In a selective attention paradigm, participants are asked to respond to target items infrequently embedded in a stream of non-target items in an attended channel as they ignore all the items in an unattended channel. Non-target stimuli also may vary on other dimensions -- e.g., contain meaning relations between consecutive items -- and the question is the extent to which semantic processing is impacted by where attention is directed. Initial studies showed that attentional selection either eliminates (for selection based on visual location) or severely attenuates (for selection based on color) visual and auditory N400 priming effects, perhaps more effectively in the visual modality. McCarthy and Nobre (1993) observed semantic and identity priming effects on the N400 only for words appearing in the attended spatial location, inconsistent with an automatic N400. Subsequent studies factorially combined semantic priming with attentional status to create four conditions: both prime and target attended, both unattended, or either the prime or the target attended while the other is not. N400 repetition and semantic priming effects were most consistently observed for targets whether or not they
