LSCP Emmanuel Dupoux
email: dupoux@lscp.ehess.fr.

Christophe C Pallier

Kazuhiko Kakehi

Jacques Mehler

Fushimi, Kakehi Dupoux

Mehler

New evidence for prelexical phonological processing in word recognition

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Most models of spoken word recognition postulate that the acoustic signal is transformed into a prelexical representation, typically a string of phonemes, and that this representation is used to access the lexicon. Such models have to spell out how the acoustic signal is transformed into a prelexical representation and whether this representation is enhanced by one's lexical knowledge. Many studies have established that the mapping between the signal and the prelexical representation is not simple. In their famous "Perception of the Speech Code" paper, [START_REF] Liberman | Perception of the speech code[END_REF] stressed the complexity of the relationships between the acoustic signal and the phonetic message: neighbor phones interact so much that a single acoustic stretch is often ambiguous and requires a larger context to be interpretable (see, for example, [START_REF] Miller | Some effects of later-occurring information on the perception of stop consonants and semivowels[END_REF]Mann and Repp, 1981;[START_REF] Whalen | Vowel and consonant judgements are not independent when cued by the same information[END_REF]. Their proposed solution was that listeners use their knowledge of how speech sounds are produced to decode the speech signal (for example, to compensate for coarticulation). A second source of information is lexical knowledge. Indeed, numerous studies have demonstrated lexical influences on phoneme identification (e.g. [START_REF] Ganong | Phonetic categorisation in auditory word perception[END_REF]Samuel, 1981a[START_REF] Samuel | Lexical uniqueness effects on phonemic restoration[END_REF][START_REF] Frauenfelder | Lexical effects in phonemic processing: Facilitatory or inhibitory ?[END_REF]. The phenomenon of phonemic restoration attests that lexical knowledge can yield the perception of a phoneme that is not present in the signal (even if acoustics play an important role in the phenomenon; cf. Samuel 1981b). A third source of information that can be used by the speech perception apparatus is phonotactic knowledge. There exists some empirical evidence that listeners tend to assimilate illegal sequences of phonemes to legal ones [START_REF] Massaro | Phonological constraints in speech perception[END_REF][START_REF] Hallé | Processing of illegal consonant clusters: a case of perceptual assimilation?[END_REF]. Thus, French listeners tend to hear the sequence /dl/, which is illegal in French, as /gl/, which is legal [START_REF] Hallé | Processing of illegal consonant clusters: a case of perceptual assimilation?[END_REF]).

Among the three above cited sources of information, the influence of phonotactics is the less well established. Both the Massaro & Cohen and the Hallé et al. studies used stimuli in only one language. Therefore, it cannot be excluded that the observed effects were due to universal effects of compensation for coarticulation: it could be that /dl/ is universally harder to perceive than /gl/. A more convincing demonstration of phonotactic effects must involve a cross-linguistic manipulation. A second difficulty is the potential confound between phonotactic and lexical informations. It can be argued that nonwords containing illegal sequences of phonemes typically have fewer lexical neighbors than legal nonwords. As a matter of fact, [START_REF] Mcclelland | The TRACE model of speech perception[END_REF] interpreted the phonotactic effects of Massaro and Cohen as the result of top-down influences from the lexicon (a "lexical conspiracy" effect). They reported on unpublished data where an apparent phonotactic effect (the preference for /dw/ over /bw/ in nonwords with an ambiguous initial phoneme) was reversed in the presence of a strong lexical candidate: /?wacelet/, yielded the perception of /bwacelet/ (from bracelet) instead of /dwacelet/ (despite the illegality of the /bw/ cluster). The authors argued that the typical preference of /dw/ over /bw/ is due to the predominance of /dw/ words in the lexicon. They simulated these data, as well as [START_REF] Massaro | Phonological constraints in speech perception[END_REF] "phonotactic" result, with a "lexical conspiracy" effect in the TRACE model (but see [START_REF] Pitt | Is compensation for coarticulation mediated by the lexicon?[END_REF] for arguments against this interpretation).

In this paper, we wish to revisit the relative role of phonotactics and lexical knowledge by building on an effect which has been well documented cross-linguistically: the perception of illusory vowels in Japanese. [START_REF] Dupoux | Epenthetic vowels in Japanese: A perceptual illusion?[END_REF] have demonstrated that Japanese listeners, but not French listeners, perceive an /u/ vowel between consonants forming illegal clusters in Japanese (e.g. between /b/ and /z/)1 . These data show that the perceptual system of Japanese listeners insert an illusory vowel between adjacent consonants in order to conform to the expected pattern in this language. We called this phenomenon "vowel epenthesis". It suggests that the role of phonotactics is so important as to produce the illusion of a segment which is not actually present in the signal.

Though Dupoux et al. (1999) attributed this effect to phonotactic knowledge, it is not a priori excluded that the illusion results from top-down lexical influences. One may imagine that many Japanese words contain sequences /C 1 uC 2 / in which C 1 C 2 represent consonant clusters present in the nonword stimuli used in the experiment. It could then be argued that the activations of such lexical items conspire to produce the perception of /u/. Thus, the potential existence of real Japanese words, phonetic neighbors of the non-word stimuli, may have induced participants to report a vowel that is not present in the signal. Some may find excessive the proposal that lexical effects can be as strong as to blindly insert a vowel that is not present in the signal. However, as we noted above, there are well documented demonstrations that the lexicon can fill in missing speech sounds, at least when the underlying signal is degraded or ambiguous [START_REF] Warren | Perceptual restoration of obliterated sounds[END_REF]Samuel, 1981a). In the present case, the signal is clear, but it contains sequences of phonemes that are illegal for Japanese speakers. The influence of lexical knowledge in such a situation is an open question.

