selective attention and n400 attenuation with spoken word repetition

Selective attention and N400 attenuationwith spoken word repetition

TSUNETAKA OKITA a and TETSUYA JIBUb

aDepartment of Science of Behavior, Hyogo College of Medicine, Nishinomiya, JapanbDepartment of Psychology, Kwansei Gakuin University, Nishinomiya, Japan

Abstract

In two experiments, event-related brain potentials were recorded to word pairs simultaneously presented to both ears,with instructions to attend to one ear and detect occasional nonwords in that ear. This attentional manipulation yieldedfour patterns of word repetition on successive trials: first and second presentations attended~AA !, both unattended~UU!,and across ears~AU and UA!. A prominent attenuation of N400 due to immediate repetition of words was observed onAA trials. However, when first presentations were ignored on UU and UA trials, no repetition effect was obtained. Thesefindings indicate that the repetition effect on N400 depends on attentional processing of first presentations.

Descriptors: Selective attention, Auditory N400, Event-related brain potential, Repetition effects

Much recent event-related brain potential~ERP! research has useda slow negative wave peaking at about 400 ms poststimulus~N400!as an electrophysiological measure to address psycholinguistic andmemory questions. An important feature of N400 is that its am-plitude is modulated by repetition, semantic, and contextual prim-ing. In word repetition paradigms, for instance, a number of studieshave shown that a large N400 elicited by first presentations ofrepeating words is attenuated for second presentations~for re-views, see Rugg & Doyle, 1994; Van Petten, Kutas, Kluender,Mitchiner, & McIsaac, 1991!.

Although the functional significance of N400 attenuation ispresently unclear, recent interpretations have related the repetitioneffect to attentionally controlled postlexical processes rather thanautomatic lexical access per se. One hypothesis is that N400 re-flects processes of integrating an item with ongoing context andthat the amplitude of N400 is inversely proportional to the ease ofcontextual integration~Bentin & McCarthy, 1994; Rugg & Doyle,1994!. The ease of integration for repetition is produced by there-occurrence of an item within the same context~Rugg & Doyle,1994!. Another hypothesis proposes that the N400 attenuation re-flects an interruption of semantic processing that items triggerautomatically. When an item is immediately repeated, the repeti-tion effect occurs because the repeating access to the semanticrepresentation is forestalled by the availability of the decision re-

lated to that item on the initial presentation~Bentin & McCarthy,1994!.

Manipulation of selective attention is one way to examine thecontributions of controlled processes to the N400 repetition effect.To date, only a few N400 studies of selective attention have beenreported. McCarthy and Nobre~1993! presented words to the leftor right of visual fixation while participants focused attention toone field. A large N400 attenuation with repetition~and also se-mantic priming! was observed for attended words. In contrast,when words were repeated in the unattended location, little N400was elicited even for the initial presentation and no N400 attenu-ation due to repetition was observed. This finding supports theinterpretation that the processes underlying N400 are not auto-matic but are dependent on the allocation of spatial attention.However, when such a small N400 is measured for unattendedwords, a question arises as to whether there is scope for the atten-uation with repetition. If N400 is truly not sensitive to processesthat are performed without attention, the degree of processing forunattended items should be tested by N400 modulation for at-tended repetition of those items.

Otten, Rugg, and Doyle~1993! adopted conditions in which thefirst and second presentations of words could occur in either theattended or unattended location, so that the word repetition acrossthe attentional locations was included. Words~or nonwords! in twocolors were presented simultaneously above and below fixation.When the direction of attention was cued by color, a positive shiftof ERPs~including N400 attenuation! was observed only for wordrepetitions within the attended location. However, when attentionwas manipulated by color and a precue, a smaller repetition effectwas additionally obtained in the cases in which only one of the firstand second presentations was attended. These findings were dis-cussed in relation to the view that visual selective attention actsmore to attenuate than to eliminate the processing of unattendeditems.

This research was supported by a Grant-in-Aid for Scientific Research~06610088 and 08610097! from the Ministry of Education, Science andCulture of Japan to T. Okita.

We thank Dr. S. Bentin, Dr. C.M. Brown, and an anonymous reviewerfor helpful comments on an earlier version of this paper.

Address reprint requests to: T. Okita, Department of Science of Be-havior, Hyogo College of Medicine, 1-1, Mukogawa-cho, Nishinomiya,Hyogo 663, Japan.

Psychophysiology, 35~1998!, 260–271. Cambridge University Press. Printed in the USA.Copyright © 1998 Society for Psychophysiological Research

260

In a study using a semantic priming procedure, Kellenbach andMichie ~1996! also observed the attenuation of N400 for primedtargets, irrespective of the attentional direction, when the preced-ing primes were attended. They further suggested the additionalmodulation of N400 by overlapping effect of attention.

In the auditory modality, no report investigated the influence ofselective attention on the N400 repetition effect. In a semanticpriming situation, Bentin, Kutas, and Hillyard~1995! presentedwords simultaneously in the attended and unattended ears. Con-sistent with the visual ERP studies described above, the N400attenuation for semantically primed words was observed in theattended but not in the unattended ear. Correspondingly, sub-sequent testing revealed that recognition memory was better forattended than for unattended words. However, there were no dif-ferences in the percentage of false alarms for new words~or “lures”!that were semantic associates of initially attended or unattendedwords. Furthermore, reaction times in a lexical decision task wereshortened equally by stimulus repetition, whether the first pres-entation of the repeated words had been attended or unattended.These behavioral findings were interpreted to suggest that se-mantic information in the unattended ear was automatically pro-cessed and the effect of semantic processing persisted over time. Intheir ERP study, however, crossed attention conditions were nottested.

