cognitive representations underlying the n400 priming effect

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Cognitive Brain Research 12 (2001) 487–490 www.elsevier.com / locate / bres Short communication Cognitive representations underlying the N400 priming effect a,b, a,b * Mika Koivisto , Antti Revonsuo a Centre for Cognitive Neuroscience, University of Turku, FIN-20014 Turku, Finland b Department of Psychology, University of Turku, Turku, Finland Accepted 19 June 2001 Abstract The present study explored whether the N400 semantic priming effect is based on semantic similarity or lexical associations, or both. The event-related potentials showed N400 priming effects for both semantically similar and lexically associated word pairs in the 250–375 ms time-window. However, the effect lasted for a longer time for lexical associates, particularly in frontal and central electrode sites in the 375–500 ms time-window, suggesting that different types of processing may contribute to the N400 priming effects evoked by the two types of relationships. 2001 Elsevier Science B.V. All rights reserved. Theme: Neural basis of behavior Topic: Cognition Keywords: ERP; N400; Priming; Semantics Lexical decisions are faster when the target is preceded between the words in a mental lexicon or to similarity by a semantically related prime word than when preceded between the semantic representations of the words. by an unrelated word. This priming effect is observed for The purpose of the present experiment was to examine many different types of prime-target relationships, and the cognitive representations underlying the N400 semantic most researchers agree that it can be generated either by priming effect for word pairs. We studied whether the automatic or controlled processing mechanisms, or both N400 semantic priming effect is based on similarity of the [8]. A neurophysiological counterpart of the semantic meanings of words in semantic memory or associations priming effect is the N400 priming effect which is between lexical items in the word recognition system, or observed in event-related potentials (ERPs) around 400 ms both. Three types of word pairs were used as stimuli. after the target onset: the amplitudes to unrelated word Lexically associated pairs ( n555) were compound words targets are more negative than those to related targets (e.g., wind-mill) which were strongly lexically associated [4,10]. This amplitude difference between related and but did not belong to the same semantic category and thus unrelated targets is the N400 priming effect. did not share many semantic features. Most of them were Most of the basic research on the N400 priming effect in semantically transparent. Semantically similar pairs ( n5 visual word recognition has used strongly related word 55) (e.g., sofa-bed) were not strongly lexically associated pairs as stimuli, for example category–instance relations but they belonged to the same semantic category and like ‘bird–robin’ [4,10] or categorical associates such as shared semantic features. Unrelated word pairs ( n555) ‘cat–dog’ or ‘nurse–doctor’ [3,5–7]. These types of were not associated and did not belong to the same stimuli are not only strongly associated but they also share category. In addition, 110 word / nonword pairs were used many semantic features. Thus, the priming effects for such as fillers. The nonwords were orthographically legal and stimuli may be based either on the strong association pronounceable pseudowords. The primes in the lexically associated, semantically similar, unrelated, and word / non- word pairs were matched for word length (mean54.9 letters for each type of stimulus pair) and lemma frequency *Corresponding author. Tel.: 1358-2-333-8554; fax: 1358-2-333- [9]. Similarly, the targets were matched for length in all 5060. E-mail address: [email protected] (M. Koivisto). the pair types (mean55.1 letters) and the word targets for 0926-6410 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0926-6410(01)00069-6

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Page 1: Cognitive representations underlying the N400 priming effect

Cognitive Brain Research 12 (2001) 487–490www.elsevier.com/ locate /bres

Short communication

Cognitive representations underlying the N400 priming effecta,b , a,b*Mika Koivisto , Antti Revonsuo

aCentre for Cognitive Neuroscience, University of Turku, FIN-20014 Turku, FinlandbDepartment of Psychology, University of Turku, Turku, Finland

Accepted 19 June 2001

Abstract

The present study explored whether the N400 semantic priming effect is based on semantic similarity or lexical associations, or both.The event-related potentials showed N400 priming effects for both semantically similar and lexically associated word pairs in the250–375 ms time-window. However, the effect lasted for a longer time for lexical associates, particularly in frontal and central electrodesites in the 375–500 ms time-window, suggesting that different types of processing may contribute to the N400 priming effects evoked bythe two types of relationships. 2001 Elsevier Science B.V. All rights reserved.

