Task modulates ERP effects of orthographic neighborhood for pseudowords but not words.

Words and pseudowords from high-density orthographic neighborhoods elicit larger amplitude N400s than similar items from low-density orthographic neighborhoods in the lexical decision task. This pattern could be interpreted as an increase in the amount of lexico-semantic information to be processed or as an increase in difficulty identifying a word (or rejecting a pseudoword) amongst many co-activated alternatives. In order to dissociate between these mechanisms, we compared neighborhood effects between a lexical decision task (LDT) and a letter search task (LST). Behaviorally, we found the standard neighborhood and lexicality effects in the LDT, but no significant effects in the LST. Thus, behavioral responses were sensitive to the decisions required by the respective tasks. Electrophysiologically, we found similar N400 neighborhood effects between tasks for words, but the N400 neighborhood effect for pseudowords was only present in the LDT. Moreover, the effect of neighborhood in the LDT occurred earlier for words than for pseudowords. These nuanced differences in the time course and automaticity of word and pseudoword neighborhood effects lend insight into the processes that underlie N400 effects of orthographic neighborhood and how they unfold over time. We propose that the early neighborhood effects for words across tasks were driven by highly automatized word identification processes that were sensitive to the lateral inhibition generated by orthographic neighbors. In contrast, the later neighborhood effects for pseudowords in the LDT could have been driven by task-specific processes tied to how global lexical activity is used to make a lexical decision.