Surface distribution of steady-state cortical potentials evoked by visual half-field stimulation.

The surface distribution of steady-state cortical potentials (VECP) evoked by visual half-field stimulation was analyzed by using the fast Fourier transform (FFT). For all ten normal subjects, checkerboard stimulation of the lower visual half-field evoked the largest response at electrode location Oz, with stimulation for the upper visual half-field at Pz. Recording at Pz, a phase difference of approximately 180 degrees was observed for cortical potentials evoked by lower and upper half-field stimulation, the response being bigger for upper field than for full-field stimulation. Comparing the VECP elicited by stimulation of the right and the left half-field, six of ten subjects displayed the highest amplitude at the contralateral cortex (anatomical type), three subjects at the midline (central type), and one subject at the ipsilateral cortex (paradoxical lateralization type). At Oz there was little phase difference in the VECP with stimulation of the right and left visual half-field, and the amplitude was about half that for full-field stimulation. However, with the electrode placed at the temporal region of the skull there was a big phase difference of VECP signals; consequently, the sum of the amplitudes did not amount to the amplitude for full-field stimulation. The dipole theory of scalp distribution of VECP signals was found to be also applicable to conditions of steady-state stimulation, including polarity reversal for upper and lower visual half-field stimulation.