Orthographic and associative neighborhood density effects: what is shared, what is different?

Words with many orthographic neighbors elicit a larger N400 than words with few orthographic neighbors. This has been interpreted as stronger overall semantic activation due to orthographic neighbors activating their semantic representations. To investigate this claim, we manipulated the number of associates of words (NoA), a variable directly affecting overall semantic activation, and compared this to the ERP effect of the number of orthographic neighbors (N) in a lexical decision task. Words with high NoA and with high N produced a very similar increase of the N400. In addition, a higher N increased the amplitude of the Late Positive Complex. The common N400 effect suggests that N affects semantic activation, like NoA does. The late positive effect specific to N could occur because words with few orthographic neighbors initially elicit little activity in the orthographic system, thereby resembling nonwords, which leads to distinct processing.