Theme: Neural basis of behavior

Topic: Cognition

Keywords: ERP; N400; Priming; Semantics

Lexical decisions are faster when the target is preceded between the words in a mental lexicon or to similarityby a semantically related prime word than when preceded between the semantic representations of the words.by an unrelated word. This priming effect is observed for The purpose of the present experiment was to examinemany different types of prime-target relationships, and the cognitive representations underlying the N400 semanticmost researchers agree that it can be generated either by priming effect for word pairs. We studied whether theautomatic or controlled processing mechanisms, or both N400 semantic priming effect is based on similarity of the[8]. A neurophysiological counterpart of the semantic meanings of words in semantic memory or associationspriming effect is the N400 priming effect which is between lexical items in the word recognition system, orobserved in event-related potentials (ERPs) around 400 ms both. Three types of word pairs were used as stimuli.after the target onset: the amplitudes to unrelated word Lexically associated pairs (n555) were compound wordstargets are more negative than those to related targets (e.g., wind-mill) which were strongly lexically associated[4,10]. This amplitude difference between related and but did not belong to the same semantic category and thusunrelated targets is the N400 priming effect. did not share many semantic features. Most of them were

Most of the basic research on the N400 priming effect in semantically transparent. Semantically similar pairs (n5

visual word recognition has used strongly related word 55) (e.g., sofa-bed) were not strongly lexically associatedpairs as stimuli, for example category–instance relations but they belonged to the same semantic category andlike ‘bird–robin’ [4,10] or categorical associates such as shared semantic features. Unrelated word pairs (n555)‘cat–dog’ or ‘nurse–doctor’ [3,5–7]. These types of were not associated and did not belong to the samestimuli are not only strongly associated but they also share category. In addition, 110 word/nonword pairs were usedmany semantic features. Thus, the priming effects for such as fillers. The nonwords were orthographically legal andstimuli may be based either on the strong association pronounceable pseudowords. The primes in the lexically

associated, semantically similar, unrelated, and word/non-word pairs were matched for word length (mean54.9letters for each type of stimulus pair) and lemma frequency*Corresponding author. Tel.: 1358-2-333-8554; fax: 1358-2-333-[9]. Similarly, the targets were matched for length in all5060.

E-mail address: [email protected] (M. Koivisto). the pair types (mean55.1 letters) and the word targets for

0926-6410/01/$ – see front matter 2001 Elsevier Science B.V. All rights reserved.PI I : S0926-6410( 01 )00069-6

Page 2: Cognitive representations underlying the N400 priming effect

488 M. Koivisto, A. Revonsuo / Cognitive Brain Research 12 (2001) 487 –490

lemma frequency. Ten independent subjects rated the A one-way ANOVA with repeated measures on meanrelated pairs for associative strength and semantic feature response times showed a significant effect for the targetoverlap on a seven-point scale (15very weak, 75very type (F 512.41, P,0.001). Lexically associated (6732,22

strong). The lexically associated pairs were more strongly ms) and semantically similar (680 ms) targets were pro-associated (mean55.1, S.D.50.6) than the semantically cessed faster than unrelated targets (713 ms)(Ps,0.003),similar pairs (mean53.5, S.D.50.7)(t 512.31, P, showing that behavioural priming effects were present for109

