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Research Report

Mismatch negativity (MMN) elicited by changes in phonemelength: A cross-linguistic study

Sari Ylinena,b,⁎, Anna Shestakovaa, Minna Huotilainena,c,Paavo Alkud, Risto Näätänena

aCognitive Brain Research Unit, Department of Psychology, University of Helsinki, P.O. Box 9, FIN-00014,Finland, and Helsinki Brain Research Centre, Helsinki, FinlandbDepartment of Languages, University of Jyväskylä, P.O. Box 35, FIN-40014, FinlandcHelsinki Collegium for Advanced Studies, University of Helsinki, FinlanddLaboratory of Acoustics and Audio Signal Processing, Helsinki University of Technology,P.O. Box 3000, FIN-02015 HUT, Finland

A R T I C L E I N F O

⁎ Corresponding author. Cognitive Brain ReseFax: +358 9 191 29 450.

E-mail address: sari.ylinen@helsinki.fi (S.

0006-8993/$ – see front matter © 2005 Elsevidoi:10.1016/j.brainres.2005.12.004

A B S T R A C T

Article history:Accepted 3 December 2005Available online 19 January 2006

Speech sounds representing different phonetic categories are typically easier todiscriminate than sounds belonging to the same category. This phenomenon isreferred to as the phoneme boundary effect. We aimed to determine whether, atneural level, this effect is indeed due to crossing the phoneme boundary. Themismatch negativity (MMN) brain response was measured for across- and within-category changes in Finnish phoneme length in native speakers and second-languageusers of Finnish as well as non-Finnish-speaking subjects. The results showed thatthe MMN amplitude was enhanced in the native speakers in comparison with thetwo non-native groups which, in turn, did not differ from each other in MMNamplitude. The response pattern to across- and within-category changes, however,was the same in all groups regardless of whether or not they had the phonemecategories. Thus, the responses could not be determined by crossing the phonemeboundary. Rather, the enhancement of MMN amplitude in the native speakers islikely to be due to the activation of native-language phonetic prototypes. The second-language users, however, did not seem to have automatic access to Finnishprototypes.

© 2005 Elsevier B.V. All rights reserved.

Keywords:Mismatch negativity (MMN)Phoneme boundary effectPhonetic categoryPhonological quantitySpeech–sound durationSecond language

1. Introduction

The mapping of acoustically variable speech sounds ontolinguistically significant categories requires that language-specific distinctive features are utilized, whereas linguisti-

arch Unit, Department of

Ylinen).

er B.V. All rights reserved

cally irrelevant acoustic variance is ignored. Therefore, thediscrimination of the phonologically relevant acoustic fea-tures of speech sounds is typically more accurate betweenphonetic categories when a phonetic boundary is crossedthan within a category. This phenomenon is known as the

Psychology, P.O. Box 9, FIN-00014 University of Helsinki, Finland.

.

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phoneme boundary effect or, to use stricter terms, categoricalperception.1 The research on categorical perception andphoneme boundary effect has aimed to reveal the mecha-nism underlying the perception of speech sounds. Thefindings of early experiments were interpreted to suggestthat categorization is based on phonemic labels (Liberman etal., 1957). Further studies indicating that also infants showphoneme boundary effect were interpreted as evidence foran innate processing mechanism specialized for speech(Eimas et al., 1971). However, these views were challengedby Kuhl (1981) and Kuhl and Padden (1983) who demon-strated that non-human animals show phoneme boundaryeffect, suggesting that sometimes the effect may be due tononlinearity in auditory processing. Nonetheless, sincedifferent languages have different phonetic features andphoneme boundary effects are language-specific (e.g., Miya-waki et al., 1975), it is clear that phonetic categories and,consequently, category-related perceptual effects are pri-marily determined by language experience.

Traditionally, perceptual effects related to phonetic cate-gories have been studied with behavioral identification anddiscrimination tasks using a stimulus continuum spanningtwo or more phonetic categories (e.g., Liberman et al., 1957).However, since these tasks involve decision making that canbe based on different cognitive strategies, it is difficult todetermine to what extent they tap the neural organization ofperceptual space (see, e.g., Massaro, 1987; Schouten et al.,2003). More recently, several studies have suggested thatcategory-related effects are reflected not only in the subject'sbehavioral response, but also in electrophysiological andmagnetic brain measures which do not require subject'sattention or reaction, namely, in the mismatch negativity(MMN) response (Dehaene-Lambertz, 1997; Näätänen et al.,1997; Sharma and Dorman, 1999, 2000; Winkler et al., 1999a,b)and itsmagnetic equivalent (Phillips et al., 2000; Shestakova etal., 2002). The MMN is a component of the auditory event-related potential (ERP) that reflects automatic, pre-attentivechange detection in any repetitive aspect of auditory stimula-tion (Näätänen et al., 1978; for reviews, see Näätänen, 2001;Picton et al., 2000). It is elicited by an acoustic change (e.g.,change of frequency, duration, intensity, location, or pattern)in a stimulus streamof speechornon-speech soundswhen thechange is discernible. Typically, the MMN peaks at 100–250msafter change onset.