This paper aims at determining the source of the vowel epenthesis effect in Japanese. We try to determine whether the illusory vowel is inserted during the first stage of speech processing, under the influence of phonotactic constraints, or whether it comes from the participants' lexical knowledge. To this aim, we created non-words containing illegal consonant clusters in Japanese. These items produce only one lexical neighbor when a vowel is inserted between the consonants. Specifically, for some items, the lexicon biases for the insertion of the vowel /u/ (like in sokdo -> sokudo, speed). In other items, the lexicon call for the insertion of a vowel other than /u/ to generate a word, (like mikdo -> mikado, emperor). How do Japanese listeners perceive these illegal nonwords? If perceptual processes insert the vowel /u/ within the /kd/ cluster irrespective of the lexical status of the outcome, then they should report hearing an /u/ inside both /sokdo/ and /mikdo/. If, in contrast, their perception is influenced by the nearest real Japanese word, we expect them to report an /u/ and an /a/ respectively.

Experiment 1

Stimuli with a CVCCV pattern were selected so that there is only one possible vowel that can be inserted in the consonant cluster to produce a real Japanese word. The stimuli were then split into two sets, according to the vowel that yields a Japanese word. In the u-Set, the vowel that produced a word was /u/, like in sokdo->sokudo. In the non-u-Set, the vowel was /a/, /e/, /i/ or /o/, like in mikdo->mikado. Participants were tested on two tasks, transcription and lexical decision. In the transcription task, participants were presented with stimuli and asked to transcribe them into the Roman alphabet. In the lexical decision task, participants decided whether the items were words in Japanese or not. If epenthesis is lexically driven, one expects a strong effect of lexical neighborhood on the perception of the items containing a consonant cluster. In particular, items in the u-Set (e.g. sokdo) should be perceived with an epenthetic /u/, and items in the non-u-Set (e.g. mikdo) should be perceived with a vowel other than /u/ since that is the only way to obtain a Japanese word. In other words, items in both sets should behave similarly in the lexical decision experiment, since all items can produce Japanese words once the appropriate vowel is inserted. In contrast, if vowel epenthesis arises before lexical access, both sokdo and mikdo items should produce the perception of an illusory /u/, and consequently, only sokdo will be processed as a Japanese word. Indeed, while sokdo becomes sokudo (a word in Japanese) after /u/ epenthesis, mikdo becomes mikudo (a non-word in Japanese). In order to assess the performance on the test items, we included control words like sokudo and mikado and non-words like sokado and mikudo in the lists.

Method

Materials: Two sets of 19 triplets were constructed. The first set was called the u-Set and contains triplets with the following structure: (illegal non-word, word, legal non-word). The first item was a (C)VCCV disyllable containing an illegal consonant cluster in Japanese (e.g., sokdo). The other two items had identical segments as the first except for the insertion of a vowel between the middle consonants. The second item of the triplet was a Japanese word obtained by the insertion of an /u/ (e.g., sokudo). The third item was a non-word obtained by the insertion of one of the following three vowels: /a/, /e/, /i/ or /o/ (e.g., sokado). The second set (the non-u-Set) was similar in all respects to the u-Set, except that the vowels that yielded a word and a non-word, respectively, were swapped around. That is, the illegal non-word mikdo yielded the word mikado through the insertion of an /a/ and yielded the non-word mikudo through the insertion of an /u/. In all cases, there was only one possible way to make a word in Japanese through the insertion of a vowel in the consonant cluster of the first element of a triplet. The items are listed in the Appendix. An additional list of 78 filler items was also constructed, consisting of half words, half legal nonwords.

The stimuli were recorded by a female native speaker of Japanese, trained in phonetics, who had been instructed to produce the cluster stimuli without any intervening vowel. All stimuli were checked and when a few glottal pulses appeared between the consonants, these pulses were deleted by digital editing.

Procedure: In the phonetic transcription task, all the items from the 38 triplets were presented through headphones in a different pseudo-random order for each participant. Participants were instructed to type the transcription of the items in the Roman alphabet on a computer keyboard. In the lexical decision task, the same items, with the filler items, were presented through headphones using the EXPE software on a PC computer [START_REF] Pallier | Expe: An expandable programming language for on-line psychological experiments[END_REF]. Lists were presented in a different pseudo-random order for each participant. Participants were instructed to classify the stimuli into real Japanese words versus non-existent words as fast as they could. If no response was given within 4 seconds after the presentation of an item, the next stimulus was presented. Participants were given a practice session of 10 trials with feedback, followed by the main experiment during which no feedback was given.

Participants: Fourteen native speakers of Japanese were tested on the lexical decision experiment. Seven of them also performed the phonetic transcription task (after completing the lexical decision task). They were all Japanese volunteers recruited in Paris.

Results and Discussion

In this experiment, four triplets in the u-Set and three in the non-u-Set had to be removed because of more than 50% transcription errors or more than 40% lexical decision errors on the words or non-words. Most of these errors fell on /h/-initial words, whose first segment was misperceived as /f/ or /r/, or deleted. This left 15 triplets in the u-Set and 16 triplets in the non-u-Set. No participant was rejected.

First, the phonetic transcription results for the consonant cluster items were analyzed separately for items in the u-Set and items in the non-u-Set. Seventy-nine percent of the transcriptions of the consonant cluster items in the u-Set contained an epenthetic /u/ inserted between the consonants. Only 1% of the responses contained a different vowel (/i/ instead of /u/ in one item). The remaining 20% of the transcriptions reproduced the consonant cluster. In the non-u-Set, 75% of the transcriptions contained an epenthetic /u/, 1% are cases of consonant deletion (/juSi/ instead of /jurSi/), and 24% reproduced the consonant cluster. The difference in /u/ transcription between the two items sets was not significant (X 2 (1)=0.10, p>0.10).

