LEFT-RIGHT ASYMMETRY OF VISUAL-EVOKED POTENTIALS IN BRAIN-DAMAGED PATIENTS - A MATHEMATICAL-MODEL AND EXPERIMENTAL RESULTS

The left-right asymmetry in the potential amplitude on the scalp was s tudied in poststroke patients by using flash visual evoked potential ( VEP) and a numerical two-dimensional model of the head. The left-right asymmetry of the VEP was measured in three patients after thrombosis, in one after hemorrhage, and in one healthy subject. The numerical mo del used computed tomography images to define the different compartmen ts of the head. The volume conductor equation for the potential distri bution created by a dipole source in the occipital region was solved n umerically with use of a finite volume method: Left-right asymmetry wa s calculated with several values of conductivity of the damaged region . The experimental results revealed a negative asymmetry in the three patients after thrombosis (i.e., the potential amplitude over the isch emic hemisphere was smaller than that over the intact hemisphere), whe reas., in the patient after hemorrhage, a positive asymmetry was found . Nonsignificant left-right asymmetry was found in the healthy subject . The numerical model revealed that the electrical conductivity of the damaged tissue has a major effect on the left-right asymmetry. Negati ve asymmetry, such as that found for patients after thrombosis, was ob tained when the conductivity of the damaged region was greater than th at of the brain, whereas positive asymmetry (hemorrhage patient) was o btained when that conductivity was smaller than that of the brain. Thi s finding indicates that the left-right asymmetry in the scalp VEP of patients after brain damage may be a result of changes in the conducti vity of the volume conductor (the ischemic region) between the source and the electrodes.