Studies have shown that the distribution of N400 is different for different types of auditory stimuli. For example, the N400 elicited by concrete words tends to have a more anterior distribution compared to those elicited by abstract words (Holcomb et al., 1999; Kounios and Holcomb, 1994). On the other hand, both hemispheres are said be involved in different types of semantic processing (Federmeier and Kutas, 1999; Federmeier and Kutas, 2002). These differences point out that the effects observed with different types of stimuli are from non-identical neural generators, which in turn implies that semantic information is not stored in a modality-independent manner (Kutas and Federmeier, 2000). Intracranial recording studies suggest that the scalp-recorded N400 is associated with waves of activity across multiple brain areas, such as ventrolateral prefrontal cortex, inferotemporal cortex, superior temporal sulcus, medial temporal lobe and hippocampus (Halgren et al., 1994a; Halgren et al., 1994b; McCarthy et al., 1995; Nobre and McCarthy, 1995). Therefore, the N400 recorded at the scalp is an outcome of coordinated activity in multiple brain regions.
