Averaged Visual Evoked Potentials Research Articles

Coherence of Visual-Evoked Gamma Oscillations Is Disrupted by Propofol but Preserved Under Equipotent Doses of Isoflurane.

Previous research demonstrates that the underlying state of the brain influences how sensory stimuli are processed. Canonically, the state of the brain has been defined by quantifying the spectral characteristics of spontaneous fluctuations in local field potentials (LFP). Here, we utilized isoflurane and propofol anesthesia to parametrically alter the spectral state of the murine brain. With either drug, we produce slow wave activity, with low anesthetic doses, or burst suppression, with higher doses. We find that while spontaneous LFP oscillations were similar, the average visual-evoked potential (VEP) was always smaller in amplitude and shorter in duration under propofol than under comparable doses of isoflurane. This diminished average VEP results from increased trial-to-trial variability in VEPs under propofol. One feature of single trial VEPs that was consistent in all animals was visual-evoked gamma band oscillation (20–60 Hz). This gamma band oscillation was coherent between trials in the early phase (<250 ms) of the visual evoked potential under isoflurane. Inter trial phase coherence (ITPC) of gamma oscillations was dramatically attenuated in the same propofol anesthetized mice despite similar spontaneous oscillations in the LFP. This suggests that while both anesthetics lead to loss of consciousness (LOC), elicit slow oscillations and burst suppression, only the isoflurane permits phase resetting of gamma oscillations by visual stimuli. These results demonstrate that accurate characterization of a brain state must include both spontaneous as well as stimulus-induced perturbations of brain activity.

Optical Coherence Tomography in Optic Nerve Hypoplasia: Correlation With Optic Disc Diameter, Nerve Fiber Layer Thickness, and Visual Function.

The correlation between optic disc diameters (DDs) with average retinal nerve fiber layer thickness (RNFLT) and visual function in children with optic nerve hypoplasia (ONH) having nystagmus is unknown. Data were obtained from a retrospective review of 28 children (mean age: 9.4 years; ±5.1). Optic DD was defined as the maximal horizontal opening of Bruch membrane with spectral optical coherence tomography combined with a confocal laser ophthalmoscope. Average RNFLT was obtained from circumpapillary b-scans. RNFLT was also remeasured at eccentricities that were proportionate with DD to rule out potential sampling artifacts. Visual function was assessed by visual acuity at last follow-up and by visual evoked potentials (VEP) in 11 patients. The eye with the larger DD, which had better visual acuity, was analyzed to exclude potential effects of amblyopia. DD was correlated with average RNFLT (r = 0.61), visual acuity (r = 0.32), and VEPs (r = 0.66). The relationship between RNFLT and DD was as follows: average RNFLT (μm) = 0.074 * DD (μm) - 18.8. RNFLT also correlated with the ratio of horizontal optic DD to macula-disc-margin distance (DD:DM; r = 0.59). RNFLT measured at eccentricities proportionate with DD showed progressive decrease in thickness only for DDs <1,100 μm. All patients with DD <1,000 μm had subnormal visual acuity, whereas those with DD <1,200 μm had subnormal VEPs. DD correlates with average RNFLT and with visual function in children with ONH. Using OCT imaging, DD can be obtained in children with nystagmus and provides objective information.

Subadditive responses to extremely short blue and green pulsed light on visual evoked potentials, pupillary constriction and electroretinograms

BackgroundThe simultaneous exposure to blue and green light was reported to result in less melatonin suppression than monochromatic exposure to blue or green light. Here, we conducted an experiment using extremely short blue- and green-pulsed light to examine their visual and nonvisual effects on visual evoked potentials (VEPs), pupillary constriction, electroretinograms (ERGs), and subjective evaluations.MethodsTwelve adult male subjects were exposed to three light conditions: blue-pulsed light (2.5-ms pulse width), green-pulsed light (2.5-ms pulse width), and simultaneous blue- and green-pulsed light with white background light. We measured the subject’s pupil diameter three times in each condition. Then, after 10 min of rest, the subject was exposed to the same three light conditions. We measured the averaged ERG and VEP during 210 pulsed-light exposures in each condition. We also determined subjective evaluations using a visual analog scale (VAS) method.ResultsThe pupillary constriction during the simultaneous exposure to blue- and green-pulsed light was significantly lower than that during the blue-pulsed light exposure despite the double irradiance intensity of the combination. We also found that the b/|a| wave of the ERGs during the simultaneous exposure to blue- and green-pulsed light was lower than that during the blue-pulsed light exposure. We confirmed the subadditive response to pulsed light on pupillary constriction and ERG. However, the P100 of the VEPs during the blue-pulsed light were smaller than those during the simultaneous blue- and green-pulsed light and green-pulsed light, indicating that the P100 amplitude might depend on the luminance of light.ConclusionsOur findings demonstrated the effect of the subadditive response to extremely short pulsed light on pupillary constriction and ERG responses. The effects on ipRGCs by the blue-pulsed light exposure are apparently reduced by the simultaneous irradiation of green light. The blue versus yellow (b/y) bipolar cells in the retina might be responsible for this phenomenon.

