Abstracts

Rationale: Scalp EEG recording consists of electrical circuits to detect AC generators through the skull table which have electrical resistivity and capacitance which work as low pass filter (LPF) which should have explicit cut-off frequency according with the frequency response characteristic of the circuit. Therefore, the upper limit frequency should be limited by the electrical property. The purpose of the study is to demonstrate the cut-off frequency in the SR-HF-EEG to estimate CR constant by =1/CR, f= /2 .Methods: Experimentally determined values of skull bone conductivities are available in multiple studies. These values have significant variation from 0.006 S/m to 0.015 S/m (Geddes, 1967; Konsterich, 1983; Oostendorp, 2000). The resistivity of the skull is defined as the reciprocal of conductivities. The resistance of skull table immediately below individual 10 mm diameter electrodes can be calculated using cylindrical model in which the length of the conductor is equal to the thickness of the skull. Data of the skull thickness is taken as 6 mm in average (Gonzalez A, 2006). Using these parameters, the variation range of skull resistance was calculated 5.1 k-Ohm to 12.7 k-Ohm. Two ictal EEG recordings out of sleep state were manually analyzed. The EEGs were recorded using standard 10-20 system with 10mm scalp electrodes with sampling frequency of 1000Hz. The seizure onset was characterized with appearance of sustained SR-HF-EEG. Results: EEG Case 1: This was electrographic subclinical seizure. The onset was detected by 120-125 Hz sustained HF originating from Fp1. The frequency was highest at onset and rhythmic epileptiform property has been maintained until disappeared in 19.8 sec. Simultaneous Eye leads ruled out EM artifacts. CR constant was estimated as 1.27-1.33x10-3 when cut-off frequency is 120-125 Hz EEG Case 2: Sustained electrographic rhythmic discharges originating from F4 with frequency of 120Hz, which resulted into secondary generalization. At the onset, sporadic ultra-high gamma discharges about 300Hz were seen immediately before the onset of sustained 120Hz electrographic HF at F4. This patient underwent right frontal lobe resection with good outcome. CR constant was estimated as 1.3x10-3 and capacitance, C=0.1 uF as cut-off frequency was 120Hz. If 300Hz is used for cut-off frequency, C=0.04 uF. Sporadic fast ripple may occur by fluctuation of the capacitance while bone resistance stays constant.Conclusions: SR-HF-EEG as seen in the ictal EEG assisted determining the CR value and capacitance of skull table. This is the first attempt to explain availability of SR-HF-EEG less than the frequency between ripples (>80 Hz) and fast ripples (>250 Hz), that may be the cut-off frequency determined by the brain-skull CR circuit. Cut-off frequency increases if CR decreases. Sporadic recording of fast ripple HF may be attributable to local fluctuation of capacitance by move of CSF, blood flow or other nonspecific local phenomena.