Shapes of direct cortical responses vs. short-range axono-cortical evoked potentials: The effects of direct electrical stimulation applied to the human brain

•Electrical stimulation in white matter induces delays in the evoked response due to slow conduction velocity.•The waveforms from white matter and cortical stimulation remain generally identical.•Responses to white matter and cortical stimulation differ on response times.•The relaxation of the N1 component is longer during cortical stimulations.•There is probable activation of intra-cortical axons during cortical stimulation. Direct cortical responses (DCR) and axono-cortical evoked potentials (ACEP) are generated by electrically stimulating the cortex either directly or indirectly through white matter pathways, potentially leading to different electrogenic processes. For ACEP, the slow conduction velocity of axons (median ≈ 4 m.s−1) is anticipated to induce a delay. For DCR, direct electrical stimulation (DES) of the cortex is expected to elicit additional cortical activity involving smaller and slower non-myelinated axons. We tried to validate these hypotheses. DES was administered either directly on the cortex or to white matter fascicles within the resection cavity, while recording DCR or ACEP at the cortical level in nine patients. Short but significant delays (≈ 2 ms) were measurable for ACEP immediately following the initial component (≈ 7 ms). Subsequent activities (≈ 40 ms) exhibited notable differences between DCR and ACEP, suggesting the presence of additional cortical activities for DCR. Distinctions between ACEPs and DCRs can be made based on a delay at the onset of early components and the dissimilarity in the shape of the later components (>40 ms after the DES artifact). The comparison of different types of evoked potentials allows to better understand the effects of DES.