TOPOLOGY OF THE VISUALLY EVOKED POTENTIAL

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The relationship between the scalp topology of the visually evoked potential (VEP) and the stimulus paradigm that causes it will be studied using a new method that allows noninvasive localization of active cortical areas in humans with a resolution of about 1 cm. The method is based on estimating the Laplacian of the VEP field on the scalp by recording second differences of the potential field along orthogonal directions from a multielectrode array. The following problems of basic neurophysiological interest will be addressed: (1) cortical localization of patterned stimuli presented to different parts of the visual field (retinotopic map of the cortex). (2) cortical localization of activity subserving visual object recognition. The retinotopic map will be used to approach several related clinical problems: (1) the selection of electrode placements to discriminate foveal signals in the VEP to wide field stimuli as an aid in the evaluation of patients with opaque media, infants, brain-damaged and mentally retarded patients, (2) the selection of electrode placements to discriminate signals from visual field zones 10 to 25 degrees from fixation as an aid to the objective detection and monitoring of field defects in glaucoma, (3) the objective measurement of eccentric fixation in amblyopia and (4) the objective detection of hemianopic and quadrantanopic field defects in neuroophthalmological problems.