Perceptual magnet effect in the light of behavioral and psychophysiological data

Perceptual magnet effect in the light of behavioraland psychophysiological data

Olli AaltonenCentre for Cognitive Neuroscience and Department of Phonetics, University of Turku, FIN-20014 Turku,Finland

Osmo EerolaCentre for Cognitive Neuroscience, University of Turku, FIN-20014 Turku, Finland

Ake HellstromDepartment of Psychology, Stockholm University, S-106 91 Stockholm, Sweden and Centre for CognitiveNeuroscience, University of Turku, FIN-20014 Turku, Finland

Esa UusipaikkaCentre for Cognitive Neuroscience and Department of Statistics, University of Turku, FIN-20014 Turku,Finland

A. Heikki LangCentre for Cognitive Neuroscience, University of Turku, FIN-20014 Turku, Finland and Department ofClinical Neurophysiology, Turku University Central Hospital, FIN-20014 Turku, Finland

~Received 19 July 1993; revised 22 April 1996; accepted 16 October 1996!

Finnish speaking adults categorized synthetic vowels, varying in the frequency of the secondformant (F2), as either /y/ or /i/. Two subject groups emerged: ‘‘good’’ and ‘‘poor’’ categorizers.In a /i/ rating experiment, only the good categorizers could consistently label their best /i/~theprototype, P!, being low in theF2 continuum. Poor categorizers rated /i/’s with highF2 values asPs. In a same/different~AX ! discrimination experiment, using the individual Ps and nonprototypes~NPs!, it was more difficult for good categorizers to detect smallF2 deviations from the P than froman NP~the ‘‘perceptual magnet effect’’!. For poor categorizers, the opposite effect was found. Thesame stimuli were used to record the mismatch negativity~MMN !, an ERP component reflectingpreattentive detection of deviations from a standard sound. For the good categorizers the MMNswere lower for Ps than for NPs; for the poor categorizers the MMNs for Ps and NPs did not differsignificantly. The results show that individual listeners behaved differently in categorization andgoodness rating but in the same way in attentive~AX ! discrimination, being the poorest at about thesameF2 location. The perceptual magnet effect was indicated in the good categorizers both bybehavioral and psychophysiological~MMN ! discrimination data. ©1997 Acoustical Society ofAmerica.@S0001-4966~97!05502-1#

PACS numbers: 43.71.Es, 43.71.An, 43.64.Qh@RAF#

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INTRODUCTION

In a series of papers, Kuhl and her collaborators~e.g.,Kuhl, 1991; Kuhlet al., 1992; Kuhl, 1993a! have suggestedthat speech perception is governed not only by the functioprinciples of the auditory system, but also by speech protypes in long-term memory. The proponents of the prototyapproach~e.g., Eimas and Corbit, 1973; Massaro and Od1989; Kuhl, 1991! hold that classification into a phonetcategory is based on a mental representation that coninformation about its characteristic and ideal attributrather than about its boundaries against other catego~Rosch and Mervis, 1975; Mervis and Rosch, 1981; Meand Barsalou, 1987!. The application of prototype theory hadiverted the emphasis of speech perception from the bouaries of categories to their centers.

In the boundary-based approach, the mental represetion of a category is thought to hold information only aboits boundaries~e.g., Studdert-Kennedyet al., 1970; Maddoxand Ashby, 1993!. Hence, discrimination is assumed po

1090 J. Acoust. Soc. Am. 101 (2), February 1997 0001-4966/97/10

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sible only for pairs of sounds whose members can be idtified as belonging to different phoneme categories. Thislation between identification and discrimination has beinterpreted as a consequence of categorial perception, wis also reflected as an increased sensitivity to stimulus difences at the boundaries. In contrast, the prototype-basedproach holds that members of a given phonetic categorydiscriminable in terms of their goodness as category explars, that is, some are perceived to be better exemplarsothers~e.g., Grieser and Kuhl, 1989!. Reaction times to iden-tification judgements of stimuli within a category dependthis goodness~Massaro, 1987!. It also appears that membeof a category can vary in their perceptual potency, providan internal structure to the category. For example, someegory exemplars are more effective than others in selecadaptation~Samuel, 1982; Milleret al., 1983! and in dichoticcompetition~Repp, 1977!.

Kuhl ~1991! provided the first evidence suggesting thdiscrimination is affected not only by physical factors balso by stimulus typicality. She showed that a vowel judg

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to be a prototype of its category was more difficult to dcriminate from its neighbors than a less prototypical onenonprototype. The recent experiments of Iverson and K~1995!, using signal detection theory and multidimensionscaling of choice reaction times, convincingly demonstrhow perceptual distances within a category shrink astretch in the regions of the best and the worst instanrespectively. The effect is even stronger for vowels in a wcontext than for isolated ones~Eyraud and Kuhl, 1994!. Withrespect to discrimination, the prototype thus seems to acta ‘‘perceptual magnet’’ by pulling other stimuli toward itseand by that means it strengthens category coherence. Mover, the magnet effect was found in infants of only smonths of age, while monkeys failed to show it~Kuhl,1991!. Therefore, Kuhl~1991, 1993a! concludes that themagnet effect must be caused by mental representaunique to humans rather than to general auditory onshared by humans and animals. Kuhl suggests that infeither are biologically endowed with phonetic prototypes,develop such prototypes through experience with spokenguage.

To test the language-experience hypothesis, Kuhlet al.~1992! synthesized a set of vowels surrounding, in formaspace, the American English prototypic /i/ and anothersurrounding the Swedish prototypic /y/.~American English/i/ is not prototypical of any Swedish vowel; neither is Sweish /y/ prototypical of any American English vowel.! BothAmerican and Swedish six-month-old infants showed poodetection of deviations from their native vowel than from tforeign one. Similar results were obtained by Polka aWerker ~1994! with American and German infants. Thuphonetic prototypes seemed to develop in infants priorword acquisition, as a result of their exposure to ambispeech. Kuhl~1993a, 1993b! hypothesized that this occurbecause infants are neurophysiologically prepared to respto linguistic stimuli.

Recent evidence questions some of Kuhl’s findinFirst, the subjects’ qualitative ratings of category goodnmay not be stable, although the location of the prototyseems to be determined by long-term exposure to languThus, rating experiments may not provide accurate measof the true internal structure of a phonetic category. Forample, vowels with highF2 frequencies, rather than thoscentral in the category, were rated with highest prototypicity in the experiments of Lively~1993!, Iverson and Kuhl~1995!, and Sussman and Lauckner-Morano~1995!. Also,Lively ~1993! found that the prototype location varied asfunction of context. Attempts to replicate the magnet effeusing Kuhl’s vowels and the subjects’ own prototypes, hafailed ~Lively, 1993!. Furthermore, Sussman and LaucknMorano ~1995! argued that results usually interpreteddemonstrating the magnet effect can be explained by varauditory resolution over the formant space, as found by Mmillan et al. ~1988!.

The findings of Kuhl and her colleagues are of gresignificance for evaluating the traditional division of thspeech perception process into distinct auditory and phonstages. According to this dual-process approach, the phostage is needed to transform the initial auditory represe

1091 J. Acoust. Soc. Am., Vol. 101, No. 2, February 1997

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tion into a categorical one that corresponds to the percesegments of the utterance~e.g., Fujisaki and Kawashima1970; Pisoni, 1973; Sawuschet al., 1980!. Auditory codingprocesses are sensitive to physical differences, whereasnetic processes depend on such factors as learning, linguexperience, memory, and attention~Repp, 1987!. In light ofthis evidence, it has been proposed that the acoustic proties of vowels favor auditory coding, whereas those of cosonants do not~Studdert-Kennedy, 1976, 1993!.

