Three-dimensional human pattern visual evoked potentials. II. Multiple sclerosis patients

Pattern visual evoked potentials were obtained from 46 patients with definite relapsing/remitting multiple sclerosis, using both a conventional 5-channel occipital array and a 3-D recording technique consisting of three bipolar derivations approximating the three dimensions of space. These three orthogonal wave forms were displayed as a 3-D Lissajous trajectory for each subject. Two of the 15 patients with completely normal conventional pattern VEPs had abnormalities of the orientation of the B-C curvilinear segment of the 3-D pattern VEPs. Delays in the first major occipital positive component (P100) were evident using both techniques; the correlation between P100 latency and the latency of the corresponding trajectory apex was r = 0.99 (P < 0.01). Post-chiasmal MRI abnormalities were associated with 3-D VEP orientation abnormalities. Three-dimensional pattern VEPs are moderately more sensitive than conventional pattern VEPs at detecting dysfunction posterior to the optic chiasm in demyelinating disease and do not require the use of eccentric fixation to do so.     

Dipole tracing of visual evoked potentials in human brain

3D tracing of equivalent current dipoles (ECDs) of averaged human visual evoked potentials (VEP) by their distribution across a 34-electrode array was obtained under short presentation of pattern-onset stimuli (sets of 45 horizontal, vertical bars or crosses). Using a 2-dipole spherical three-layer model, we dynamically (step of 1 ms) localized dipoles in four healthy subjects. Dipole locations were matched to anatomical brain regions visualized in structural MRI. Best-fitting source parameters were superimposed on MR images of each subject to identify the anatomical structures giving rise to the surface patterns. It was found that during 50-300 ms following the onset of the stimuli, the ECDs in all subjects were localized in the occipital cortex and demonstrated reliable systematic shift in localization. Two local (1-2 cm3) zones of the preferable dipole attendance were found at 5-6 cm behind zero line: the first one was localized near the midline of the brain, whereas the other zone was situated in the right hemisphere at a distance of 6-7 cm from the first zone. Their localization and strength of activation were reliably different for crosses and lines and changed during VEP generation. Zones of relatively rare dipole attendance were found also. The data are discussed in relation to localization of initial and endpoint of ECDs trajectories, as well as with sensitivity of the visual cortex to line crossing and branching.     

Effects of age and sex on pattern electroretinograms and visual evoked potentials

Pattern-electroretinograms (P-ERGs) and visual evoked potentials (VEPs) were simulataneously recorded in 112 normal individuals aged 20–75. Two sized checks subtending 15′ and 31′ were used as stimuli. A weighted regression analysis was used to determine which of the variables, sex or age, was significant. The latency of the a and b wave of the P-ERGs showed a progressive increase with age but no difference between sexes. The effect was statistically significant for both 15′ and 31′ checks. There was no statistically significant aging effects for VEPs elicited by 31′ checks. Aging, however, affected N70, P100, and the interpeak interval between b wave to N70 and b wave to P100 for responses to 15′ checks. Shorter VEP latencies were noted in females for both 15′ and 31′ checks.The simultaneous recording of P-ERGs and VEPs has demonstrated that aging is a major variable at the retinal level. The effects on the a and b waves are mostly due to optic changes with aging and only partially to aging changes in the neuronal retinal circuitry. The effect of aging on VEPs is different for different size stimuli. The cause is a random neuronal cell loss in the visual pathways from the optic nerve to the visual cortex as the individual ages.The difference in VEP data between sexes may be related to anatomical size and hormonal influences.     

Delayed visual evoked potentials are independent of pattern orientation in macular disease

Visual evoked potentials (VEPs) and contrast sensitivity (CS) were studied in patients affected by maculopathy. VEP delays and CS reduction were demonstrated in each affected eye. In distinction to patients affected by multiple sclerosis (MS), in maculopathy patients VEP latency is independent of the orientation of the grating stimulus. It is proposed that stimulating with more than one pattern orientation is useful in the differential diagnostic use of VEPs.     

Contrast adaptation and representation in human early visual cortex

The human visual system can distinguish variations in image contrast over a much larger range than measurements of the static relationship between contrast and response in visual cortex would suggest. This discrepancy may be explained if adaptation serves to re-center contrast response functions around the ambient contrast, yet experiments on humans have yet to report such an effect. By using event-related fMRI and a data-driven analysis approach, we found that contrast response functions in V1, V2, and V3 shift to approximately center on the adapting contrast. Furthermore, we discovered that, unlike earlier areas, human V4 (hV4) responds positively to contrast changes, whether increments or decrements, suggesting that hV4 does not faithfully represent contrast, but instead responds to salient changes. These findings suggest that the visual system discounts slow uninformative changes in contrast with adaptation, yet remains exquisitely sensitive to changes that may signal important events in the environment.     

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.     

