Multichannel visual evoked potentials in migraine

Multichannel recordings of visual evoked potentials (VEPs) have proved to be useful in the evaluation of visual field defects. We studied the topographic distribution of transient VEPs in 15 migraine patients (8 with visual aura and 7 without) and 15 age-matched controls during the migraine-free interval. All the subjects included in the study had normal visual fields. VEPs were recorded from 9 electrodes placed on the posterior scalp. Stimuli were full-field and hemifield reversing square wave grating patterns of medium spatial frequency (4 c/deg). The groups did not show significant differences in latencies and amplitudes of the major components (N70, P100) recorded from the midline. However, migraine patients with visual hemianopic aura showed definite asymmetries in the VEP amplitude distribution. Significantly reduced, absent or polarity-invered VEP responses were recorded ipsilateral to the side of the prodromic visual symptoms. Direct comparison of affected and unaffected hemispheres by partial field stimulation confirmed these findings. According to the VEP cortical generator theory, these abnormalities suggest a functional anomaly consistent with the clinical syndrome and detectable also in the migraine-free interval. None of the migraine patients without aura or the controls showed VEP amplitude asymmetries. We conclude that multichannel VEP recordings may discriminate between different subtypes of migraine and contribute important physiopathological information to the study of this disease.     

Scalp-recorded oscillatory potentials evoked by transient pattern-reversal visual stimulation in man

Replicable oscillatory potentials, time-locked to pattern stimuli (9.0° central; counterphase reversal at 2.13 Hz) were dissociated from conventional, broad-band VEPs recorded in healthy volunteers at occipital scalp locations by high-pass digital filtering at 17.0–20.0 Hz. Nine consecutive wavelets were identified with a 56.4 ± 8.4 msec mean latency of the first replicable wavelet and mean peak-to-peak amplitude varying between 0.9 and 2.0 μV. The first 2 wavelets had significantly shorter latencies than wave N70 of unfiltered VEP, whereas the last 2 wavelets had longer latencies than N145. Latency and amplitude values varied as a function of contrast and spatial frequency of the stimulus, with shorter latencies and larger amplitudes at 60–90% contrast level and tuning of amplitude at 5.0 c/deg. All wavelets were correlated with wave P100 of unfiltered VEP, while a correlation with N70 of VEP was observed only for those wavelets with latencies in the range of wave P100. Two patients with documented brain lesions involving the visual system are described as examples of oscillatory responses occurring irrespective of filter bandpass and instead of the expected conventional VEP when the generation of these is interfered with by brain pathology. A substantial cortical contribution to the origin of the oscillatory response is conceivable. It is suggested that the oscillatory response to pattern-reversal stimulation reflects events in the visual system that are parallel to, and partly independent of, the conventional VEP, with potential application in research or for clinical purposes.     

Contrast dependency of VEPs as a function of spatial frequency: the parvocellular and magnocellular contributions to human VEPs

The present study investigated the contrast dependency of visual evoked potentials (VEPs) elicited by phase reversing sine wave gratings of varying spatial frequency. Sixty-five trials were recorded for each of 54 conditions: 6 spatial frequencies (0.8, 1.7, 2.8, 4.0, 8.0 and 16.0 c deg(-1)) each presented at 9 contrast levels (2, 4, 8, 11, 16, 23, 32, 64 and 90%). At the lowest spatial frequency, the waveform contained mainly one peak (P1). For spatial frequencies up to 8 c deg(-1), P1 had a characteristic magnocellular contrast response: it appeared at low contrasts, increased rapidly in amplitude with increasing contrast, and saturated at medium contrasts. With increasing spatial frequency, an additional peak (N1) gradually became the more dominant component of the waveform. N1 had a characteristic parvocellular contrast response: it appeared at medium to high contrasts, increased linearly in amplitude with increasing contrast, and did not appear to saturate. The data suggest the contribution of both magnocellular and parvocellular responses at intermediate spatial frequencies. Only at the lowest and highest spatial frequencies tested did magnocellular and parvocellular responses, respectively, appear to dominate.     

