Averaging in time-frequency domain reveals the temporal and spatial extent of seizures recorded by scalp EEG

Abstract

To demonstrate the importance of averaging in time-frequency space and the added localizing value in time and space in a case of cortical myoclonus. One hundred myoclonic jerks were averaged in time series and in spectral domain. For the latter, we chose 100 (10-second) segments from interictal background and used the unpaired t-test for the jerk-related and control spectral data to obtain the t value and corresponding p value at each pixel. We corrected for multiple comparisons using false discovery rate procedure. We generated maps of spectral significance per electrode. All insignificant t-values were converted to 0 for easier visual analysis. Standard back-averaging of 100 jerks disclosed a single spike preceding EMG activity by 19-27 milliseconds. No other definite ictal patterns were discernible in the time domain. Statistical analysis of the same 100 epochs in the time-frequency domain disclosed a greater temporal extent of the seizure, as well as a more detailed rendering of rhythms and frequencies involved. Valproate was added and led to substantial improvement. Averaging in the spectral domain may reveal frequency-specific changes that may not be otherwise appreciated in the time domain. Future studies may elucidate the effect on the sensitivity of diagnosis of simple partial seizures and auras.

Keywords: cortical myoclonus; localization; myoclonic jerk; seizure; space; statistical spectral averaging; time.