Kuhl ~1991! suggested, however, that the magnet efffor vowels, which was found only in humans, not in animamust reside at the phonetic stage, subsequent to the audanalyses. In contrast, categorical perception, shared bymans and animals, has to have the characteristics of theditory stage~e.g., Kuhl and Miller, 1975!. Thus, in Kuhl’sview, the magnet effect offers a tool for behaviorally dissciating the two distinct and ordered analysis stages. Therhowever, a methodological problem in realizing this dissciation. Standard behavioral measures are based on theoutput of the categorization process~e.g., Meyeret al., 1988!and cannot yield information about its components. In adtion, these measures require that the subjects attend tostimuli. Attending affects discrimination performance bway of numerous factors such as strategies, previous leing, and the nature of the experimental task~Creelman andMacmillan, 1979; Pisoni and Luce, 1986! and thus make themeasures hard to interpret.

Looking for the magnet effect in subjects who are ignoing the stimuli should therefore help to clarify the role of thauditory component in speech perception. In conditiowhen attention is not being paid to the stimuli there maytoo little spread of activation from the acoustic detectorsthe neurons underlying phonetic perception~Maiste et al.,1995!. Thus, ignoring the stimuli has the advantage thatauditory processing is not affected by the phonetic proceing, which otherwise would always be superimposed onconcordant auditory processing. If the magnet effect coulddemonstrated by examining sensitivity to ignored stimuthis would support the view~e.g., Massaro, 1987; Rosner anPickering, 1994! that auditory prototypes, rather than moabstract ones, are used by listeners in categorizing spsounds. In the developing brain, auditory prototypes mibe built up by synaptic modifications resulting from somform of simple unsupervised learning of phonetic categor~e.g., Hebb, 1949; Lieberman, 1984; Kluender, 1994!. Thehuman auditory system thus becomes customized to huspeech sounds, just as those of animals become customto species-typical animal sounds~Konishi, 1985; Suga,1988!. The role of common auditory coding processes information of category prototypes for speech sounds is aindicated by animal studies of categorization. For exampKluender et al. ~1987! have shown that birds~quails! canlearn to categorize speech sounds in a way that parallelsfound in studies of human categorization~for a review, seeKluender, 1994!. Furthermore, the recent results of Sussmand Lauckner-Morano~1995! using a paradigm less conducive to phonetic processing than that of Kuhl~1991! suggestan important auditory factor behind the perceptual mageffect.

1091Aaltonen et al.: Magnet effect in the light of new data

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One promising method of measuring cortical informtion processing ‘‘on line’’ is to record event-related potetials ~ERPs!, transient voltage fluctuations in brain tissugenerated by neural events. There are two general classERP components: exogenous and endogenous~Hillyard andKutas, 1983!. According to Meyeret al. ~1988! the exog-enous components occur within 100 ms or so after the stilus onset, and they are believed to reflect relatively perieral sensory mechanisms, while the endogenous componoccurring 100 ms or more after stimulus onset, are thoughreflect more central processes such as perception, attenand decision.

One of the endogenous ERP components, the mismnegativity ~MMN !, is of particular interest for the study oauditory processes, because it reflects preattentive deteof deviations from sounds in sensory memory, and can thfore be elicited even when the subject ignores the stimuli~forreviews, see Naätanen, 1990, 1992; Ritteret al., 1995!. Thus,the MMN makes it possible to study automatic auditory dcrimination without influences from higher-level psychologcal factors.

Naatanen ~1992! claims that each auditory stimululeaves a memory trace which is formed automatically, wiout conscious perception or selective attention, and thattrace accurately represents the physical features of the stlus. In an automatic comparison process, with a neuromemory trace left by a repetitive standard stimulus,MMN process is triggered by an occasional stimulus tdeviates from the standard~the oddball paradigm!. This neu-ronal mismatch process, reflected by the MMN, is capablecausing an attention switch. The MMN consists of ancreased negativity in the 100–200 ms poststimulus rangthe ERP, its amplitude increasing with the magnitude ofdeviation. Magnetoencephalographic recordings~Hari et al.,1984; Samset al., 1991! and source analyses of scalrecorded electrical potentials~Scherget al., 1989! suggestthat the MMN is mainly generated in the supratempoplane~for a review, see Alho, 1995!. It can be obtained fromsimple sounds as well as from speech signals such as voand consonant–vowel syllables~e.g., Aaltonenet al., 1987,1992; Langet al., 1990; Samset al., 1990; Sharmaet al.,1993; Lawson and Gaillard, 1981; Maisteet al., 1995!. Thecorrelation of the MMN with reported behavioral discrimnation measures is high even with stimulus differences cto detection thresholds~Aaltonenet al., 1994; Krauset al.,1992!.

MMN recordings from guinea pigs suggest that certspeech features require processing at the cortical level~e.g.,rapid spectral changes typical to stop consonants!, whereasothers~e.g., slow varying spectral changes typical to supsegmental aspects of speech! are processed at the thalamlevel ~Kraus et al., 1994!. Thus, different acoustic featureunderlying speech are represented by different neuralcesses in the auditory system. Likewise, results from aphpatients~Aaltonen et al., 1993! indicate that the MMN tovowels, but not to sine tones, disappears after left tempparietal lesions.

The present study is a series of four experiments wthe same Finnish-speaking subjects, using synthetic vo


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stimuli varying only in the second formant (F2). In experi-ment 1, the subjects categorized the stimuli as either /i/y/. In experiment 2, the stimuli previously categorized cosistently as /i/ were rated for their goodness as exemplar/i/. For each subject, the best scoring exemplar was denated the prototype~P! and the lowest scoring the nonprototype ~NP!. In experiment 3, the individual P and NP weused as referents, and they were discriminated, usinsame–different procedure~AX discrimination!, from devi-ants spaced in equal mel steps. Finally, in experimenusing the same P, NP, and deviants, preattentive audidiscrimination was studied by MMN recordings.

I. EXPERIMENT 1: CATEGORIZATION

It is essential that in locating the prototype of a phonecategory, all employed stimuli represent this category~Kuhl,1991!. However, in earlier studies of the categorizationFinnish vowels~e.g., Aaltonen and Suonpaä, 1983!, in groupdata the phoneme boundary between /i/ and /y/ was not sbut gradual, indicating that listeners differ in their categozation of the same sounds. Therefore, the purpose ofexperiment was to determine what stimuli are consistencategorized as /i/ by speakers of Finnish. Thequality of theFinnish closed front vowels /i/ and /y/ is mainly dependeon the frequencies ofF2 andF3. To make the interpretationof the results straightforward and comparable to our earexperiments studying the effects ofF2 on the MMN and onthe labeling of synthetic vowel stimuli~e.g., Aaltonenet al.,1987!, we used stimuli varying only in the frequency ofF2.

A. Method

1. Subjects

Thirteen Finnish-speaking young adults with normhearing~aged 21–32 years, mean age 23.5 years, 9 fema4 males! volunteered. All spoke the modern educated Finnof the Turku area. Before the experiment, they were screefor possible hearing defects by routine audiometric tests.

2. Stimuli

Nineteen long vowels~duration 500 ms! in the Finnish/y/–/i/ continuum~Aaltonen and Suonpaä, 1983! were syn-thesized by a parallel mode speech synthesizer~Klatt, 1980!embedded in a UNIX workstation. The duration of eastimulus was 500 ms with a linearly ramped 30-ms onsetoffset in the beginning and at the end of the stimulus. Tsynthesis parameters given by Kuhl~1991! were followedwith the exception that only theF2 value varied from 1520Hz to 2966 Hz in steps of 30 mels~Table I!. The otherformants were fixed at the following frequencies:F15250Hz, F353010 Hz,F453300 Hz,F553850 Hz. A 100-msrise–400-ms fall contour was used forF0; that is, it rosefrom 112 Hz to 132 Hz in 100 ms and dropped to 92 Hduring the remaining 400 ms of the vowel duration.