1 According to the definition of categorical perception, discri-mination performance should be determined by the ability tocategorize stimuli, i.e., discrimination sensitivity should beenhanced at the phoneme boundary, but within-category dis-crimination should be at a chance level (Studdert-Kennedy et al.,1970; see also Liberman et al., 1957). The criterion on chance-levelwithin-category discrimination, however, has usually not beenmet in perceptual experiments on vowels and some consonantfeatures, suggesting that their discrimination is constrained butnot entirely determined by categorization (for a review, seeStrange, 1999). In line with Wood (1976) and Iverson and Kuhl(2000), the term phoneme boundary effect is used in the presentstudy to refer to a discrimination peak at the boundary andreduced, but not chance-level, sensitivity within the category tohighlight the difference to Studdert–Kennedy's and colleagues'definition of categorical perception.

To elicit an MMN, stimuli are usually presented in anoddball paradigm where repetitive, standard, stimuli areoccasionally replaced with deviant stimuli differing from thestandard in some acoustic feature. According to Näätänen(1990), the repetitive standard stimuli form and maintain asensory memory representation of the repetitive features ofthe stimuli and, further, if a deviant stimulus occurs duringthe lifetime of this representation, then an MMN is elicited(see also Schröger, 1997). Generally, the MMN amplitudeincreases when the acoustic discrepancy between the deviantand standard stimuli increases. With phonetic stimuli,however, this regularity is not as straightforward as describedabove. The neural networks encoding speech information aremodified by exposure to ambient language, and this isreflected in the mismatch process (for reviews, see Krausand Cheour, 2000; Näätänen, 2001): stimuli representingnative-language phonemes elicit an enhanced MMN responsein comparison with non-native speech sounds (Dehaene-Lambertz, 1997; Näätänen et al., 1997) and, similarly, mean-ingful words elicit an enhanced MMN compared to thatelicited by pseudowords (Pulvermüller et al., 2001, 2004;Shtyrov and Pulvermüller, 2002). Moreover, the typicality orfamiliarity of speech sounds facilitates the formation ofsensory memory representations (Huotilainen et al., 2001).Together, these results suggest that even at the pre-attentivelevel, long-term memory representations automatically par-ticipate in the processing of speech features in the brain, andthe MMN can be used as a tool to probe them.

Even though several behavioral and electrophysiologicalstudies report the language-specific phoneme boundaryeffect, it is not always clear whether crossing the boundarybetween two phonetic categories is, in fact, the critical factorin eliciting the effect at a neural level. Since phoneticcategories are internally organized according to the typicalityof instances within a category (Iverson and Kuhl, 1996; Kuhl,1991; Miller et al., 1983; Samuel, 1982), the extent of typicalitymay sometimes account for the phoneme boundary effect (seeIverson and Kuhl, 2000). Thus, in the present study, our firstaimwas to determine whether, as indicated by theMMN brainresponse, the phoneme boundary effect is reflected in the pre-attentive processing of the Finnish quantity categories2 (i.e.,categories for phoneme length), and whether the effect isindeed induced by crossing a phoneme boundary. In a recentstudy, we found that in behavioral categorization anddiscrimination tasks native speakers of Finnish showed aphoneme boundary effect for the Finnish quantity, whereasnon-native listeners did not (Ylinen et al., 2005). If, at theneural level, the effect is due to the crossing of the phonemeboundary which results in the activation of neural representa-tions for different phonemes, the MMN elicited by an across-category quantity change is expected to be larger in amplitudethan that elicited by a within-category change of the sameduration difference. This should, however, hold only for thosesubjects who know the target language and are thus supposed

2 The term quantity is used when referring to a phonologicaldistinction between short and long sounds, and the term durationis used when referring to a physical sound feature. In Finnish, theprimary cue of quantity is the duration of speech sounds.

Fig. 1 – The waveforms of the stimuli. In the Categorizationexperiment, Stimuli 1–7 were used. In the oddball blocks ofthe MMN experiments, Stimulus 5 was presented as theStandard, Stimulus 2 as the Across-category deviant, andStimulus 8 as theWithin-category deviant. The duration (ms)of each manipulated vowel has been rounded to the nearestmillisecond and given below the vowel in question. Thedurations of the other sounds are given below the 8th step ofthe Word continuum. Dashed vertical lines illustrate thesegmentation criteria.

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to have categories for quantity and, consequently, a phonemeboundary between the quantity degrees.

In Finnish, quantity is quite a frequent and importantphenomenon. For example, all vowels can occur as eithershort or long in any position of the word, and most of theconsonants can be short or long within the word. Due tothe fact that the quantity degrees can distinguish differentwords (e.g., tuli /tuli/ ‘fire’ and tuuli /tu:li/ ‘wind’), the correctcategorization as well as the comprehensible realization ofthe quantity degrees can be essential in the perception andproduction of spoken Finnish. Second-language (L2) users,however, often have difficulties with quantity (Han, 1992;Hayes-Harb, 2005; Hirata, 2004b; McAllister et al., 2002). Tobecome a competent L2 user, one must modify thephonological system established for the native language(L1) during the first year of life and possibly establish newcategories for those sounds that cannot be mapped onto L1categories (see Speech Learning Model by Flege, 1995). Themodification of the phonological system should also involvethose features that are used contrastively in L2 but not inL1. McAllister et al. (2002) have proposed in their FeatureProminence Hypothesis that L2 phonological contrastsinvolving such features are more difficult to acquire thanthose involving features relevant for L1. Their hypothesiswas supported by results suggesting that L2 users' perfor-mance in the Swedish quantity contrast varied as afunction of the relevance of duration in the L1. In contrast,Bohn (1995) has proposed that duration cues are easilyaccessible to non-native listeners regardless of the role ofduration in L1. In our earlier studies (Nenonen et al., 2003;Ylinen et al., 2005), we investigated the perception ofduration cues and quantity in Russian L2 users of Finnishwhose L1 uses duration cues differently than Finnish;Russian does not use duration in a phonological quantitydistinction, but as a cue for word stress. The findings ofthese studies are consistent with views of both McAllister etal. (2002) and Bohn (1995) in a sense that even thoughnative speakers of Finnish are more sensitive to durationcues in speech sounds, also non-native listeners perceivethem (Nenonen et al., 2003; Ylinen et al., 2005). However,non-native listeners' access to duration cues may not resultin the establishment of quantity categories without extensiveexposure to L2 (Ylinen et al., 2005). With regard to this, thesecond aim of the present study was to determine whetherlanguage learning is reflected in Russian L2 users' behav-ioral and brain responses to quantity by comparing L2 userswith native speakers of Finnish and native speakers ofRussian who cannot speak Finnish (hereafter referred to asnaive Russians). In particular, we aimed to reveal whetherL2 users show similar category-related effects as nativespeakers do.

