Is the N400 effect a neurophysiological index of associative relationships?

The N400 is one of the most widely studied ERP components and has come to be viewed as an index of the semantic processing that relates distinct stimuli. In this study, we examine whether the N400 is sensitive to the associative relationship between distinct stimuli, and not the degree to which the stimuli share semantic features. We used previously established norms to parametrically vary the strength of linguistic association between words within word-pairs, while holding constant their degree of semantic congruency. This manipulation allowed us to compare N400s elicited by unrelated prime-target word-pairs (e.g. mirror-thumb) with N400s generated by related prime-target word-pairs of either moderate (e.g. camera-lens) or high (e.g. cherry-tree) degrees of association. We observed that larger N400 effects occurred for highly associated versus moderately associated pairs despite the fact that no differences in terms of semantic congruency existed between pairs belonging to the highly and moderately associated conditions. These findings demonstrate that the N400 can be modulated by associative relationships quite independently of semantics, and suggest that the N400 effect reflects processes sensitive to the contiguity of distinct elements within one's past experience and not their semantic properties per se