3. Apparatus

The stimuli were played with a NeuroStim PC-basstimulus presentation device at 10-kHz playback rate. Abit digital-to-analog converter with an integrated reconstr


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tion filter fed the stimuli through calibrated insert earphon~Ear Tone 3A! at a sound-pressure level of 75 dB~A!. Theaudio system was calibrated with a Bruel & Kjaer artificear ~type 4152! and a precision sound level meter~type2230!. Like in all experiments, the subjects sat in a qusoundproof room~sound-pressure level of ambient noilower than 40 dB!.

4. Procedure and analysis

Each of the 19 vowel variants was randomly repeatedtimes, i.e., each variant occurred in a random order 15 timin a series of 285 vowel variants. The subjects were to idtify each variant as /y/ or /i/ by pressing one of two buttoduring a 2000-ms period after the stimulus. Pressing theton paced the next stimulus. The maximum interstimulusterval ~ISI! was thus 2000 ms, i.e., those responses giafter the 2000-ms period were not counted, but the nstimulus was played instead. Half of the subjects usedleft thumb for /y/ and the right thumb for /i/, and the othhalf vice versa.

To evaluate the reliability of the identifications, Subjec3, 4, 5, and 12 were selected at random to repeat thewithin a few days from the first recording.

B. Results and discussion

The categorization data are presented in Fig. 1. Thecentage of /y/ and /i/ categorizations in the 15 occurrenceeach variant is plotted against theF2 frequency.

All the subjects were able to make the categorizatiThe categorization changed in an orderly manner: Stimwith low F2 values were predominantly called /y/, and thowith higherF2 values were called /i/. At aF2 of about 2000Hz the subjects categorized stimuli equally often as /i/ and/y/ Considerable interindividual variation can be seen in

TABLE I. The F2 values of the stimuli used in the experiments in theand mel scales. Center frequencies of the critical bands, according to S~1970!, associated with theF2 frequency range. Frequency differencesadjacent stimuli in the Hz and mel scale.

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1 1520 1290 14802 1582 1320 62 303 1646 1350 64 304 1712 1380 1720 66 305 1780 1410 68 306 1850 1440 70 307 1922 1470 72 308 1996 1500 2000 74 309 2071 1530 75 3010 2149 1560 78 3011 2230 1590 81 3012 2313 1620 2320 83 3013 2400 1650 87 3014 2488 1680 88 3015 2578 1710 90 3016 2670 1740 2700 92 3017 2766 1770 96 3018 2864 1800 98 3019 2966 1830 102 30


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steepness of the categorization function and in the boundlocation. For example, subjects 3 and 10 consistently caterized the stimulus withF252000 Hz as /y/, whereas thsame stimulus was categorized as /i/ by subjects 5 andThe retest results for subjects 3, 4, 5, and 12 demonstratethe categorization functions were quite stable.

The plot shapes indicate that subjects differed greatlythe consistency with which they made the categorizatioOn the basis of the categorization consistency and the boary width ~defined as the difference between theF2 valuesyielding 75% /i/ and 75% /y/!, the subjects were classified agood categorizers~subjects 1, 2, 3, 5, 9, 11, and 12! and poorcategorizers~subjects 4, 6, 7, 8, 10, and 13!. The meanboundary width of the good categorizers was 112 Hz~s.d.39.5 Hz, range 68–182!. Thus, they made phonemic distinctions with F2 differences much smaller than the standacritical bandwidth, one bark~e.g., Zwicker and Feldtkeller1967!. The poor categorizers had a mean boundary width339.5 Hz~s.d. 115.5 Hz, range 250–363! indicating that theyneeded more than one bark of difference inF2 to be certainabout the phonemic quality of the vowel stimuli used in texperiment. Subject 4 was inconsistent in categorizing mof the stimuli. Subject 7 was least consistent in categorizvowels as /y/, for this subject there were no clear exempof /y/ and some of the stimuli in the boundary area wereresponded to at all within 2000 ms~hence the less than 50%in Fig. 1!. Subject 10 behaved in the opposite way; for thsubject there were more clear exemplars of /y/ than ofThe individual differences in the location and width of thboundary resulted in a gently sloping categorization functfor /i/ in the group data, with a boundary width of approxmately 1.2 bark.

As shown in Fig. 1, there is no clear relationship btween the location of the boundary and its width. Forstance, subjects 3 and 5 were equally consistent in categing the stimuli in the range of 1750–2300 Hz although thdid this in opposite ways. The phonetic category boundardefined by the point at which the stimuli were labelequally often as /i/ and /y/, were at the same locationsubjects 3 and 7. However, the boundary was sharpersubject 3.

Three main findings emerge. First, the location of t/y/–/i/ boundary in theF2 continuum varied greatly betweesubjects. However, it was quite stable within each subjewhich is consistent with the results of Ganong and Zato~1980!. It is generally agreed that several factors influenthe location of phonetic category boundaries on physstimulus continua. For example, subjects may be differensensitive to effects of the stimulus sequence, their intercategory representations may be different, or they may hadopted different criteria for deciding between two comping categories~for a review, see Repp and Liberman, 1987!.

Second, some subjects made distinctions forF2 differ-ences much smaller than one bark, indicating that the sdardized critical bandwidth does not limit individual categrization performance~cf. Weitzman, 1992!. Third, thefrequency ofF2 alone was a sufficient cue for the goocategorizers to classify the vowels as /i/ or /y/. For the pocategorizers the stimuli apparently were not natural enou

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which may have led these subjects to feel uncertain aboucategorization task. This may have been due to the absof the information which in natural vowels is provided by thhigher formants. In a naturally spoken /y/, the lip roundilowers the frequencies of the higher formants~Fant, 1960!.These~especiallyF3! contribute to the perceptual distinctiveness of /y/ and /i/~e.g., Aaltonen, 1984!. Furthermore,many researchers~e.g., Carlsonet al., 1970; Chistovichet al., 1979! have suggested that the higher formants are pceptually integrated withF2, forming a single ‘‘effective’’formant ~F28!, which is higher for /i/ than for /y/.

II. EXPERIMENT 2: GOODNESS RATING

While it is a long-standing tradition to focus on subjecrelative inability to detect within-category differences, tprototype approach, as discussed above, holds that stiwithin a category differ in their representativeness as cegory members~e.g., Eimas and Corbit, 1973; Oden aMassaro, 1978; Samuel, 1982; Miller and Volaitis, 1989!. Ifthe prototype location, defined as the location of the tokwith the highest rated goodness, is determined by long-t

FIG. 1. Individual categorization percentages as a function ofF2 in thefrequency range of 1520–2966 Hz. Repetitions for subjects’ 3, 4, 5, anare shown in the corresponding frames. The averaged categorization daall subjects are presented in the lowermost panel on the right. TheF2 stepsize between successive stimuli is 30 mels. TheF2 scale ticks indicateintervals of 125 Hz. Poor categorizers are marked by* .


