As the N400 came to be characterized, it quickly became clear that it was the amplitude of the response that was most susceptible to manipulation (becoming smaller when factors rendered information more expected and thus easier to process) and most likely to vary with many of the same factors that influence RT measures (see overview in Kutas & Federmeier 2009). At a physiological level, amplitude reductions might reflect smaller post-synaptic potentials in the same neurons, activation of fewer neurons in a population, and/or less temporal synchrony among the generating neurons. N400 latency, by contrast, was generally quite stable - a fact whose theoretical significance we are just beginning to appreciate (see theory section). Characterizing N400 topography across the scalp has proven more difficult because a stable distribution was seen to visual words across manipulations, but temporally and functionally similar responses to other stimulus types had overlapping, but dissociable, scalp topographies. Whether this means there is more than one “N400” is difficult to answer based on surface potentials alone because of possible temporal overlap with other responses. At a deeper level,