Second, the lexical decision data was analyzed in terms of percent 'word' responses. The data are displayed in Table 1. As one can see, words were labeled as words in 94% of the cases, and non-words in only 7% of the cases. The results of the cluster items depended on set type. 71% of the cluster items in the u-Set are labeled as words, a score significantly different from chance (p<.007). Planned contrasts showed that this score differed significantly from the score obtained with control non-words (F1(1,13)=222, p<.001; F2(1,14)=120, p<.001) and control words (F1(1,13)=23, p<0.001; F2(1,14)=8, p<0.02). In contrast, the cluster items in the non-u-Set were classified as words in only 8.0% of the trials, a score significantly below chance (p<.001). This score was significantly different from the score with the control words (F1(1,13)=868, p<.001; F2(1,15)=1329, p<.001), but did not differ from the score with the control non-words (both Fs<1). Third, we analyzed the RTs for responses falling into the `predicted' category. For items in the u-Set the predicted category was "Word" for both the Japanese words and for illegal cluster items. For the non-u-Set items the predicted category was "Non-Word" for Japanese non-words and for illegal cluster items. The reaction times are shown in Table 2. An ANOVA revealed that the control words yielded significantly faster latencies than control non-words (F1(1,13)=28, p<.001; F2(1,30)=49, p<.001). Cluster items in the u-Set tended to yield faster RTs than control non-words (F1(1,13)=3.3, .05<p<.1; F2(1,14)=4.5, p=.053), but did not differ significantly from control words (both Fs<1) In contrast, cluster items in the non-u-Set yielded slower RTs than the control words (F1(1,13)=23, p<.001; F2(1,15)=62, p<.001), but did not differ significantly from the control nonwords (F1(1,13)=2.9, p>.1; F2(1,15)=3.3, .05<p<.1). Thus, the latencies to respond to cluster items depended on the items in the lexical category to which they are assimilated. The overall pattern of results for this experiment was straightforward. The transcription task yielded similar rates of /u/ insertion irrespective of the presence or absence of a neighboring lexical item with that particular vowel. We found no insertion of a vowel other than /u/ (except for one participant who reported an /i/ in "namda" producing the Japanese word "namida"; this amounts to 1% of the responses). Thus, globally, the presence of biasing lexical neighbors did not influence transcriptions. The same conclusion arose from the analysis of the lexical decision data. Participants consistently classified cluster items as if they had inserted an /u/ prior to lexical access.

The reported results could be attributed to the fact that the stimuli were recorded by a native Japanese speaker. Even though the speaker was trained in phonetics, at times she could not avoid inserting short vowels between the cluster consonants. Her speech was digitally edited to remove pitch pulses corresponding to an inter-consonantal vowel. However, it could be that traces of coarticulation remained in the adjacent consonant allowing Japanese participants to reconstruct the underlying vowel. To control for this possibility, in the next experiment, we used a speaker whose maternal language allows consonant clusters.

Experiment 2

In order to remove the possibility that the effects obtained in the previous experiment are due to potential traces of /u/ produced by the Japanese speaker, the stimuli were recorded anew, but this time they were spoken by a native speaker of French. He was instructed to imitate words and non-words produced by a Japanese speaker, and produced the cluster stimuli without any intervening vocalic element. If the effects obtained in the previous experiment were due to coarticulation, much less vowel epenthesis should obtain in the new experiment. If, in contrast, the results were a true phonologically-driven phoneme restoration, the same results should obtain.

Method

The method was identical to the one employed in Experiment 1.

Materials:

The same sets of u-and non-u-triplets as in the previous experiments were used. The stimuli were recorded by a native speaker of French who imitated a native speaker of Japanese as follows: The Japanese speaker first read a given triplet, and the French speaker repeated, trying to imitate the segmental and suprasegmental features of Japanese except in the cluster condition, where the consonant cluster was pronounced as such. Each triplet was recorded three times, and the best tokens were selected by a French and Japanese listener. The stimuli were then digitally recorded (16kHz).

Procedure: The procedures for the phonetic transcription task and lexical decision task were the same as in the previous experiment.

Participants: Fifteen native speakers of Japanese, recruited in Paris, participated in the lexical decision experiment. Seven of them also performed the phonetic transcription task.

Results and Discussion

Overall, 5 triplets in the non-u-Set and 5 triplets in the u-Set were removed because the words or non words yielded more than 40% errors or are incorrectly transcribed by more than half of the participants. This leaved 14 triplets in the u-Set and 14 triplets in the non-u-Set (the items are listed in the Appendix). Two participants who made more than 50% errors on the non-words (both for distractors and test items) were removed from subsequent analysis.

First, the phonetic transcription results for the consonant cluster items were analyzed separately for items in the u-Set and items in the non-u-Set. In the u-Set, sixty-five percent of the transcriptions of the consonant cluster items contained an epenthetic /u/ inserted between the consonants. Only 1% of the responses contained a different vowel (/o/ instead of /u/ in one item), and in 3% the second consonant was deleted. The remaining 30% of the transcriptions reproduced the consonant cluster. In the non-u-Set, fifty-nine percent of the transcriptions contained an epenthetic /u/, 9% a different vowel (the vowel /i/), and 31% reproduced the consonant cluster response. The /i/ response arose in two items: rekSi and rikSi. It is interesting to note that the /i/ responses turns these two items into real Japanese words (rekiSi and rikiSi, respectively). Note also that in these items, 21% of the responses were still /u/ insertions. The difference in /u/ insertion rates between the u-Set and the non-u-Set did not reach significance (X²(1)=0.52, p>0.10).