In the present study, lists of spoken words and nonwords weresimultaneously delivered to the attended and unattended ears, andERPs were recorded for trials including pairs of immediately re-peated words in either the attended or unattended ear. That is,immediate repetition effects for words was manipulated within theattended ears, within unattended ears, and across ears. The primarygoal was to extend the knowledge regarding the processing ofspoken words without attention. There is relatively scarce litera-ture, especially on selective attention to spoken words using dichoticpresentation. The question was whether the first word presentationin the unattended ear produced the attenuation of N400 to the im-mediately repeated presentation in the attended ear. The N400 mod-ulation for repeated words in the attended ear should reflect thedegree of processing received by the first word presentation in theunattended ear. This auditory version of ERP studies of selective at-tention also allowed for the comparison with the previous investi-gations using written words.

EXPERIMENT 1

Methods

ParticipantsFourteen adults~10 men, 4 women!, aged 21–23 years old, servedas paid volunteers. All participants were right handed and nativeJapanese speakers with normal hearing.

StimuliThe stimulus presentation is illustrated in Figure 1. Japanese words~and occasionally nonwords! were presented simultaneously to theleft and right ears through stereo head phones. The participants wererequired to attend a stimulus sequence in one ear for nonword de-tection. Two types of immediate repetition on successive trials wereprepared:~a! both first and second presentations of a repeating wordwere in the same ear, left or right;~b! the first presentation of a re-peating word was followed by the second presentation to the oppo-

site ear, left to right or right to left. Thus, the combination of atten-tional direction and trial type enabled us to test four patterns of trialpairs:~a! both first and second presentations of a repeating word werein the attended ear~AA trials!; ~b! both first and second presenta-tions were in the unattended ear~UU!; ~c! the first presentation wasin the attended ear and the second presentation in the unattended ear~AU !; and~d! the first presentation was in the unattended ear and thesecond presentation in the attended ear~UA!.

Four lists of 130 trials each were assembled. Each list contained10 first and 10 second presentations of repeating test words to theleft and the right ear in the same and opposite conditions~for atotal of 80 trials!, 5 nonwords to each ear~for a total of 10 trials!,and 40 filler trials. Each item of test words and nonwords wasaccompanied by a simultaneously presented filler word on eachtrial. The filler trials contained neither a test word nor a nonword

Figure 1. Schematic diagram of the stimulus sequence during the dichoticlistening task. Three-syllable Japanese words and nonwords, whose sylla-bles are separated by dots in the figure, were presented simultaneously tothe left and right ears. Participants were asked to focus their attention toone ear and detect occasionally interspersed nonwords~NWs; e.g.,komote!in the attended ear. Four patterns of repetition trials~AA, UU, AU, and UA!were varied in terms of attentional direction to first and second presenta-tions of repeating test words; for example, on an AU trial, the first presen-tation of a repeating test word was in the attended ear and the secondpresentation was in the unattended ear.

Selective attention and autidory N400 261

in both ears. The test words and nonwords, except the filler words,1

were never repeatedly used across the four lists. The order of trialsin each list was randomized and the stimulus onset asynchrony~SOA! was 1.5 s. An additional list was similarly prepared forpractice.

All the words and nonwords were three syllables. The wordswere Japanese nouns and verbs selected in a range of 23–39 oc-currences per million~National Language Research Institute, 1962!.The nonwords were speech sound without meaning in Japanese.Each item, recorded by a male voice, was stored on a Macintoshcomputer using a MacRecorder at a sampling rate of 22 kHz. ASound Designer II program was used to edit the sound resourcesand assemble critical and filler items into play lists on two tracks.On the third track, each trigger code was also synchronized atonset with the critical and filler items. The mean duration of editeditems was 490 ms~SD5 65 ms!. In these editorial processes, theword onset and offset were defined as the first and last marks ofacoustic energy, respectively. A Deck II program was used to playback the four lists, which were recorded onto three channels of afour-track digital tape deck. In addition, to facilitate subjectiveseparation of stimulus sequences in the left and right ears, passagesfrom a novel spoken in two male voices were recorded onto thefourth channel and reproduced to be subjectively localized in themidline between the two ears. The intensity for all the stimuli wasset to be slightly soft~approximately 35–40 dB SL!.

ProcedureParticipants were seated in a reclining chair inside a sound-attenuated, electrically shielded chamber. The participants weretold that their task was to attend to one ear and to respond bypressing a button with the thumb whenever they detected non-words in the designated~or attended! ear. They were also asked tominimize eye movements and blinks during task performance.

Following practice, each participant received four test runs. Intwo of these, the attended ear was left~L!, and in the remainingtwo runs, it was right~R!. The administration order of attentionaldirection was LRRL for half of the participants and RLLR for theother half. The four word lists were used in the different runs, andthe ordering of Lists 1–4 and the stimulus presentation to the leftand right ears were fixed across all participants. Therefore, through-out the experiment, each item served equally as an attended stim-ulus. The response hand was left in the first two runs and right inthe last two runs for half of the subjects, and the reverse for theother half. A rest pause of about 1 min was taken between runs.