Simultaneous EEG–fMRI at ultra-high field: Artifact prevention and safety assessment

The simultaneous recording of scalp electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) can provide unique insights into the dynamics of human brain function, and the increased functional sensitivity offered by ultra-high field fMRI opens exciting perspectives for the future of this multimodal approach. However, simultaneous recordings are susceptible to various types of artifacts, many of which scale with magnetic field strength and can seriously compromise both EEG and fMRI data quality in recordings above 3T. The aim of the present study was to implement and characterize an optimized setup for simultaneous EEG–fMRI in humans at 7T. The effects of EEG cable length and geometry for signal transmission between the cap and amplifiers were assessed in a phantom model, with specific attention to noise contributions from the MR scanner coldheads. Cable shortening (down to 12cm from cap to amplifiers) and bundling effectively reduced environment noise by up to 84% in average power and 91% in inter-channel power variability. Subject safety was assessed and confirmed via numerical simulations of RF power distribution and temperature measurements on a phantom model, building on the limited existing literature at ultra-high field. MRI data degradation effects due to the EEG system were characterized via B0 and B1+ field mapping on a human volunteer, demonstrating important, although not prohibitive, B1 disruption effects. With the optimized setup, simultaneous EEG–fMRI acquisitions were performed on 5 healthy volunteers undergoing two visual paradigms: an eyes-open/eyes-closed task, and a visual evoked potential (VEP) paradigm using reversing-checkerboard stimulation. EEG data exhibited clear occipital alpha modulation and average VEPs, respectively, with concomitant BOLD signal changes. On a single-trial level, alpha power variations could be observed with relative confidence on all trials; VEP detection was more limited, although statistically significant responses could be detected in more than 50% of trials for every subject. Overall, we conclude that the proposed setup is well suited for simultaneous EEG–fMRI at 7T.

Effects of light deprivation on visual evoked potentials in migraine without aura

BackgroundThe mechanisms underlying the interictal habituation deficit of cortical visual evoked potentials (VEP) in migraine are not well understood. Abnormal long-term functional plasticity of the visual cortex may play a role and it can be assessed experimentally by light deprivation (LD).MethodsWe have compared the effects of LD on VEP in migraine patients without aura between attacks (MO, n = 17) and in healthy volunteers (HV, n = 17). Six sequential blocks of 100 averaged VEP at 3.1 Hz were recorded before and after 1 hour of LD. We measured VEP P100 amplitude of the 1st block of 100 sweeps and its change over 5 sequential blocks of 100 responses.ResultsIn HV, the consequence of LD was a reduction of 1st block VEP amplitude and of the normal habituation pattern. By contrast, in MO patients, the interictal habituation deficit was not significantly modified, although 1st block VEP amplitude, already lower than in HV before LD, further decreased after LD.ConclusionsLight deprivation is thought to decrease both excitatory and subsequent inhibitory processes in visual cortex, which is in line with our findings in healthy volunteers. The VEP results in migraine patients suggest that early excitation was adequately suppressed, but not the inhibitory mechanisms occurring during long term stimulation and habituation. Accordingly, deficient intracortical inhibition is unlikely to be a primary factor in migraine pathophysiology and the habituation deficit.