Three experiments were included in the present cross-linguistic study. First, a behavioral test of quantity categori-zation in a word context as well as in isolated vowels wascarried out to determine the individual boundaries betweenthe categories for each subject and to compare the consis-tency of categorization and boundary location between thegroups (see Fig. 1 for stimuli and Experimental procedure fordetails). According to our previous study (Ylinen et al., 2005),the consistency of categorization may reveal to what extent

the quantity categories are accessed. Second, to assess theinvolvement of phonetic prototypes in the processing ofquantity, typical vowel durations for Finnish and Russianwere determined on the basis of word-production data.Third, an MMN experiment was conducted to investigatepossible differences in the processing of the across- andwithin-category changes of quantity degrees at the neurallevel in each group. Also in the MMN experiment, thechanges were introduced in a word context as well as inisolated vowels (see Fig. 1 for stimuli and Experimentalprocedure for details).

2. Results

2.1. Categorization experiment

According to a two-way analysis of variance (ANOVA) testingdifferences in the consistencyof categorization,maineffects of

Fig. 2 – The categorization of the Finnish quantity in native speakers of Finnish, Russian second-language (L2) users of Finnish,and non-Finnish-speaking (naive) Russians in the two conditions.

3 Amplitude differences in the early exogenous components ofthe ERPs (see Fig. 3) reflect a different state of refractoriness,caused by a constant stimulus onset asynchrony (SOA) (Imada etal., 1997). Since the refractoriness effects are observed in earlycomponents only, i.e., before the MMN time range (Imada et al.,1997; Jacobsen and Schröger, 2003), it is implausible that theydistorted the MMN.

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Group (Native speakers vs. L2 users vs. Naive Russians) [F(2,36) = 3.40, P b 0.05] and Condition (Word vs. Isolated vowel) [F(1,36) = 12.86, P b 0.001] were significant, whereasGroup × Condition interaction was non-significant [F(2,36) b 1]. A post hoc test for the Group main effect revealedthat the native speakers' categorization was significantlymore consistent than that of the naive Russians (P b 0.05). Asimilar trend was found between the native speakers and theL2 users as well, but it was only marginally significant(P b 0.06). The consistency of categorization did not differbetween the L2 users and the naive Russians (n.s.). Accordingto the post hoc test for the Condition main effect, thecategorization was more consistent in the Word than in theIsolated vowel condition.

The category boundary between short and long vowelsoccurred between Stimuli 2 and 5 in all subject groups (seeFig. 2). For the category boundary locations, an ANOVAshowed a significant Group main effect [F(2,36) = 7.73,P b 0.01]. According to a post hoc test, the native speakers'category boundary (120 ms) occurred at a significantlyshorter duration than that of the L2 users (135 ms; P b 0.01)and the naive Russians (135 ms; P b 0.01), whereas theboundaries of the L2 users and the naive Russians did notdiffer from each other (n.s.). In addition, the main effect ofCondition was significant [F(1,36) = 5.52, P b 0.05], indicatingthat in all groups, the category boundary occurred at shorterdurations in the Word condition than in the Isolated vowelcondition. Group × Condition interaction was non-significant[F(2,36) b 1].

2.2. Production experiment

For the assessment of prototypical duration of Finnish andRussian vowels, duration ratios were calculated to eliminatethe effect of the speaking rates of the individual subjects onsound durations (see Experimental procedure for details).Given that an average (short) sound in an utterance had value1, the value of Finnish short vowel produced by nativespeakers of Finnish was 0.90, and that of long vowel was1.83. The value of the Russian vowel produced by nativespeakers of Russian was 1.32. All values differed significantlyfrom each other [F(2,24) = 149.87, P b 0.001; for all post hoccomparisons P b 0.001].

2.3. MMN experiment

Figs. 3 and 4 show the ERPs3 and the difference waveforms,respectively. In the three subject groups, a significant MMNwas elicited in all conditions at the frontal and centralelectrodes (P b 0.05) within a time range that is typical toMMN, i.e., 100–150 ms from change onset (see Fig. 3 forillustration on timing with regard to change onsets that aremarked with arrows).