RecordingsThe electroencephalogram~EEG! was recorded from Ag0AgClelectrodes placed at Fz, Cz, Pz, C5, and C6~International 10-20system!, and approximately over Wernicke’s area~midway be-tween C3 and T5: Wl! and its right homologue~midway betweenC4 and T6: Wr! sites, all referred to the linked earlobes~A1–A2!.The vertical electrooculogram~EOG! was recorded from the in-fraorbital ridge of the left eye, also referred to the linked earlobes.In addition, the horizontal EOG was recorded bipolarly from theouter canthi. The EEG and EOG were amplified with a bandpassset at 0.05 Hz~time constant5 3 s! and 30 Hz and stored on a harddisk with stimulus trigger codes and response pulses after an A0Dconversion at a rate of 100 Hz.

Data AnalysesAverage ERPs were computed separately on each electrode site foreight classes of trials containing the test words, that is, for first andsecond presentations of repeating words in the four combinations~AA, UU, AU, and UA! of attentional direction~attended vs. un-attended! and trial type~same vs. opposite!. The ERPs were com-bined over the left and right ears. The averaging epoch began200 ms before stimulus onset and lasted for 1200 ms. Trials con-taining EOG artifacts, amplifier saturation, or incorrect responseswere automatically excluded from the average. On average, 31trials ~range5 19–39! composed ERP waveforms for first andsecond presentations, with no substantial differences among AA,UU, AU, and UA trials.

N400 was measured as the mean amplitude relative to a 200-msprestimulus baseline at the 300–900-ms period after stimulus on-set. In addition, to achieve more detailed characterization of theN400 modulation by repetition and attention, the mean amplitudesover three consecutive 200-ms periods of 300–500 ms, 500–700 ms, and 700–900 ms were measured. The statistical analysesfor ERP measures were carried out by using repeated-measuresanalyses of variance~ANOVAs!. Because fillers were employedrepeatedly across word lists and because N400 elicited by attendedfillers was predicted to be attenuated even slightly, ANOVAs wereperformed separately when test words were presented to the at-tended ear~i.e., first presentations of AA and AU trials and secondpresentations of AA and UA trials! and when they were presentedto the unattended ear~i.e., first presentations of UU and UA trialsand second presentations of UU and AU trials!. The separate anal-yses could avoid confusing the repetition N400 effect of test wordswith the N400 attenuation for attended fillers. Findings that reachedthe .05 level after Greenhouse–Geisser correction were consideredas significant. Where post hoc pairwise comparisons were re-quired, the Tukey HSD test was employed. The scalp distributionsof ERP were analyzed on values scaled by vector length~cf. Mc-Carthy & Wood, 1985!.

Button-press responses 200–1,500 ms after each onset of non-words in the attended ear were scored as hits. All other responseswere considered to be false alarms.

Results

ERPs on Trials Containing Test WordsThe grand average ERP waveforms over all 14 participants arepresented separately for attended and unattended test word trials inFigures 2 and 3, respectively, in which ERPs to first and secondpresentations of test words are superimposed. All ERPs to attendedand unattended test words contained two negative deflections, arelatively small wave~N1! peaking at about 150 ms and a largeslow wave ~N400! lasting 250–900 ms and peaking at about550 ms poststimulus. A prominent attenuation of N400 was seenfor the second presentation of AA trials but not for the remainingcomparisons. The difference in N400 between first and secondpresentations, or the repetition effect, is maximal over the midlinecentroparietal region.

For the statistical evaluation of the repetition N400 effect, athree-way ANOVA was performed for the mean amplitudes overthe 300–900 ms period~see Table 1!, with variables of trial type~same vs. opposite!, repetition~first vs. second presentation!, andelectrode site. For trials containing attended test words, in additionto significant main effects of trial type,F~1,13! 5 5.24,p , .04;repetition,F~1,13! 5 5.98,p , .03; and electrode site,F~6,78! 513.12,p , .001,E5 0.569; the interactions between trial type and

1Because of the limitation of the available word materials, the fillerswere used repeatedly across lists.

262 T. Okita and T. Jibu

repetition,F~1,13! 5 6.41,p , .03; between repetition and elec-trode site,F~6,78! 5 3.63,p , .009,E5 0.752; and the three-wayinteraction,F~6,78! 5 3.53,p , .02, E 5 0.629, were obtained.In contrast, for trials containing unattended test words, only aneffect of electrode site was significant,F~6,78! 5 24.11,p , .001,E 5 0.563.

Planned two-way ANOVAs for each trial type at each attentionlevel revealed that, when both first and second presentations of testwords were attended on AA trials, a main effect of repetition,F~1,13! 5 10.91,p , .006, and the interaction between repetitionand electrode site for the same condition,F~6,78! 5 5.50, p ,

.001,E 5 0.669, were significant, reflecting the above-mentionedN400 attenuation. In addition, a slight N400 attenuation for theunattended second presentation of test words on AU trials, whichis shown in the lower panel of Figure 3, approached significancewhen compared with the first presentation of UA trials,F~1,13! 54.49, p , .06. For this comparison, the interaction between rep-etition and electrode site was not significant~F , 1!. The remain-ing two comparisons revealed neither a repetition effect~Fs , 1!nor the interaction between repetition and electrode site~Fs , 1!.

