Brain activities immediately after finding rare targets

It is generally assumed that the prior knowledge about the frequency of the target occurrence would influence the identification of a stimulus. Nevertheless, whether this influence occurs immediately after finding a rare target is unknown as yet. To clarify this issue, Event-related brain potentials were recorded in the present study while subjects were presented with thousands of pictures and were required to indicate whether a categorical target such as a tool was in the display. The results showed that participants responded more quickly to the posttarget nontargets (PNTs, i.e. the nontargets that immediately followed the rare targets) than to the common nontargets (CNTs, i.e. the nontargets that did not immediately follow the rare targets). Compared with CNTs, PNTs elicited enhanced amplitudes at N1, N2, and parietal P2 components. Moreover, the N2 latency was longer overall for PNTs than for CNTs. These results suggested that the prior knowledge did not affect observer's judgment immediately; instead, the event of finding a rare target strengthened the processing of PNTs.