Dipole Tracing Examination for the Electric Source of Photoparoxysmal Response Provoked by Half‐Field Stimulation

Abstract

Purpose: Dipole tracing (DT) is a computer‐aidcd noninvasive method used to estimate the location of epileptic discharges from the scalp EEG. In DT equivalent current dipoles (ECDs), which rcflcct the electric source in the brain, are rcsponsible for the potential distribution on the scalp EEC. Thercfore, the DT method is useful to estimatc the focal paroxysmal discharges. In this study we examined the location of the clectric source of photoparoxysmal response (PPR) using scalpskull‐brain dipolc tracing (SSB‐DT) after hal[‐field stimulation, which produced focal PPR on the scalp EEG. Methods: We studied 4 cases of photoscnsitive epilepsy. Wc performed 20 Hz red flicker and flickcring dot pattern half‐ficld stimulation to provoke PPR. In this method, the loci of gcnerators corresponding to the paroxysmal discharges were estimated as ECDs by I ‐ and 2‐dipole analyses. Each location of the ECDs was estimated by iterative calculation. Algorithms minimizing the squarcd difference betwccn the electrical potentials recorded from the scalp EEG and those calculated theoretically from the voluntary dipoles were uscd. In the SSB model, the scalp shell was reconstructed from the helmet mcasurements, and the shapc of the skull and brain was 3‐dimcnsionally reconstructed from CT images. A dipolarity larger than 98% w the accuracy of the estimation. We recorded thcir 2 I channel monopolar scalp EEG. Each spike was sampled analyzed at 10 points around the peaks of at least 10 spikes in each patient using the SSB‐DT method. The ECDs were then supcrimposed on thc MRI of each palient to idcntify the more cxact anatomical region. Results: This study showed the location of cach focus and a dipolarity of greater than 98% in all cases, although the results from the 2‐dipole method showed scattered location. We considered that the analyzed signals were generated from single source. PPR was elicitcd cross‐lateral to the field stimulated. By red flicker half‐field stimulation, EEG revealed eithcr focal spikes and waves in the contralatcral occipital, temporo‐occipitel region, or diffuse spikes and wave complex bursts, sccn dominantly at the contralateral hcmisphere. The supcrimposed ESDs on MRI were located at the occipital or inferior temporal lobe. PPR, provoked by flickering dot pattern half‐field stimulation, werc focal spikes and waves, mainly in the occipital, parieto‐occipital region, or diffuse spikes and wave complcx bursts, seen dominantly at thc contralateral hcmiaphere. The ECDs of their PPRs were located in the occipital, inferior temporal, or inferior pirietal lobules on MRI. Conclusion: Our findings suggest that the inferior temporal and inferior parictal lobules which are important for the processing sequence of the visual system in addition to the occipital lobc, might he responsible for thc mechanism of PPR by half‐ficld stimulation, espccially for electric source expansion.