Unattended distractor-induced priming in a visual selective attention task - N400 effects in the absence of RT effects

Event-related potentials (ERPs) were recorded from 24 subjects in a visual selective-attention task in which two words were each presented both in a prime display and in a probe display with the target word defined by color. Subjects' task for the prime display was a physical one (target word presented in upper or lower case letters), while the probe display task was a lexical decision. In addition to a neutral condition, four conditions were realized by varying the target probe: The target probe word was (1) a repetition of the target prime (attended repetition), (2) a semantically associated word to the target prime (attended semantic), (3) a repetition of the distractor prime (unattended repetition), or (4) a word semantically associated to the distractor prime (unattended semantic). An attended semantic and an attended repetition priming effect was observed by means of both RT and N400. The N400 differentiated between these two attended priming conditions while RT did not. No unattended priming effects were found with behavioral data. The N400 amplitude modulation, however, was also present for unattended priming but was attenuated compared to the attended condition. The data suggest (1) that automatic processes are sufficient to evoke an N400 effect, and (2) that the N400 effect is a more sensitive indicator for priming effects than response times.