Intermittent Photic Stimulation Research Articles

P9-20 Study of connectivity by means of Partial Directed Coherence on EEG signals in photosensitive patients during 14Hz intermittent photic stimulation

P9-20 Study of connectivity by means of Partial Directed Coherence on EEG signals in photosensitive patients during 14Hz intermittent photic stimulation

Maturational changes of 5 Hz SSVEPs elicited by intermittent photic stimulation

We investigated the development of the magnitude and phase alignment of steady-state visual evoked potentials induced by 5 Hz intermittent photic stimulation in 46 children (3 to 16 years) and 8 adults, as a function of age. We found that, over the occipital region, magnitude values were the highest in 8–11-year old children, but decreased with age over all other cerebral regions. Phase alignment values increased with age over the occipital, parietal and frontal cerebral regions. We interpret these findings in terms of the development of functional interactions between different cortical areas involved in the processing of visual stimuli.

EEG Evidence of Posterior Cortical Disconnection in PD and Related Dementias

ABSTRACTElectroencephalogram (EEG) reactivity to eyes opening and 12-Hz photic stimulation was investigated in 14 healthy elderly subjects, 21 parkinsonian patients (PD), 7 demented parkinsonian patients (PDD), and 10 patients with Lewy body dementia (LBD) using global field synchronization (GFS). During eyes closed Theta GFS was increased in Parkinson's disease and patients and alpha1 GFS was decreased in LBD subjects. During 12-Hz intermittent photic stimulation (IPS), reactivity of posterior electrodes was decreased in PD and LBD patients. No reactivity was observed in PDD. Results are consistent with a graded posterior cortical disconnection in parkinsonian syndromes and with a model of dopamine-modulated thalamocortical interplay in visual processing.

Congenital long QT syndrome presenting with a history of epilepsy: Misdiagnosis or relationship between channelopathies of the heart and brain?

A 60-year-old man with a long history of epilepsy was referred for cardiologic evaluation. An earlier diagnosis of epilepsy was made on the basis of his clinical manifestation of tonic-clonic seizure. Electroencephalography (EEG) demonstrated paroxysmal slow waves in response to intermittent photic stimulation. However, electrocardiography (ECG) revealed bradycardia (heart rate, 48 bpm) and marked QT prolongation (QTc 477 ms). ECG monitoring confirmed remarkable QT prolongation; ventricular ectopy triggering torsades de pointes was recorded during seizure. The patient underwent temporary antitachycardia pacing, and an implantable cardioverter defibrillator (ICD) was finally implanted. Long QT syndrome (LQTS) genetic testing was conducted and a diagnosis of LQT2 was confirmed by the identification of mutation in KCNH2 (HERG). LQTS is associated with abnormal channel function due to mutations in ion channel genes. Epilepsy, a disorder of neural function, is also associated with abnormal channel function. The possibility that some channelopathies can manifest as both LQTS and epilepsy is discussed.

Photosensitive epilepsy: Spectral and coherence analyses of EEG using 14 Hz intermittent photic stimulation

To evaluate the EEG recorded in photosensitive idiopathic generalised epilepsy (IGE) patients at rest and during 14Hz IPS, frequency capable of inducing photoparoxysmal responses (PPRs). Power spectrum density and coherence profiles were estimated using a block autoregressive parametric model (AR) in 28 patients and 22 controls. At rest, the intra- and inter-hemispheric coherence spectra showed a significantly larger number of coherence peaks in the gamma band in patients with respect to controls. During intermittent photic stimulation (IPS), coherent gamma activity is mainly presented as IPS frequency harmonics; moreover, the patients' mean coherence values significantly increased. In six patients re-evaluated with IPS after putting on glasses with Z1 blue lens (which counteracts PPR) the spectral and coherence profiles tended to return to the resting ones. Patients are endowed with inherited EEG hyper-synchrony as shown by the large number of coherence peaks detectable under resting conditions, whereas IPS enhances intra- and inter-hemispheric mean coherence values in the gamma band. The persistence of alpha activity peaks during IPS in most controls but not in patients suggests that the alpha generating network plays a significant role in counteracting PPR. Both gamma and alpha EEG generators are involved in the PPR generation and in the widespread synchronisation characterising the IGE-associated photosensitivity.