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exposure to language~Kuhl et al., 1992! it should be rela-tively stable across subjects and contexts. In Kuhl’s~1991!study, the prototype was found in the center of the categand rated goodness decreased symmetrically from that pOn the other hand, the results of Lively~1993!, Iverson andKuhl ~1995!, and Sussman and Lauckner-Morano~1995! in-dicate that prototypes can reside at more extreme locationthe vowel space. The location of the prototype may be inenced by for instance, the particular set of stimuli used~Iver-son and Kuhl, 1995!. Moreover, Lively~1993! found that thesubjects’ prototypes may be unstable, whereas the subjecIverson and Kuhl had a stable prototype location acrossals, although their identification responses were influenby the context and also showed individual differences.nally, Sussman and Lauckner-Morano~1995! suggest thatexperienced listeners may judge the ‘‘goodness’’ of stimin a different way from naive listeners, because knowledof phonetics plays a role in identification. Altogether, thecontradictory results support the notion of graded interstructure of vowel categories, but they also call into questwhether rating experiments yield accurate measures ofstructure.

In experiment 2, the subjects rated the stimuli previoulabeled as /i/, using a ‘‘category goodness’’ metric. We epected the good categorizers, with their steeper categotion functions, to give higher ratings than the poor categoers, as was found by Na`beleket al. ~1993!. We also expectedthat the poor categorizers, who could reliably categorize ostimuli high in theF2 continuum as /i/, would tend to selecthose stimuli as the best instances of /i/. For the goodegorizers, a wider range of stimuli received a high perceage of /i/ responses, and therefore they should have mchoices.

A. Method

1. Subjects and stimuli

The same subjects as in experiment 1 participated. Esubject evaluated those vowels that were consistently~inmore than 75% of the cases! categorized by him/herself as /~see Table I!. Because there was large variation in the invidual location of the /y/–/i/ boundary, the number ofvariants differed between subjects.


The stimuli were played via the same stimulus presention device as in experiment 1. The minimum ISI was 20ms although stimulus presentation was self-paced. The vants were presented in random order, 15 times each.stimuli were rated using the scale 1–7~15poor categoryexemplar, 75good category exemplar! and the results weremarked on a form. The ratings were to be done with refence to a good pronunciation of /i:/ in the Finnish wordtiili/ti:li/ ‘‘brick.’’ To check the reliability of the ratings, the teswas repeated with subjects 1, 3, 4, 5, 7, and 10 within adays from the first session. The individual ratings were plted against theF2 frequency~see Fig. 2!. For each subjectthe stimulus with the highest rating was called the P and

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one with the lowest the NP. The significance of the diffeence in mean rating between the P and NP wast tested foreach subject.

B. Results

The individual rating data are presented in Fig. 2, aTable II. The Ps and the NPs obtained average ratings oand 2.1, respectively. Although goodness is a relative msure usable only within a given stimulus set, the rating valobtained are lower than those reported by Kuhl~1991!, usingthe same scale. This is probably, at least partly, due tosubjects’ inexperience with synthetic speech. The differein goodness rating between the Ps and the NPs is substaand comparable to that found by Kuhl. Apart from thethemselves, ratings were highest for their closest neighband consistently decreased with increasing distance fromPs.

Individual goodness ratings show that, as was hypoesized on the basis of the categorization data, the perceorganization of the /i/ category was not the same acrossjects. Subjects 4, 6, 7, 8 and 10~the high-P group! selectedthe stimulus with highestF2 as the best /i/ with ratings decreasing with lowerF2 values. Thus, they showed a caegory structure similar to that reported by Lively~1993!.However, except for subject 6,t tests for the difference between P and NP in rated goodness showed no significafor the subjects in the high-P group. Subjects 1, 2, 3, 11,13 ~the Down group! yielded an opposite pattern. For themstimuli close to the boundary against /y/ were rated asbest /i/ and those with highestF2 as the worst. Subjects 5, 9and 12 ~the Hill group! had similar P positions but lowecategory boundaries; therefore the stimuli they rated asbest /i/ were close to their category center. Thus, their ratiindicate a category structure for /i/ similar to that obtainedKuhl ~1991!. The Down and Hill groups~henceforth, thelow-P group! showed significant differences in rated gooness between the P and the NP, which indicates that, unthe high-P group, they could reliably distinguish the bestfrom the worst. The replications for subjects 1, 3, 4, 5, 7, a10 yielded virtually identical results.

The goodness ratings and categorization data were cpared. As expected, the subjects in the high-P group werepoor categorizers of experiment 1. The subjects of the lowgroup were the good categorizers@with subject 13, a poorcategorizer~boundary width 272 Hz!, as an exception#. Themean width of the boundary for the low-P group was 132~s.d. 67 Hz, range 68–272! and for the high-P group 353 H~s.d. 124 Hz, range 250–363!. The lower the boundary, thlower was the location of the P~r50.67,p,0.05!. Also, thesharper the boundary, the lower was the P~r50.67,p,0.05!. The location of the NP correlated significantly onwith the boundary width~r520.59,p,0.05!.

C. Discussion

Our subjects’ ability to perceive and label goodness dferences within-category was related to their performancthe identification task. The good categorizers were more ato distinguish the best category exemplars from the p


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ones than the poor categorizers. As indicated by the goness ratings, only the good categorizers exhibited an intecategory structure. However, for the Down~sub!group thisstructure was not concordant with that reported by Ku~1991!, because the vowels close to their boundary wrated as having the highest prototypicality, not those incenter of the category.

It can be doubted, however, whether goodness ratingsynthetic stimuli is an adequate method for revealing the tinternal structure of vowel categories. The P and NP demined in this way are most likely based on a rather abstlevel of information processing guided by experience awith principles varying between subjects. The subjects inhigh-P group may have been chasing after the most distexample of /i/~as compared to /y/!, instead of the most natural /i/, because they could categorize with certainty onlyextreme stimuli. SettingF2 close toF3 typically increasesthe distinctiveness of /i/ sounds, but at the cost of losing thstatus as typical members of the Finnish /i/ category. Ad

FIG. 2. Individual goodness ratings in the scale of 1–7~7 is the best rating!for the stimuli categorized by the subject as /i/ in more than 75% ofcases. Dashed lines represent repetitions. The individualF2 values~Hz! ofprototypes~P! and nonprototypes~NP! are shown in Table II as well as thep-value of the significance of their difference. TheF2 step size betweensuccessive stimuli is 30 mels. TheF2 scale ticks indicate intervals of 125Hz. Poor categorizers are marked by* . The /i/ category boundary is indi-cated by a thick line.


TABLE II. The F2 values and the mean goodness ratings for the best rated~prototype, P! and the worst rated stimuli~nonprototype, NP! for each subject.t andp values for the differences between mean ratings of P and NP. Poor categorizers are marked by* . Ns5not significant~p.0.05!.

Subject

F2 value~Hz! Mean goodness ratings on scale 1–7

t d f pP NP P s2 NP s2

1 2230 2966 6.00 6.34 1.00 0.00 7.692 28 ,0.0012 2313 2966 4.87 1.84 2.00 0.00 8.200 28 ,0.0013 2400 2966 6.87 1.84 1.00 0.00 16.771 28 ,0.0014* 2864 2313 5.53 8.21 3.00 38.40 1.435 28 ns5 2230 2966 6.00 0.00 1.07 1.01 18.926 28 ,0.0016* 2766 1996 6.27 7.35 1.93 28.98 2.789 28 ,0.017* 2864 2400 5.47 8.21 4.00 23.44 1.012 28 ns8* 2488 2149 4.27 37.45 3.07 22.33 0.601 28 ns9 2230 2966 5.87 14.70 1.13 1.84 5.790 28 ,0.00110* 2864 2578 5.13 4.06 3.4 40.34 1.006 28 ns11 1996 2966 6.20 8.89 1.53 6.14 4.664 28 ,0.00112 2230 2966 6.60 3.90 3.23 2.52 5.196 28 ,0.00113* 2230 2966 4.60 10.33 1.00 0.00 4.337 28 ,0.001

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tional information of thevowel quality, and thus for the suf-ficient contrast between thenatural /i/ and /y/, is hiddenamong higher formants, althoughF1 andF2 are consideredto convey more information than any other formant in cegorizing natural vowel sounds.

