Abstracts

Rationale: Cortico-cortical evoked potentials (CCEPs) are brain responses to direct cortical stimulation, used to study effective connectivity between regions. They help map functional networks in epilepsy surgery patients. However, it remains unclear whether interhemispheric CCEP responses differ by stimulation direction. This study found asymmetries in amplitude and latency, particularly in the early P1 and N1 components, by stimulating one frontal hemisphere and recording from the opposite side to compare response strength and timing.

Methods: ECoG data were collected from four epilepsy patients undergoing clinical monitoring at two hospitals in Japan. Subdural electrode grids were implanted over both frontal cortices based on clinical needs. CCEPs were analyzed during two interhemispheric stimulation paradigms: Left-to-Right (stimulating left, recording right) and Right-to-Left (stimulating right, recording left). Electrical stimulation consisted of constant-current, monophasic square wave pulses (5 mA, 0.3 ms duration) delivered at 1 Hz. Stimulus polarity was alternated to minimize stimulus artifacts. Stimulation triggers were synchronized using transistor-transistor logic (TTL) pulses. CCEPs were extracted by calculating the mean of 60 trials time-locked to the stimulus onset. Each trial included a baseline period of −100 ms and a post-stimulus period of 500 ms. For peak analysis, the early positive (P1) and negative (N1) deflections of the CCEP waveform were identified in each condition. The amplitude and latency of P1 and N1 were extracted and compared between the two stimulation directions.

Results: Over four patients, 318 CCEPs were identified for the Left-to-Right stimulation condition and 422 for the Right-to-Left condition. Significant differences were observed in both amplitude and latency of the early CCEP components (P1 and N1) between the two stimulation directions. The mean P1 amplitude was significantly lower in the Left-to-Right condition (93 ± 67 µV) compared to the Right-to-Left condition (127 ± 85 µV; p < 0.005). Similarly, the mean N1 amplitude was significantly more negative in the Left-to-Right condition (−124 ± 88 µV) than in the Right-to-Left condition (−101 ± 65 µV; p < 0.005).  Latency analysis also revealed significant asymmetries. The P1 latency was shorter in the Left-to-Right condition (19.53±7 ms) compared to the Right-to-Left condition (27.6±5 ms; p < 0.005), and similarly, the N1 latency was earlier in the Left-to-Right direction (34.3±6 ms) than in the Right-to-Left direction (41.0±7 ms; p < 0.005). These findings indicate consistent interhemispheric asymmetries in the strength and timing of CCEP responses.