Neural sources of focused attention in visual search

Previous studies of visual search in humans using event-related potentials (ERPs) have revealed an ERP component called 'N2pc' (180-280 ms) that refle cts the focusing of attention onto potential target items in the search arr ay. The present study was designed to localize the neuroanatomical sources of this component by means of magnetoencephalographic (MEG) recordings, whi ch provide greater spatial precision than ERP recordings. MEG recordings we re obtained with an array of 148 magnetometers from six normal adult subjec ts, one of whom was tested in multiple sessions so that both single-subject and group analyses could be performed. Source localization procedures reve aled that the N2pc is composed of two distinct neural responses, an early p arietal source (180-200 ms) and a later occipito-temporal source (220-240 m s). These findings are consistent with the proposal that parietal areas are used to initiate a shift of attention within a visual search array and tha t the focusing of attention is implemented by extrastriate areas of the occ ipital and inferior temporal cortex.