The scalp distributions of N400 for first and second presenta-tions were analyzed separately for the midline and lateral electrode

Figure 2. Grand average~n 5 14! ERPs for trials containing attended test words at different electrode sites in Experiment 1. Thewaveforms are depicted separately for trials on which first presentations of repeating test words were followed by second presentationsto the same~upper panel! and opposite~lower panel! ears, and are overlapped for the first~solid lines! and second~dotted lines!presentations.


sites on the scaled mean amplitudes of the 300–900-ms period onAA trials, on which the N400 attenuation associated with repetitionwas evident. For the midline data, a two-way ANOVA with vari-ables of repetition and electrode site yielded a significant electrodesite effect,F~2,26! 5 12.63,p , .001,E 5 0.871, and the inter-action between repetition and electrode site,F~2,26! 5 7.34,p ,.007, E 5 0.773. Post hoc pairwise comparisons indicated thatN400 for the first presentation was slightly dominant over thefrontocentral region~Fz . Pz: p , .04; Cz. Pz: p , .07!, andfor the second presentation N400 was frontally focused~Fz . Cz:p , .03, Fz. Pz: p , .001!. Further comparisons on the differ-ence of mean amplitudes between first and second presentations~Table 1! revealed that an electrode site effect approached signif-icance,F~2,26! 5 3.45, p , .06, E 5 0.757, reflecting that the

N400 attenuation with repetition was dominant over the centro-parietal region.

For the lateral data, a three-way ANOVA with variables of rep-etition, hemisphere~left, right!, and electrode site~C, W! revealedthe interaction between repetition and electrode site,F~1,13! 517.05,p , .002, reflecting a larger attenuation of N400 at the Wl0rsite than the C506. A main effect of electrode site was also signif-icant,F~1,13! 5 20.81,p , .001. However, there were no signif-icant effects associated with hemisphere.

Analyses for the 200-ms Time WindowsFor each trial type, a three-way ANOVA was performed for themean amplitudes at the three consecutive periods, with variables ofperiod~300–500, 500–700, 700–900 ms!, repetition~first vs. sec-

Figure 3. Same as Figure 2, but for unattended test word trials.


ond presentation!, and electrode site. Because these analyses wereinterested in an assessment of the time course of the N400 devel-opment and the repetition effect, the effects associated with thevariable of period will be mainly noted.

On AA trials, the interaction between period and repetition washighly significant,F~2,26! 5 16.22,p , .001,E 5 0.916, but notreliable for the remaining trial types of UU, AU, and UA. None ofthe four trial types revealed the reliable three-way interaction. Posthoc pairwise comparisons for the first presentation on AA trialsindicated that N400 developed maximally at the 500–700-ms period~23.9 mV ! ~500–700. 300–500,p , .02; 500-700. 700–900,p , .001! and the 300–500-ms period~22.8 mV ! was larger thanthe 700–900-ms period~21.8 mV ! ~ p , .04!, whereas N400 forthe repetition was maximal at the 300–500-ms period~22.1 mV !~300–500. 500–700,p , .02; 300–500. 700–900,p , .001!and gradually declined at the 500–700-~20.9 mV ! and 700–900-ms periods~0.2 mV ! ~ p , .03!.

Separate two-way ANOVAs at each period revealed that therepetition effect on AA trials was evident over the three consecu-tive periods: the interaction between repetition and electrode sitewas significant over the three consecutive periods of 300–500,500–700, and 700–900 ms,Fs~6,78! 5 5.15, 6.09, and 3.89;ps,.001, .001, and .01;es5 0.643, 0.656, and 0.616, respectively, anda significant repetition effect was found at the periods of 500–700and 700–900 ms,Fs~1,13! 5 18.36 and 9.14,ps , .001 and .01,respectively. Pairwise comparisons on AA trials revealed that therepetition effect was larger at the 500–700-ms period~23.0 mV !than at either the 300–500-ms~20.7 mV ! ~ p , .001! or 700–900-ms~22.0 mV ! ~ p , .009! period. The difference betweenperiods of 500–700 and 700–900 ms also approached significance~ p , .06!.

To assess the topographical shift of the repetition effect acrossthe three periods, a three-way ANOVA for the midline scaled meanamplitudes on AA trials was carried out with the same variables asin the above analysis. The interactions were obtained betweenrepetition and electrode site,F~2,26! 5 6.25,p , .02,E 5 0.753,

and between period and electrode site,F~4,52! 5 4.79,p , .03,E 5 0.376. The three-way interaction between period, repetition,and electrode site also approached significance,F~4,52! 5 2.81,p , .10, E 5 0.396. A two-way analysis for the first presentationverified an electrode site effect,F~2,26! 5 4.07, p , .04, E 50.814, but not the interaction between period and electrode site,F~4,52! 5 2.37,p . .13, E 5 0.336, suggesting that the fronto-centrally dominant distribution of N400 lasted over all three peri-ods. In contrast, for the second presentation the interaction betweenperiod and electrode site was reliable,F~4,52! 5 4.13,p , .05,E 5 0.381, reflecting that at the later periods the frontal focusingof N400 tended to increase and the positive shift was elicited at Czand especially at Pz. A four-way ANOVA, adding the forth variableof hemisphere, for the lateral data revealed no interactions includ-ing variables of period, repetition, and electrode site~or hemi-sphere!.