An ANOVA showed that the MMN amplitude differedbetween the groups (see Table 1 and Figs. 3 and 4): themain effect of Group was significant [F(2,34) = 5.38, P b 0.01].A post hoc test revealed that the effect was due to largerMMN amplitudes in the native speakers than in the L2users (P b 0.01) or in the naive Russians (P b 0.05). In con-trast, the Russian L2 users and the naive Russians did notsignificantly differ from each other. The main effects ofCondition [F(1,34) = 6.75, P b 0.05] and Electrode were alsosignificant [F(8,272) = 56.57, P b 0.001]. The main effect ofCondition was due to the larger MMN amplitude in theIsolated vowel than in the Word condition and that ofElectrode to the maximal amplitudes at the frontal andcentral scalp sites. In addition, the Condition × Deviant-typeinteraction was significant [F(1,34) = 16.24, P b 0.001]. It wasnot of interest, however, because Condition and Devianttype did not interact with Group, suggesting that theinteraction was not language-specific.

3. Discussion

The present study aimed to determine whether the phonemeboundary effect induced by the crossing of a phonemeboundary is reflected in the MMN brain response to changesof phonological quantity degrees. In addition, our goal was toinvestigate the possible group differences between native

Fig. 3 – The ERP responses elicited at Fz electrode in the two conditions of the MMN experiment in native speakers of Finnish,Russian second-language (L2) users of Finnish, and non-Finnish-speaking (naive) Russians. Stimulus 2 represented anacross-category change and Stimulus 8 a within-category change in the oddball blocks. The 100% responses were elicited bythe same stimuli but presented with a 100% probability. Change onset is marked with an arrow.

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speakers and L2 users of Finnish in comparison with naivesubjects in the behavioral andMMN responses. The purpose ofthe Categorization experiment was to define the individualcategory boundaries between the quantity categories and,accordingly, to establish which stimulus changes constitute

Fig. 4 – The MMN responses elicited by the across- andwithin-category quantity changes in native speakers ofFinnish, Russian second-language (L2) users of Finnish, andnon-Finnish-speaking (naive) Russians in the twoconditions of the MMN experiment. Difference waveformsat Fz electrode, showing maximal MMN amplitude.

across- and within-category changes for each subject. Inaddition, we aimed to reveal possible differences betweenthe groups in the consistency of quantity categorization aswell as phoneme boundary location.

The consistency of categorization and the categoryboundary locations differed between the two conditions inall subject groups. Since the same acoustic variation wasintroduced in both conditions, the differences between themindicate that subjects did not respond on the basis of theacoustic cues only. The fact that in all groups, the categoryboundary occurred at shorter durations in the Word

Table 1 – MMN amplitudes (μV, ±SD) at Fz in nativespeakers of Finnish, Russian second-language (L2) usersof Finnish, and non-Finnish-speaking (naive) Russians inthe two conditions of the MMN experiment

Word condition Isolated-vowelcondition

Across-category

Within-category

Across-category

Within-category

Nativespeakers

−3.50 (±2.35) −2.47 (±2.34) −3.39 (±2.20) −4.19 (±1.79)

Russian L2users

−2.10 (±1.68) −1.07 (±1.27) −1.60 (±1.68) −2.16 (±1.59)

NaïveRussians

−2.24 (±1.27) −1.28 (±1.41) −2.24 (±1.81) −2.93 (±1.73)

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condition than in the Isolated vowel condition might be dueto the language-general tendency of vowel durations to beshorter within a word than in a word-final position (Oller,1973). The result that the categorization was more consistentin the Word than Isolated vowel condition, in turn, suggeststhat quantity categorization was facilitated when the vowelwas embedded in a word. This might be due to the increasedinformation with regard to the speaking rate provided by theword context.

The categorization results were in line with those of ourprevious study (Ylinen et al., 2005) showing that in the Russiangroups, the boundary between the short and long vowelsoccurred at longer durations than in the native speakers ofFinnish. In the naive Russians, however, this cannot be a realphoneme boundary, since they could not have categories forthe Finnish quantity. On the basis of our earlier study, it is alsoimplausible that they mapped the stimuli onto L1 word stresscategories regardless of the fact that in Russian vowelduration is associated with word stress (for details, see Ylinenet al., 2005). Still, the Russians' boundary location might beaffected by their L1, because they may have used a strategybased on the L1. Since in a word-stressed position, Russianvowels are typically longer in duration than the Finnish shortvowels (see Production experiment), using the mental repre-sentation of a typical Russian CVCV word as a reference andevaluating stimulus vowels as long when they were overlongin comparison with L1 vowel could result in such a boundaryas observed in the present as well as in the earlier study. Thus,the naive Russians boundary could reflect a response strategyrather than a true phoneme boundary (see Massaro, 1987).Importantly, the native speakers also categorized the stimulimore consistently than the Russians did (reaching onlymarginal significance, however, between the native speakersand the L2 users). The differences in the boundary locationsand especially in the consistency of categorization betweenthe groups suggest that the native speakers of Finnish mayhave benefited from the mapping of stimuli onto categoriesfor L1 quantity degrees. However, the location of the categoryboundary or the consistency of categorization did not differbetween the naive Russians and the Russian L2 users,suggesting that the L2 users probably did not benefit fromthe quantity categories. Thus, the L2 users' categorizationmight be determined by L1 rather than L2. Interestingly, if thedifference between the native and non-native subjects wasindeed due to the mapping of stimuli onto Finnish quantitycategories in the former but not in the latter groups, then thenative speakerswere able tomap also the isolated vowels ontothe categories regardless of the lacking word context, becauseno significant Group × Condition interaction for the consis-tency of categorization was found.

