G. F. A. Harding

Photic- and Pattern-induced Seizures : A Review for the Epilepsy Foundation of America Working Group

Summary:  Purpose: This report summarizes background material presented to a consensus conference on visually provoked seizures, convened by the Epilepsy Foundation of America.

Characteristics of a unique visual field defect attributed to Vigabatrin

Summary: Purpose: Vigabatrin (VGB) therapy is associated with a loss of peripheral vision. The characteristics and prevalence of VGB‐attributed visual field loss (V‐AVFL) and associated risk factors were evaluated in patients with epilepsy.

Standard for Visual Evoked Potentials 1995

A revised ISCEV standard for clinical electro-retinography has been published (1995). It is our concern that similar standards should be adopted for visual evoked potentials (VEP). The VEP is an electrophysiological signal that is recorded at the scalp, using standard EEG techniques. When used with other visual electrophysiological techniques the VEP can provide important diagnostic information with regard to the functional integrity of the visual system. Our intention is that the standard method and responses for each of the stimuli should be used widely, but not to the exclusion of additionaltests that individuallaboratoriesmay chooseor may continue to use. It is not essential that all stimuli shouldbe used for every investigationon every patientor normal subject. In addition, we propose a standard nomenclature for the various types of recording. These standards will be reviewed at approximately 4 year intervals. The organisationof this report is as follows:

Localization and Functional Analysis of Human Cortical Area V5 Using Magneto-Encephalography

Using a multi-channel SQUID-based neuromagnetometer, we have determined the location, temporal dynamics and functional response properties of the human homologue of the primate cortical area V5 (MT). We provide evidence that area V5 in humans is located near the occipito-temporal border in a minor sulcus immediately below the superior temporal sulcus. This area is selective for low spatial frequencies ( ≤4.0 c/deg), responds to a wide range of temporal frequencies ( ≤ 35 Hz) and shows response saturation for stimulus contrasts greater than 10%. In addition, we find that this area is not responsive to purely chromatic patterns but is responsive to motion-contrast stimuli. Our results are consistent with the hypothesis that area V5 in humans represents a stage of processing within the magnocellular pathway. We discuss our results in relation to the widespread belief that area V5 in humans is specifically concerned with motion perception.

The influence of age on the electroretinogram and visual evoked potential

Changes in the ERG and VEP across the life span were investigated. The dark adapted and scotopic ERGs both showed a progressive increase in the implicit times of the A and B waves and a reduction in the amplitude of the AB configuration. There was also an increase in the implicit times of the oscillatory potentials of the photopic ERG.The flash and pattern onset-offset VEP both showed changes in waveform with age whilst the waveform of the pattern reversal VEP was constant. The amplitudes of the components of the flash and pattern reversal VEP were very high in the teenage group, but once reduced, were constant from the twenties onwards, showing no further consistent age changes.The latencies of the components of the pattern VEPs showed an increase with age which could be accounted for by the reduction in retinal illuminance due to the decrease in pupil diameter with age. However, the increase in the latency of the flash major positive (P2) component was greater than that expected from the decrease in retinal illuminance alone, suggesting that this is due to neural factors.

A controlled study of the effect of sodium valproate on photosensitive epilepsy and its prognosis.

The effect of sodium valproate on photosensitive epilepsy was studied. The range of flash rates of intermittent photic stimulation to which patients were sensitive (photosensitive range) was tested prior to treatment, and the reliability of this measure established by repeated tests on 70 patients. The photosensitive range was measured on 50 patients prior to and during treatment with sodium valproate. In 27 patients photosensitivity was abolished and in a further 12 patients photosensitivity was significantly reduced. A group of 167 patients followed without treatment did not show significant improvement over a 7 year period. Sixteen patients had drug treatment withdrawn, and in 7 months their photosensitive range had returned to its predrug level.

Photic Stimulation: Standardization of Screening Methods

Purpose: Differences in methodology of intermittent photic stimulation within and between countries in Europe make collaborative research and interpretation of results difficult.

