IDENTIFICATION OF THE HEMISPHERE ACTIVATED BY HEMIFIELD VISUAL-STIMULATION USING A SINGLE EQUIVALENT DIPOLE MODEL

Topographic amplitude distribution of the hemifield pattern visual evo ked potential (PVEP) shows substantial intersubject variability. Many subjects have larger P100 amplitudes paradoxically over the hemisphere ipsilateral to the stimulated field, whereas others show larger respo nses over the stimulated hemisphere. The present study was designed to determine whether a single equivalent dipole model could correctly id entify the field of stimulation, and therefore the hemisphere activate d, under conditions in which the surface distribution is variable. Und er conditions used in the present study, visual examination of the sur face amplitude distribution of the P100 peak could not be reliably use d to identify the hemifield that was stimulated. Of 28 hemifield PVEPs , obtained from 14 normal subjects, only 13 showed higher amplitude ip silateral to the field of stimulation. Thus, neither examination of EP wave forms nor topographic maps provided an accurate means for determ ination of which hemifield was stimulated or which hemisphere was acti vated. The single equivalent dipole model correctly identified the sti mulated hemisphere for 25/28 hemifield PVEPs. Orientation of the equiv alent dipole accounted for much of the variability in surface amplitud e distribution, with tangential orientations obtained in subjects with ipsilaterally predominant P100 surface topography. Although the dipol e model improved identification of the stimulated hemisphere, dipoles were located anterior or inferior to the occipital lobe in some subjec ts. Results suggest that dipole modeling can provide useful informatio n regarding the source of surface recorded potentials.