Empirical mode decomposition of field potentials from macaque V4 in visual spatial attention

Empirical mode decomposition (EMD) has recently been introduced as a local and fully data-driven technique for the analysis of non-stationary time-series. It allows the frequency and amplitude of a time-series to be evaluated with excellent time resolution. In this article we consider the application of EMD to the analysis of neuronal activity in visual cortical area V4 of a macaque monkey performing a visual spatial attention task. We show that, by virtue of EMD, field potentials can be resolved into a sum of intrinsic components with different degrees of oscillatory content. Low-frequency components in single-trial recordings contribute to the average visual evoked potential (AVEP), whereas high-frequency components do not, but are identified as gamma-band (30-90 Hz) oscillations. The magnitude of time-varying gamma activity is shown to be enhanced when the monkey attends to a visual stimulus as compared to when it is not attending to the same stimulus. Comparison with Fourier analysis shows that EMD may offer better temporal and frequency resolution. These results support the idea that the magnitude of gamma activity reflects the modulation of V4 neurons by visual spatial attention. EMD, coupled with instantaneous frequency analysis, is demonstrated to be a useful technique for the analysis of neurobiological time-series.

Empirical mode decomposition: a method for analyzing neural data

Almost all processes that are quantified in neurobiology are stochastic and nonstationary. Conventional methods that characterize these processes to provide a meaningful and precise description of complex neurobiological phenomenon may be insufficient. Here, we report on the use of the data-driven empirical mode decomposition (EMD) method to study neuronal activity in visual cortical area V4 of macaque monkeys performing a visual spatial attention task. We found that local field potentials were resolved by the EMD into the sum of a set of intrinsic components with different degrees of oscillatory content. High-frequency components were identified as gamma band (35–90 Hz) oscillations, whereas low-frequency components in single-trial recordings contributed to the average visual evoked potential (AVEP). Comparison with Fourier analysis showed that EMD may offer better temporal and frequency resolution. The EMD, coupled with instantaneous frequency analysis, may prove to be a vital technique for the analysis of neural data.

PEV e BAEP in bambini affetti da Neurofibromatosi

Visual evoked potentials (VEP) and brain stem acoustic evoked potentials (BAEP) were studied in 19 children with neurofibromatosis (13 males and six females aged from seven months to 17 years; average age 6.3 years) comparing them with those of a control group of 43 healthy children (25 males and 18 females). The patients had an increased average VEP latency (III wave = p &lt; .01559; IV wave = p &lt; .00001) and I-V interpeak of BAEP (right I-V interpeak = p &lt; .04083; left I-V interpeak ? p &lt; .00852). Impaired responses were relatively common in both VEP (40%) and BAEP (35%), with a moderate but not constant relation to neuroradiological findings. Altered potentials were also present in patients without structural abnormalities. Evoked potentials serve as a sensitive test for early diagnosis of neurofibromatosis.

Cerebroid Ganglion is the Presumptive Pacemaker of the Circadian Rhythm of Electrical Response to Light in the Crayfish

One of the most widely studied circadian rhythms in invertebrates is that of light responsiveness whose underlying mechanisms seem to involve different groups of oscillators which act as pacemakers. Although, in crayfish, there are clear circadian rhythms in the electroretinogram (ERG) amplitude, the precise location of the pacemaker system driving this rhythm is uncertain. Some data suggest that the circadian pacemaker could be located in a group of neurosecretory cells of the supraesophageal ganglion (the cerebroid ganglion or brain) and that the sinus gland plays a determinant role in the generation and expression of this rhythm through periodic release of pigment-dispersing hormone (PDH). The aim of this work is to examine the role of the brain in the expression of the ERG circadian activity. The hypothesis we test is that the electrical activity at the brain level has a circadian behavior in the firing pattern of spontaneous multiunit activity (MUA) and in visual evoked potentials (VEPs). The results indicate that there are robust circadian rhythms in both MUA, recorded from several regions of the brain, and in the averaged VEPs recorded from the protocerebrum area. These rhythms are 180° out of phase to one another. The rhythm of VEPs showed a main peak at midnight which was in close phase relationship with the ERG amplitude rhythm.

Single P100 visual evoked potential analyses in man.

Standards evoked potential averaging leads to a loss of information inherent in the moment to moment variability of the amplitude and latency of single evoked potentials. In order to extract this information, attempts were made to develop procedures which would allow recording of single visual evoked potentials in at least a selected group of subjects. In 11 out of 25 non-selected subjects, 43% to 86% of all stimuli (reversal checkerboard pattern elicited single visual evoked potentials (VEP) which were recognized as such by a group of independent observers. The mean amplitude and latency obtained by directly measuring the peak to peak amplitudes and peak latencies of these single VEPs (arithmetic averaging) were compared to those obtained following conventional time-locked averaging of the same data. During long-term continuous stimulation, the arithmetic averaged VEPs increased in amplitude to a steady state while the time-locked averages of the same sets of responses decreased in amplitude. This reduction was found to be closely related to the latency jitter. It may provide a better understanding of the phenomenon of habituation. This finding was confirmed in model single VEPs obtained by summing on-going pre-stimulus EEG activity with a time-locked average VEP stationary wave form. The variability of the true single VEPs was found to be less than the variability of the model single VEPs. The latency and amplitude parameters of the true single VEPs were strongly correlated with each other while those of the model single VEPs were not. These findings show that single VEPs have an inherent variability which may reflect brain processing.