A two-way ANOVA for the midline scaled difference of meanamplitudes between first and second presentations revealed thatthe Period3 Electrode site interaction was not reliable,F~4,52! 52.20,p . .13,E5 0.502, reflecting that the centroparietal dominanceof the repetition N400 effect on AA trials was consistent over thethree periods. The analysis for the lateral data, however, yieldedthe interaction between period and electrode site,F~2,26! 5 8.23,p , .007, E 5 0.666. The following analysis on the C506 dataindicated a significant period effect,F~2,26! 5 8.81, p . .002,E 5 0.873. Pairwise comparisons revealed that the slightly en-hanced repetition N400 effect at the 300–500-ms period was sub-stantially different from the attenuation effect at the later periods of500–700 and 700–900 ms~ ps , .002 and .007, respectively!. Incontrast, the period effect at the Wl0r data was not reliable~F , 1!,suggesting that the magnitude of the N400 attenuation with rep-etition was consistent over the three periods.

PerformanceThe mean percentage of hit responses and the mean reaction timesfor nonwords in the attended ear were 67.5%~SD 5 15.8%! and

Table 1. Mean Amplitudes (inmV) Over 300–900 ms Poststimulus for ERPs at Various Electrode Sites

Electrode

Presentation Fz Cz Pz C5 C6 Wl Wr

AttendedSame

First presentation on AA 24.2 24.0 23.1 22.7 22.4 21.7 21.4Second presentation on AA 22.4 20.8 0.2 21.6 21.7 0.4 20.3Repetition effect 1.8 3.2 3.3 1.1 0.7 2.1 1.1

OppositeFirst presentation on AU 23.7 23.4 22.6 22.2 22.2 21.2 21.0Second presentation on UA 23.8 23.5 22.7 22.7 22.4 21.3 21.4Repetition effect 20.1 20.1 20.1 20.5 20.2 20.1 20.4

UnattendedSame

First presentation on UU 23.7 23.4 22.3 22.4 22.3 21.2 20.6Second presentation on UU 23.9 23.4 22.8 22.4 22.5 21.2 21.5Repetition effect 20.2 0.0 20.5 0.0 20.2 0.0 20.9

OppositeFirst presentation on UA 24.0 23.8 22.7 22.3 22.6 21.0 21.5Second presentation on AU 23.4 23.1 21.5 21.5 22.3 20.4 21.0Repetition effect 0.6 0.7 1.2 0.8 0.3 0.6 0.5

Note:Amplitudes listed are for first and second presentations of test words when attended and unattended stimuli are in the sameand opposite conditions of Experiment 1.


1019 ms~SD 5 107 ms!, respectively. The mean percentage offalse alarms was 2.0%~SD 5 3.9%! on trials containing un-attended nonwords and 2.5%~SD 5 1.5%! on word-only trials.The difference in false alarms between these trials was insignificant,t~13! , 1.

Discussion

The slow negative wave we identified with N400 developed at250–900 ms and peaked at about 500 ms poststimulus. Thesefeatures are consistent with recent observations of the auditorilyelicited N400~e.g., Anderson, & Holcomb, 1995; Bentin, Kutas, &Hillyard, 1993; Bentin et al., 1995; Praamstra & Stegeman, 1993;Rugg, Doyle, & Wells, 1995!. The centroparietally dominant at-tenuation of N400 is also consistent with the classical N400 elic-ited by semantic incongruity~Kutas & Hillyard, 1980!.

In agreement with recent findings by Bentin et al.~1995!, Mc-Carthy and Nobre~1993!, and Otten et al.~1993!, the N400 mod-ulation was sensitive to direction of selective attention. The reliableN400 attenuation was observed for the word repetition in the at-tended ear~i.e., on AA trials!, whereas the repetition in the un-attended ear~on UU trials! did not attenuate N400.

More interesting results are derived from the opposite condi-tion. The N400 attenuation effect was absent for attended repeatedwords when the initial presentation was in the opposite unattendedear ~on UA trials!. In contrast, when the first presentation of arepeating test word was in the attended ear~on AU trials!, thefollowing second presentation to the unattended ear gave rise to aslight attenuation of N400.

However, a problem arises as to the observed N400 attenuation.Van Petten et al.~1991! pointed out the possibility that a partici-pant could tag any repeated items as words when nonwords arenever repeated. In Experiment 1 in which nonword repetition trialswere used only three times throughout the test lists, use of thisstrategy might have facilitated the task-relevant decision to re-peated words. In other words, participants would be thinking some-thing like “I’ve heard that sound pattern before, so it must be aword, given that the experimenters seem to scarcely repeat thesound patterns of nonwords.” The quick decision would forestallthe repeating access to the semantic representation, which might bereflected by N400. A second experiment was designed to confirmthe above main findings in a situation preventing participants fromusing the possible strategy.

Another related problem concerns the marginal repetition effecton N400 for unattended second words on AU trials. This finding ishardly understood if N400 is related to controlled processing forwords in the attended ear. A possibility is a momentary shift ofattention when second presentations of repeating words are crossedto a to-be-unattended ear from an attended ear. In Experiment 2, anattempt was made to reduce this possible attentional shift by in-creasing time-stress for the task-relevant processing with a shorterSOA of 1 s.

EXPERIMENT 2

Methods

ParticipantsTwelve adults~7 men, 5 women!, aged 18–26 years old, served aspaid volunteers. All participants were right handed and native Jap-anese speakers with normal hearing. None had participated in Ex-periment 1.

StimuliThe stimuli were mostly identical to Experiment 1, except for thefollowing modifications.

