Early Components Of Visual Evoked Potentials Research Articles

Characteristics of visual evoked potentials related to the electro-clinical expression of reflex seizures in photosensitive patients with idiopathic occipital lobe epilepsy

Seizures provoked by visual stimuli may be induced by abnormal responses to light (photosensitivity) and structured patterns (patternsensitivity). In this study, we analysed visual evoked potentials (VEPs) in three different samples: i) 38 photosensitive patients (21 males, 17 females; mean age 10.0 ± 2.9 years) with idiopathic occipital lobe epilepsy and reflex seizures (RS); ii) 13 non-photosensitive patients (6 males, 7 females; mean age 11.7 ± 5.3) with idiopathic occipital lobe epilepsy; 20 healthy controls (12 males, 8 females; mean age 10.0 ± 3.4). After written informed consent, all subjects underwent a standard procedure of visual stimulation with intermittent light and pattern stimulation, under digital video-EEG recording. The EEG signal was processed off-line by averaging analysis for each stimulus to obtain the corresponding VEP. Comparisons among groups showed no significant differences for P100 latency. Higher P100 amplitude as well as higher after-discharge (AD) were found in photosensitive patients with RS. Thirty-seven of these patients had one or more RS during the procedure of stimulation for a total of 66 episodes. Significant increases of P100 amplitude and higher values of AD amplitude were found in relation to the occurrence of photoparoxysmal response (PPR) and/or seizures during full-field pattern stimulation. The increase in amplitude of the AD was higher when PPR was associated with seizures. The high amplitude of early VEP components confirms the abnormal hyperexcitability in the cortex of photosensitive patients with occipital lobe epilepsy. Moreover, the AD amplitude appears to be related to electro-clinical expression, being greater when PPR evolves into clinically evident seizures.

ID 197 – Altered recovery from inhibitory repetitive transcranial magnetic stimulation (rTMS) in subjects with photosensitive epilepsy

To investigate functional changes underlying photosensitivity, we studied the response of the visual cortex to low-frequency, inhibitory repetitive transcranial magnetic stimulation (rTMS) in patients with photosensitive seizures and healthy volunteers. Visual evoked potentials (VEPs) triggered by grating stimuli of different contrasts were recorded in both hemispheres before and after functional inactivation of the occipital cortex of one side via low-frequency rTMS (0.5 Hz for 20′). VEPs were recorded in before (T0), immediately after (T1) and 45′ following the completion of rTMS (T2). Baseline amplitudes of the early VEP components (N1 and P1) were enhanced in photosensitive subjects. At T1, rTMS produced an inhibitory effect on VEPs amplitudes at all contrasts in the targeted side, and a concurrent facilitation of responses in the contralateral hemisphere. While the relative variation with respect to pre-rTMS values was similar in photosensitive patients and controls at T1, robust changes appeared in the recovery phase (T2). Visual responses recovered more quickly in the stimulated hemisphere, and disinhibition persisted in the contralateral side of photosensitive subjects. The rapid recovery of excitability and the persistent transcallosal disinhibition following perturbation of cortical activity may play a role in the pathophysiology of photosensitive epilepsy.

Differential effects of low-frequency rTMS at the occipital pole on visual-induced alpha desynchronization and visual-evoked potentials

Visual-induced alpha desynchronization (VID) and visual-evoked potentials (VEPs) characterize occipital activation in response to visual stimulation but their exact relationship is unclear. Here, we tested the hypothesis that VID and VEPs reflect different aspects of cortical activation. For this purpose, we determined whether VID and VEPs are differentially modulated by low-frequency repetitive transcranial magnetic stimulation (rTMS) over the occipital pole. Scalp EEG responses to visual stimuli (flashed either to the left or to the right visual field) were recorded for 8 min in six healthy subjects (1) before, (2) immediately following, and (3) 20 min after left occipital rTMS (1 Hz, 10 min). The parameters aimed to reduce cortical excitability beyond the end of the TMS train. In addition, simple reaction times to visual stimulation were recorded (left or right hand in separate blocks). In all subjects, VID was significantly and prominently reduced by rTMS ( P = 0.0001). In contrast, rTMS failed to modulate early VEP components (P1/N1). A moderate effect was found on a late VEP component close to manual response onset ( P = 0.014) but this effect was in the opposite direction to the VID change. All changes were restricted to the targeted left occipital cortex. The effects were present only after right visual field stimulation when a right hand response was required, were associated with a behavioral effect, and had washed out 20 min after rTMS. We conclude that VID and early VEPs represent different aspects of cortical activation. The findings that rTMS did not change early VEPs and selectively affected VID and late VEPs in conditions where the visual input must be transferred intrahemispherically for visuomotor integration (right visual field/right hand) are suggestive of rTMS interference with higher-order visual functions beyond visual input. This is consistent with the idea that alpha desynchronization serves an integrative role through a corticocortical “gating function.”