Second, the lexical decision data were analyzed in terms of percent 'word' response. The data are displayed in Table 3. As one can see, words were labeled as words (93% of the cases), and non-words were not (8% of the cases). The results of the cluster items depended on the type of set. An average 70% of the cluster items in the u-Set were labeled as words, a score significantly different from 50% (p<.007). Planned contrasts showed that this score was significantly different from the score with the control non-words (F1(1,12)=103, p<.001; F2(1,13)=108, p<.001) and with the control words (F1(1,12)=12, p<0.003; F2(1,13)=13, p<0.001). In contrast, the cluster items in the non-u-Set were classified as words in only 18.7% on average (significantly below 50%, p<.001). This score was significantly different from the score with the control words (F1(1,12)=341, p<.001; F2(1,13)=109, p<.001), and from the control non-words although the difference was significant only for the participants analysis (F1(1,12)=11.7, p<.005; F2(1,13)=1.7, p>.1). Third, we analyzed the RTs for the responses in the dominant category, that is, responses as 'word' for cluster items in the u-Set and responses as 'non-word' for items in the non-u-Set. These responses are shown in Table 4, together with the RTs for the control words and non-words. An Anova revealed that the control words yielded significantly faster latencies than control non-words (F1(1,12)=16, p<.002; F2(1,13)=91, p<.001). Cluster items in the u-Set did not differ significantly from control words (both Fs<1), but yielded significantly faster RTs than the control non-words in the participants ' analysis (F1(1,12)=4.7, p<.05; F2(1,13)=2.7, p>.1). In contrast, cluster items in the non-u-Set yielded slower RTs than the control words (F1(1,12)=28.0, p<.001; F2(1,13)=56.5, p<.001), but did not differ significantly from the control non-words (F1(1,12)=1.9, p>.1; F2(1,13)=3.0, p>.1). In other words, the latencies to respond to cluster items were again similar to the latencies for the items in the lexical category to which they are assimilated. In the above analysis of the transcription task, we found that the stimuli recorded by the French speaker produced 10% more cluster responses than in the previous experiment (a significant difference, p<.001). This suggests that the /u/ epenthesis effect can be enhanced when coarticulation cues remain in the adjacent consonants. Yet, the overall /u/ response rate was still very high (62%), a rate quite similar to that reported in Dupoux et al. (1999). In addition, two items in the non-u-Set yielded an identification of /i/ instead of /u/ (namely, rekSi and rikSi). A post-hoc analysis showed that the lexical decision responses for these two items were 77% and 54% "word" responses, respectively, whereas all the other items in the non-u-Set yielded a majority of non-word responses. Could it be that these two items reflect a lexical influence on the epenthesis effect? If so, the insertion of /i/ is due to the presence of the Japanese words rekiSi and rikiSi. However, none of the other items in the non-u-Set behaved similarly. Despite the existence of lexical items with nonu vowels between the medial consonant, all the other items elicited the perception of an /u/ and a non-word response. An alternative interpretation for rekSi and rikSi is that there are cases in Japanese of /i/ epenthesis. [START_REF] Shinohara | Analyse phonologique de l'adaptation japonaise de mots étrangers[END_REF] discussed some of these cases, and while she argued that /i/ epenthesis may not be a productive phenomenon, it is worthwhile noting that the majority of the existing cases of loan words with /i/ insertion arise in the context of voiceless stop-fricative clusters (i.e. textile -> /tekisutairo/), hence the same context as in rikSi. Interestingly, there were 5 items in the non-u-Set where the insertion of /i/ should have yielded a word. None of these items had a voiceless stop-fricative cluster, and none of them gave rise to the perception of an /i/. This suggests that the insertion of /i/ in rekSi and rikSi may also have a prelexical origin. Further research should uncover whether these cases are bona fide prelexical epenthesis effects or lexical influences.

General Discussion

We created Japanese non-words that contained illegal consonant clusters, and manipulated their lexical neighborood. The stimuli were either digitally edited utterances produced by a native Japanese talker (Experiment 1), or natural utterances produced by a native speaker of a language that allows consonant clusters (Experiment 2). In both cases, and irrespective of the lexical status of the perceptual outcome, Japanese listeners reported hearing a vowel /u/ between the consonants. For example, they transcribe sokdo as `sokudo' (a real Japanese word), and mikdo as `mikudo' (a nonword), taking no account of the existence of a lexical neighbor with a different vowel (mikado). In a speeded lexical decision task, participants classified the stimuli like sokdo as real words, and the stimuli like mikdo as non-words. Moreover, lexical decision times with the stimuli containing the illegal consonant cluster were as fast as with the stimuli with an original vowel /u/.

These results allow us to reject a purely lexical account of vowel epenthesis, and support the interpretation of epenthesis as a prelexical process. Interestingly enough, our data do not reveal any influence of lexical information at all. Lexical influences should have produced more epenthesis in items from the u-Set than in items from the non-u-Set, but this was not the case : the rates of /u/ epenthesis were not greater in items with /u/ neighbors (sokdo) than in items with non-/u/ neighbors (mikdo). This interpretation, however, is made more complex because of high vowel devoicing in Japanese. In several dialects, high vowels (i.e., /i/ and /u/) are often devoiced between voiceless obstruents, or utterance finally after voiceless obstruents. Several factors seem to influence the likelyhood of devoicing: speech rate, pitch accent placement, consonant type (stop versus fricative), as well as sociological effects (see [START_REF] Kondo | Mechanisms of vowel devoicing in Japanese[END_REF][START_REF] Varden | On high vowel devoicing in standard modern Japanese: implications for current phonological theory[END_REF]. Similarily, the precise phonetic implementation of devoicing seems to be non-homogeneous, ranging from something close to a wispered vowel (with formant structure), to fricative noise (without formant structure), going even to pure deletion [START_REF] Tsuchida | The phonetics and phonology of japanese vowel devoicing[END_REF][START_REF] Varden | On high vowel devoicing in standard modern Japanese: implications for current phonological theory[END_REF]. Abstracting away from these details, Japanese listeners may encounter words like sokudo in phonetic forms that are similar to /sokdo/. Could it be that this phenomenon accounts for our results? Two arguments can be offered to counter this view. First, a post-hoc analysis revealed that the eight items containing voiceless obstruent clusters did not yield more epenthesis of /u/ than the others items (Experiment 1: 78% vs. 76%, respectively, p>0.10; Experiment 2: 65% vs. 61%, p>0.10). Second, the vowel /i/ devoices as readily as the vowel /u/. Nevertheless, /i/ epenthesis is quite infrequent, both in our experimental results and in the pattern of foreign word borrowings (see for instance [START_REF] Shinohara | Analyse phonologique de l'adaptation japonaise de mots étrangers[END_REF]. If high vowel devoicing were the sole basis for the vowel epenthesis effect in perception, we should have found equal amounts of /u/ and /i/ epenthesis. Nevertheless, let us assume, counterfactually, that high vowel devoicing applies only to /u/ and within all the consonant clusters used in our material. One may then suppose that items in the u-set (like sokudo) have two possible phonetic forms (e.g. /sokdo/ and /sokudo/) stored in the mental lexicon of Japanese. It is certainly not the case, however, that /mikdo/ is stored as a variant mikado (because /a/ does not devoice). Therefore, a lexical feedback hypothesis cannot explain the results of the transcription task where participants transcribed /mikdo/ as mikudo as readily as they transcribed /sokdo/ as sokudo. In brief, the existence of vowel devoicing in Japanese does not alter our conclusion that it is the phonological context, not the lexical context, that triggers the illusory perception of /u/.