0.001). Semantically similar pairs shared more features both lexically related (40 ms) and semantically similar (33(mean54.8, S.D.50.7) than lexically associated pairs ms) stimuli. However, the lexically associated and semanti-(mean51.8, S.D.50.7) (t 522.54, P,0.001). A N400 cally similar targets did not differ from each other (P5109

priming effect for the semantically similar targets would 0.45). The error rates were low (on average 2.1%) and theysuggest that semantic similarity relation is a sufficient did not differ between the targets types (F 52.51).2,22

cognitive basis for the effect to emerge, whereas a N400 Statistical analyses on the N400 priming effects werepriming effect for the lexically associated targets would performed on the basis of mean amplitudes in the latencypropose that a lexical association is sufficient for the effect range of 250–500 ms post target. Because visual inspec-to occur. tion of the ERPs suggested that the time-course of the

Subjects were 12 right-handed healthy volunteers (five priming effects may be different for the lexical andmales) with the mean age of 23.5 years. All were native semantic stimuli, the latency range was divided into theFinnish speakers and had normal or corrected vision. The 250–375 ms and 375–500 ms time-windows which weresubjects sat in a comfortable chair in a dimly lit registra- analysed separately. Electrodes Fz, Cz, Pz, and Oz weretion room. There was a constant fixation point in the centre selected for statistical analyses as the effects were mostof a 210 computer screen and the subjects were asked to clearly observable in midline. Fig. 1 displays the Grandfixate on it throughout the experiment. The stimuli were average ERP curves for the three stimulus types.displayed in black on a white background and their size 250 –375 ms. A 3 (Stimulus Type: lexical, semantic,was 0.58 vertically and 1.3–3.78 horizontally, depending on unrelated)34 (Electrode: Oz, Pz, Cz, Fz) ANOVA on theword length. Each trial consisted of the following se- mean amplitudes revealed a significant main effect forquence: (1) the prime for 150 ms, (2) blank screen for 565 Stimulus Type (F 55.07, P,0.02), and pairwise com-2,22

ms, (3) the target for 150 ms, (4) response interval during parisons showed that amplitudes to unrelated targets (1.8which the subject responded to the target (2 s at maxi- mV) were more negative than to lexically associated targetsmum), and (5) 3 s interval before the next trial was (3.4 mV)(P,0.05) or to semantically similar targets (3.2initiated. The subjects were asked to attend to both the mV) (P,0.05). In other words, N400 priming effects wereprime and target, and to respond immediately when they observed for lexical (1.6 mV) and semantic (1.4 mV)knew whether the target was a word or a nonword. Word stimuli. The main effect for Electrode was only approach-responses were made with the index finger and nonword ing significance (F 53.14, P,0.10). The absence of a3,33

responses with the middle finger, by pressing one of two Stimulus Type3Electrode interaction (F 51.58) sug-6,66

buttons in the response box. Half of the subjects responded gests that the magnitude of the N400 priming effects waswith the right hand and half with the left hand. Following similar across the electrodes.practice stimuli, the experimental stimuli were presented in 375 –500 ms. A 334 ANOVA on the mean amplitudesa random order in one block of about 20 min. revealed a main effect for Electrode (F 55.52, P,0.05).3,33

EEG was recorded using 20 Ag/AgCl scalp electrodes, Pairwise comparisons of the electrodes suggested that thearranged according to the 10/20 system of electrode amplitudes were less positive in Oz (2.6 mV) than in Pzplacement. Two EOG electrodes, one at the upper canthus (7.4 mV) (P,0.01) or in Cz (8.8 mV)(P,0.02) and lessof the right eye and the other at the lower canthus of the positive in Fz (6.6 mV) than in Cz (P,0.02). The mainleft eye, were used for detecting eye movements and effect for Stimulus Type was significant (F 54.33, P,2,22

blinks. All electrodes were referred to linked ears. The 0.05), and pairwise comparisons of the different stimulusimpedance of the electrodes was held below 5 kV. The types showed a N400 priming effect for lexically associ-NeuroScan 386 Scan 3.0 acquisition system with a Brain- ated targets (1.34 mV) (P,0.01) but not for semanticallytronics CNV/ISO-1032 amplifier, with a low-frequency similar targets (0.6 mV) (P,0.26). In addition, thecut-off of 0.3 Hz and a high-frequency cut-off of 70 Hz, Stimulus Type3Electrode interaction was significantwas used for recording continuous EEG. The EEG data (F 53.50, P,0.025), suggesting that the magnitude of6,66

