Temporal variability of the N2pc during efficient and inefficient visual search.

Efficient and inefficient visual search are characterized by the difference in the time required to find the target. Efficient "popout" search time is relatively unaffected by increases in the number of search items, whereas inefficient "non-popout" search time requires more time increases in duration. Electrophysiological investigations of the neural correlates of visual search have revealed a component of the event-related potential (ERP) known as the N2pc. The N2pc is thought to reflect the orienting of attention to the target during visual search. If the N2pc is more closely associated in time with the moment of target selection than the onset of the search display, it may be predicted that the N2pc would be delayed or more temporally variable during inefficient compared to efficient visual search. In the present study, we contrasted efficient "popout" search with inefficient "non-popout" search to investigate the hypothesis that the N2pc is temporally associated with the moment of search completion, and this would be reflected as a delayed or attenuated N2pc during non-popout search compared to popout. In Experiment 1, we observed a robust N2pc during popout search, but not during non-popout search. However, sorting trials by the N2pc latency recovered the N2pc during non-popout search. In Experiment 2 we replicated this observation and demonstrated that popout and non-popout search reflected differences in the temporal variability of the N2pc latency. Further investigation using time-spectral analysis suggests that evoked posterior-contralateral theta (4-6 Hz) underlies the N2pc during both popout and non-popout search.