Subregions of human MT complex revealed by comparative MEG and direct electrocorticographic recordings

Objective: To locate the visual motion complex (MT+) and study its response properties in an epilepsy surgery patient. Methods: A 17-year-old epilepsy patient underwent invasive monitoring with subdural electrodes in the right temporo-parieto-occipital area. MT+ was investigated by cortical electric stimulation and by epicortical visual evoked potentials time-locked to motion onset of sinusoidal gratings (motion VEP). Motion-related visual evoked magnetic field (motion VEF) was also recorded before the electrode implantation to complement the invasive recording. Results: Motion VEPs revealed two subregions within MT+, generating early and late potentials respectively. The early activity with a peak around 130 ms was localized at a single electrode situated immediately caudal to the initial portion of the ascending limb of the superior temporal sulcus (AL-STS). The late activity, peaking at 242-274 ms, was located ventro-rostrally over three electrodes. Among the four electrodes with motion VEPs, cortical stimulation at the most caudal pair elicited motion-in-depth perception involving the whole visual field. In addition to two subregions revealed on the gyral crown, magnetoencephalography (MEG) demonstrated another subregion with a late motion VEF in AL-STS immediately rostral to the electrode with the early motion VEP. Conclusions: In combination with MEG recording, the present invasive exploration demonstrated human MT+ in a focal area of the temporo-parieto-occipital junction and delineated possible three subregions as indicated by the different latencies and distributions of the motion VEP/VEFs. Significance: Comparative MEG and direct electrocorticographic recordings delineated possible subregions within the human MT complex.