Dynamics of magnetic cortico-cortical responses evoked by single-pulse electrical stimulation

Intracranial single-pulse electrical stimulation (SPES) can elicit cortico-cortical evoked potentials. Their investigation with intracranial EEG is biased by the limited number and selected location of electrodes, which could be circumvented by simultaneous non-invasive whole-scalp recording. This study aimed at investigating the ability of magnetoencephalography (MEG) to characterize cortico-cortical evoked fields (CCEFs) and effective connectivity between the epileptogenic zone (EZ) and non-epileptogenic zone (i.e., non-involved [NIZ]).

A total of 301 SPES trains (at 0.9 Hz during 120 s) were performed in 10 patients with refractory focal epilepsy. MEG signals were denoised, epoched, averaged, and decomposed using independent component analysis. Significant response deflections and significant source generators were detected. Peak latency/amplitude were compared between each different cortical/subcortical structure of the NIZ containing more than five SPES, and then between the EZ and corresponding brain structures in the NIZ.

MEG detected and localized polymorphic/polyphasic CCEFs, including one to eight significant consecutive deflections. The latency and amplitude of CCEFs within the NIZ differed significantly depending on the stimulated brain structure. Compared with the corresponding NIZ, SPES within the extratemporal EZ demonstrated delayed CCEF latency, whereas SPES within the temporal EZ showed decreased CCEF amplitude. SPES within the EZ elicited a significantly higher rate of CCEFs within the stimulated lobe compared with those within the NIZ.

This study reveals polymorphic CCEFs with complex spatiotemporal dynamics both within the NIZ and EZ. It highlights significant differences in effective connectivity of the epileptogenic network. These cortico-cortical evoked responses could thus contribute to increasing the yield of intracranial recordings.