The lack of additional information may have made thesubjects particularly susceptible to the ‘‘hyperspace effediscovered by Johnsonet al. ~1993!: Listeners’ perceptuavowel space is expanded relative to their production spareflecting hyperarticulated vowels rather than vowels of nmal effortless speech. Johnsonet al. also found that good-ness ratings of synthetic stimuli can be affected by thestructions given. Subjects who were asked to find the ‘‘bevowel for each word were more critical to the stimuli thathose instructed to find the vowel sound most closely maing their own pronunciation. Thus, our subjects may hainterpreted the instructions in two different ways: The sujects in our high-P group may have meant as the ‘‘best’’‘‘most distinct,’’ and consequently, rated the stimuli wireference to more abstract linguistic representations thansubjects in the low-P group did, for whom the vowels proably sounded more natural or closer to their own pronuntion. For the low-P group the goodness ratings at the pnetic level may be based on an internal category structurthe auditory level, as was earlier suggested by the adaptastudy of Miller et al. ~1983!.

III. EXPERIMENT 3: DISCRIMINATION

As was discussed earlier, Kuhl~1991! found that detect-ing deviations from the P was more difficult than from tNP; thus, discrimination performance was determinedonly by the mel difference but also by the prototypicalitythe referent. A signal detection analysis of the discriminatresults of Sussman and Lauckner-Morano~1995, p. 551!partly supports this conclusion, but these authors also sgest that ‘‘an auditory processing component may be anportant part of the ‘perceptual magnet’ effect’’~p. 551!. Incontrast, Lively ~1993! found that discrimination performance, as measured by the percentage of misses, was sfor the P and the NP, and depended solely on the mel difence. Experiment 3~the same–different discrimination tes!


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studied the relation between the discriminability ofF2 dif-ferences for /i/ tokens and their rated goodness.

A. Method

1. Subjects

The same subjects as in experiments 1 and 2 parpated.

2. Stimuli

The discrimination performance of each subject wstudied using a same–different~AX ! paradigm. The indi-vidual Ps and NPs, and stimuli differing from them by 360, 90, or 120 mels inF2 ~for theF2 values in Hz see TableI! were used as stimulus pairs. The presentation of the stilus pairs is indicated in Table III. Different sessions weused for Ps~rows 1–4 in Table III! and NPs~rows 6–10 inTable III!. All P and NP stimuli served as referents@thecolumns marked ‘‘A~standard!’’ in Table III # as well as de-viants @the columns marked ‘‘X~deviant!’’ in Table III #. Inaddition X could be identical to A, i.e., the difference wasmel. When the referent was high inF2 ~close toF3! onlylower deviants were used; when it was low~close to theboundary! only higher ones. The AX pairs were presentedrandom order, 15 times each in each direction, thatA5referent/X5deviant and A5deviant/X5referent. Thus,each subject heard 8315 pairs of different stimuli and 15pairs of equal stimuli in two different sessions of randomordered pairs.


The stimuli were presented with a pause of 1000between stimuli, and 2000 ms between stimulus pairs.responses were collected using the NeuroStim reactionkeypad. The subjects were asked to press, as quickly assible, the ‘‘same’’ button when they heard the stimulibeing the same and the ‘‘different’’ button when they heathem as different. Half of the subjects used the left thumb‘‘same’’ and the right thumb for ‘‘different,’’ and the othehalf vice versa.


ding

1097 J. Acoust. S

TABLE III. The stimulus pairs in experiment 3~AX discrimination!, and experiment 4~MMN recordings!. Inboth experiments two separate sessions were used: the prototype session~cases 1–5! and the nonprototypesession~cases 6–10!. In the AX discrimination sessions nine stimulus pairs were used. In the MMN recorsessions only eight stimulus pairs were used because equal stimuli~P-P or NP-NP! do not elicit MMN re-sponses.

A ~standard! X ~deviant! A ~standard! X ~deviant!

1 prototype prototype2 prototype prototype130 mel prototype130 mel prototype3 prototype prototype160 mel prototype160 mel prototype4 prototype prototype190 mel prototype190 mel prototype5 prototype prototype1120 mel prototype1120 mel prototype6 nonprototype nonprototype7 nonprototype nonprototype130 mel nonprototype130 mel nonprototype8 nonprototype nonprototype160 mel nonprototype160 mel nonprototype9 nonprototype nonprototype190 mel nonprototype190 mel nonprototype10 nonprototype nonprototype1120 mel nonprototype1120 mel nonprototype

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B. Results

Table IV gives, for each subject, the number of rsponses ‘‘different’’ and ‘‘same’’ for each referent and dviant combination~P5P, PÞP, and NP5NP, NPÞNP! alongwith the values ofd8, read off from the tables of Kaplanet al. ~1978! for AX experiments. As recommended by Mamillan and Creelman~1991!, percentages of hits and falsalarms below 1% were changed to 1%, and those aboveto 99%.

A MANOVA ~multivariate approach to repeated mesures! on thed8 values with factors referent~P and NP! andP level~low-P and high-P! yielded a nonsignificant effect oreferent@F~1,11!53.00, ns#. The effect of the P level wassignificant@F~1,11!510.06;p,0.01#, which is due to higherd8 values in the low-P group~mean54.54, s.d.50.53! thanin the high-P group~mean53.54, s.d.50.59!. Individual d8values correlated significantly with the width of the bounary: r520.68,p,0.05 ~two tailed!.

The referentx P level interaction was highly significan@F~1,11!569.38; p,0.001#. To explore the nature of thisinteraction in more detail, for each subject the relative dference ind8 between NP and P,

Dd852@dNP8 2dP8#/@dNP8 1dP8# ~1!

and the correspondingF2 difference in Hz between NP anP,

DF25F2NP2F2P ~2!

were computed. An effect ofF2 on discriminability willshow up in the regression term, and an effect of the desigtion of the referents~the one with higherF2 as P and the onewith lower F2 as NP, or vice versa! in the intercept~thevalue ofDd8 that obtains forF2NP5F2P!. The linearity ofthis plot, and thus also the estimate of its intercept, is qinsensitive to a possible nonlinearity of the relation betwed8 andF2.

The computed regression line was:

Dd8520.02210.5013DF2 , r50.912. ~3!

The regression coefficient differed highly significantfrom zero@t~11!57.39; p,0.001# whereas the intercept dinot @t~11!520.48, ns#. This implies that the detectability o

oc. Am., Vol. 101, No. 2, February 1997

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deviations increased with theF2 frequency of the referentbut was independent of its designation as P or NP. Thus,each group, discrimination was easiest for the referent wthe highestF2 frequency: for the low-P group, NP; for thhigh-P group, P.

C. Discussion

Our aim with experiment 3 was to investigate wheththe discriminability of the /i/ tokens is related to their rategoodness. In other words, are equal mel deviations moreficult to detect from the P than from the NP? The answfrom the analysis of the data as a whole seems to be ntive. For both the high-P and low-P groups, the poorest dcrimination occurred at about the same location in theF2continuum, which made the best-rated /i/ for the lowgroup, and was therefore used as their P, but which wasthe worst-rated /i/ for the high-P group and used as their NThus, only the data from the low-P group are possibleexplain as resulting from a prototype-based magnet effThe high-P group showed an opposite effect. For these sjects, it was more difficult to detect deviations from their Nthan from their P.

