Although some ERP responses are named for their presumed function, the N400 is not, and its functional characterization (like that of all cognitive measures) is in a continual state of fine-tuning. Its identity is some function of its morphology, timing, and behavior under certain experimental manipulations. Some electrophysiologists have argued for a precise neuroanatomical characterization, but that is not so straightforward in practice, especially given that the same functional operations may be carried out in different neuroanatomical substrates. Accordingly, we do not view “the N400” as an undifferentiable, localizable (or lesoniable) neural entity that indexes one particular mental operation. Instead, we use the term N400 as a heuristic label for stimulus-related brain activity in the 200–600 ms post-stimulus-onset window with a characteristic morphology and, critically, a pattern of sensitivity to experimental variables – and hence a common functionality; next, we discuss what these variables are and how they impact human brain activity as well as our comprehension of sensory input.