Interhemispheric asymmetry in EEG photic driving coherence in childhood autism

Examination of the EEG photic driving coherence during intermittent photic stimulation in autistic patients with relatively intact verbal and intellectual functions in order to enhance the likely latent interhemispheric asymmetry in neural connectivity. Fourteen autistic boys, aged 6-14years, free of drug treatment, with I.Q. 91.4+/-22.8, and 19 normally developing boys were subject to stimulation of 12 fixed frequencies of 3-27Hz. The number of high coherent connections (HCC) (coherence >0.6-0.8) was estimated among 7 leads in each hemisphere. In contrast to the spectral characteristics showing the right hemisphere deficit in the photic driving reactivity, the number of HCC differentiated the groups only in the left hemisphere where it was higher in autistics at the EEG frequencies corresponding to those of stimulation at 6-27Hz without asymmetry at other frequencies, the left-side prevalence increasing with frequency. No asymmetry was observed in the resting state. Spectral and coherence characteristics of the EEG photic driving show different aspects of latent abnormal interhemispheric asymmetry in autistics: the right hemisphere "hyporeactivity" and potential "hyperconectivity" of likely compensatory nature in the left hemisphere. The EEG photic driving can reveal functional topographic alterations not present in the spontaneous EEG.

FMRI activation during spike and wave discharges evoked by photic stimulation

Photoparoxysmal response (PPR) is an electroencephalographic (EEG) trait characterized by the occurrence of epileptiform discharges in response to visual stimulation. Studying this trait helps to learn about mechanisms of epileptogenicity. While simultaneous recordings of EEG and functional MRI (EEG–fMRI) in patients with spontaneous generalised spike-wave discharges (GSW) have revealed activation of the thalamus and deactivation in frontoparietal areas, EEG–fMRI studies on evoked GSW such as PPR are lacking. In this EEG–fMRI study, 30 subjects with reported generalised PPR underwent intermittent photic stimulation (IPS) in a 3 T MR scanner. PPR was elicited in 6 subjects, four diagnosed with idiopathic generalised epilepsy and two with tension-type headache. Because PPR is preceded by synchronization of cortical gamma oscillations, blood oxygenation level-dependent (BOLD) signal changes were analysed at the onset of the PPR (standard regressor) and 3 s before the onset of PPR (early regressor) in one model. In all subjects, IPS led to a significant activation of the visual cortex. Based on the early regressor, PPR associated activation was found in the parietal cortex adjacent to the intraparietal sulcus in five and in the premotor cortex in all 6 subjects. The standard regressor revealed deactivation in early activated areas in all subjects and thalamic activation in one subject. In contrast to spontaneous GSW, these results suggest that PPR is a cortical phenomenon with an involvement of the parietal and frontal cortices. Pronounced haemodynamic changes seen with the early regressor could mirror gamma activity that is known to precede PPR.

Mapping brain activity on the verge of a photically induced generalized tonic–clonic seizure

In a photosensitive patient intermittent photic stimulation (IPS) accidentally provoked a generalized tonic-clonic seizure during simultaneous recordings of electroencephalography (EEG) and functional magnetic resonance imaging (fMRI). Before seizure onset, IPS consistently induced generalized photoparoxysmal responses (PPRs). These PPRs were associated with increases in blood oxygen level dependent (BOLD) signal in the visual cortex, the thalamus, and both superior colliculi, and a decrease in BOLD signal in the frontoparietal areas. The BOLD signal in the visual cortex increased in magnitude during consecutive epochs of IPS associated with PPRs. We propose that repeated IPS led to an excessive amount of neuronal activity in the visual cortex that evoked PPRs and finally exceeded a critical threshold and triggered a generalized seizure.

A study of 63 cases with eyelid myoclonia with or without absences: Type of seizure or an epileptic syndrome?