The simplest conclusion, then, from the evidence clected so far seems to be that discriminability, rather thbeing ruled by the prototypicality of the referent as indicatby goodness ratings, depends on its location in theF2 con-tinuum. Furthermore, our results can neither be explainedcategorical perception, which predicts that discriminationidentically labeled stimuli should be equally poor~Fry et al.,1962; Pisoni and Tash, 1974; Reppet al., 1979!, nor by thelisteners’ strategy to use such stimuli that are easy to laconsistently ~‘‘perceptual anchors’’! for discriminatingspeech stimuli~Macmillan et al., 1988!. Although the theo-ries of categorical perception, perceptual magnets, andceptual anchors are based on fundamentally differentproaches to speech perception, they all predict peaksdiscrimination near the category boundary. Thus, nonethese theories can explain the discrimination behavior ofFinnish listeners, who were more sensitive to stimulus dferences far from the boundary than close to it.

An important point, as made by a reviewer, is that f


1098 J. Acoust. S

TABLE IV. Number of responses in categories ‘‘different’’ and ‘‘same’’~total for all deviants!, the d8 esti-mates, and differences ofd8 estimates for NP and P referents.

SubjectNo. ~Type! Referent Deviant Different Same d8 dNP8 2dP8

1 ~low-P! P 5 5 67 3.21P Þ 80 40 12.28NP 5 0 75 5.49NP Þ 104 11

2 ~low-P! P 5 0 68 4.42P Þ 80 33 10.92NP 5 0 75 5.34NP Þ 108 14

3 ~low-P! P 5 2 73 4.61P Þ 102 18 10.89NP 5 3 72 5.50NP Þ 116 4

11 ~low-P! P 5 8 67 2.80P Þ 77 43 11.52NP 5 3 72 4.32NP Þ 101 19

13 ~low-P! P 5 21 53 3.03P Þ 103 17 12.19NP 5 9 66 5.22NP Þ 117 2

5 ~low-P! P 5 3 72 3.75P Þ 87 33 11.29NP 5 1 74 5.04NP Þ 103 17

9 ~low-P! P 5 0 66 4.33P Þ 112 8 11.52NP 5 1 74 5.85NP Þ 114 6

12 ~low-P! P 5 10 65 3.81P Þ 106 14 12.06NP 5 5 69 5.87NP Þ 119 1

4 ~high-P! P 5 11 61 3.64P Þ 96 14 20.61NP 5 14 71 3.03NP Þ 110 32

6 ~high-P! P 5 17 58 3.45P Þ 106 13 20.60NP 5 8 66 2.85NP Þ 79 41

7 ~high-P! P 5 8 67 3.85P Þ 104 16 21.21NP 5 14 61 2.64NP Þ 83 34

8 ~high-P! P 5 7 67 3.78P Þ 101 19 20.82NP 5 10 64 2.96NP Þ 87 33

10 ~high-P! P 5 0 60 5.55P Þ 82 8 21.93NP 5 2 58 3.62NP Þ 60 30

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similar F1 frequencies the Finnish listeners in our stushowed a discrimination pattern opposite to that of Americlisteners: Finnish listeners are least sensitive to acousticferences for /i/ tokens with lowF2; American listeners forthose with highF2 ~Kuhl, 1991; Iverson and Kuhl, 1995Sussman and Lauckner-Morano, 1995!. Thus, the pattern odifferential sensitivity along theF2 continuum observed inthe present study is not due to common auditory processas suggested by Sussman and Lauckner-Morano, but ra

oc. Am., Vol. 101, No. 2, February 1997

nif-

g,her

seems to result from the specific linguistic experience ofFinnish-speaking subjects. Macmillanet al. ~1988! have sug-gested that listeners use perceptual anchors for discriming speech stimuli, which leads to increased discriminatnear perceptual anchors and poor discrimination far frthem. According to this, our discrimination results suggthat Finnish listeners have perceptual anchors located atrather than low frequencies ofF2 for /i/ tokens. Increasedauditory resolution for higher formant frequencies is need


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in Finnish, which has an opposition between /i/ and /y/ baon higher versus lower values ofF2 andF3 ~or of F28!. Nosuch opposition is found in English, which instead hasopposition~not found in Finnish! between /i/ and /I/ based oF1 andF2 ~Wiik, 1965!.

Sussman and Lauckner-Morano~1995! suggest that thedirection of the intrapair change in formant frequency playrole in determining differential sensitivity, and that this mhave contributed to the perceptual magnet effect they fouSubjects who made frequency comparisons in pairs witchange from high to low showed better sensitivity than thwho heard the same standard but with an upward frequechange. Dooley and Moore~1988! found the same asymmetry in glide detection. These results suggest that, inpresent study, the measured sensitivity toF2 changes in theregions close toF3 might have been somewhat overesmated due to the fact that only downward changes inF2were used. Nord~1980! also showed that Swedish listeneare more sensitive to anF2 fall than to a rise, whereas iFlanagan’s~1955! study the same type of stimuli producethe opposite pattern with American listeners. Thus, linguisexperience as well as acoustic factors may make listemore sensitive in their vowel discrimination to a shift offormant frequency in one direction than in the other.

IV. EXPERIMENT 4: MMN RECORDINGS

According to Kuhl’s~1991, 1993a! hypothesis, the perceptual magnet effect is restricted to a phonetic process cponent that uses abstracted mental representations of ptypical speech sounds. An alternative hypothesis isauditory ~sensory! prototypes are linked with phoneticallgood tokens. The assumption is supported, at least partlytheories that emphasize the role of lower-level articulatand auditory constraints in shaping phonetic inventor~e.g., Liljencrants and Lindblom, 1972; Stevens, 1989; Diet al., 1990!. To test the latter hypothesis, auditory discrimnation was measured by recording MMNs using the sam~Pand NP! referents~in MMN literature: standards! and devi-ants as in experiment 3. In order to rule out attention effeduring the MMN recording, the subjects were instructedwatch a silent subtitled movie and to ignore the vowstimuli.

A. Method

1. Subjects and stimuli

The same subjects and the same stimuli as in experim3 were used. Using the oddball paradigm, either the P orNP served as a standard and vowels deviating from it by60, 90, or 120 mel inF2 as deviants, or vice versa~see TableIII !. For each subject there were two sessions, eacheight stimulus pairs, which were separately randomizedPs and NPs. The duration of each stimulus was 500 ms;time interval from the beginning of one stimulus to the newas 850 ms, so the presentation of one block~100 deviantsand 900 referents! took about 15 min. The duration ofrecording session was thus about 2 h excluding a 30-minpreparing time for electrode fittings. The subject watchesilent, subtitled movie~of their own choice! on a 21-in. TV


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screen 2.5-m apart and was instructed to ignore the vostimuli presented binaurally in earphones. All subjects hnormal vision.

2. Equipment and procedure

Continuous EEG was recorded and stored using a Brtronics 32-channel EEG-amplifier connected to the NeScan EEG data acquisition and analysis computer. Ag/Aelectrodes were used, and they were placed according tointernational 10/20 system, using linked ears as the reence. The electrodes were placed atFp1, Fp2, F7, F3, Fz,F4,F8,T3,C3,Cz, C4,T4,T5, P3, Pz, P4,T6,O1,Oz,andO2 ~Figs. 4 and 5!. The electrode impedance was reglarly checked during the sessions in order to keep it belowkV. At the beginning of each session the amplifiers wecalibrated. The amplifier bandwidth was set to 0.1–70~23 dB! and a sampling frequency of 200 Hz was used. TEEG averaging was triggered by the stimulus onset, andsponse epochs of 800-ms duration were averaged off-and stored in separate files for standard and deviant stimA 50-ms pretrigger period was used to evaluate the Ebackground activity, which was subtracted as a dc basefrom the response epoch. A level-sensitive~650-mV! artifactrejection was applied prior to the summing of the trials. Taveraged ERP responses to standard and deviant stwere digitally filtered~using a time-domain bandpass filter24 dB/octave in a band of 0.1–21 Hz! to further reducenoise.