Four lists were constructed. The critical items composing eachlist were 15 repeating test words to the left and right ears in thesame and opposite ears~for a total of 120 trials!, 3 repeating and3 nonrepeating nonwords to each ear~18 trials!, and 6 pairs ofwords and nonwords, which were matched in the first two sylla-bles, to each ear~24 trials!. For half of the pairs words precedednonwords, and the presentation order was reversed for the remain-ing half. The nonword repetition and the syllable matching trials ofwords and nonwords were used to prevent participants from tag-ging repeated items as words.

Each list of 162 trials was divided into two blocks, and eachblock was preceded by three start-up trials. The SOA was 1.0 s.

All the words, including fillers simultaneously presented to theopposite ear of critical items, and nonwords were three syllables ofvoiceless sound. The words were Japanese nouns selected in arange of 2.25–4.75 familiarity values~Chihara & Tsujimura, 1985!.The recording voice was male.

ProcedureWith the exception that each participant received eight test runs,the procedure was similar to that of Experiment 1.

Recordings and Data AnalysesThe EEG recording sites were Fz, Cz, Pz, F3, F4, C3, C4, P3, andP4, and an additional lateral pair of Wl and Wr. Each site wasreferred to an average of the left and right earlobes calculateddigitally off-line. Eye movements and blinks were monitored bi-polarly from the infraorbital ridge and the outer canthus of the lefteye. ERP waveforms contained 50~range5 34–60! trials for firstand second presentations of AA, UU, AU, and UA trials.

For the statistical evaluation of N400, the mean amplitudesrelative to a 100-ms prestimulus baseline were calculated at the300–800-ms period after stimulus onset and at the consecutive200-ms periods during 300–900 ms.2

All other aspects of recordings and data analyses were as inExperiment 1.

Results

ERPs on Trials Containing Test WordsThe grand average ERPs for attended and unattended test wordtrials are shown separately in Figures 4 and 5. Similarly to theprominent repetition effect in Experiment 1, a large centropari-etally maximal attenuation of N400 was observed for the secondpresentation of AA trials. The duration of the N400 repetitioneffect appears to be shorter than that of Experiment 1.

For trials containing attended test words, a three-way ANOVAfor the mean amplitudes at the 300–800-ms period~see Table 2!revealed a main significant effect of electrode site,F~10,110! 52.76,p , .05,E 5 0.363, and of the interaction between repetitionand electrode site,F~10,110! 5 3.66, p , .02, E 5 0.410. Fortrials containing unattended test words, no significant effects wereobtained.

Planned two-way ANOVAs for each trial type again revealeda main effect of repetition,F~1,11! 5 8.81, p , .02, and the

2The baseline of this experiment with the 1-s SOA was estimated at a100-ms prestimulus period to reduce a possible contamination due to theoverlap of the late ERP waves to the previous stimulus.


interaction between repetition and electrode site on AA trials,F~10,110! 5 3.61,p , .01, E 5 0.441, reflecting the prominentN400 attenuation for the attended second presentation in the samecondition. However, in disagreement with Experiment 1, the in-teraction between repetition and electrode site was insignificant forthe unattended second presentation of test words on AU trials,F~10,110! 5 1.81,p . .10, E 5 0.596, when compared with thefirst presentation of UA trials. For the two remaining comparisons,the attended second presentation of UA versus the first presenta-tion of AU trials and the first versus second presentations of UU

trials, no significant effects including the repetition variable wereobtained.

A midline distribution analysis with the scaled mean ampli-tudes of the 300–800-ms period on AA trials revealed no signifi-cant effects of repetition and electrode site and the interaction. Ananalysis for the difference of mean amplitude between first andsecond presentations~Table 2! revealed a marginal effect of elec-trode site,F~2,22! 5 2.73,p 5 .09,E 5 0.901. Post hoc pairwisecomparisons indicated that the N400 attenuation with repetitionwas larger at the Pz than at the Fz site~ p , .07!.

Figure 4. Grand average~n 5 12! ERPs for trials containing attended test words at different electrode sites in Experiment 2. Thewaveforms are depicted separately for trials on which first presentations of repeating test words were followed by second presentationsto the same~upper panel! and opposite~lower panel! ears, and are overlapped for the first~solid lines! and second~dotted lines!presentations.


The lateral data revealed a main effect of electrode site~F,C,W,P!,F~3,33! 5 5.45,p , .009,E 5 0.772, and the interaction betweenrepetition and electrode site,F~3,33! 5 7.17,p , .003,E5 0.760,but no effect of hemisphere was observed.

Analyses for the 200-ms Time WindowsA three-way ANOVA on AA trials revealed the three-way inter-action between period, repetition, and electrode,F~20,220! 5 5.32,p , .001,E5 0.222, and the interaction between period and repeti-tion, F~2,22! 5 6.95,p , .02, E 5 0.675. However, none of theremaining trial types of UU,AU, and UArevealed such interactions.

A two-way ANOVA for the first presentation on AA trials ver-ified a period effect,F~2,22! 5 18.97, p , .001, E 5 0.772.Pairwise comparisons revealed that, similarly to Experiment 1,N400 developed maximally at the 500–700-ms period~23.3 mV !~500–700. 300–500,p , .03; 500–700. 700–900,p , .001!and that the 300–500-ms period~22.2 mV ! was larger than the700–900-ms period~21.0 mV ! ~ p , .01!. The analysis for thesecond presentation revealed the interaction between period andelectrode site,F~20,220! 5 7.61,p , .001,E 5 0.261. Pairwisecomparisons indicated that the N400 development was signifi-cantly larger during the 300–500-ms and 500–700-ms periods than

Figure 5. Same as Figure 4, but for unattended test word trials.


during the 700–900-ms period at the Fz, F3, and C3 sites~300–500. 700–900:ps , .006, .004, and 007; 500–700. 700–900:ps , .01, .03, and 03, respectively!.

