HUMAN CORTICAL-EVOKED FIELDS DURING DETECTION, LOCALIZATION, AND IDENTIFICATION OF POP-OUT TARGETS

We investigated human cortical activity during four 'effortless-pop-ou t' visual search tasks with the use of magnetoencephalography. The sea rch display, which was identical across all the tasks, consisted of ve rtical line segments, one of which was rotated abruptly 45 degrees clo ckwise or counterclockwise. In the passive-viewing task the observers gave no response to the search display. In the target-detection task t hey responded to the onset of the target motion irrespective of its lo cation and direction. In the target-localisation task the observers re ported whether the line rotation appeared above or below the fixation point while ignoring the direction of the rotation. In contrast, in th e target-identification task they indicated the direction of the line rotation, and the location of the rotation in the array was irrelevant . Cortical activity was recorded with a whole-scalp magnetometer while the observers were performing each task. In addition to the expected activation of the occipital and somatomotor cortical regions, two othe r active cortical areas were consistently identified in both hemispher es: one in the occipito-temporal area, probably corresponding to the m otion-specific V5 complex, and another in the parieto-temporal region. The activation of the right occipito-temporal source depended on the task. The maximum amplitude was smallest for the passive viewing, incr eased for the detection task, and was largest for the localisation and identification.