Purpose Eyelid myoclonia and absences (EMA) induced by eye closure associated with brief, fast, and generalized paroxysms of polyspikes and waves was considered as an epileptic syndrome and a type of seizure as well. We analyzed the electroclinical features and evolution of EMA, and tried to determine if it represents a well-defined epileptic syndrome or a non-specific condition associated to other epilepsies. Methods Between June 1994 and June 2005, 63 patients who met diagnostic criteria of EMA were enrolled in the study and have been followed up to the present time. Results Two main groups could be identified. The first group was divided into two subgroups. One subgroup of 28 patients presented EMA associated with infrequent generalized tonic–clonic seizures (GTCS), and the other 1 of 9 patients presented early-onset EMA refractory to antiepileptic drugs (AEDs), associated or not with GTCS and mental retardation. Four of them had self-induced seizures. The second group included 26 patients with EMA associated with GTCS and/or massive myoclonias, or GTCS induced by intermittent photic stimulation. All these patients had electroclinical features compatible with idiopathic generalized epilepsies. Conclusion In the first group, EMA should be considered as a photosensitive idiopathic epileptic syndrome. A subgroup of early-onset of EMA refractory to AEDs, associated or not with GTCS and mental retardation should also be considered as a variant or a distinct photosensitive idiopathic epileptic syndrome. Finally, in the second group EMA may correspond to a type of seizures in idiopathic generalized epilepsies.

Ictal Headache and Visual Sensitivity

Migrainous headache is reported by patients with photosensitive epilepsy, whereas their relatives complain more often about headache than the relatives of patients with other types of epilepsy. We therefore investigated whether headache itself could be an epileptic symptom related to photosensitivity. Four probands with headache and photosensitive epilepsy were selected. Their first-degree family members were studied using video-EEG with extensive intermittent photic stimulation and pattern stimulation. Nine of the 12 subjects (10 female and two male, mean age 30 years, range 14-46 years) proved to be photosensitive with either focal (n = 5) or generalized (n = 4) epileptiform discharges. In two subjects an ictal recording of headache occurred after visual stimulation. We found evidence that, in specific patients, headache could be an ictal sign of epilepsy. Photic stimulation during EEG recording can contribute to correct diagnosis and lead to the best care and management of the patient.

Incidence of photosensitive epilepsy in unselected Indian epileptic population.

The recognition of photosensitivity depends upon the finding of an abnormal EEG response to intermittent photic stimulation (IPS). Photosensitive epilepsy being the most common form of reflex epilepsy, has been studied most extensively and is reported to account for 5-10% of the epileptic population in general. One thousand unselected epileptics hailing from different parts of Northern India were screened for photosensitivity, both clinically and on a standard protocol of IPS-provoked EEG recordings. Six patients (mean age 14.5 +/- 3.56 yr) were found to be photosensitive: 4 had generalized, 1 had complex partial and 1 mixed seizures; 3 had history of seizures provoked by visual stimuli. The baseline EEG in 4 patients showed generalized and 2 partial, with secondary generalized, epileptiform discharges. On IPS, similar EEG findings were obtained with a wide range of stimulus frequency (6-60 cycles/s). There is a low incidence of photosensitivity in our epileptic population, for which we believe, genetic factors are responsible.

The influence of racial and environmental factors on EEG photoparoxysmal responses in Zimbabwe.

This study examined the influences of environmental and racial factors on the incidence of EEG photoparoxysmal responses (PPRs) to intermittent photic stimulation (IPS) in a multi-racial population exposed to seasonal variations in sunshine intensity. The records of patients referred for EEG examination in Harare, Zimbabwe were screened for the presence of PPRs to IPS. The age, sex and racial classification of patients manifesting PPRs were studied. The relationship between sunshine duration and intensity and the frequency of PPRs was also studied. EEG records of 16,496 patients referred for EEG examination were studied. Of these, 136 patients (0.8%) showed PPRs to photic stimulation. The highest age-specific PPR (2.0%) occurred in the 16-20 year age group. The race-specific PPRs were 1.5% for white patients, 1.18% for Asian patients, 0.36% for colored patients and 0.09% for black patients (chi3[2] = 95.21, P = 0.001). The incidence of PPRs did not show any significant variation with the mean monthly sunshine duration (P = 0.8254) or with the mean monthly sunlight radiation (P = 0.2631). The findings of this study support a role for genetic rather than environmental factors in the relative rarity of PPRs in black Zimbabweans.

