Horizontal scalp distribution of steady-state pattern visual evoked cortical potentials in response to quadrant field stimulation.

Scalp profiles of steady-state pattern visual evoked cortical potentials (VECPs) along the horizontal line at Pz (for upper field stimuli) and Oz (for lower field stimuli) were studied in 12 normal subjects by simultaneous five channel recordings from electrode positions 3 cm and 6 cm laterally on either side of, and at, Oz or Pz. Each electrode was referred to Fz or to the right earlobe. The amplitude and phase of the averaged VECPs (n = 20) were calculated by means of Fast Fourier Transform. With smaller check size the amplitude data verified that the scalp profile of the lower visual field peaked at the hemisphere contralateral to the quadrant stimulated. With larger check size the peak shifted to the ipsilateral hemisphere. With upper field quadrant stimulation, position of the reference electrode at Fz was found to be unsuitable since the VECP amplitude at the level of Pz was greatly reduced. Phase and apparent latency also varied with the field stimulated as well as with the position of electrodes.