The experimenter~an experienced EEG laboratory tecnician! continuously monitored the subject’s EEG and pformance~via a video system!. Eyeblinks, drowsiness, analfa activity were specifically controlled. In a few cases, texperiment was stopped because a subject did not watchmovie or got tired. The session was then restarted after aminutes’ break.

For each stimulus block, the response epoch to the sdard preceding the deviant was subtracted~point-by-point!from that to the deviant, and the MMN peak amplitude alatency were measured from the difference curves~from on-set to peak! manually by two scorers blind as to the behaioral performance of the subject. Both were professioEEG laboratory technicians experienced with MMN expements. The amplitude values were fed to a computerstatistical analysis.

B. Results

1. MANOVA

The analysis was done with SAS software usingMANOVA approach to repeated measures~Bock, 1975!.This method eliminates interindividual differences normapresent in ERP recordings~e.g., Langet al., 1995!. TheMMN peak amplitudes and latencies from the same subin different conditions and in different electrodes were trepeated measures. These were structured by within-facreferent ~P,NP!, position ~standard; deviant!, distance~30,60, 90, and 120 mel!, laterality @left (F3,C3); middle(Fz,Cz); right (F4,C4)#, and sagittal@front (F3,Fz,F4);


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central (C3,Cz,C4)#. The rest of the electrodes were ecluded from the statistical analysis, because they showeMMN.

The subjects were grouped by two between-subjecttors, gender~male; female! and P level@low-P ~8 subjects!;high-P ~5 subjects!#. Distance had a significant effect oMMN amplitude @F~3,7!517.31;p,0.01# and on MMN la-tency @F~3,29!57.98; p,0.001#. The MMN amplitude in-creased and latency decreased with the difference betwstandard and deviant. The linear effect of distance on MMamplitude was highly significant@F~1,9!545.25;p,0.0001#.The MMN latency was not affected by any other factor thdistance.

For MMN amplitudes, the effects of position and sagitfactors were nonsignificant, whereas that of laterality facwas highly significant@F~2,6!547.50;p,0.001#. The meanMMN amplitudes were22.65 mV for the left (F3,C3),22.79mV for the middle (Fz,Cz), and22.68mV for theright (F4,C4) electrodes. Thus, the significance resulfrom a difference between the middle electrodes and thosthe left and to the right.

There was also a significant main effect on MMN amplitude due to gender@F~1,7!512.03; p,0.05#. The meanMMN amplitudes were higher for females~22.79mV! thanfor males~22.49mV!. The main effect of referent was nosignificant@F~1,9!50.61#. This effect was significant for thelow-P group@F~1,9!56.79; p,0.05# but not for the high-Pgroup @F~1,9!51.65#. The referent3P level interaction wassignificant@F~1,9!57.26;p,0.05#.

The mean MMNs for P and NP as a function of distanare plotted in Fig. 3. As shown in the low-P group the meMMN amplitude is systematically less negative for P~mean522.77 mV! than for NP~mean523.40 mV!, reflecting amagnet effect. The MMN becomes less affected by stimuprototypicality as the difference between the standard~refer-ent! and deviant increases. The MMN data from the highgroup are less systematic than those from the low-P groThe difference curves in Figs. 4 and 5 illustrate the diffence between the P and the NP as a standard~referent! atdifferent electrodes in the low-P and high-P groups whendeviation was 30 mels. For the low-P group, with the NPreferent, there is a clear MMN recorded at the F and C etrodes, peaking around 200 ms. No such peaks are visiblethe P. The low-P group shows no P300~an ERP componenreflecting attentive discrimination in an oddball paradigm!,which indicates that the 30-mel deviation from the P did ntrigger attentional processes. As illustrated by Fig. 5, thera different MMN pattern in the high-P group: The MMNsthe F and C electrodes are clearly larger for the P than forNP. In addition, aP3 component is visible, indicating thafor the same stimulus differences the mismatch processsensitive enough to cause an attention switch in the higgroup but not in the low-P group. There is also a clear netive peak at a latency of 400 ms in the low-P group, butsuch component is visible in the high-P group~Figs. 6 and7!. This late negative peak may thus be related particularlpreattentive discrimination. However, these conclusionsbased only on the qualitative inspection of the grand averdifference curves.


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2. Regression analysis

In analogy with the analyses for thed8 values from ex-periment 3@see Eqs.~1! and~2!#, the within-subject relativedifference in MMN between NP and P,

DMMN52@MMNNP2MMNP!/@MMNNP1MMNP# ~4!

for each electrode position, and the correspondingF2 differ-ence in Hz,

DF25F2NP2F2P ~5!

were calculated~using amplitudes as well as latencies!. In alinear regression of MMN onDF2, an effect of theF2 dif-ference between the standards~referents! would show up inthe regression term, and an effect of the designation ofstandard~referent! with the higher and lowerF2 as P and

FIG. 3. Mean MMN amplitudes for the low-P and high-P groups recordfrom electrodesF3, F4, C3, C4, Fz andCz in the P and NP conditions.

FIG. 4. Difference curves for P and NP at different electrodes for the lowgroup with 30-mel deviance.


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NP, or vice versa, in the intercept~the value ofDMMN forF2NP5F2P!. The correlations were higher for the amplitudthan for the latencies. The computed correlation coefficie~two-tailedp values in parentheses! betweenDMMN ~absoluteamplitudes! andDF2 were:F3: 0.346~ns!; Fz: 0.430 ~ns!;F4: 0.531 ~p,0.10!; C3: 0.534 ~p,0.10!; C4: 0.570 ~p,0.05!. For C4, which showed the highest correlation, tregression line was:

DMMN50.000417.443DF2 , r50.569. ~6!

The regression coefficient differed significantly frozero~p,0.05! whereas the intercept did not. Thus, similaras for the behavioral discrimination measures, for the subgroup as a whole, only theF2 frequency of the referent, noits designation as P or NP, determined the MMN evokedthe deviants.

Also, for each electrode, the correlations were compubetween the subjects’ meand8 values and their mean MMNlatencies and amplitudes. The coefficients and two-tailepvalues were for the MMN latencies:F3: 0.494 ~p,0.10!;Fz: 0.520 ~p,0.10!; F4: 0.512 ~p,0.10!; C3: 0.523 ~p

FIG. 5. Difference curves for P and NP at different electrodes for the higgroup with 30-mel deviance.

FIG. 6. Difference curves for P and NP atFz for the low-P group with30-mel deviance.


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,0.10!; Cz: 0.514 ~p,0.10!; C4: 0.507~p,0.10!. For theMMN amplitudes all correlations were small and nonsigncant.

The correlations between MMNs and category widtwere also computed; none of these correlations was sigcant.

C. Discussion

The MMN recordings showed that preattentive proceing is sensitive to differences within a vowel category. TMMN amplitude increased with the difference between stdard ~referent! and deviant. The subjects’ MMN latenciecorrelated with their discrimination performance as measuby d8, which suggests that conscious and preattentivecrimination of vowel sounds works in much the same waThe MMN amplitudes were highest in the middle of thhead, and tended to be higher for females than for males.d8, there was no appreciable gender difference.