It turns out that the transcription task provides a critical piece of evidence against lexical involvement. This task could be criticized as being rather off-line and metalinguistic ; without denying that, we note that Dupoux et al (1999) found very similar results with off-line transcription and more on-line tasks such as speeded ABX discrimination. Furthermore, Dehaene-Lambertz, [START_REF] Dehaene-Lambertz | Electrophysiological correlates of phonological processing: a cross-linguistic study[END_REF], using evoked potentials in an oddball paradigm, found that Japanese participants showed no mismatch negativity response (MMN) to a change from ebzo to ebuzo, whereas French participants show an early MMN reponse to the same change (140-280msec after the offset of /b/). This further supports our hypothesis that phonotactic constraints modulate the brain response at a very early processing stage, prior to lexical access.

Overall, our results suggest that models of speech perception must take phonotactic information into consideration. Questions still remain about what could the mechanisms be. We can foresee two alternative accounts.

The first account postulates a rule-based process that inspects the phonetic representation of utterances [START_REF] Church | Phonological parsing and lexical retrieval[END_REF][START_REF] Frazier | Structure in auditory word recognition[END_REF] and inserts a vowel whenever there is an illegal sequence of consonants (as in ( 1)). The rules would look like those typically proposed in phonological accounts of loan word adaptations.

(1) ∅ → u / C_C A problem with this account is that there is no independant evidence for this rule in the phonology of standard Japanese. It is thus not clear how it would be acquired by children given that they are not presented with pairs of foreign words together with their Japanese adaptations.

The second account postulates a pattern matching process that assimilates foreign sounds to the phonetically closest prototypes of the native language. It is similar to the Perceptual Assimilation Model [START_REF] Best | The emergence of native-language phonological influence in infants: A perceptual assimilation model[END_REF], see also [START_REF] Takagi | A peceptual basis for the systematic phonological correspondences between Japanese loan words and their English source words[END_REF], with the important modification that whole syllables rather than individual segments, are used as prototypes. Phonetic pattern matching explains why /u/ is used as an epenthetic vowel, instead of /i/ or /a/. Indeed, /u/ tends to be the shortest vowel [START_REF] Beckman | Segment duration and the « mora » in Japanese[END_REF] and the one with the most formant variability [START_REF] Keating | Vowel variation in Japanese[END_REF]. Hence, /ku/ is the closest syllabic prototype that matches the segment /k/ in /mikdo/.

Of course, cross-linguistic research is needed to further work out and test these alternative accounts and also to understand how and when phonotactic constraints are acquired by young infants.

Table 1 :

 1 percent "word" response to cluster and control items in Experiment 1.

		Non-words	Words	Clusters
	u-Set
	Percent " word " response 2.8%	91.0%	71.4%
	Example	sokado	sokudo	sokdo
	non-u-Set
	Percent " word " response 10.3%	96.9%	8.0%
	Example	mikudo	mikado	mikdo

Table 2 :

 2 Reaction times (ms), standard error, and percent error to cluster and control items in

	Experiment 1.
		Non-words		Words			Clusters
		RT	SE	Err	RT	SE Err	RT	SE Err
	u-Set	1223 33	2.8%	1046 33	9.0%	1093 77 28.6%
	non-u-Set	1315 68	10.2%	933	31	3.1%	1234 61 8.0%

Table 3 :

 3 percent "word" response to cluster and control items in Experiment 2.

		Non-words	Words	Clusters
	u-Set
	Percent " word " response 6.0%	91.8%	70.4%
	Example	sokado	sokudo	sokdo
	non-u-Set
	Percent " word " response 10.4%	96.1%	18.7%
	Example	mikudo	mikado	mikdo

Table 4 :

 4 Reaction times (ms), standard error, and percent error to cluster and control items in

	Experiment 2.
		Non-words		Words			Clusters
		RT	SE	Err	RT	SE Err	RT	SE Err
	u-Set	1231 78	6.0%	1055 40	8.2%	1084 37 29.6%
	non-u-Set	1241 64	10.4%	949	34	3.8%	1323 86 18.7%

In japanese, the syllabic types are restricted to C(y)V, V, and C(y)VN. As a result, the only legal consonant clusters are of the nasal plus consonants type.

Appendix

Items in the u-Set bakro-bakuro-bakaro, harka-haruka-haraka, hirma-hiruma-hiroma, huksi-hukusi-hukisi, hikme-hikume-hikime, kaksa-kakusa-kakasa, kokdo-kokudo-kokado, kokmu-kokumu-kokamu, kokso-kokuso-kokaso, kokti-kokuti-kokiti, komgi-komugi-komigi, magro-maguro-magaro, samsa-samusa-samosa, sokdo-sokudo-sokido, soksi-sokusi-sokasi, tuksi-tukusi-tukesi, yakba-yakuba-yakaba, yakza-yakuza-yakaza, yursi-yurusi-yurisi Items in the non-u-Set arsi-arasi-arusi, hiktu-hiketu-hikutu, huksa-hukasa-hukusa, kikme-kikime-kikume, kormo-koromo-korumo, mikdo-mikado-mikudo, namda-namida-namuda, nikbi-nikibi-nikubi, omsa-omosa-omusa, reksi-rekisi-rekusi, riksi-rikisi-rikusi, sakba-sakaba-sakuba, sekri-sekiri-sekuri, sikti-sikiti-sikuti, takra-takara-takura, taksa-takasa-takusa, tegru-tegaru-teguru, wakme-wakame-wakume, waksa-wakasa-wakusa


Emmanuel Dupoux

Kazuhiko Kakehi

Yuki Hirose

Christophe C Pallier

Jacques Mehler

¡ Y Hirose

Epenthetic Vowels in Japanese: a Perceptual Illusion?

come   L'archive ouverte pluridisciplinaire

Languages use different repertoires of distinctive sounds. Some languages use more than 50 distinct consonants, while others use only 5 [START_REF] Crystal | The cambridge encyclopedia of language[END_REF]. In addition, languages differ in the way phonemes can be arranged in sequences, i.e., they respect different phonotactic constraints. For instance, some languages (French, English) allow complex strings of consonants, while others favor a quasi regular alternation of consonant and vowel (Japanese).

