Semantic priming modulates the N400, N300, and N400RP
Introduction
Semantic priming is the facilitated processing of a target word when it is preceded by a semantically/associatively related prime word, relative to when it is preceded by a totally unrelated prime word (McNamara, 2005, Neely, 1991). Cognitive psychologists have used semantic priming to gain insight into how people use semantic context to access a word’s meaning and to shed light on how semantic memory is organized and accessed (Meyer and Schvaneveldt, 1976). A three process model of priming, consisting of automatic spreading activation (ASA), expectancy, and post-lexical semantic integration, has been used to account for the wide array of effects seen in semantic priming experiments (Neely and Keefe, 1989).
One method for isolating these three processes is the manipulation of the direction of association between the prime and the target. In forward associative priming, there is an association from the prime to target, but not the other way around (e.g., prime-FRUIT, target-FLY; prime-STORK, target-BABY). Forward priming is presumably produced by ASA at short SOAs and expectancy at longer SOAs. Backward associative priming (Koriat, 1981) occurs as a result of a strong associative link from target to prime but not the other way around (e.g., prime-FLY, target-FRUIT; prime-BABY, target-STORK). Because there is a backward association from the target fruit to the prime fly, the semantic matching mechanism could detect this target-prime relation, whose presence and absence is correlated with the target being a word or a nonword, thereby producing a priming effect in the lexical decision task.
Event-related potentials (ERPs) have been a useful tool in examining processes involved in priming. Of particular interest is the N400, an ERP component that appears to be sensitive to the congruity of a stimulus with the current semantic context (Kutas and Van Petten, 1994). Opinions regarding what the N400 reflects vary widely. Some argue that the N400 reflects controlled processes and not ASA (e.g., Chwilla et al., 1998) whereas others argue that it reflects both (e.g., Deacon et al., 1999). Overall, evidence suggests that the N400 reflects all three priming mechanisms, ASA (Deacon et al., 2004, Deacon et al., 2000, Kiefer, 2002, Kiefer and Spitzer, 2000, Kutas and Hillyard, 1989), expectancy (Kutas et al., 1984, Silva-Pereyra et al., 1999), and semantic matching (Chwilla et al., 1998, Holcomb, 1993), although the case for expectancy priming is not as strong. Although the relatively late time course of the N400 with respect to the presumed time course of semantic analysis (Sereno et al., 1998) might seem to tilt the evidence towards N400 being a manifestation of post-lexical semantic matching and/or integration, the P300 literature provides a clear precedent for a late acting component to index an expectancy process.
Some existing reports suggest the possibility that the different priming mechanisms are indexed by different ERP components. One such paper (Hill et al., 2002) conducted a lexical decision task with both short (150 ms) and long (700 ms) SOAs. In addition to the conventional centro-parietal N400 effect, there was a frontal negativity at about 300 ms that was sensitive to semantic incongruity only at the short SOA, as might be expected for an effect that responds to ASA. Such a frontal effect has been proposed elsewhere as potentially indexing ASA (Pritchard et al., 1991). Hill and colleagues (2002) also reported a right posterolateral N400 effect that was significant only at the long SOA, mirroring an ERP component in another study (Dien et al., 2006) that responded to both backward and forward associative priming and was therefore proposed to reflect semantic matching.
The first goal of this study was to try to provide more information on how the N400 (and other related components) relates to ASA, expectancy, and semantic matching. To isolate the operation of these mechanisms, this study manipulated SOA and direction of association. The empirical question to be answered is whether the four combinations of short vs. long SOA and forward vs. backward priming will be differentially associated with the N400. A long, 500-ms prime-target SOA and asymmetrically associated word pairs were used to isolate semantic matching, through backward priming, and expectancy, through forward priming. In addition, conventional symmetrically associated prime pairs were included for purposes of comparison.
A short, 150-ms SOA was also utilized to investigate whether ASA-based priming modulates the N400. This 150-ms SOA also permits us to observe the operation of a fourth potential priming mechanism that accounts for backward priming at short SOAs. It is speculated that there might be differences in the N400 effects seen for backward priming at the long and short SOAs based on behavioral evidence for different mechanisms of backward priming at short and long SOAs (Kahan et al., 1999).
An earlier study by Chwilla and colleagues (Chwilla et al., 1998) had much in common with the present study, including examining backward priming at both short and long interstimulus intervals (ISI) of 0 and 500 ms each, but using auditory stimuli. Reaction time and N400 priming effects occurred at both SOAs and were interpreted as supporting the position that the N400 reflects semantic matching/integration and possibly expectancy.