Pattern ERG: effects of reference electrode site,stimulus mode and check size

We studied monocular pattern ERG (PERG) in 10 normal subjects and a patient with optic neuritis. No clinically significant PERG could be recorded from the occluded eye with any reference (ipsilateral ear or temple, or midfrontal), indicating that cross-contamination is not present with binocular testing. Ipsilateral temple reference minimized VEP (P100/N100) contribution to the PERG N95 which occurred with ipsilateral ear or midfrontal reference. The conclusions were confirmed by results from the patient, who had marked monocular delay of a normal amplitude P100. Twenty-four subjects were tested with monocular and binocular stimulation using an ipsilateral temple reference. There were differences in PERG latencies and amplitudes although the interside amplitude ratio showed smaller differences with binocular stimulation. Increasing check size (17, 35 and 70 min) decreased P50 and N95 latencies and increased P50 amplitude.     

Normal variability of the amplitude and phase of steady-state VEPs

The present study quantifies the amplitude and phase variability of steady-state VEPs (S-VEPs) and compares this variability between subjects and between individual runs. The S-VEPs were recorded repeatedly in 14 normal subjects with varying spatial and temporal frequencies of sinusoidal gratings; 6 spatial frequencies (range 0.5–8.0 c/deg) with 3 temporal frequencies (4, 6 and 8 Hz) were used. A total of 75 responses were averaged and analyzed by the Fourier method. Four recordings were obtained in each spatio-temporal combination.In general, the phase data showed small inter- and intrasubject variability. As anticipated, the amplitude data showed a large degree of intersubject variability, although the intrasubject variability was very small. In addition, in some stimulus conditions the inter- and intrasubject variability increased, which thus suggested the existence of an optimal spatio-temporal combination. Therefore, these stimulus parameters should be taken into consideration when S-VEPs are applied in clinical practice.     

Visual event-related potentials evoked by using a virtual reality display

This study aimed at investigating whether a virtual reality display (VRD) is an appropriate tool for evoking visual event-related potentials (VEPs). VEPs evoked by VRD stimuli were highly similar in form to VEPs evoked by using a computer monitor, both having two dominant peaks, labeled P100 and N200. Monitor and VRD N200 latencies and amplitudes were highly correlated. However, peak latencies were longer and the peaks were broader when stimuli were presented on the VRD. Besides, VRD P100 amplitude was smaller, and an N75 peak could be seen usually only on monitor VEPs.     

Clinical relevance of phase of steady-state VEPs to P100 latency of transient VEPs

Pattern visual evoked potentials (VEPs) to transient and steady-state stimulation were recorded in 10 normal subjects at 4 levels of luminance (180, 57, 22 and 11 cd/m²). VEPs were also recorded in 5 patients with optic neuropathy at a fixed luminance (180 cd/m²). The relationship between P100 latency of transient VEPs (T-VEPs) and the phase of steady-state VEPs (S-VEPs) was analyzed. As luminance decreased in normal subjects, P100 latency was prolonged and the phase lag increased. A significant linear relationship between the P100 latency and phase was found. Patients showed both the prolonged P100 latency and the delayed phase. The simple linear regression line of the phase-P100 latency function of normal subjects closely matched the patients' values. These results suggest that changes in the phase may be equivalent to changes in the P100 latency. S-VEPs, therefore, may be clinically useful in assessing visual function.     

Effect of spatial frequency on simultaneous recorded steady-state pattern electroretinograms and visual evoked potentials

Pattern electroretinograms (P-ERGs) and visual evoked potentials (VEPs) to 4 Hz alternating square-wave gratings were simultaneously recorded in 23 subjects. Responses were Fourier analyzed and amplitude and phase of the 2nd and 4th temporal harmonics were measured.The spatial frequency-amplitude function of the P-ERG 2nd harmonic component displayed either a bandpass tuning behavior, or a low-pass behavior. The peak amplitude for subjects with bandpass tuning was at 1.5 c/deg. The phase of the P-ERG 2nd harmonic decreased monotonically as spatial frequency increased. The VEP 2nd harmonic had a bimodal spatial frequency function with a peak at 3 c/deg and a second increase at spatial frequencies below 1 c/deg, regardless of the P-ERG characteristics. The phase of VEP 2nd and 4th harmonic had an inverted U-shaped function with peak at 3 c/deg and 1.5 c/deg respectively.Comparison of simultaneously recorded P-ERG and VEP spatial frequency functions demonstrated different tuning behavior for cortical and retinal responses. It is concluded that the proposed technique permits the separate analysis of retinal and cortical processing of visual information. The 2nd and 4th harmonic components of VEP behave independently of each other suggesting they may be generated by different subsystems.     