It is well known that differences in the phonemic repertoire of a language have a strong impact on the language processing by speakers of that language. For instance, it is well-known that Japanese speakers find it difficult to distinguish the sound of the English /r/ from /l/ [START_REF] Goto | Auditory perception by normal japanese adults of the sounds 'r' and 'l[END_REF][START_REF] Mann | Distinguishing universal and language-dependent levels of speech perception: Evidence from japanese listeners' perception of english [l] and [r][END_REF][START_REF] Miyawaki | An effect of linguistic experience: the discrimination of /r/ and /l/ by native speakers of japanese and english[END_REF]. Many studies have shown that people are strongly influenced by their own native phoneme categories when perceiving and producing foreign sounds [START_REF] Best | Examination of perceptual reorganization for nonnative speech contrasts: Zulu click discrimination by englishspeaking adults and infants[END_REF][START_REF] Kuhl | Innate predispositions and the effets of experience in speech perception: the native language magnet theory[END_REF][START_REF] Trehub | The discrimination of foreign speech contrasts by infants and adults[END_REF]Werker & Tees, 1984a, 1984b;[START_REF] Flege | Effects of age of second-language learning on the production of english consonants[END_REF].

The influence of phonotactic constraints on processing have been much less studied. However, the following observations suggest that they may play a role as important as that of the phonemic repertoire. In Spanish, there is no word starting with an /s/ followed by a consonant; in this language, /s/+consonant clusters are systematically preceded by a vowel. Accordingly, Spanish speakers of English typically say especial instead of special, estimulus instead of stimulus, esport instead of sport, etc. Spanish speakers even maintain that they hear an [e] vowel in /s/+consonant-initial English words. In this paper, we explore a similar, but more spectacular kind of phenomenon in Japanese.

Consonant clusters are not allowed in Japanese utterances which are composed of regular alternations of consonants and vowels1 . This is certainly not the case in languages such as French or English in which words can have very complex onsets and almost equally complex codas, e.g., spleen and facts. Linguists have remarked that native speakers of Japanese tend to incorporate foreign words with consonant clusters into their own vocabulary after inserting a vowel between the consonants (see 1;Ito, 1993). This process of vowel insertion is referred to as vowel epenthesis.

(1) 'fight' ¦ faito 'festival' ¦ fesutibaru 'sphinx' ¦ sufiNkusu 'Zeitgeist' ¦ tsaitogaisuto Why does epenthesis occur? One obvious possibility is orthography. All kanji orthographic characters, by and large, are pronounced as either /n/, a V (vowel) or a CV (consonant-vowel). Hence, there is no Japanese character or combination of characters (in the kanji system) that can spell an item like [sfinks] or any other item with a consonant cluster that does not include nasals. In contrast, [sufinkusu] can easily be spelled in Japanese. Thus, it may be that Japanese speakers modify foreign words by inserting epenthetic vowels so that they can be spelled in their language.

Undoubtedly, orthography has to be taken into account, although it seems unlikely that this factor alone explains vowel epenthesis. For instance, in the examples above, vowel epenthesis seems to follow a rather regular pattern: an [u] is inserted in all cases, except after a dental stop [t] or [d], in which case, an [o] is inserted. It is far from obvious how to explain this pattern on purely orthographic grounds. At any rate, if epenthesis is just an epiphenomenon of orthography, it should disappear (or greatly diminish) in a purely perceptual task. This is one of the questions that we will address in this paper.

Alternative factors that may determine epenthesis are based on phonology. Speakers of Japanese know that their language is a CV-language. When they are confronted with foreign words that contain consonant clusters, they may experience production and/or perceptual problems. Perhaps Japanese speakers have to some extent lost the ability to articulate consonant clusters, and therefore tend to insert vowels to trigger the more practiced CV motor programs. Another possibility, which is one that we explore in this paper, is that the problem originates in perception: namely, that Japanese speakers really hear illusory [u] vowels inside words such as sphinx. If this is true, it would support the view that the phonotactic constraints of one's native language play an important role in speech processing: indeed, one so strong that it produces illusory phonemes. The production-and perception-based accounts of epenthesis are not necessarily exclusive.

Do the phonological properties of one's native language, other than the phonemic repertoire influence speech perception? Psycholinguists have explored this question for the last fifteen years. For instance, studies have shown that listeners who perform various detection tasks are influenced by the syllabic structures of their language (Cutler, [START_REF] Cutler | A language specific comprehension strategy[END_REF][START_REF] Kolinsky | Intermediate representations in spoken word recognition: A cross-linguistic study of word illusions[END_REF][START_REF] Otake | Mora or syllable? speech segmentation in japanese[END_REF][START_REF] Pallier | Attentional allocation within syllabic structure of spoken words[END_REF][START_REF] Sebastian-Galles | Contrasting syllabic effects in catalan and spanish[END_REF][START_REF] Zwitserlood | The role of the syllable in the perception of spoken dutch[END_REF]. Other studies have revealed that language-specific strategies are used to postulate word boundaries in continuous speech recognition [START_REF] Cutler | The role of strong syllables in segmentation for lexical access[END_REF][START_REF] Cutler | The monolingual nature of speech segmentation by bilinguals[END_REF][START_REF] Suomi | Vowel harmony and speech segmentation in finnish[END_REF]. Such crosslinguistic studies are important for at least two reasons. First, if speech processing is influenced by phonological properties beyond the phonemic repertoire, models of speech perception have to be revised to incorporate such higher order structures, and, cross linguistic studies are necessary to establish a general model of speech perception that holds across all human languages. Second, psycholinguists will have to explain how the child acquires the perceptual adjustments necessary to perceive his or her native language [START_REF] Best | The emergence of native-language phonological influence in infants: A perceptual assimilation model[END_REF][START_REF] Jusczyk | Infant speech perception and the development of the mental lexicon[END_REF][START_REF] Mehler | Constraining models of lexical access: The onset of word recognition[END_REF][START_REF] Polka | Developmental changes in perception of nonnative vowel contrasts[END_REF].

