Event-related potential indices of semantic priming using masked and unmasked words: evidence that the N400 does not reflect a post-lexical process.

Several authors have contended that the N400 is a reflection of a post-lexical event such as that proposed by Neely and Keefe [J.H. Neely, D.E. Keefe, Semantic context effects on visual word processing: a hybrid prospective/retrospective processing theory, in: G.H. Bower (Ed.), The Psychology of Learning and Motivation: Advances in Research and Theory, Vol. 23, Academic Press, New York, 1989, pp. 207-248.], whereby the subject compares the word on the current trial to the "context" provided by the word on the preceding trial [M. Besson, M. Kutas, The many facets of repetition: A cued-recall and event-related potential analysis of repeating words in same versus different sentence contexts, Journal of Experimental Psychology: Learning, Memory and Cognition, 19 (5) (1993), 1115-1133; C. Brown, P. Hagoort, The processing nature of the N400: Evidence from masked priming. Journal of Cognitive Neuroscience, 5(1) (1993), 34-44; P.J. Holcomb, Semantic priming and stimulus degradation: Implications for the role of the N400 in language processing, Psychophysiology 30 (1993), 47-61; M.D. Rugg, M.C. Doyle, Event-related potentials and stimulus repetition in indirect and direct tests of memory, in: H. Heinze, T. Munte, G.R. Mangun (Eds), Cognitive Electrophysiology, Birkhauser Boston, Cambridge, MA, 1994]. A study which used masked primes to directly test this possibility has been reported by Brown and Hagoort [C. Brown, P. Hagoort, The processing nature of the N400: evidence from masked priming. Journal of Cognitive Neuroscience, 5(1) (1993), 34-44]. When the primes were masked, no priming effect was observed on the N400. When behavioral data were collected in the same paradigm, from another group of subjects, the usual priming effect on RT was obtained. Considered together, the data from the two groups of subjects indicated that activation of semantic representations had occurred without conscious awareness. As no N400 priming effect was observed, it was suggested that N400 must reflect a post-lexical process. This interpretation, however, is at odds with the findings of other studies which have reported N400 priming effects under conditions where post-lexical processes would not be thought to operate[J. Anderson, P. Holcomb, Auditory and visual semantic priming using different stimulus onset asynchronies: an event-related brain potential study. Psychophysiology 32 (1995), 177-190; J. Boddy, Event-related potentials in chronometric analysis of primed word recognition with different stimulus onset asynchronies, Psychophysiology 23 (1986), 232-245; D. Deacon, T. Uhm, W. Ritter, S. Hewitt, The lifetime of automatic priming effects may exceed two seconds, Cognitive Brain Research 7 (1999), 465-472; P.J. Holcomb, Automatic and attentional process: an event-related brain potential analysis of semantic priming. Brain and Language 35 (1998) 66-85]. The present study replicated Brown and Hagoort using a repeated measures design, a shorter SOA (stimulus onset asynchrony), and a slightly different threshold setting procedure. Significant priming effects were obtained on the mean amplitude of the N400 regardless of whether the words were masked or unmasked. The findings imply that the processing subserving the N400 is not postlexical, since the N400 was manipulated without the subjects being aware of the identity of the words.