Trajectories of shifting dipole sources of visual evoked potentials across the human brain.

Visual evoked potentials in response to images of a set of horizontal and vertical lines or crosses were recorded from the brains of 18 human subjects in 34 leads. Inverse EEG analyses were used for the dynamic location of the dipole current sources of the N1, P1, and N2 waves using a two-dipole spherical model with a 1-msec step. The occipital lobes of all subjects showed significant displacement of the dipoles of evoked potential waves along predominantly arc-shaped trajectories (75.8% of cases). Trajectory durations (average about 25 msec) were characterized by insignificant interindividual variability and were independent of the type of stimulus and the phase of the evoked potential. A characteristic (occurring in 85% of cases) "jump" in the coordinates of the dipole, which constituted a rapid, sharp, and significant medial displacement, was seen between the first and second trajectories of the equivalent current dipoles (at 110-120 msec after stimulus onset). The possible significance of these data for understanding the dynamics and kinetics of processing of local image features in the human visual cortex is discussed.