The N400 was labeled as such because it was a relative negativity peaking around 400 ms. More precisely, it is negative-going at particular scalp locations relative to a specific reference derivation (e.g., posterior sites relative to recordings behind the ears), relative to a 100 ms pre-stimulus baseline. Indeed, the N400 to an unexpected item need not be negative in absolute terms. It is thus typically examined in cross-condition comparisons and routinely instantiated in a difference ERP created via a point-by-point subtraction of, e.g., a congruent ERP from an incongruent one. This difference – or N400 effect -- is a monophasic negativity between 200 and 600 ms, largest over centro-parietal sites, with a slightly right hemisphere bias (at least for written words in sentences).