Although the study by Chwilla and colleagues (1998) was an important step towards better characterizing the N400, it left some issues unresolved. The present report provides new information beyond Chwilla and colleagues’ (1998) original report in the following ways: (1) because visual stimuli have a discrete onset time, the present study had stronger control over SOA timing issues; (2) the present study used a high-density recording montage that could detect potentially new semantic effects; (3) the high-density montage might make it possible to separate out P300 effects.
A potential issue with the experimental design is that the lexical decision task is likely to superimpose a P300 on the resulting waveforms. The study by Chwilla and colleagues (1998) sought to minimize the presence of the P300 by replicating the experiment with the instruction to press a button to nonwords only. However, the P300 responds primarily to stimulus probability rather than target status (Donchin, 1981) so simply designating the non-words as being the target will not eliminate the P300 from the word targets, although it will reduce it. In order to maximize comparability with the published behavioral priming literature this study attempted the strategy of capitalizing on a combination of high-density topographical information and principal components analysis (PCA) to separate the P300 and the N400, an approach that has been previously reported to work (Polich, 1985).
In summary, our procedures allow us the opportunity to achieve both psychophysiological and cognitive goals. The first psychophysiological goal is to try to better characterize the N400 with respect to the three priming mechanisms, separate from the P300. The second psychophysiological goal is to identify and characterize other semantic effects, especially the frontal effect and the right posterolateral effect. The cognitive goal is to obtain further information about whether backward priming at short SOAs is mediated by the same priming mechanism that produces backward priming at long SOAs.
Section snippets
Participants
Fifty-two, right-handed, native English speaking, Tulane University introductory-level psychology students, without neurological history or psychotropic medications, participated in the study for course credit. An additional 59 participants participated at the University of Kansas. Decisions were made prior to data analysis to exclude from analysis a participant’s data when they were likely to be unreliable and/or invalid, due to too many trials in which there were measurement problems due to
Behavioral results
As seen in Table 1 and Fig. 2, in the 2 (SOA) × 5 (Priming) factorial design, there was a borderline trend for the RTs to be faster for the long than for the short SOA (TWJt/c[1, 41.95] = 3.86, p = .055). There was also a significant main effect for priming (TWJt/c[4, 32.32] = 22.62, p < .0001) and a significant priming × SOA interaction (TWJt/c[4, 32.32] = 2.99, p = .030). However, contrasts of backward, forward, and symmetrical priming interactions with SOA were not at all significant, with all p-values > .33.
Discussion
The behavioral results reveal significant RT priming effects for all conditions. The forward and backward priming RT priming effects were roughly numerically equivalent at the 500- and 150-ms SOAs. These data are similar to the significant effects found in a larger sample tested by Kahan and colleagues (1999) and allow an investigation of the ERPs with the knowledge that priming was indeed occurring in all conditions. As for the three goals, the effort to separate the N400 from the P300 appears
Acknowledgements
The research in this paper was done in partial fulfillment of the first author’s requirements for the M.S. degree at Tulane University. Portions of this research were presented at the Cognitive Neuroscience Society, April 2003, in New York City. We thank Kyle Malone for assistance with the behavioral data. We also thank the members of the first author’s thesis committee, Gary Dohanich, Paul Colombo, and Dave Corey, for their helpful comments and support throughout the study. The preparation of
References (61)
- et al.
Event-related potentials and the semantic matching of pictures
Brain Cogn
(1990) - et al.
A theoretical justification of the average reference in topographic evoked potential studies
Electroencephalograph Clin Neurophysiol
(1985) - et al.
Evidence for a new conceptualization of semantic representation in the left and right cerebral hemispheres
Cortex
(2004) - et al.
Event-related potential indices of semantic priming using masked and unmasked words: evidence that the N400 does not reflect a post-lexical process
Cogn Brain Res
(2000) - et al.
The lifetime of automatic semantic priming effects may exceed two seconds
Cogn Brain Res
(1999) - et al.
EEGLAB: an open source toolbox for analysis of single-trial EEG dynamics including independent component analysis
J Neurosci Meth
(2004) - et al.
Optimizing principal components analysis of event-related potential analysis: matrix type, factor loading weighting, extraction, and rotations
Clin Neurophysiol
(2005) - et al.
Is “blank” a suitable neutral prime for event-related potential experiments?
Brain Lang
(2006) - et al.
Parametric analysis of event-related potentials in semantic comprehension: evidence for parallel brain mechanisms
Cogn Brain Res
(2003) - et al.
Picture the difference: electrophysiological investigations of picture processing in the two cerebral hemispheres
Neuropsychologia
(2002)