Separate two-way ANOVAs at each period on AA trials re-vealed that the interaction between repetition and electrode sitewas significant at 300–500 and 500–700 ms,Fs~10,110! 5 5.86and 3.15,ps , .001 and .002,es 5 0.382 and 0.421, respectively,in addition to a significant repetition effect at 500–700 ms,F~1,11! 5 18.23,p , .001. The repetition effect during the 300–500-ms period was significant only at the parietal sites of Pz, P4,and Wr,Fs~1,11! 5 8.83, 6.85, and 6.08,ps , .02, .03, and .04,respectively, whereas during the 500–700 ms the repetition effectwas reliable at all electrode sites except at F3,Fs~1,11! 5 7.86–20.50,ps , .02 for all. At the 700–900-ms period, there was nosignificant repetition effect. Furthermore, although on AU trials theinteraction between repetition and electrode site approached sig-nificance at the 700–900-ms period,F~10,110! 5 2.42,p , .06,E 5 0.418, separate analyses revealed no reliable repetition effectat any electrode site.

The midline data of the scaled mean amplitudes on AA trialsrevealed that the three-way interaction between period, repetition,and electrode site approached significance,F~4,44! 5 3.57,p ,.06, E 5 0.446. The two-way analysis for the first presentationindicated neither an electrode site effect,F~2,22! 5 2.59,p . .10,E 5 0.904, nor a Period3 Electrode Site interaction,F~4,44! 52.61,p . .10,E5 0.458, suggesting that the equipotential midlinedistribution of N400 for the first presentation lasted over the threeperiods. In contrast, for the second presentation, the interactionbetween period and electrode site was reliable,F~4,44! 5 6.18,p , .02, E 5 0.359, reflecting that the frontally dominant N400at the 300–500-ms and 500–700-ms periods steeply declined atthe 700–900-ms period. The lateral data revealed no interac-tions including variables of period, repetition, and electrode site~or hemisphere!.

The analyses for the midline and lateral data of the scaleddifference of mean amplitudes between first and second presenta-tions revealed no interaction between period and electrode site.

PerformanceThe mean percentage of hit responses and the mean reaction timesfor attended nonwords were 70.6%~SD 5 11.7%! and 840 ms~SD 5 34 ms!, respectively. The mean percentage of false alarmswas 1.1%~SD5 1.1%! on trials containing unattended nonwordsand 1.1%~SD 5 0.8%! on word-only trials. There was no signif-icant difference in false alarms between these trials,t~11! , 1.

Discussion

The large attenuation of N400 was again observed when the wordrepetition was in the attended ear. However, the duration of N400was shorter than that in Experiment 1, and the repetition effectdisappeared at the 700–900-ms period. In addition, the marginalrepetition effect for AU trials observed in Experiment 1 was furtherreduced. As discussed by McCarthy and Nobre~1993!, even aslight difference in task parameters appears to change the atten-tional effects on N400. Especially as the shorter SOA in Experi-ment 2 required the earlier termination of nonword-detectionprocessing, attention might be more focused to the relevant ear andthus momentary shifts of attention to task-irrelevant inputs mightbe reduced.

GENERAL DISCUSSION

A main objective of the present two experiments was to investigatewhether spoken words delivered in the unattended ear have aninfluence on the ERP elicited by identical words immediately re-peated in the attended ear. The results clearly showed no influenceof unattended words, in contrast to the prominent N400 repetitioneffect when words were repeated in the attended ear. In light of theinterpretation that the N400 repetition effect reflects the relativeease of contextual integration~Bentin & McCarthy, 1994; Rugg &Doyle, 1994!, the absence of N400 repetition effect on the presentUA trials indicates that unattended words are not integrated withtheir contexts. This finding is consistent with the argument that the

Table 2. Mean Amplitudes (inmV) Over 300–800 ms Poststimulus for ERPs at Various Electrode Sites

Electrode

Presentation Fz Cz Pz F3 F4 C3 C4 P3 P4 Wl Wr

AttendedSame

First presentation on AA 22.3 23.1 23.0 21.7 22.2 22.8 23.0 21.5 22.6 22.2 22.6Second presentation on AA 21.4 21.6 20.8 21.7 21.8 21.4 21.5 20.2 20.6 20.9 20.9Repetition effect 0.9 1.5 2.2 0.0 0.4 1.4 1.5 1.3 2.0 1.3 1.7

OppositeFirst presentation on AU 22.1 22.5 22.3 21.8 21.3 22.5 22.4 21.9 22.0 22.3 22.3Second presentation on UA 22.3 22.5 22.4 22.2 22.0 22.5 22.2 21.8 21.9 21.6 21.9Repetition effect 20.2 0.0 20.1 20.4 20.7 0.0 0.2 0.1 0.1 0.7 0.4

UnattendedSame

First presentation on UU 21.5 22.5 22.3 21.5 21.5 22.1 22.4 21.8 22.4 21.8 22.3Second presentation on UU 21.8 22.3 22.3 21.7 21.6 22.1 22.0 21.7 21.6 21.9 21.8Repetition effect 20.3 0.2 0.0 20.2 20.1 0.0 0.4 0.1 0.8 20.1 0.5