After validating across- and within-category changes forFinnish-speaking groups, the question whether crossing of aphoneme boundary affects pre-attentive processing can beaddressed. Previously, it was hypothesized that if the MMNreflects a pre-attentive phoneme boundary effect, then theacross-category change would elicit a larger MMN than thatelicited by the within-category change in native speakers ofFinnish, but not in naive Russians who lack the categories forquantity. However, the patterns of the MMN results of thethree groups did not differ from each other, as indicated by the

lack of interactions involving the Group factor. Thus, it seemsunlikely that the crossing of the phoneme boundary affectedthe MMN amplitude in the Finnish-speaking groups.

Nevertheless, it still is possible that the Group main effectof the MMN experiment was category-related. According tothis effect, the MMN amplitudes for duration changes werelarger in the native speakers than in the L2 users or the naiveRussians, whereas the Russian L2 users and the naiveRussians did not differ significantly from each other. Thismight be due to the fact that the two deviant stimuli used inthe experiment matched the L1 prototypes of the nativespeakers (see Production experiment) but did not match thoseof the Russian groups. The duration proportion of the vowel inStimulus 2, i.e., the Across-category deviant (0.92), matchedthe proportion of the Finnish short vowel pronounced by thenative speakers in the Production experiment (0.90). For theRussians, the match was not as good, since the durationproportion of the vowel in the Russian word was 1.32 in theProduction experiment. Similarly, the vowel proportion ofStimulus 8, i.e., the Within-category deviant (2.17), matchedmore closely the proportion of the Finnish long vowelpronounced by the native speakers (1.83) than that of theRussian vowel pronounced by the Russians (1.32).

Even though one might expect that language-specificeffects in the processing of quantity would be stronger inthe Word condition than in the Isolated vowel condition dueto minimal speaking-rate information in the latter, in theMMN experiment, the native speakers differed from theRussian groups throughout the deviant types and conditions,suggesting a similar degree of facilitation for the processing ofwords and isolated vowels. Correspondingly, in the Catego-rization experiment, the native speakers differed from theRussians in consistency of categorization in both conditionsand seemed to be able to map the words as well as theisolated vowels onto quantity categories. Possibly, an exper-imental design with constant stimulation generates a suffi-cient context for the activation of phoneme prototypes.Moreover, even though there are no V vs. VV minimal-pairwords in Finnish, there is a phonological distinction consist-ing of V vs. VV syllables (e.g., /u/ vs. /u:/ in uni /uni/ ‘dream’ vs.uuni /u:ni/ ‘oven’) as well as one-syllable minimal-pair words[e.g., te /te/ ‘you (plural)’ vs. tee /te:/ ‘tea’]. In the latter case, ifthe words are presented in isolation, one must recognize thequantity degree with no speaking-rate information from theadjacent sounds in order to recognize the word. Therefore, itis possible that some kind of long-term memory representa-tions for V vs. VV exist and, further, that even the MMNelicited by isolated vowels may be affected by the typicality oftheir duration.

As an alternative to the prototype explanation, thedifference found between the native speakers of Finnish andthe Russian groups in the MMN experiment could also be dueto native Finnish speakers' general sensitivity to durationchanges in speech sounds. This interpretationwould be in linewith our previous finding (Nenonen et al., 2003) on durationchange detection in native speakers and advanced L2 users ofFinnish: even though a duration change in a non-speechsound elicited closely resembling MMN responses in the twosubject groups of that study, the MMN to duration change in aspeech sound was larger in the native speakers of Finnish,

4 Unlike the other studies that found an enhanced MMNresponse to L2 speech sounds, Peltola et al. (2003) observed adiminished response to L1 sounds in L2 learners.

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suggesting the speech-specific tuning of duration processingin this group. However, Dehaene-Lambertz et al. (2000)demonstrated that the MMN reflects the parsing of pseudo-word syllable structure into L1 prototypes. Thus, the activa-tion of the L1 prototypes, rather than the tuning of durationprocessing, may account for the present results as well (seealso Näätänen et al., 1997).

In previous studies, the comparison of MMNs elicited byacross- andwithin-category changes has revealed thatMMN isenhanced by across-category changes at least in vowels(Winkler et al., 1999b), place of articulation of consonants(Dehaene-Lambertz, 1997), and voice onset time (VOT) of stopconsonants (Sharma and Dorman, 1999). It is well establishedthat phonetic categories have an internal structure in terms ofthe degree of typicality (Iverson and Kuhl, 1996; Kuhl, 1991;Miller etal., 1983;Samuel, 1982). Thus, asnotedbyWinkleretal.(1999b), their results on across- and within-category vowelchanges may be affected by the typicality of the stimuli. Thesame may hold for consonant changes as well. In an MMNexperiment by Sharma and Dorman (2000), syllables startingwith pre-voiced stop consonants with VOT values of −10 and−50 ms were used as a standard and a deviant, respectively.Behaviorally, native English listeners categorized all of thestimuli as /ba/,whereasHindi listeners categorized−10msVOTas /pa/ and −50 ms VOT as /ba/. A significant MMN wasobserved only in the Hindi listeners. This difference betweenthe groups may, however, be due to a higher degree oftypicality of the stimulus with −50 ms VOT to Hindi thanEnglish listeners, since short-lag VOT is more typical toEnglish voiced stops than pre-voicing (Lisker and Abramson,1964). In the study by Dehaene-Lambertz (1997), across-category /ba/ vs. /da/ distinction elicited an MMN in Frenchlisteners, whereas dental vs. retroflex distinction, notphonemic in French, did not. Again, the Hindi retroflexwas less prototypical to French listeners than the stimuliused in the across-category change, suggesting that theresults may be affected by typicality.