Interocular interaction assessed by VEPs to pattern-onset, -reversal,and -offset in normally sighted and amblyopic subjects

The extent of interocular interaction reflected in sequentially averaged VEPs to checkerboard onset, reversal and offset stimulation was investigated to assess the relative efficacy of the three modes of pattern stimulation. Thirty-one controls and 18 amblyopic children were studied. Components on the side of the scalp ipsilateral and contralateral to the stimulated left half-field were measured for checksizes 12′, 20′, 50′ and 80′. Binocular:monocular amplitude ratios for normals were compared with `binocular:good eye' amplitude ratios for amblyopes. The reversal P100 ratio was found to differ significantly between normals and amblyopes for 12′, 20′ and 50′ checks. Ipsilateral (CII) and contralateral (P105) onset components also differed significantly but for the smallest 12′ checks only. In controls, onset components (P105 and CIII) and, reversal components (N80 and P100) showed significantly shorter binocular as compared with monocular latencies. These latency differences were not found in amblyopes. Our results show that interocular interaction in normals is best shown by potentials which predominantly reflect macular pathway activation, and are most conspicuous for reversal N80 and P100 components. Similarly, these components demonstrated the clearest differences when comparing binocular interaction effects between controls and amblyopic subjects.     

Influence of physiological changes of glycaemia on VEPs and visual ERPs

Since hypoglycemia is known to influence cognitive functions, we checked whether the physiological changes in glycemia (after fasting or exertion) can explain the rather high intra-individual variability of event-related potentials (ERPs). Besides the ERPs to "change in coherence of a moving pattern" with reaction time (RT) recording, binocular pattern reversal VEPs and motion-onset VEPs (to linear and radial motion) were also examined in 14 healthy subjects prior to and after 24-h fasting that decreased glycemia from 5.3 to 3.9 mmol/l on the average. We only found one significant change in the latencies and amplitudes of VEPs and ERPs (with no change of RT). The N160 peak in the motion-onset VEPs to radial (expansive) motion (EM-VEPs) showed a larger amplitude at lower glycemia. For evaluation of the exertion influence, we tested glycemia prior to and after 90 min long exercise -- bicycle ergometry with the load set to 2 W/kg in women and 2.5 W/kg in men (average age-related values for W170/kg index). The changes of glycemia to exertion were, however, less distinct than those to fasting. We conclude that in healthy subjects the glycemia decrease due to 24-h fasting or intensive time-limited exercise never reaches the critical value to change the VEP, ERPs and RTs.     

Retinal Neurodegeneration in Wilson’s Disease Revealed by Spectral Domain Optical Coherence Tomography

Background/Objective

In addition to cirrhosis of the liver, Wilson’s disease leads to copper accumulation and widespread degeneration of the nervous system. Delayed visual evoked potentials (VEPs) suggest changes to the visual system and potential structural changes of the retina.

Methods

We used the latest generation of spectral domain optical coherence tomography to assess the retinal morphology of 42 patients with Wilson’s disease and 76 age- and sex-matched controls. We measured peripapillary retinal nerve fiber layer (RNFL) thickness and total macular thickness and manually segmented all retinal layers in foveal scans of 42 patients with Wilson’s disease and 76 age- and sex-matched controls. The results were compared with VEPs and clinical parameters.

Results

The mean thickness of the RNFL, paramacular region, retinal ganglion cell/inner plexiform layer and inner nuclear layer was reduced in Wilson’s disease. VEPs were altered with delayed N75 and P100 latencies, but the N140 latency and amplitude was unchanged. An analysis of the laboratory parameters indicated that the serum concentrations of copper and caeruloplasmin positively correlated with the thickness of the outer plexiform layer and with N75 and P100 VEP latencies.

Conclusion

Neuronal degeneration in Wilson’s disease involves the retina and changes can be quantified by optical coherence tomography. While the VEPs and the thickness of the outer plexiform layer appear to reflect the current copper metabolism, the thicknesses of the RNFL, ganglion cell/inner plexiform layer, inner nuclear layer and the total paramacular thickness may be the best indicators of chronic neuronal degeneration.     

