Abstracts

The term [ldquo]synchronization[rdquo] means significant matching between two oscillatory activities. EEG (brain wave) synchronization has been used to investigate various types of brain function such as memory coding, learning, attention, cognition, and epileptic seizures. Several lines of evidence from animal studies suggest that increased neuronal synchronization occurs either locally or between two distant functional brain areas during object perception and/or motor action. One study with human intracranial EEG showed increased coherence between the primary motor and supplementary sensori-motor cortices during voluntary movements. The role of synchronization in memory or learning function is well studied in both animal and human subjects. A number of studies with human scalp EEG revealed global increase of synchronization during many types of cognitive tasks. However, no previous studies systematically investigated synchronization during language process using intracranial human EEG. Two patients who underwent chronic implantation of subdural electrodes on the left hemisphere participated in the study. Subdural EEG was recorded during resting condition, during language task, and during pseudo-language task. The language task included passive listening of English speech and word generation task. The pseudo-language tasks included passive listening of the time reverse play of the English speech. The duration of each task was minimum 15 seconds.
The analysis was performed for three different frequency bands (10-30Hz, 30-50Hz, 50-70Hz). The [ldquo]phase[rdquo] of the EEG waveform on each channel was calculated using the Hilbert transform. Continuous trend of the phase was derived by unwrapping the phase cycle. Then, the difference of the phase trend between any pairs of electrode was calculated by subtraction. Highly persistent synchronization between the two channels was expressed as a stable or invariable time-course of the phase difference (low variance). A low degree of synchronization, on the other hand, was expressed as an unstable or highly variable course of the phase difference (high variance). Mean values of pair-wise synchrony were generally higher during the language task than during the resting or pseudo-language task. Significant synchronization occurred regionally between distant electrode pairs, but rarely between nearest neighbor pairs. It was possible to identify several electrodes as [ldquo]key nodes[rdquo] that have significant synchronization divergently to multiple electrodes located in a target area. Phase synchronization analysis revealed the EEG changes specific to the language process. The long-range synchrony occurs in the area wider than the presumed [ldquo]language[rdquo] area which is typically mapped with direct cortical stimulation or functional MRI. The synchronization analysis is useful to elucidate dynamic aspects of the brain function.