Presenile dementia—the use of the flash and pattern vep in diagnosis

Flash, pattern reversal and pattern onset-offset VEPs were recorded in 17 patients with a clinical diagnosis of presenile dementia. The results were compared with those from two control groups--17 normal volunteers of equivalent ages and 17 patients from the same psychiatric hospital with affective disorders but no clinical evidence of dementia. The flash P2 component was found to be significantly delayed in the group of patients with dementia, but the latency of the P1 component and pattern VEP components were all within normal limits. This unusual finding presents a useful index for the diagnosis of presenile dementia.

Topographic mapping of cortical potentials evoked by distension of the human proximal and distal oesophagus

We describe cortical potentials evoked by balloon distension of the proximal and distal oesophagus in 8 healthy right handed volunteers. Oesophageal stimulation was performed using a pump which rapidly inflated a 2 cm silicone balloon positioned either 3 cm distal to the upper oesophageal sphincter or 5 cm proximal to the lower oesophageal sphincter, at a frequency of 0.2 Hz, using inflation volumes which produced a definite but not painful sensation. Oesophageal evoked cortical potentials were recorded in all subjects with an initial negative and positive component (N1 and P1), followed by a second negative and positive component (N2 and P2) in 6 subjects. The morphology and the scalp topography of the N1 component elicited by proximal and distal oesophageal stimulation suggests activation of the primary somatosensory cortex and/or the insular. There was also evidence for hemispheric dominance for the N1 potential which was independent of handedness. The frontal emphasis of the proximal oesophageal N1 component, in contrast to the central emphasis of the distal oesophageal N1 component, suggests that different neuronal populations were activated by stimulation of the two sites. The frontal emphasis of the ensuing P1 component from both oesophageal sites suggests that it originates in a separate precentral source. The topography of the N2 components obtained by stimulation of either oesophageal site was similar to that of the N1 component, suggesting that they originate in similar areas of the cortex. The P2 component evoked by stimulation of both oesophageal sites was localised at the vertex. The inter- and intra-subject variation in the morphology of the N2 and P2 components suggests that secondary cortical processes related to cognition may be involved in their generation.

Televised Material and Photosensitive Epilepsy

It has long been recognised that in Europe, the television (TV) monitor is the most common provocative stimulus for photosensitive seizures (1). More recent evidence confirms this view, whether the TV monitor is used for broadcast material, video playback, or electronic screen games. The monitor is an inherently provocative stimulus because its basic frequency causes flicker or changes in luminance. In Europe the frequency of this change is at 50 Hz (PAL), whereas for most of the world, the rate is 60 Hz (NTSC). At 50 Hz, 49% of patients are sensitive and will produce photoparoxysmal responses (PPRs) in the EEG, whereas at 60 Hz, only 15% of patients are sensitive (2). This marked difference in sensitivity may explain the relatively low incidence of TVinduced seizures in North America. The TV picture is made by a system called interlaced scanning. The picture is made up of a series of thin strips or “lines.” Normally 625 lines are used, and the scan begins at the top left corner of the picture and continues until the bottom right-hand corner is reached. When TV standards were established, it was realised that apparent flicker could be reduced if the 625 lines were divided into two 312.5-line pictures, made up in one picture of even-numbered lines and in the other picture of oddnumbered lines. These two pictures are called fields. The alternating lines can be resolved only at relatively short viewing distances, typically around 1 m. At these short distances, spatial flicker is therefore present at 25 Hz for PAL and 30 Hz for NTSC TV systems. There are similar probabilities of inducing PPRs associated with these rates of flicker, 76% at 25 Hz and 72% at’30 Hz (3). In Europe, photosensitive epilepsy has an incidence of 1.1 : 100,000 per annum of the population, but between the ages of 7 and 19 years, it is 5.7 times as common (4). This incidence correlates well with the onset of seizures being maximal around puberty (2). Photosensitive epilepsy is more common in girls (2: 1) and has a marked genetic factor; 25% of children of photosensitive mothers show PPRs in the laboratory (5). Although the condition usually manifests itself around puberty, 90% of patients having their first seizure before