However, typicality seems a less likely account for anotherstudy by Sharma andDorman (1999), where VOT values typicalto English were used. The results suggested that an across-category change from 30 ms VOT to 50 ms VOT elicited anMMN,whereas awithin-category change from60msVOT to 80ms VOT did not. Additionally, in the study by Dehaene-Lambertz (1997), a within-category change between two /ba/syllables elicited a smaller response than an across-categorychangedespite the fact that thedeviant stimuluswas the samefor both changes. Since also the typicality of standard stimulusmay affect the MMN (Huotilainen et al., 2001), one could askwhether there were any differences in the typicality of thestandards, given that the stimulus continuum used in thestudy was originally designed for English speakers by Werkerand Lalonde (1988). However, since in behavioral tasksconsonants have been observed to show stronger and morerobust category-related effects than vowels (see Strange, 1999,for a review), it is possible that the neural representations ofvowels and consonants differ from each other. The pre-attentive processing of consonant features, such as VOT andplace of articulation, may be less susceptible to typicalityeffects than that of vowels and some other contrasts, such asquantity, that show a phoneme boundary effect but not

categorical perception (in a strict sense) in behavioral tests.Unfortunately, it is difficult to assess the effect of stimulustypicality on some of the abovementioned results, since nodata on this issue were reported. As some other studies havesuggested that the MMN is enhanced by the typicality ofspeech stimuli (e.g., Huotilainen et al., 2001; Näätänen et al.,1997), it should be taken into account in future studieswith anytype of speech sounds.

In addition to category-related effects, the present studyaddressed L2 learning. Interestingly, theMMNresponses of thenaive Russians and the Russian L2 users did not differsignificantly from each other. This suggests that learningFinnish as L2 had not affected the pre-attentive processing ofchanges in quantity that is cued by duration. Aswith theMMN,no evident learning effect was observed in the L2 users'behavioral categorization of quantity when compared withnative speakers and naive subjects. Thus, the present resultssuggest that behavioral categorization as well as pre-attentiveprocessing of quantity degrees were largely determined by thesubjects' L1 rather than L2. The results are in contrast withthose of previous studies (e.g., Cheour et al., 2002; Peltola et al.,20034; Shestakova et al., 2003; Winkler et al., 1999a) that showthe effects of language learning on the MMN elicited bysegmental-level phonetic stimuli. Since the age of acquisitionaffects L2 learning, it is noteworthy that in the studies byCheour et al. (2002) and Shestakova et al. (2003), subjects were3- to 6-year-old children and thus exposed to L2 markedlyearlier than in the present study. The study by Winkler et al.(1999a), however, is comparable with the current one, becausethe age of the onset of L2 exposure in their study was close tothat in the present study (according to Winkler et al., 13–32years, with the exception of one participant who was 7 yearsold at the onset of L2 learning).

The lack of the language-learning effect in the presentstudy as opposed to Winkler et al. (1999a) might be due to thefact that the present study involved durational rather thanspectral characteristics of speech sounds. McAllister et al.(2002, p. 256) stated in their Feature Prominence Hypothesisthat “re-attunement of the L1 phonological system to newfeatures, in this case duration, can be difficult for those L2learners whose L1 does not exploit the feature in question.”Furthermore, according to the hypothesis, these kinds of L2learners may be better able to attune their phonologicalsystem to spectral rather than to durational cues of L2, sincethe spectral cues are used in L1 phonological contrasts,whereas the durational cues are not. This may be due todifferent optimal periods for acquisition of different phono-logical features (Werker and Tees, 2005). Infants are sensitiveto rhythmic properties of language, and they learn torecognize the prosodic properties of their L1 before 5 monthsof age (see Nazzi and Ramus, 2003, for a review). Thus, theperception of the rhythmic features of speech is attuned to L1earlier than that of sound segments. Werker and Tees (2005)have suggested a cascading model of optimal periods oflanguage acquisition, where each level of processing rein-forces and constrains preceding and following sensitivities.

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Therefore, if rhythmic features are acquired at an earlier phasethan sound segments in a cascading system, these language-specific sensitivities may be more difficult to change afterhigher levels of L1 processing have been established. Thishypothesis also implies that the learning of different L2features may be differently constrained by the age of acquisi-tion (see Mueller, 2005). Our earlier studies (Nenonen et al.,2003, 2005) disclosed no native-like attuning of durationprocessing in 10- to 14-year-old advanced L2 users of Finnishwho started to learn L2 on average at the age of 7 years (i.e.,earlier than the adult subjects of Winkler et al., 1999a), whentarget sounds could be processed via the L1 phonologicalsystem. Therefore, the discordance between our results andthose by Winkler et al. (1999a) might suggest that in compar-ison with the processing of spectral cues, that of durationalcues is to a lesser degree prone to plastic changes during L2learning.

