Moving potential field of frontal midline theta activity during a mental task

The movement of potential field (potential flow) of frontal midline theta activity (Fm theta) was examined by using optical flow detection techniques in image processing in 6 normal subjects. Electroencephalograms (EEGs) over the fronto-central region were recorded from 13 electrodes around Fz. The potential flow of Fm theta was estimated on a frame which consisted of a square grid of 19 x 19 points in which each of the 13 electrodes was positioned with Fz at the center of grid. The peak-to-trough amplitude distribution was rounded or oval with a maximum just anterior to Fz. In each hemisphere, potential field moved medially in a region anterior to Fz, posteriorly near or at the midline and laterally in a region posterior to Fz. In the temporal sequence of potential field, at each point velocity directions tended to change progressively clockwise or counterclockwise. In the whole frame, a rotary movement of potential flow was found with a center at the middle on each side. Our findings suggested that a potential maximum at the midline may be due to overlapping of fields of opposite direction over the frontal scalp, arising from sources of similar orientation in both hemispheres. It can be assumed that Fm theta propagates in a clockwise direction in the left hemisphere and in a counterclockwise direction in the right hemisphere.