SPATIAL SELECTIVE ATTENTION AFFECTS EARLY EXTRASTRIATE BUT NOT STRIATE COMPONENTS OF THE VISUAL-EVOKED POTENTIAL

The effects of spatial selective attention on sensory processing in vi sual cortical areas were investigated by means of visual evoked potent ial (VEP) recordings and source localization techniques. Patterned sti muli were rapidly presented in random order to the left and right visu al fields while subjects maintained central fixation and attended to o ne visual field at a time. Attended stimuli evoked enhanced P1 (100-13 0 msec) and N1 (120-200 msec) components of the VEP, whereas no effect s of attention were observed on the C1 (50-100 msec) or P2 (200-240 ms ec) components. Spatiotemporal dipole modeling of the early VEP source s was carried out in relation to MRI-defined cortical anatomy. The dip olar generator of the CI component was found to lie in calcarine corte x, the human homologue of area Vl,whereas the attention-sensitive P1 g enerator was localized to ventral-lateral occipital cortex, within ext rastriate area 19. These results support the hypothesis that spatial a ttention does not affect the initial activity evoked in area V1 but ra ther produces an enhancement within extrastriate visual areas of senso ry signals arising from stimuli at attended locations.