Enhancement of visual evoked potentials by stimulation during low prestimulus EEG stages.

A new algorithm for averaging of visual evoked potentials (VEPs) is introduced and compared with the conventional method. The root mean square (RMS) values of the alpha and theta band EEG activities at the vertex 1-s preceding stimulus presentation were computed: stimuli were blocked during phases of high prestimulus activity. Stimulation was applied selectively during periods of low activity. Light stimuli of 1-s duration were given to twelve healthy, voluntary subjects. The main results were: (1) The averaged VEPs at the vertex showed an increase in the N1-P2 amplitude of about 35% in comparison to experiments with conventional stimulation. This increase was in the range of 25-30% when an ISI correction was taken into account. Frontal, temporal and parietal locations showed also increases of the N1-P2 amplitude in evoked potentials. (2) VEPs were also affected qualitatively depending on the frequency content of the prestimulus activity.

Postnatal Development of Flash Visual Evoked Potentials in the Jaundiced Gunn Rat

In this study, the postnatal development of flash visual evoked potential (VEP) has been monitored in the jaundiced (jj) Gunn rat model of neonatal hyperbilirubinemia to determine whether this evoked response is affected by bilirubin-induced neurotoxicity. VEP could first be recorded at 16 d of age. The jj rats exhibited prolonged wave latencies and lower wave amplitudes during the 3rd wk of postnatal life, when compared with their nonjaundiced littermates. There was no correlation at 21 d of age between VEP parameters and either bilirubin levels or body weight. About one third of the jj animals died between 21 and 28 d of age. The average VEP wave latencies at 21 d of age of the rats who were to die was prolonged compared with those of rats who survived till at least 28 d of age. Thus, the latency of VEP waves at the age of 21 d appears to be related to the further outcome of jj Gunn rats. Although wave amplitudes were lower in jj as compared with nonjaundiced 21-d-old animals, there were no amplitude differences between the jj rats who would die and those who would survive during the 4th wk of life. These findings may contribute to the understanding of the pathogenesis of bilirubin encephalopathy in the neonatal period.

Cross-orientation inhibition in cat is GABA mediated.

Visual evoked potentials (VEPs) were recorded from cat cortex (area 17) before, during and after application of the GABA blocker bicuculline (iontophoretic or topical). The stimuli comprised a test sinusoidal grating, and a mask grating oriented either parallel or orthogonal to the test. Both test and mask alternated in contrast at different temporal frequencies. VEPs were averaged in synchrony with the test contrast reversal, so the mask did not contribute directly to the averaged VEP response. Before application of bicuculline, both parallel and orthogonal masks attenuated the amplitude of VEPs and changed the phase response, but in different ways. Orthogonal masks lowered the slope of the contrast response curve without affecting extrapolated threshold, while parallel masks caused the curve to shift to the right. Orthogonal masks increased the phase advance, while parallel masks eliminated it. During application of bicuculline, neither parallel nor the orthogonal masks attenuated VEP amplitudes. The results suggest that although the mechanisms for the action of parallel and orthogonal masks are clearly distinct, both are mediated by the GABA-ergic inhibitory system. Given this evidence, measurement of VEP contrast response curves may provide a simple non-invasive technique for monitoring visual inhibition in humans.

Effect of therapeutic level ultrasound on visual evoked potentials in the hypoxic cat

The effect of 1.0 MHz ultrasound at an intensity of 1.0 W/cm 2 SATA on brain function of anaesthetized cats was assessed using Visual Evoked Potentials (VEPs). Ultrasound alone did not significantly modify the averaged VEP signals. However, acute hypoxia depressed the VEP response. When the brain was exposed to ultrasound during the hypoxic episode, the VEP response remained normal. Raising brain temperature by whole body heating could not mimic the beneficial effect of ultrasound on the VEP response during hypoxia. This suggests that therapeutic ultrasound may have a protective effect on hypoxic tissues and may have clinical applications.

