Such manipulations might thus affect the size of N400 effects observed. Van Berkum (2009) also emphasizes that semantic retrieval can be “intensified” by attention. On the view we are building here, one way that this could happen is by effects of selective attention on the sensory input. Much ERP work details how selective attention to space, objects, and various perceptual features of objects can modulate the amount of feedforward activity elicited by an incoming stimulus (Luck et al 2000). To the extent that the N400 is part of this feedforward stream, selective attention would be expected to correspondingly modulate even baseline N400 amplitudes. However, just as selective attention generally does not eradicate sensory ERP components, it would seem unlikely to eliminate all signs of N400 activity, and, since the nature of attentional effects differs across modalities, N400 modulations by attention would likely as well. We suggest that attention can thus serve to alter the balance between the contributions of feedforward and topdown activity to the processing of meaning. That balance also seems to be importantly different between the two cerebral hemispheres (Federmeier 2007), with concomitant impact on the N400 seen when processing is biased toward the LH and RH (which
