Abstracts

Our objective was to study changes in phase synchronization and EEG power spectral densities (PSDs) during covert object naming tasks in subjects with epilepsy. Noninvasive lateralization or localization of essential language areas using high density EEG recordings for presurgical planning may eventually be possible with use of these techniques., The trials required each subject to provide the name of 80 common objects presented every 4 sec on slides. Each trial data consisted of 1.0 second before and 3.0 seconds after initial object. EEG data of four subjects with refractory epilepsy was acquired during the covert object naming tasks while patients were recorded using a 256 electrode array. The EEG data was analyzed in frequency bands of 25-30 Hz, 30-35 Hz, 35-40 Hz, 40-45 Hz, 45-50 Hz. The data were filtered in each band with a Remez band-pass filter. The power spectra densities (PSDs) for baseline and during tasks were calculated after taking the Fourier transform of the data. The PSDs were averaged over trials.
The Hilbert transform was applied on the pairs of EEG traces (3 seconds length) with a sliding window of 80 ms duration which is long enough to encompass several cycles of EEG activity. The analysis was repeated by stepping the window at 10 ms intervals. The synchronization and de-coherence indices were computed for each pair of EEG traces. An averaged synchronization index for each channel was computed by pairing with the nearby six channels. Contour plots of the PSDs and synchronization indices were constructed using a montage of the layout of the 256 channel electrode positions., The contour plots reveal that PSDs during object naming were 2-3 times higher than the baseline values for all frequency bands in all four patients. Lateralization and localization of focal regions of maximal PSDs to left hempshere was observed. Coincident to changes in PSD was the observation that the phase synchronization was lowered during the task as compared to the baseline., These data suggest that PSDs and the phase synchronization of EEG changes during object naming tasks, and that these changes can be localized using high density EEG recordings. It can be stipulated that the phase synchronization between different regions of the brain is destroyed, at least transiently, when the language task of naming is presented.,