Prior evidence suggested that the MMN reflectsphysical parameters of the stimuli, and not their phonecategories~e.g., Aaltonenet al., 1992; Samset al., 1990;Sharmaet al., 1993; Maisteet al., 1995!. For example, in-creased sensitivity at phonetic boundaries, reflectingegorical perception, has not been reported for MMN; neitare there any differences between MMNs elicited by phoncally relevant ~e.g., F2! and irrelevant stimuli~e.g., F0!~Aaltonenet al., 1994!. Typically, the choice of stimuli inearlier studies was based on the boundary-oriented appr~using stimuli from opposite categories or from the boundarea as standards!, whereas in the present study it was guidby the prototype-oriented approach~using stimuli within onecategory!. The most interesting result of experiment 4, theis that the position of the stimuli in the phonetic continuuplays a role in the generation of the MMN. Equal mel dferences between standard and deviant stimuli elicited laMMNs at the higher end of theF2 continuum than at thelower end. Earlier MMN studies indicate equal discrimintion both across and within categories. Moreover, inlow-P group the MMN amplitudes not only reflected the dference between referent and deviant, but also correla

PFIG. 7. Difference curves for P and NP atFz for the high-P group with30-mel deviance.


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with the rated goodness of the stimuli. This gives furthsupport to the hypothesis that the prototypes of the higgroup were based on more abstract levels of processingsuch prototypes as yield a perceptual magnet effect. Tmight also explain the attention shift, which was not shoby the low-P group.

An important feature of the MMN is that its amplitudincreases with training~Kraus et al., 1995!. Perhaps theMMNs were larger in the good categorizers~the low-Pgroup! because these had properly learned to discriminthe stimuli. In the high-P group, as well as in the lowgroup, the MMN tended to be diminished for stimuli in thlow F2 region. Applying the perceptual magnet theory, tcould mean that this region has features that are relatestimulus prototypicality—although subjects may not reconize this—and therefore impair discrimination.

V. GENERAL DISCUSSION

In earlier studies it has generally been assumed thatprocesses underlying the MMN are based primarilyacoustical stimulus differences~e.g., Naatanen, 1992; Aal-tonenet al., 1994!. Our results suggest that this may nottrue. The low-P group demonstrated what may be interpreas a perceptual magnet effect, not only behaviorally but ain terms of MMN amplitudes. For this group, equal mel dferences between standard~referent! and deviant stimuli elic-ited lower MMNs when the standard was the P than whewas the NP. On the other hand, the high-P group behavethe opposite way: Both as indicated by MMN amplitudes abehaviorally, it was easier for them to detect differencfrom their Ps than from their NPs.

The MMN reflects aspects of sensory processing thatassumed to be relatively independent of cognitive, linguisand attentional factors. Nevertheless, according to oursults, it is modified by the listener’s experience with spoklanguage. One interpretation is that on the auditory procing level this experience is organized into clusters of simifrequently heard speech sounds. The ability to discriminsounds within the same cluster is impaired—the percepmagnet effect—and at the subsequent phonetic proceslevel the category limits are adjusted to fit the patternclusters.

Morais and Kolinsky~1994! suggest that there are twfunctionally distinct phonological systems underlying laguage processing: an unconscious one for speech perceand production, and a conscious one for reading and writOne explanation for the results of the high-P group mightthat these subjects used unconscious~auditory! phoneme rep-resentations for discrimination, resulting in interindividuaconsistent MMNs andd8 values but conscious~phonetic!representations, which are more difficult to use, for caterization and goodness rating, yielding inter- and intrainvidually inconsistent results.

In the study of Langet al. ~1990! the MMN elicited bya given set of pure-tone stimuli varied greatly between nmally hearing listeners; as in the present study, one-thirdtheir subjects showed lower MMN amplitudes than the oers. The MMNs correlated with frequency discriminatioperformance. It is possible that, for some reason, the mem


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traces generating the MMN were not equally well developin our poor categorizers as in our good ones. To compenfor this deficit at the preattentive level of perception, the pocategorizers may have been inclined to rely on abstractresentations of the vowels, different from the auditory onused by the good categorizers; therefore, attentional pcesses were triggered by the poor categorizers to activatecorresponding neuronal assemblies.

One counterargument against the interpretation we ofor our results might be that the mechanism that generthe MMN may not be fully independent of attention; subjecmay have attended to the stimuli although they were askeignore them. However, there is a bulk of literature on tMMN ~e.g., Naatanen, 1992! indicating that the MMN rep-resents preattentive processes. This view is supported byfact that the MMN is elicited even when the subject concetrates on a difficult problem solving task~Lyytinen et al.,1992!, or tries to recognize visual stimuli~Woods et al.,1992! or stimuli in another auditory modality~Paavilainenet al., 1993!. A strong piece of evidence for the indepedence of the MMN from the direction of the subject’s attetion comes from a recent study by Cheour-Luhtanenet al.~1995! who recorded an MMN-like response to vowelsquietly sleeping newborns. In the present study, the lacktheP3 component in the low-P group strongly suggests tthey did not attend to the stimuli.

Our study also showed that listeners are not all alikeidentifying vowels and in rating their category goodneAbout one-third of our subjects~the poor categorizers! wereuncertain about the phonetic quality of the stimuli. In adtion, their mean MMN amplitude over all electrodes~22.1mV! was lower than that of the other subjects~22.9 mV!.However, this difference was not significant, probably bcause of the small number of subjects~Lang et al., 1995!.Nevertheless, separatet tests for each electrode showed dferences approaching statistical significance betweengroups at Fz @t~11!522.05, p,0.10# and F4 @t~11!521.86, p,0.10#. As expected, the meand8 also differedsignificantly ~2.94 for the high-P group, 3.39 for the low-group! @~9.10! ~unequal variances!52.90,p,0.05#.

High goodness ratings require that most formants~F1,F2,F3, etc.! are close to their prototypical values. Howeveas indicated by our results, the perceptual magnet efworks at a lower level of processing than the goodnessings. The level of the magnet effect may be also lower ththat where the formant cues are integrated to form a sinpercept of a vowel. If so, then the magnet effect canelicited when the stimulus vowel has the sameF2 frequencyas the prototype~cf. Lively, 1993!, not requiring a match tooccur forF3 as well. The latter match may depend on inteindividually varying principles. Our stimuli turned out tsound quite unnatural for many subjects, and for those sjects theF3 match may not have occurred. The elicitationthe magnet effect by a single formant match reminds ofprinciple in visual perception that the presence of a sindepth cue can evoke size scaling to achieve constancy, wout eliciting any conscious experience of depth~see Corenet al., 1994!.


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VI. CONCLUSIONS

Taken together, our results show that individual listenwere inconsistent in categorization and goodness ratingconsistent in discrimination. For all our subjects, the poordiscrimination occurred at about the same location in theF2continuum, but only the data from the good categorizerspossible to explain as resulting from a prototype-based mnet effect. In speech perception research, the absence ocisive evidence often leads to a long-standing competibetween phonetic explanations and more general and simauditory ones~Diehl et al., 1990!. In the case of the perceptual magnet effect, more evidence from electrophysiologias well as, behavioral measures is obviously needed bethe locus of the effect can be unambiguously determinHowever, for similarF1 frequencies the Finnish listenersour study show a discrimination pattern opposite to thatAmerican listeners. Thus, the pattern of differential sensiity along theF2 continuum observed in the present stuseems to result from language experience affecting basicsory capabilities of listeners, as demonstrated by our MMrecordings.

ACKNOWLEDGMENTS

The study was supported by the Academy of Finlaand by the Swedish Council for Research in the Humaniand Social Sciences~Ake Hellstrom!. We wish to thank JariRiitala, M.Sc., for vowel synthesization, Tuula Ja¨rnstedt,Mia Ek, M.Sc., and Anne Hjort for making the recordingPasi Hakulinen for writing the analysis software for the bhavioral experiments, and Lea Heinonen-Eerola, M.A.,editing the English language of the manuscript. Dr. ASalmivalli, M.D., has kindly allowed the hearing tests of tsubjects at the Hearing Centre of Turku University Hospi

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