What evidence exists that constraints on possible sequences of phonemes play a role in perception? Psychologists have established that humans are sensitive to statistical regularities in sequences of units [START_REF] Miller | The intelligibility of speech as a function of the context of the test materials[END_REF][START_REF] Miller | Language and communication[END_REF]. [START_REF] Jusczyk | Infants' sensitivity to the sound pattern of native language words[END_REF]Jusczyk (1993), Jusczyk, Luce, and[START_REF] Jusczyk | Infants' sensitivity to phonotactic patterns in the native language[END_REF] have shown that at 9 months infants become sensitive to the phonotactic patterns of the words in their language. Indeed, some researchers have argued that such regularities could be useful in helping the child to discover words [START_REF] Hayes | Experiments on the segmentation of an artificial speech analogue[END_REF][START_REF] Brent | Distributional regularity and phonotactic constraints are useful for segmentation[END_REF].

Adults have rather clear intuitions about permissible sequences, e.g. English speakers know that "mba" is not a possible English word. However, the only study we are aware of, that has specifically investigated the influence of phonotactic constraints in phoneme perception is that of [START_REF] Massaro | Phonological constrainst in speech perception[END_REF]. They used the fact that /sri/ and / § li/ are not allowed in English while /sli/ and / § ri/ are allowed. They synthesized a series of stimuli ranging from /s/ to / § / and presented them to subjects in the / li/ and / ri/ context. There was a significant shift in the identification functions between the two contexts demonstrating that subjects tend to hear segments that respect the phonotactics of their language (see also [START_REF] Mcclelland | The TRACE model of speech perception[END_REF][START_REF] Massaro | Integration versus interactive activation: The joint influence of stimulus and context in perception[END_REF].

Notice, however, that this study only demonstrates an effect on ambiguous stimuli. We believe that it would be desirable to demonstrate the influence of phonotactics on endpoint (unambiguous) stimuli. Second, this study was conducted with a single language, leaving open the possibility that some of the effects might be found in all speakers regardless of their native language.

Here, we further explore the role of phonotactics on perception by using a methodology that involves non-degraded speech stimuli and a cross-linguistic design. We investigate the perceptual reality of epenthesis using an off-line phoneme detection task (Experiment 1), and two speeded ABX tasks (Experiments 2 and 3). We test the same stimuli on two populations: Japanese native speakers and French native speakers. As in English, French has complex syllabic structures and was used as a control because it typically does not exhibit epenthetic effects.

Experiment 1:

The aim of this experiment is to assess the extent of the epenthesis effect in a purely perceptual task. We created nonword stimuli that formed a continuum ranging from trisyllabic tokens like ebuzo to disyllabic tokens like ebzo by progressively removing acoustic correlates of the vowel from the original stimuli. We selected our materials in such a way that the clusters could always potentially yield an epenthetic [u] (that is, the first consonant of the cluster was never a dental stop). Subjects were then asked to decide whether or not the vowel [u] was present in the stimuli, so that no overt production of the stimuli was needed. If the epenthesis effect has a perceptual basis, Japanese subjects should be more reluctant than the French listeners to say that the [u] vowel is absent.

Method

Materials Ten sequences of © © ¨(¨: four Japanese vowels excluding [u], © : voiced and voiceless stops, © : nasals and voiced obstruents) uttered by a male Japanese speaker were used as stimulus items (see the Appendix). None of the stimulus items constituted a meaningful word either in French or Japanese.

The stimuli were digitized on a PC Compatible computer using an OROS AU22 A/D board. Five different files were then created from each original item by splicing out pitch periods of the medial vowel [u] at zero crossings. Stimulus 1 contained no vowel [u] at all (most of the transitions in and out of the vowel were also removed). Stimulus 2 contained the two most extreme pitch periods of the vowel (i.e., one from the transition of the first consonant to the vowel [u], and another from the end part of [u] into the following consonant). Stimulus 3 contained the fout most extreme pitch periods (two on each side), and similarly, Stimulus 4, 6 pitch periods, and Stimulus 5, 8 pitch periods. Stimulus 6 was the original stimulus in which the number of pitch periods contained in the vowel [u] in each item varied from 10 to 13 (10.7 periods in average.) The average overall duration of one pitch period in the [u] vowels in each item was 9.06 msec. There were a total of 60 stimuli in one session.

French Subjects

Japanese Subjects

% [u] detection

Vowel Duration Procedure Subjects were instructed to listen to the stimuli through headsets and make a judgment as to whether there was a [u] vowel in the middle of each stimulus word. The stimuli were played on a SONY DAT player. The subjects were provided with an answer sheet and asked to draw a circle for "Yes" and a cross for "No". We emphasized that the experiment was not aimed at measuring their linguistic skills, and that the number of Yes and No answers need not be balanced. Japanese subjects in particular were told that the [u] target in the experiment was not meant to be equivalent to the kana syllabary " " which represents the single vowel [u], but rather to the phoneme [u] that was a part of another CV syllabary. Each subject heard the list three times with all the stimulus sets differently randomized each time.

The French subjects were given a similar setup and instructions, except that stimulus presentation and response recording was performed on a PC Compatible with a Proaudio Spectrum 16 D/A Board. Subjects were required to press the [O] key for yes ('oui') responses and the [N] key for no ('non') responses.

Subjects Ten Japanese and ten French native speakers volunteered to participate in Experiment 1. All the subjects were college students. None of the Japanese subjects had studied French and none of the French subjects had studied Japanese.

