Visual evoked cortical magnetic responses to checkerboard pattern reversal stimulation: A study on the neural generators of N75, P100 and N145

In an attempt to elucidate the neural generators of pattern reversal visual evoked potentials (PR-VEPs), we measured the visual evoked magnetic fields (PR-VEFs) using a 37-channel magnetoencephalography in six healthy young adults. A half-field checkerboard pattern was phase-reversed at a rate of 1 Hz to stimulate the right or left visual half-field, thus yielding 12 PR-VEFs in total from the six subjects. The simultaneously recorded scalp PR- VEPs showed three distinct components of N75, P100 and N145. Three corresponding components were also identified in the PR-VEFs with similar peak latencies (N75m, P100m and N145m). P100m and N145m were clearly identified in all 12 PR-VEFs, whereas N75m was observed in only nine of 12 PR-VEFs. The equivalent current dipoles (ECDs) of N75m, P100m and N145m were located closely to each other in the occipital cortex around the calcarine fissure contralateral to the stimulated visual field, when they were overlaid on the MRI. The reliability of dipole estimation was highest in P100m, followed by N145m while N75m showed the least reliability. The direction of the current flow of ECDs of N75m and N145m was from the medial to the lateral in the occipital cortex when viewed in a coronal section, whereas that for P100m was toward the medial. The ECD location of P100m changed according to the retinotopic organization when the upper or lower quadrant of the visual field was stimulated, with the ECDs being located in the lower or upper part, respectively, of the visual cortex. Our results therefore indicate that the neural origins of N75m, P100m and N145m of PR-VEFs are in the primary visual cortex on the contralateral side of the stimulated visual half-field, while the three components are physiologically distinct.