Effect of brain damage and source location on left–right asymmetry of visual evoked potentials in a realistic model of the head

The left–right asymmetry of visual evoked potentials (VEP) was studied using a realistic three-dimensional model of the head, constructed from CT scans. The integral equation describing the potential distribution due to an inner current-dipole (simulating the VEP source), inside the biological volume conductor (the head) was solved numerically using the finite volume method. The effect of several structural parameters, such as the natural head geometry, the location and orientation of the current source, and conductivity changes (simulating a brain damage) on the VEP asymmetry was examined. The results revealed that the natural anatomical asymmetry presented in a normal head induces some degree of VEP asymmetry (up to 0.42% at the occipital electrodes), yet the major cause of left–right asymmetry is due to asymmetrical location of the source: up to 6.53% in the O2–O1 electrodes for an angular shift of 3° to the left. It was also found that conductivity changes inside the head have a smaller effect on the VEP asymmetry (up to 3.35% in hemorrhaged brain compared to 1.96% in the normal brain at the C4–C3 electrodes). These findings may help in better understanding VEP sources of asymmetry, originating not only due to cerebral functionality, but from structural parameters as well.