OppositeFirst presentation on UA 21.6 22.0 22.1 21.1 21.2 22.2 22.1 21.7 22.0 22.1 22.2Second presentation on AU 21.9 22.1 21.7 21.5 21.3 21.7 22.0 20.9 21.5 21.2 21.4

Repetition effect 20.3 20.1 0.4 20.4 20.1 0.5 0.1 0.8 0.5 0.9 0.8

Note:Amplitudes listed are for first and second presentations of test words when attended and unattended stimuli are in the same and opposite condi-tions of Experiment 2.


integration process is not automatic but rather attentionally con-trolled. This finding also corresponds to a recent proposal thatattention at the time of semantic encoding helps the correct bindingof events to the context in which they occurred~Cowan, 1995!.Furthermore, this suggestion is consistent with a similar argumentbased on the attention-sensitive ERP repetition effect in a visualmodality ~Otten et al., 1993!.

As mentioned earlier, Bentin and McCarthy~1994! hypoth-esized that N400 reflects semantic access that is automaticallytriggered by the initiation of word analysis. The N400 attenuationis assumed to be elicited by repeated words because the immediateavailability of the task-related decision made for the first presen-tation interrupts the automatic access to semantic memory. Thishypothesis is also compatible with the present finding that theprominent repetition effect was observed on AA trials in which thelexical decision was required for both first and repeated presenta-tions of words but not on UA trials in which the decision wasunnecessary for the first word presentations in the unattended~ortask-irrelevant! ear.

The N400 wave for first presentations of spoken words wasdiffusely distributed with the central maximum, whereas the N400attenuation with repetition on AA trials was relatively focused overthe central and parietal sites. This contrast feature suggests thatnegative components other than the repetition-sensitive centropari-etal negativity are present in the N400 latency range. The mostprobable component is a sustained frontal negativity~SFN!, whichwas observed in tasks such as auditory recognition memory~Chao,Nielsen-Bohlman, & Knight, 1995! and short-term memory scan-ning ~Chao & Knight, 1996! and which was related to performanceeffort or allocation of cognitive resources. The frontally maximalnegative wave, which was observed for repeated words and mea-sured here as N400, appears to receive a larger contribution fromthe SFN than the centroparietal N400. Furthermore, the repetitioneffect on the initial phase of N400, which was slightly enhanced atthe frontal ~Fz0304! and central~C506! sites, suggests the exis-tence of another negativity. The enhanced negativity for repetitionmight correspond with the N2 component, which has a similarfrontocentral distribution and which is enhanced by immediatelyrepeated sounds~Chao et al., 1995!. Moreover, a recent study~Rugg et al., 1995! has suggested that the auditory repetition-sensitive N400 itself includes at least two components, one ofwhich is modality sensitive and the other lexically sensitive. Thus,these repetition effects suggest that the broad, large late negativityidentified as N400 includes multiple components. The present find-ing of no N400 repetition effect on UA and UU trials indicates that

unattended words do not modulate any component of the broadN400 wave elicited by immediate repetition.

As noted by Otten et al.~1993!, the slight N400 repetition effecton AU trials in Experiment 1 might reflect either a small repetitioneffect for unattended words on most of the trials or a clear repeti-tion effect on just a few trials. The first possibility suggests anattenuated or partial processing carried out on unattended items,whereas the second is assumed to be due to a failure of selection.The fluctuation of attention between the two ears might be relatedto an occasional reduction of processing demands for unattendedwords within the same context as the previous attended words.When semantic representations specific to unattended words areactivated by the previous attended words, the amount of informa-tion necessary for their identification might be reduced~Bentinet al., 1995!. The partial processing of unattended words appears tobe occasionally adequate to gain full lexical access. The full accessof repeated words in the unattended channel might recruit attention~i.e., trigger attentional shifts to those items! and induce attention-based further processing. As briefly discussed earlier, the reductionof the repetition effect on AU trials in Experiment 2 implies that anincrease of the processing demand due to short SOAs prevents amomentary shift of attention to an irrelevant input.

The repetition effect observed in UU trials by Otten et al.~1993! was reversely associated with a more negative shift. Thisnegative repetition effect, which they called thenegative shift, wasinterpreted in terms of the active inhibition of the processing ofunattended objects. That is, given that the inhibition associatedwith first presentations can be transferred to the second presenta-tions, the negative shift for the unattended repetition may reflecteither increased inhibition of the processing of the unattended itemor the consequential facilitated processing of the accompanyingattended item. In Experiments 1 and 2, however, no substantialrepetition effect was found on UU trials. This discrepancy may berelated to a modality difference in the inhibitory process of selec-tive attention.

Thus, robust repetition effects on the broad, large late negativ-ity, analyzed as N400, was observed only across word pairs withinthe attended ear, and no repetition effects were found if the firstword presentation was unattended. These findings indicate thatwords to the unattended ear do not receive enough processing tomodulate the N400 to immediately repeated words and that atten-tion plays an important role in producing the immediate repetitioneffects on the N400. However, it is difficult to assess the preciseextent of semantic encoding for unattended items because the cog-nitive processes underlying the N400 are not yet clear.

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~Received August 26, 1996;Accepted August 26, 1997!


selective attention and n400 attenuation with spoken word repetition

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