To conclude, the phoneme boundary effect was notreflected in the pre-attentive processing of quantity asdetermined by using theMMN. Nevertheless, larger amplitudeMMN responses were elicited by both across- and within-category changes of Finnish quantity in the native speakers ofFinnish in comparison with the two Russian groups (Finnish-speaking and non-Finnish-speaking). This may be due to thepre-attentive activation of Finnish phonetic prototypes in thenative, but not in the non-native subjects. Correspondingly,the native, but not the non-native, subjects seemed to benefitfrom phonetic categories for quantity in the behavioralcategorization task.

4. Experimental procedure

4.1. Categorization experiment

The categorization of the vowel quantity was investigated in twoconditions – Word and Isolated vowel – using stimulus continuawith variable vowel duration (see Fig. 1, Stimuli 1–7). Since thecategorization of quantity is based on relative, rather thanabsolute, duration in the speech context (Hirata, 2004a; Lehto-nen, 1970; Pickett et al., 1999; Pind, 1995), the Word condition wasincluded to provide sufficient contextual information for thecategorization. The Isolated vowel condition introducing thesame vowel without the word context served as a control ofacoustic variation. The continua consisted of seven steps, thevowel duration being the shortest in Stimulus 1 and the longestin Stimulus 7. In the Word condition, the continuum wassynthesized from pseudoword tuuku [tu:ku], pronounced by afemale native speaker of Finnish. Only the duration of the first-syllable vowel was varied between the end points tuku [tuku](with a short vowel) and tuuku [tu:ku] (with a long vowel). Theduration was varied by concatenating the fundamental cycles ofthe vowel [u] at the end of the first syllable [tu]. The vowelquality and duration were identical in the Word and Isolatedvowel conditions, for the isolated vowel was cut from the wordcontext for each step of the continuum. In both conditions, theduration change between the adjacent steps of the continua wasca. 20.5 ms, corresponding to 5 fundamental cycles.

Three subject groups were studied: native speakers of Finnish,non-Finnish-speaking, naive Russians, and Russian L2 users of

Finnish. The native speaker group consisted of 13 native speakersof Finnish with monolingual familial background (19–34 years old,on average 24.6 years, 2 males). The group of the naive Russiansconsisted of 12 subjects (19–41 years old, on average 27.5 years, 4males) with Russian as their L1. They reported having nocompetence in Finnish except for a few words. The L2 usergroup, in turn, consisted of 14 subjects (19–33 years old, on average24.9 years, 2 males) with Russian as their L1 and Finnish as theirL2. On average, the L2 users had lived in Finland for 7 (±3) yearsand had spoken Finnish for 8 (±4) years. Their age at onset oflearning Finnish was 17 (±3) years, and the age on moving toFinland was 18 (±4) years. Three L2 users had been exposed to theFinnish language in childhood because they had Finnish-speakingrelatives, but they reported that they could not speak orunderstand Finnish before they started to study Finnish. All L2users spoke Finnish fluently, but most of them were not native-like bilinguals. On a 1-to-5 rating scale, they evaluated their L2skills from 3 (moderate) to 5 (native-like), with the mean ratingbeing 3.6. All subjects reported that they had no language, speech,or hearing impairment, and were right-handed, with the excep-tion of one left-handed or ambidextrous subject in each group.

Subjects participated in a two-alternative, forced-choice cate-gorization task with two conditions: Word and Isolated vowel,respectively. They were instructed to respond to each stimulus bymarking on an answer sheet whether they heard the word /tuku/or /tu:ku/, or isolated /u/ or /u:/. The stimuli were presented viaheadphones at a 65-dB hearing level (HL) in an acousticallyshielded room. Before each condition, subjects heard a practicesequence of 3 stimuli (Stimuli 1, 4, and 7) twice. During therepetition of the practice sequence, they were asked to mark theperceived vowel length on the answer sheet. Before starting thetest sequence, subjects were asked whether they understood thetask or had any questions. In both conditions, the 7 stimuli of theduration continuum were presented 10 times each in a randomorder in trains of 5 stimuli with a 2-s inter-stimulus interval (ISI;offset to onset) and a 4-s inter-train interval.

To compare the location of the category boundary and theconsistencyof categorizationbetweenthe threegroups, thenormaldistribution was fitted to each subject's categorization functions.Themeanvalueof thenormaldistributionrepresents theboundarylocation (50% cross-over point of the functions), and its standarddeviation (SD) indicates the consistency of categorization (thesmaller the SD, the steeper the categorization function and themore consistent the categorization). Themean and SD valueswereused as dependent variables in two-way ANOVAs (Between-subjectfactor: Group;Within-subject factor: Condition). LSD (Least SignificantDifference) was used as a post hoc test here and further on.

4.2. Production experiment

To evaluate the typical vowel durations in Finnish andRussian, the native speakers of Finnish were asked to readaloud Finnish sentences inwhich pseudowords tuku and tuukuwere embedded. In addition, the L2 users read Russiansentences which included a similar Russian word that hadstress on the first syllable. The vocalizations were recordedwith a digital audio tape recorder, utterances were segmented,and sound durations were measured. To eliminate the effectof different speaking rates on sound durations, duration ratioswere calculated by dividing the absolute durations of the

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vowels of interest by the mean duration of a sound in theutterance. For statistical comparison, the data were submittedto a one-way ANOVA.

4.3. MMN experiment

To ensure that stimuli of the MMN experiment represented trulyacross- and within-category changes for Finnish-speaking sub-jects, Stimuli 2 and 5 were chosen for an across-category change(see Fig. 2 for categorization of each stimulus). As a consequence,Stimulus 8 (not included in the Categorization experiment) waschosen for a within-category change to keep the duration changeequal for across- and within-category changes. Since Stimulus8 had longer vowel duration than preceding steps in thecontinuum, it is implausible that Finnish-speaking subjectscould have categorized it as having a short vowel.