Abnormal response of motor cortex to photic stimulation in idiopathic generalized epilepsy

Intermittent photic stimulation (IPS) shortens the cortical silent period (CSP) elicited by transcranial magnetic stimulation (TMS) over the primary motor hand area (M1(HAND)). This response is absent in healthy individuals with a photoparoxysmal response (PPR). Here we combined TMS of the M1(HAND) with IPS to examine whether patients with idiopathic generalized epilepsy (IGE) exhibit an abnormal cortical response pattern to IPS. In 13 PPR-positive and 12 PPR-negative patients with IGE and in 13 PPR-negative healthy controls, we used focal TMS to the M1(HAND) to study how cortical excitability is changed by concurrent IPS at 50 Hz. IPS at 50 Hz reduced the duration of the CSP in healthy PPR-negative individuals, whereas IPS had no effect on the CSP in PPR-positive and PPR-negative patients with generalized epilepsy. The failure of IPS to shorten the CSP was independent of antiepileptic medication. Single-pulse or paired-pulse TMS only without concurrent IPS showed a higher motor threshold in PPR-positive patients with epilepsy, presumably caused by antiepileptic medication. No additional differences in cortical excitability were found among groups. Because the CSP is mediated by intracortical GABAergic mechanisms, our results indicate that IGEs are associated with an altered responsiveness of GABAergic inhibitory circuits in the M1(HAND). This electrophysiological trait is independent of photosensitivity. Excitability changes at the cortical or thalamic level may mediate this abnormal cortical response pattern in patients with IGE.

Visual Pattern Adaptation in Subjects with Photoparoxysmal EEG Response: Evidence for Increased Visual Cortical Excitability

Photosensitivity, or photoparoxysmal response (PPR), is an abnormal EEG reaction to intermittent photic stimulation (IPS), consisting of spikes, spike-waves, and intermittent slow waves. Depending on the spread of the abnormal activity, PPR subgroups have been defined as having either propagating PPR or localized, occipital spikes (OS) only. Recent research suggests an enhanced excitability in the occipital cortex may underlie this reaction during IPS, but it remains unclear whether changes in excitability affect the function of the occipital cortex to other, less provocative visual stimuli. In this study, cortical function in photosensitivity was assessed using two visual aftereffects that occur after prolonged adaptation. Motion and tilt aftereffects were compared in healthy subjects with (n = 14, seven with propagating PPR, seven with OS) or without (n = 14) PPR. The duration of the motion aftereffect was shorter in the PPR group than in the control group. The size of the tilt aftereffect did not differ between the groups. Thirteen from each group had participated in an earlier study in which occipital transcranial magnetic stimulation (TMS) was used to elicit phosphenes and to suppress the perception of briefly presented letters. The TMS intensity required to elicit phosphenes correlated with the size of the tilt aftereffect (TAE) in the PPR group only. This study provides further evidence of enhanced cortical excitability in subjects with photosensitivity, which is likely to reflect changes in excitatory neurotransmission.

Does photoparoxysmal response in children represent provoked seizure? Evidence from simultaneous motor task during EEG

Objectives Acute cognitive changes during epileptiform discharges have been studied using computer assisted cognitive tasks. We aimed to demonstrate acute behavioral change (using a simple motor response task MRT) during photoparoxysmal response (PPR) in children below 18 years. Methods Children performed a simple repetitive motor task during intermittent photic stimulation (IPS). All episodes of PPR not associated with obvious clinical change (as observed by the technologist or reported by the patient) were analyzed for this study. The average time interval between two successive motor responses across a PPR (test time) was compared to the average time interval between two successive motor responses during IPS not associated with PPR (control time) using Wilcoxon signed ranks test. Results 21 children who had PPR successfully completed the MRT. The difference between the mean durations was 0.894 s ( p = 0.002). More than 50% increase compared to the control time was considered a delay in MRT during PPR. 10 children showed slowing of MRT during PPR. Conclusion By definition, acute behavioral change during generalized epileptiform discharges represent provoked seizures. Detecting subclinical seizures can have important safety implications in children (skiing, skating and driving) with PPR on EEG, but no clinical seizures. We recommend MRT during IPS.

