flash exposure increased, the amplitude of the OP response increased, the peak OP occurred earlier, and the peak OP frequency increased. OPs at brighter flashes clustered into two groups, peaking at 50ms in the 70 and 130Hz band for moderate intensities, and peaking at 20ms in the 70Hz band and 50ms in the 120Hz band for the highest intensities (>0logcdsm(-2)). These dynamics agree with physiological, pharmacological and clinical studies that suggest several distinct neural mechanisms contribute to OPs. Wavelet analysis reveals important dynamics in OP data that are not evident with traditional analytical approaches.

Oscillatory potentials of the slow-sequence multifocal ERG in primates extracted using the Matching Pursuit method

This study used the Matching Pursuit (MP) method, a time-frequency analysis, to identify and characterize oscillatory potentials (OPs) in the primate electroretinogram (ERG). When the slow-sequence mfERG from the macular region of the retina was matched with Gabor functions, OPs were identified in two distinct bands: a high-frequency band peaking around 150 Hz that contributes to early OPs, and a low-frequency band peaking around 80 Hz that contributes to both early and late OPs. Pharmacological blockade and experimental glaucoma studies showed that the high-frequency OPs depend upon sodium-dependent spiking activity of retinal ganglion cells, whereas the low-frequency OPs depend primarily upon non-spiking activity of amacrine cells, and more distal retinal activity.

ERG oscillatory potentials in infants

PURPOSE

Study the scotopic and photopic oscillatory potentials (OPs) of the electroretinogram (ERG) in 10-week old infants.

METHODS

Term-born 10-week old infants (n = 15) and adults (n = 12) were tested. Full-field ERGs were recorded under scotopic and photopic conditions. The records were filtered (75-300 Hz) to demonstrate the oscillatory wavelets. The amplitudes and implicit times of the infants' OPs were compared to those in adults and also to amplitudes of the saturated photoreceptor responses.

RESULTS

In infants, the mean OP amplitudes are similar in scotopic and photopic conditions and do not vary significantly with OP number. Infants' OPs are significantly smaller than in adults, with scotopic OPs averaging 19% of that in adults and photopic OPs averaging 47%, whereas the amplitudes of the saturated photoreceptor responses are 43% and 66% of those in adults. Mean interpeak intervals are similar in infants and adults, indicating oscillatory behavior at a frequency of 155 Hz in scotopic conditions and 135 Hz in photopic conditions.

CONCLUSIONS

In young infants, the OPs are relatively immature compared to the photoreceptor responses, with the immaturity of the scotopic responses being more marked than that of the photopic responses.

Origins of the electroretinogram oscillatory potentials in the rabbit retina

The electroretinogram (ERG) oscillatory potential (OP) is a high-frequency, low-amplitude potential that is superimposed on the rising phase of the b-wave. It provides noninvasive evaluation of inner retina function in vivo and is a useful tool in basic research as well as in the clinic. While the OP is widely believed to be generated mainly by activity of the inner retina, the exact underlying neural mechanisms are not well understood. We have investigated the retinal mechanisms that underlie OP generation in Dutch-belted rabbits. The OP was isolated by band-filtering (100-1000 Hz) ERG signals. We used pharmacological agents that block specific transmitter receptors or voltage-gated channels in order to examine contributions of various retinal mechanisms to OP generation. Our results show that the OP elicited by a bright brief flash can be classified into early, intermediate, and late subgroups that are likely generated mainly by photoreceptors, action-potential-independent, and action-potential-dependent mechanisms in the ON pathway of the inner retina, respectively. ON bipolar cells themselves make only a small direct contribution to OP generation, as do horizontal cells and neurons in the OFF pathway.

Sampling and interpolation of the a-wave of the electroretinogram

This study was undertaken (a) to determine the minimum sampling frequency required to record a-waves evoked by flashes of very high energy without significant distortion and (b) to demonstrate that data sampled at a minimally adequate frequency can be interpolated to reconstruct the original waveform.

METHODS

Dark-adapted ERGs from two anaesthetized macaque monkeys and an adult human were studied. Responses evoked by high-energy flashes that produced a-waves peaking as early as 5 or 6 ms after the flash were sampled at 5 kHz and transformed to obtain their discrete Fourier spectra. The amplitude of all spectral components above some cut-off frequency (e.g., 400 Hz) was then set to zero and the modified spectra transformed back into the time domain. The resulting computed responses, which contained no Fourier components above the cut-off frequency, were compared with the original recorded samples. To assess the validity of one method of interpolation, sample sets consisting of every fifth point of records sampled at 5 kHz (i.e., sets of 1 kHz samples) were subjected to Fourier transformation to give spectra with a frequency range of 0-500 Hz. These spectra were extended from 500 Hz up to a much higher frequency (e.g., 8 kHz) by adding zeros. The extended spectra were transformed back into the time domain to provide sets of interpolated samples at twice the chosen spectral frequency limit (i.e., 16 kHz).

RESULTS

Removing all Fourier components above 400 or 500 Hz had no significant effect upon the leading edge or peak of the a-wave. However, removing Fourier components above 500 Hz gave rise to slight distortion of the oscillatory potentials (OPs) that appeared just after the a-wave peak on the leading edge of the b-wave. Except for this small distortion, the original 5 kHz data samples corresponded very well with the interpolated curves that had been generated as described above from a 1 kHz subset of the samples. This provides further confirmation that dark-adapted ERG a-waves evoked by flashes of up to about 50,000 sc. Td sc do not contain Fourier components with frequencies above 500 Hz.

CONCLUSION

Human and macaque a-waves are completely represented by 1 kHz samples and Fourier methods can be used to reconstruct the original continuous waveform. However, to capture the OPs with complete fidelity, a higher sampling rate is necessary.