Visual evoked potentials and selective attention to points in space

Visual evoked potentials (VEPs) were recorded to sequences of flashes delivered to the right and left visual fields while subjects responded promptly to designated stimuli in one field at a time (focused attention), in both fields at once (divided attention), or to neither field (passive). Three stimulus schedules were used: the first was a replication of a previous study (Eason, Harter, & White, 1969) where left- and right-field flashes were delivered quasi-independently, while in the other two the flashes were delivered to the two fields in random order (Bernoulli sequence). VEPs to attended-field stimuli were enhanced at both occipital (O₂) and central (C_z) recording sites under all stimulus sequences, but different components were affected at the two scalp sites. It was suggested that the VEP at O₂ may reflect modality-specific processing events, while the response at C_z, like its auditory homologue, may index more general aspects of selective attention.