Effects of chlorpromazine on pattern and flash ERGs and VEPs compared to oxazepam and to placebo in normal subjects

Antidopaminergic drugs delay the pattern-reversal VEP (P-VEP) and the flash VEP (F-VEP) and, in separate studies, reductions in the amplitude and increases in the latencies of scotopic ERGs have been reported. This study investigated the effects of chlorpromazine (CPZ) on the pattern ERG (P-ERG), P-VEP, flash ERGs and VEPs and oscillatory potentials (OPs). Normal volunteers (N = 15) were administered a placebo, or a single dose of CPZ 100 mg or oxazepam (OZP) 15 mg at weekly intervals, in a double-blind crossover design. A gold foil-ipsilateral ear derivation and an Oz′-Fz derivation were used for the ERG and VEP recordings, respectively. The latencies of ‘mixed’ and cone ERGs were significantly prolonged after CPZ compared to both placebo and to OZP. Amplitudes of rod- and cone-dominated ERGs were reduced following CPZ administration. All components of the OPs were significantly delayed after CPZ administration. No significant intertreatment differences were found in the F-VEP results. The P-ERG P50 peak and the P-VEP N70 and P100 peaks were significantly delayed after CPZ in the case of 28′ checks but not 55′ checks. Retinocortical times and P-ERG and P-VEP amplitudes were not significantly affected. In contrast to CPZ, the administration of OZP had virtually no significant effects compared to placebo. These findings suggest that the antidopaminergic CPZ has a primary effect on retinal electrophysiology. Similar findings have been reported in Parkinson's disease and in animal models.     

Reliability and upper normal variability limits of pattern reversal visual evoked potential parameters.

Twenty healthy volunteers aged 21-48 years (10 males, 10 females) were submitted to pattern reversal visual evoked potentials with 15' and 30' checks. The recordings were repeated after 7 days to assess reliability and upper normal variability limits of the following parameters: latencies of N70, P100, N140 and peak-to peak amplitudes of N70-P100, P100-N140. Reliability was tested with intraclass correlation coefficient, which was excellent or good for all parameters. Test-retest variability limits were computed with = 0.01 for absolute latency differences and relative amplitude differences.     

Visual evoked potentials in response to dichoptic presentation of sinusoidal grating and noise background

Dichoptic stimulation was used in comparison of visual evoked potentials (VEPs) with those obtained with monocular stimulation (recordings made from the occipital area). 16 subjects viewed sinusoidal gratings with the right eye while a visual noise was added via a mirror for the left eye. In presence of the noise, amplitude of the early VEP components' N1, P1b, and the late component P2 decreased, P1a is not changed in presence of the noise, and the late negative wave N2 increased for all spatial frequencies. The effect of noise on the amplitude of VEPs obtained for monocular and dichoptic stimulation was similar. The data suggest that external noise is filtered by the V1 cortical neurons--matched filters for the gratings.     

Amplitude asymmetry of hemifield pattern reversal VEPs in healthy subjects

The amplitudes of transient and steady-state visual evoked potentials (VEPs) were measured during hemifield stimulation of the left eye in 10 healthy adults. Pattern reversal of a checkerboard was produced at 4 stimulation frequencies: 1, 5, 10 and 15 Hz. The amplitudes of pattern VEPs were evaluated using the paired t test to determine significant differences between right and left hemifields. The transient VEP amplitudes from midoccipital, midparietal, ipsilateral occipital and contralateral occipital electrodes were significantly greater with right hemifield stimulation. The steady-state VEP amplitudes from the midoccipital electrode during 15 Hz stimulation were significantly greater with right hemifield stimulation. Our neurophysiological data may be compatible with neuroanatomical asymmetries of the occipital lobes in humans.     

Effect of check size on the pattern reversal visual evoked potential

Modifications of the components of pattern-reversal visual evoked potentials (PR-VEP) with changes in check size of the stimulating pattern were studied in 11 healthy subjects. We made use of 8 different check size ranging between 10 and 90 min of arc. Changes in the check size modified in different manners the latencies and amplitudes of N75, P100 and N145. Two-step statistical analyses using the polynomial regression analysis method revealed significant modifications of latencies of the 3 components, but non-significant modifications of the amplitudes, except for N75. The latency and amplitude of N75 showed a significant inverse linear relationship with the logarithm of the check size, while the P100 and N145 latencies showed significant curvilinear relationships, with minimal latencies at check sizes around 35 min. These findings suggest different physiological properties of N75 from those of P100 and N145, and hence, the necessity to establish normal values for each check size of stimulation, for application in clinical studies.     

