Monocularly and binocularly evoked visual responses to patterned half-field stimulation

Visual evoked responses to half-field stimulation were recorded in 18 normal subjects to determine the most suitable techniques for use in detecting hemianopic deficits. The most consistent results were obtained by recording from a transverse array of electrodes using the midfrontal region as reference and stimulating with a reversing checkerboard pattern, each check subtending 50 min of visual angle. Responses recorded from electrodes ipsilateral to the half-field stimulated consisted of an N70, P105, N165 complex. Responses from contralateral electrodes demonstrated the presence of an N100 which divided the P105 seen ipsilaterally into a P85 and a P120. The contralateral predominance of the N100 was further documented by plotting the ratio of the N100 to the immediately preceding P wave. Consideration of the ipsilateral P105 and the contralateral N100 together was found to be more reliable in detecting lateralization of the visual evoked potential than either wave alone. Crossed and uncrossed pathways made equal contributions to the visual evoked potential produced by half-field stimulation. Good correspondence was found between algebraic summation of homonymous monocular hemifield responses when compared to binocular hemifield stimulation. This finding supports the use of binocular hemifield stimulation for the detection of subtle congruous homonymous field defects.