Symmetry detection and orientation perception: Electrocortical responses to stimuli with real and implicit axes of orientation

The present study was designed to investigate the electrocortical correlates of visual anisotropy with particular reference to the anisotropics reported for symmetrical patterns. On the basis of earlier studies of visual evoked potentials (VEPs) to lines of differing orientation, it was predicted that VEP parameters would differ according to the orientation of the axis of symmetry of a stimulus pattern. It was further predicted that a direct comparison of explicit orientation (by line presentation) and implicit orientation (via patterns symmetrical about axes differing in orientation) would yield data consistent with the notion that symmetry detection is a special case of orientation detection. The present findings indicated that for components of VEP waveforms up to 500 ms from stimulus onset, amplitude was generally greater for patterns with a vertical axis of symmetry than for patterns with a horizontal axis of symmetry, while patterns with positive and negative oblique axes evoked VEPs with the lowest amplitude components. In addition, it was found that the early VEP components exhibited latency differences for different axes of symmetry, with shortest latencies occurring for figures with vertical symmetry. The data are discussed, within the context of theories that suggest symmetry detection is an acquired process dependent on the development of cell populations responsible for particular stimulus orientations.

Rat and human visual-evoked potentials recorded under comparable conditions: A preliminary analysis to address the issue of predicting human neurotoxic effects from rat data

Pattern-onset visual-evoked potentials (VEPs) were recorded from rats and humans in order to perform cross-species comparison of neuronal functional properties reflected by the early VEP components. The spatial frequency of a sinusoidal test grating was varied in Experiment 1. For both species, amplitude of the first positive VEP component was larger at low spatial frequency and decreased as spatial frequency increased. The immediately succeeding negative component was small at low spatial frequency and was of maximal amplitude at moderate spatial frequency. The effects of stationary pattern adaptation on these components were investigated in Experiment 2. Subjects viewed either a blank field or the test grating prior to recording VEPs. For both species, adaptation had no effect on the positive component but strongly attenuated the negative component. Experiment 3, in which only humans were tested, indicated that the negative component was of cortical origin. Only cortical neurons are known to be orientation selective, and the effect of adaptation diminished as the orientation difference between the adaptation and test gratings increased. These results suggest that the early positive and negative components arise from parallel visual pathways, and that the rat components may reflect visual processes qualitatively similar to those of humans.

Visual evoked potentials to illusory reversals of the necker cube.

ABSTRACTSensory adaptation and cognitive evaluation have been proposed as explanations of illusory figure reversals. The effect of variations in the perceived orientation of a Necker cube on visual evoked potentials (VEPs) was studied to test these two hypotheses. VEPs associated with perceptual reversals and non‐reversals of a Necker cube were compared with VEPs elicited by a sequence of physically varying cubical figures. Amplitude differences in the early VEP components consistent with adaptation effects were not detected. Both types of reversals were associated with a late positive component (400–700 ms), which was smaller in amplitude in the illusory condition. The late positivity to illusory reversals was also distinguished by a broad (200–700 ms) positive component over frontal and central recording sites which was absent over Oz. These findings suggest that illusory reversals are more difficult to discriminate than physical reversals, and require additional cognitive resources for evaluation.

Maturation of evoked potentials and visual preference in 6–45-day-old infants: Effects of check size, visual acuity, and refractive error

Visual evoked potentials (VEPs) and the percentage time fixated (PTF) were investigated in response to checkerboard light flashes in 10 human infants ranging in age from 6 to 45 days. Changes in the VEPs and PTF were considered as a function of the size of check in the evoking stimulus (diffuse light, 11, 22, 45, 90 and 180 min of arc), the refractive lens strength the checkerboards were viewed through (−6 to +6 diopters), and the age og the infants (6–26 or 27–45 days). Check size significantly influenced VEP amplitude in infants as young as 6 days. The 11' checks evoked greater responses that diffuse light suggesting a visual acuity of better than 20/220. Only the 27–45-day-olds behaviorally discriminated the checks, PTF indicating an acuity of 20/120. Evoked potential refraction with spherical lenses indicated an average refractive error of −1.7. dopters. The changes in the function between VEP amplitude and check size measured from different VEP components at different ages indicated the function contained two modes or components. The first mode was inverted “U-shaped” and was obtained in response to check sizes less than 45′. It was primarily due to changes in amplitude of the early VEP components (less than 210 msec after the evoking stimulus) and was poorly correlated with the behavioral PTF measure. It was proposed that this mode reflected subcortical activity. The second mode was a linear increase in amplitude as check size was increased from 45′ to 180′. It was primarily due to changes in the amplitude of late VEP components (240–400 msec after the evoking stimulus) and was highly correlated with the percentage time the infants fixated the various check sizes. It was proposed that this mode reflected cortical activity. Age selectively influenced the late VEP components and the PTF behavioral measure, these measures being influenced by check size only in the 27–45-day-old infants. This change in responsivity of late VEP components and the transition from passive to more active and discriminating visual preference, suggest the onset of increased cortical function between 28 and 45 days of age.