Mapping of evoked magnetic field with visual stimulation: a secondary projection and processing area

Visual evoked fields (VEFs) were measured in four subjects, using magnetoencephalography (MEG) and a flashed checkerboard pattern stimulation with a duration of 20 ms. The left hemisphere was searched for the primary and the secondary projection areas with a full-field stimulus. Typical VEFs were found in the occipital area, which is considered to be the primary projection; and a secondary projection area was found in the dorsal temporal area in all subjects. The negative VEF in the secondary projection area was robust and stable, with a range of latencies from 200 to 230 ms. The amplitude was more than -400 femtotesla (fT), as high as the VEF in the occipital area. However, we were unable to localize the position of a dipole, since only one amplitude maxima was found.