Gamma frequency SSVEP components differentiate children with febrile seizures from normal controls

Gamma band electroencephalography (EEG) abnormalities have been reported in patients with epilepsy. We aimed to investigate whether patients with febrile seizures (FS) show abnormalities of the gamma frequency steady-state visual evoked potential (SSVEP) components evoked by intermittent photic stimulation (IPS). We analyzed the magnitude and phase alignment of the 50-100 Hz SSVEP components elicited by IPS from 12 FS patients, 5 siblings of FS patients, and 15 control children between 6 and 36 months of age. Patients with FS showed significantly higher SSVEP magnitude and phase alignment values when compared to both the siblings and control groups. Detected abnormalities could either represent the direct consequence of seizures or indicate a preexisting tendency to hypersynchrony in FS patients. Future prospective studies could assess whether SSVEP abnormalities are associated with complex rather than simple FS, or have a prognostic value for the development of epilepsy following FS.

Photosensitive epilepsy and image safety

Photosensitive epilepsy came to prominence in the 1950s with the advent of television. Photosensitive epilepsy occurs in 1 in 4000 of the population. The incidence is 1.1 per 100,000 per annum, however amongst 7–19 year-olds the incidence is more than five times as common. Photosensitive epilepsy is twice as common in females as in males. The onset is around puberty, but less than 25 per cent of patients lose their photosensitivity in their twenties. Patients are investigated in the EEG laboratory using intermittent photic stimulation. Peak sensitivity is between 16 and 20 flashes/s but 49 per cent of patients are sensitive to 50 flashes/s, explaining the sensitivity to PAL television systems. From 1993 the development of broadcast guidelines was developed restricting both flash rates and the areas of screen involved, as well as the use of long-wavelength red. Automatic analysis systems can now test material for compliance with guidelines in real time.

EEG photic driving: Right-hemisphere reactivity deficit in childhood autism. A pilot study

In 14 autistic boys, aged 6–14 years, free of drug treatment, with relatively intact verbal functions and without severe or moderate mental retardation (I.Q. 91.4 ± 22.8), intermittent photic stimulation at 11 fixed frequencies of 3–24 Hz revealed latent deficiency of the right hemisphere in the photic driving reactivity, predominantly at the fast alpha and beta frequencies of stimulation. The left-side prevalence was observed: 1) in the total number of driving peaks evaluated for the first four harmonics in the EEG spectra of 14 cortical areas and 2) in the driving amplitude in the spectra of the 2 occipital areas. As compared to 21 normally developing boys matched on age who did not show interhemispheric asymmetry in the driving reactivity, the autistic patients had significantly lower driving characteristics only in the right hemisphere. There were no significant differences between the autistic and control groups in the spontaneous EEG spectra of the occipital areas in the resting state.

178. Variety in epileptiform patterns induced by intermittent photic stimulation

178. Variety in epileptiform patterns induced by intermittent photic stimulation

Estimulação fótica: padronização do método de acordo com o consenso europeu

INTRODUÇÃO: Embora a fotostimulação intermitente (FEI) seja rotineiramente utilizada nos laboratórios de eletrencefalografia (EEG), raramente é utilizada de forma padronizada. A FEI é um método de ativação utilizado no EEG de rotina que pode desencadear tanto respostas fisiológicas quanto potencialmente patológicas. Historicamente, o termo fotossensibilidade se refere às respostas anormais à estimulação com luz estroboscópica durante o registro do EEG. OBJETIVO: O objetivo desta publicação é revisar os aspectos diagnósticos do procedimento da FEI, baseados no encontro de consenso realizado em Heemstede na Holanda, em 1996, com o propósito de facilitar e padronizar a detecção de pacientes fotossensíveis.