was sampled once every 5 ms. All trials showing evidence the N400 priming effect for lexically associated targetsof eye movements (.50 mV) were automatically rejected. varied as a function of the electrode. Separate t-testThe ERP components were analysed with the NeuroScan comparing amplitudes between lexically associated andequipment. Baseline correction was performed to the EEG unrelated targets in each electrode showed that the primingactivity in the 100 ms preceding the onset of the target. effect was significant in Fz (2.3 mV) (t 53.05, P,0.02)11

Separate ERPs were averaged for lexically associated, and in Cz (2.1 mV) (t 54.68, P,0.01) but not in Pz (1.011

semantically similar and unrelated targets. mV) (t 51.91) or in Oz (0.1 mV) (t 50.33).11 11

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M. Koivisto, A. Revonsuo / Cognitive Brain Research 12 (2001) 487 –490 489

Fig. 1. Grand average ERPs to lexically associated, semantically similar, and unrelated targets in electrodes Fz, Cz, Pz, and Oz. Because the subjects wererequired to make lexical decisions to the targets, a large P300 can be observed, peaking around 500 ms. The 250–375 and 375–500 ms time-windowswithin which the N400 priming effects were analysed are denoted by three vertical dotted lines. The N400 priming effect for the lexically associated targetsis the difference between the ERPs to lexically associated and unrelated targets; the N400 priming effect for the semantically similar targets is thedifference between the ERPs to semantically similar and unrelated targets.

Our main conclusion is that either lexical association or of automatic processing would not fit to the dissociationsemantic similarity is sufficient but not necessary for the between the word pair types at the later time-window.N400 priming effect to appear. However, the N400 prim- There are no convincing reasons to believe that automaticing effect for semantically similar stimuli was present only N400 priming effect would last for a longer time forin the earlier time-window (250–375 ms) but not in the lexically associated targets than for semantically similarlater one (375–500 ms), whereas the lexically associated targets in the anterior sites. Rather, it can be suggested thatstimuli elicited the effect in both time-windows. In addi- the temporally later part of the effect — observed for thetion, the N400 priming effect in the later time-window for lexically associated pairs only — was contributed mostlylexically associated stimuli was stronger at frontal and by attention-demanding controlled processing and maycentral electrode sites than at the more posterior ones. reflect the integration of the prime and target into aThese observations suggest that the longer duration of compound word and/or shifting of attention from theN400 priming effect for lexical associates is at least partly target word (e.g., stick) to the prime-target compound (e.g.,generated by different cognitive and neural processes than lipstick). The earlier part of the effects — which wasthe early part of the effect which was similar for both word similar to both word pair types — was likely to be due topair types. automatic spreading activation between associative and

At present, it is not clear whether the N400 priming semantic connections. Thus, the N400 priming effects ineffect for word pairs is produced by automatic activation the lexical decision task can be based on lexical as-and attention-demanding controlled processing [3,4,6,10] sociativity or semantic similarity, but their duration mayor mostly by controlled processing [1,2]. It seems that an vary depending on the additional (controlled) processinginterpretation of the present N400 priming effects in terms elicited by the specific relationship between the words.

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490 M. Koivisto, A. Revonsuo / Cognitive Brain Research 12 (2001) 487 –490

[5] H. Gomes, W. Ritter, V. Tartter, H.G. Vaughan, J.J. Rosen, LexicalAcknowledgementsprocessing of visually and auditorily presented nouns and verbs:evidence from reaction time and N400 priming data, Cogn. Brain

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66–85.[7] P.J. Holcomb, H. Neville, Auditory and visual semantic priming in

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