Dancers and fastball sports athletes have different spatial visual attention styles

Physical exercise and the training effects of repeated practice of skills over an extended period of time may have additive effects on brain networks and functions. Various motor skills and attentional styles can be developed by athletes engaged in different sports. In this study, the effects of fast ball sports and dance training on attention were investigated by event related potentials (ERP). ERP were recorded in auditory and visual tasks in professional dancer, professional fast ball sports athlete (FBSA) and healthy control volunteer groups consisting of twelve subjects each. In the auditory task both dancer and FBSA groups have faster N200 (N2) and P300 (P3) latencies than the controls. In the visual task FBSA have faster latencies of P3 than the dancers and controls. They also have higher P100 (P1) amplitudes to non-target stimuli than the dancers and controls. On the other hand, dancers have faster latencies of P1 and higher N100 (N1) amplitude to non-target stimuli and they also have higher P3 amplitudes than the FBSA and controls. Overall exercise has positive effects on cognitive processing speed as reflected on the faster auditory N2 and P3 latencies. However, FBSA and dancers differed on attentional styles in the visual task. Dancers displayed predominantly endogenous/top down features reflected by increased N1 and P3 amplitudes, decreased P1 amplitude and shorter P1 latency. On the other hand, FBSA showed predominantly exogenous/bottom up processes revealed by increased P1 amplitude. The controls were in between the two groups.     

Potential of the human cerebral cortex prior to antisaccades

Latencies and other parameters of presaccadic potentials preceding antisaccades and normal saccades to visual stimuli were studied in 10 right-handed healthy subjects. The EEG was recorded in F3, F4, Fz, C3, C4, Cz, P3, P4, O1 and O2 derivation. EEG records preceding saccades and antisaccades with mean latencies were selected and averaged. The latencies of the leftward antisaccades were shorter than of the rightward antisaccades. The slow presaccadic negativity (in the period of central eye fixations) and fast N -2 and P -1 potentials within the latent period were more prominent before antisaccades than normal saccades. Spatiotemporal analyses of presaccadic potentials showed that the right frontal cortex was activated to a greater extent before antisaccades than before saccades. These findings suggest that right-hemispheric dominance in the spatial attention and inhibition of automatic saccades to visual stimuli in the period of antisaccades preparation.     

The temporal frequency response function of pattern ERG and VEP: changes in optic neuritis

Steady-state pattern electroretinograms (PERGs) and visual evoked potentials (VEPs) in response to sinusoidal gratings (2 c/deg), sinusoidally counterphased at closely spaced temporal frequencies (TFs) between 4 and 28 Hz, were recorded from 11 patients with unilateral optic neuritis (ON; 11 affected eyes and 10 healthy fellow eyes) and 7 age-matched normal subjects (7 eyes). Amplitude and phase of responses' second harmonics were measured. Responses' apparent latencies were estimated from the rate at which phase lagged with TF. When compared to control values, mean PERG and VEP amplitudes of ON eyes were reduced (by about 0.4 log units) at both low (5–10 Hz) and high (16–20 Hz) TFs. Mean PERG amplitudes of fellow eyes were selectively reduced at low TFs (by about 0.3 log units). Mean PERG apparent latencies of both ON and fellow eyes were delayed (by 15 and 9 ms, respectively). Mean VEP apparent latency of ON eyes was delayed at both low and high TFs (by 24 and 30 ms, respectively), while that of fellow eyes was selectively delayed at high TFs (by 28 ms). The results in ON eyes indicate non-selective abnormalities of PERG and VEP generators responding at both low and high TFs. VEP TF losses may be in part accounted for by corresponding PERG losses. In the fellow eyes of patients, more selective PERG and VEP TF abnormalities may suggest differential impairment of retinal and postretinal subsystems responding better to low and high TFs (i.e. parvo-and magnocellular streams).