Operative memory and evoked visual potentials

Changes in human EEG and event-related potentials to the complex color stimulus were studied during performance of three cognitive tasks: passive viewing, visual searching, and memorizing and storing of the stimulus in the operative memory. It was shown that involvement of memory mechanisms resulted in the alpha rhythm suppression in both occipital and frontal brain areas. It was also accompanied by a generation of the slow positive component in the evoked potential recorded in the frontal areas (in the occipital areas this wave was manifested as negative).     

The effect of eccentricity and colour on negativity in pattern onset visual evoked potentials

The effects of stimulus size, eccentricity and colour on the amplitudes of N100 and N130 were investigated in pattern onset VEPs.For black-and-white pattern stimulation, the first set of stimuli was derived from a full dartboard pattern. Central stimulation of various extents was produced by patterns with reduced number of outer rings and for eccentric stimuli a number of central rings were removed from the full pattern. It was found that amplitude of N100 was maximal in VEPs to central stimuli and that it was greatly reduced when eccentric stimulation was applied. The amplitude of N130 showed no significant change in relation to the type of stimulus.When checkerboard stimuli of identical configuration were used for black-and-white pattern stimulation instead of dartboards, systematic changes in peak latencies of N100 were observed in relation to check size. In VEPs to centrally presented small checks the emergence of an early negative peak preceding N100 was recorded at 75 msec. In VEPs to coarse checkerboards presented centrally N100 was often observed with a delayed peak latency of 110 msec. Changes in N130 were not regular when checkerboards with different check sizes were presented centrally. For eccentric checkerboard stimulation, both negative peaks N100 and N130 were revealed. Their peak latencies were similar to those observed in the case of dartboard paracentral stimulation.In VEPs to patterns projected through red or blue filters, regular changes were observed in both negative peaks. Introduction of the red filter led to enhancement of the N100 amplitude in VEPs to dartboard and to checkerboards with fine checks, but it caused no effect on N100 in the VEPs to coarse checkerboards. Introduction of the blue filter led to a decrease in the N100 amplitude in VEPs to dartboard and fine checkerboards and to a slight increase of N100 VEPs to coarse checkerboards. Changes in N130 were observed only when the blue filter was introduced and they corresponded to those which take place when the level of illumination changes from photopic to mesopic.     

Three-channel Lissajous' trajectory of the human short latency visual evoked potentials

Three orthogonally derived voltage-time records, when plotted simultaneously on three-dimensional voltage-voltage-voltage coordinates, produce a 3-Channel Lissajous' Trajectory (3CLT). 3CLTs of short latency visual evoked potentials (SVEP) were obtained for 8 adult humans (16 eyes) in response to monocular flash stimulation.Intersubject variability of 3CLT of SVEP was small enough to enable identification along the trajectory of comparable planar-segments of approx. 3–5 ms duration across subjects. In each planar-segment, the point at which the trajectory exhibited marked bonding was noted. These points were called apices and they corresponded to maxima in the trajectory's distance from the origin (Trajectory Amplitude). Intersubject variability in apex latencies was comparable to, or smaller than, peak latency variability of single-channel records. 3CLT of SVEP consisted of 8 planar-segments in each of the latency ranges of 0–40, 40–70 and 70–100 ms.Analysis of 3CLT, combining the criteria of segment planarity, apices and trajectory amplitude peaks, enabled differentiation between components that had been considered the same in surface distribution studies. 3CLT of SVEP seemed to be influenced by the direction of propagaion along the visual pathway.     

Human auditory evoked potentials during natural sleep: The early components

The auditory brain-stem evoked potential (ABEP) was recorded from 9 female subjects during 1 night of natural sleep. Monaural click stimuli were delivered at a rate of either 11, 41 or 81/sec through a hearing-aid device. The intensity was held constant at 70 dB nHL. In other runs, the intensity was lowered to either 50, 30 or 10 dB, the rate of click presentation being held constant at 81/sec. Tympanic temperature was monitored throughout the recording session. The ABEP was unaltered during any stage of sleep regardless of the rate of presentation or stimulus intensity. Distinct peak V responses were recognizable to within 10 dB of the adult threshold in the sleeping subject. It may be concluded that sleep has no virtually on effect on ABEP morphology.     

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.     

Pattern-reversal visual evoked potentials in photosensitive epilepsy

VEPs to checkerboard pattern-reversal were recorded from 18 epileptic patients who had EEG photoparoxysmal responses to stroboscopic light. Patients were grouped according to whether seizures were precipitated by environmental light stimuli, or television viewing. Longitudinal studies were conducted on 8 patients treated with valproic acid. We concluded the following: (1) Latency of the major positive peak (P2) of the pattern-reversal VEP was shorter among photosensitive patients than among normal controls. This was especially true of television-sensitive patients. (2) Valproic acid, when effective in controlling seizures, lengthened the P2 latency and decreased VEP amplitude. Studies of drug effects on VEPs may help to elucidate neurochemical mechanisms of the visual cortex. (3) Because of overlap of values with normals, VEP measurements are not at present very sensitive in the diagnosis of photosensitivity. However, longitudinal studies in individuals parallel clinical changes and may be useful as objective measures of improvement.