The stimuli were presented in a passive oddball paradigmwitha repetitive Standard stimulus (P = 84%) and two deviant stimuli, anAcross-category deviant (P = 8%) and a Within-category deviant(P = 8%). The Standard was Stimulus 5, categorized by subjectsas CVVCV with a long vowel. Importantly, the standard stimulusthat probably determines the speaking rate and thus may serve asa reference for quantity categorization was the same for the twodeviant types (for previous results on context effects, seeSummerfield, 1981). The Across-category deviant, Stimulus 2,was 62 ms shorter in duration than the Standard and categorizedby subjects as CVCV with a short vowel. The Within-categorydeviant, Stimulus 8, was 62 ms longer in duration than theStandard, and thus represented CVVCV with a long vowel.

Similarly to the behavioral Categorization experiment, theMMN experiment consisted of two conditions, Word and Isolatedvowel conditions, where the vowels varying in duration werepresented within a word or in isolation, respectively. In the Wordcondition, the stimulus onset asynchrony (SOA) was 1000 ms. Inthe Isolated vowel condition, a 500-ms SOA was used because ofthe shorter overall duration of the stimuli. In addition, both Stimuli2 and 8, used in the oddball paradigm as deviants, were presentedwith a 100% probability in separate blocks (see Kraus et al., 1995;Sharma and Dorman, 1999, 2000). Stimulus blocks were presentedin a random order, counterbalanced between the groups.

The same subjects as those in the Categorization experimentparticipated in the MMN experiment. However, to obtain trulyacross- and within-category responses from the native speakersand the L2 users, only those subjects who had their categoryboundaries between Stimuli 2 and 5 in the Word condition of theCategorization experiment were included in the group results.These subjects thus unambiguously (N80%) categorized Stimulus 2as having a short vowel and Stimulus 5 and all further steps of thecontinuum as having a long vowel. This resulted in the exclusionof two L2 users (who found Stimulus 5 ambiguous and, therefore,whose individual category boundaries did not necessarily occurbetween Stimuli 2 and 5). These criteria were not applied to thenaive Russians as they did not have any language-specificcategories for the Finnish quantity, and thus, the Finnishboundary location could not play a role in their responses.

During the electroencephalogram (EEG) recordings, sub-jects watched a self-selected, muted movie in an acousticallyand electrically shielded room, while auditory stimuli werepresented via headphones at 65 dB HL. Subjects wereinstructed not to pay attention to sound stimuli, but rather

to the movie. The testing time for one subject was about 3h (including preparation and breaks).

Ag/AgCl electrodes were placed at F3, Fz, F4, C3, Cz, C4, P3, Pz,and P4 scalp sites and the two mastoids according to theinternational 10–20 system. Eye movements were monitoredwith electrooculogram (EOG) attached to the canthus of the eyeand below the eye. The nose-referenced EEG was recorded at a500-Hz sampling rate using NeuroScan system and SYNAMPSamplifiers. After the recording, the data were band-pass filtered(1–20 Hz, roll-off 24 dB/octave) and re-referenced to the average ofthe left and right mastoids to improve the signal-to-noise ratio. Toexclude the possibility of N2b (Näätänen et al., 1982; Näätänen andGaillard, 1983) contamination of the MMN, it was ensured beforethe re-referencing that all negative peaks interpreted as the MMNindeed showed in all subject groups a fronto-central maximumand polarity inversion at the mastoid sites. Epochs of −50–1000 msfor the Word condition and −50–500 ms for the Isolated vowelcondition were separately averaged for each stimulus type andbaseline was corrected using a −50–0 ms pre-stimulus interval.Epochs with artifacts exceeding ±75 μV at any channel, the first 10responses of each block, and 2 standards after each deviant wererejected. In order to avoid subtracting responses to physicallydifferent stimuli from each other (see Jacobsen and Schröger, 2003;Kraus et al., 1995; Sharma and Dorman, 1999, 2000), the differencewaveforms were created by using the ERP waveforms elicited bythe same stimulus in low- and high-probability positions: thewaveforms from the 100% blocks were subtracted from thecorresponding deviant-stimulus waveforms.

The mean amplitude of the MMN peak in the individualdifference waveforms was measured by centering a 20-mswindow on the latency of the grand-average difference waveformpeak for each subject group and condition. The statisticalsignificance of the MMN component was determined with one-tailed t tests. In addition, theMMN amplitude data were submittedto a four-way ANOVA (Between-subject factor: Group; Within-subjectfactors: Condition, Deviant type, and Electrode).

Acknowledgments

This work, as part of the European Science FoundationEUROCORES Programme OMLL, was supported by funds fromthe Academy of Finland (project numbers 80572, 77322, 79820,and 79821) and the EC Sixth Framework Programme underContract no. ERAS-CT-2003-980409. In addition, it was sup-ported by Langnet Graduate School in Language Studies(Finland). The authors wish to thank Drs. Teija Kujala andViola de Silva for their comments on an earlier version of themanuscript, Drs. Istvan Winkler and Thomas Jacobsen fordiscussions on the data, Eino Partanen for help with dataanalysis, as well as Hanna Anttila and Mietta Lennes for theirhelp with the Praat script.

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