Short-range axono-cortical evoked-potentials in brain tumor surgery: Waveform characteristics as markers of direct connectivity.

• During brain surgery, the measurement of axono-cortical evoked potentials can allow exploring the neural connectivity on-line. • Electrical stimulation on the white matter tracts were applied while recording the evoked response at the cortical level. • The 'hottest' spot with a direct connection to the stimulation site can be distinguished from the neighboring activity. Intraoperative measurement of axono-cortical evoked potentials (ACEP) has emerged as a promising tool for studying neural connectivity. However, it is often difficult to determine if the activity recorded by cortical grids is generated by stimulated tracts or by spurious phenomena. This study aimed to identify criteria that would indicate a direct neurophysiological connection between a recording contact and a stimulated pathway. Electrical stimulation was applied to white matter fascicles within the resection cavity, while the evoked response was recorded at the cortical level in seven patients. By analyzing the ACEP recordings, we identified a main epicenter characterized by a very early positive (or negative) evoked response occurring just after the stimulation artifact (<5 ms, |Amplitude| > 100 µV) followed by an early and large negative (or positive) monophasic evoked response (<40 ms; |Amplitude| > 300 µV). The neighboring activity had a different waveform and was attenuated compared to the hot-spot activity. It is possible to distinguish the hotspot with direct connectivity to the stimulated site from neighboring activity using the identified criteria. The electrogenesis of the ACEP at the hotspot and neighboring activity is discussed. [ABSTRACT FROM AUTHOR]