Data preprocessing applied to human average visual evoked potential P100-N140 amplitude, latency, and slope

Data preprocessing, using principal components and Varimax rotation, was applied to 26 human average visual evoked potential (AVEP) variables representing bilateral amplitudes, latencies, and slopes from the P100 and N140 peaks. Five orthogonal factors, which accounted for about 80% of the variance, were found to describe the data from control subjects, stabilized alcoholics, and chronic nonparanoid schizophrenics. Tests of orthogonality and variance explained were employed to evaluate the results of the analysis.

Visual evoked potentials in monkeys

Visual evoked potentials (VEPs) were recorded from 2 cortical sites in stump-tailed macaques. VEPs recorded from striate cortex were basically consistent between animals (especially at low sight intensity), remained remarkably stable over time, and compared favorably to VEPs reported by other investigators. We concluded that the VEP recorded from the striate cortex of day-active monkeys consists of 5 major peaks within the first 250 msec. The potentials recorded from post-central gyrus were simpler and more individualized and did not show intensity-related latency changes or increases in inter-subject variability. However, amplitudes of potentials recorded from both electrode placements increased with light intensity apparently reflecting the amplitude of individual potentials rather than the variability of these potentials from which the average VEPs were derived.

Phenothiazine effects on cerebral-evoked potentials and eye movements in acute schizophrenics.

An investigation was made of the effects of phenothiazine medication on the averaged visual-evoked potentials (AVEP) and on eye movements in hospitalized, young, acute schizophrenic patients. These results were compared with those of normal subjects who were not given medication. AVEP measures included maximum amplitude (Am), frequency of peaks (FOP'S), variability (V) and peak latencies for an early negative peak (N1) and a later positive peak (P6). Eye movement measures included percent of time looking at a stimulus slide, percent of time looking at a figure on the slide, the number of fixations and the percent of cells entered in which fixations occurred. For schizophrenics off and on phenothiazine medication, there were no consistently significant drug effects on any measure except frequency of peaks. Schizophrenics compared to normals had lower amplitudes, greater frequency of peaks, greater variability and lower eye movement scores.

Cerebral visual evoked potentials in relation to EEG

The average occipital visual evoked potentials (VEPs) in response to single flash stimulus (1 f/sec), with little or no rhythmic after-discharge, were obtained from twelve human adult subjects and repetitively summed algebraically at varying intervals of approximately 0.5-0.04 sec for comparison with the average VEP driven by repetitive flash stimulation at the same intervals. Large oscillatory contours were obtained by repetitive summation of the VEP at time intervals corresponding to the period of delta, theta and alpha frequency ranges in EEG, whereas a small amplitude fast pattern was produced by summations at intervals corresponding to the period of beta frequencies. In this situation, alpha rhythm-like configuration resulted from summations at intervals of approximately 0.1 to 0.083 or 0.07 sec. Each of the main components in the average VEP to single flash was increased by summation at intervals corresponding to delta, theta and alpha frequencies, whereas they were decreased by that at intervals corresponding to beta frequencies. By summation and stimulation at a frequency corresponding to the alpha frequency range, deflections other than the second surface negative (N 2) and third positive (P 3) deflections in the VEP were fused or synchronised respectively to the N 2 and P 3 deflections to build up an enhanced surface negative-positive diphasic (alpha rhythm-like) configuration. By further increase in the frequencies of the summation and stimulations, the diphasic pattern deteriorated to form a small amplitude fast pattern. The rhythmically driven average VEP and the summed average VEP at frequencies of approximately 2–12 c/sec resembled each other in their contours, whereas at frequencies higher than 12/sec, the former was lower than the latter so as to indicate occlusive (negative interactive) effects between adjoining flashes of light in the photic stimulation. In addition to the above, some new findings were obtained from the contour maps created by drawing equipotential lines for both the rhythmically driven VEP and summed VEP in relation to the frequency of the stimulation and summation. These findings were also affirmed in the frequency patterns revealed by power spectra and the frequency response activities of both the summed and driven VEP. The large sinusoidal wave built up in both the summed and driven average VEP at a frequency corresponding to an alpha frequency was often preceded or followed by a hump in the rising and falling phases, respectively, but quite similar configurations have often been observed in the alpha activity in the spontaneous EEG traced by the authors and other investigators.