Results

The mean percentage of vowel responses as a function of language and vowel length is shown in Figure 1. We performed two analyses of variance on percentages of vowel responses, one with subjects and one with items as random variables. Language (Japanese or French) was a between-subject factor and Vowel Length a within-subject factor (with 6 levels). In the following, and in all subsequent analyses, we report the MinF' statistics when they are significant (p .05), and the F1 and F2 statistics otherwise.

Overall, there was a significant Language effect (MinF'(1,25)=25.1, p .001), with the Japanese subjects providing more Vowel responses than the French subjects. There was also a significant Vowel Length effect (MinF'(5,100)=56, p .001), which had a significant linear component (MinF'(1,20)=152, p .001), in that longer vowels yielded more Vowel responses than shorter vowels. There was an interaction between Language and the linear component of Vowel Length (MinF'(1,26)=128, p .001), corresponding to the fact that the French subjects were much more influenced by vowel length than the Japanese. Yet, even in the Japanese subjects, the linear component of vowel length introduced a significant effect (MinF'(1,18)=11, p .005).

We ran pairwise comparisons between the two languages for each vowel length. For the first three vowel lengths (0ms, 18ms, 36ms), Japanese subjects gave significantly more Vowel responses than French subjects (all MinF' p .001)2 . For the fourth vowel length (54ms), there was only a trend in the same direction (F1(1,18)=5.4, p .04; F2(1,9)=3.67, p=.088). A significant difference between the two populations did not appear for the last two vowel lengths (72ms and full vowel).

Discussion

In this experiment, Japanese and French subjects judged the presence or absence of the vowel [u] in stimuli containing varying amounts of the acoustic correlate of the vowel. French subjects were able to judge that the vowel was absent in the ebzo case, and present in the ebuzo case, with a monotonic function for the intermediate cases. The cutoff point for the French subjects, that is, the point at which they judged the vowel to be present in 50% of the cases, can be estimated at just over 4 pitch periods (38ms) of the vowel. In contrast, Japanese subjects predominantly judged that the vowel was present at all lvels of vowel length. Like the French, Japanese vowel responses show a steady decrease as a function of decreasing vowel length -which shows that they are sensitive to manipulation in vowel length-but the slope is much less sharp. Even at the extreme of the continuum where the vowel had been completely deleted, they still reported that the vowel was present in more than 70% of the cases.

This experiment establishes that, in a task involving no overt speech production, Japanese subjects consistently report vowel between two consonants in CC clusters. This experiment alone, however, cannot firmly establish the locus of the effect in speech perception for two reasons. First, the task requires subjects to make an explicit metalinguistic judgment: subjects have to know what a vowel is in order to do the task. It is known that learning to read influences the way in which individual phonemic segments can be manipulated in a metalinguistic task (see the collection of articles in [START_REF] Bertelson | The onset of literacy[END_REF]. Because the writing systems of Japanese and French differ, it is possible that it differentially affects vowels judgments in Japanese and French subjects. Second, the task did not use a speeded or on-line judgment. Therefore, it cannot identify which of the different sources of information (the orthographic code, covert production, explicit strategies) influenced the subjects' responses. For instance, it is possible that Japanese subjects were reluctant to give a vowel-absent response simply because they knew that such stimuli do not occur in Japanese.

The next two experiments were designed to address this issue. We used an ABX paradigm that only required identity judgments, thus involving no explicit or implicit mention of vowels. We also had subjects perform a speeded response, thereby reducing the use they could make of complicated response strategies.

Experiment 2:

In this experiment, we use a speeded ABX paradigm, whereby subjects hear three stimuli and have to decide whether the third stimulus is identical to the first or to the second. If the findings of Experiment 1 have no perceptual basis but are instead a by-product of metalinguistic limitations in segment manipulation, Japanese subjects should make few errors when discriminating between ebuzo and ebzo. In fact, their performance should be indistinguishable from that of French subjects. If, in contrast, the perceptual system inserts an epenthetic vowel to break up consonant clusters, Japanese subjects should have trouble distinguishing stimuli such as ebuzo from stimuli such as ebzo, because they will in fact "hear" the same thing twice. However, ebzo may be "heard" as containing a vowel with different acoustic/phonetic characteristics from the [u] in ebuzo. For this reason, in this experiment we chose to have different talkers produce the X stimuli and the other two stimuli (A and B), thereby forcing subjects to rely on a more abstract/phonological representation rather than on an acoustic/phonetic one. Experiment 3 will specifically test the effect of talker change.

Note, however, that testing these hypotheses involves comparing the performance of different groups of subjects (that is, testing whether Japanese subjects are significantly better or worse than French subjects on a given task). Such comparisons raise many methodological issues since it is hard to match populations of subjects in all possible respects other than native language. This is why we introduced a within-population control.

This control was achieved by considering another property of the phonology of Japanese: in Japanese, vowel length is contrastive, for instance, tokei (watch) vs. tookei (statistics). The long vowel is in fact perceived as two adjacent vowels. If this is so, Japanese subjects should have no problem in performing the ABX task on an ebuzo-ebuuzo contrast. In our stimuli, the ebuuzo-ebuzo contrast had the same difference in acoustic duration as the ebuzo-ebzo contrast (about 90ms). One can thus see our design as an extension of the continuum of vowel length used in Experiment 1.

An interesting feature of this design is that it predicts a cross-over interaction between contrast and native language. In French, vowel length has no contrastive function, i.e., no pairs of French words can be distinguished purely on the basis of the length of one vowel. The hypothesis under examination is that language listeners impose the phonology of their native language on unfamiliar linguistic stimuli, regardless of whether the stimuli are native or foreign. Hence, we predict that French subjects might have trouble in making the ebuzo-ebuuzo contrast whereas the Japanese should have no problem at all.

Method

Materials Sixteen triplets of the form (ebzo, ebuzo, ebuuzo) were constructed (see the Appendix). All triplets conformed to the model ! " # $ ! " # $# $ . The first consonants were from the set [b, k, g, % ], the initial and final vowels were from the set [e, i, a, o], and the second consonants were from the set [z, d, g, n, m, % , t]. All stimuli were non-words in both French and Japanese. All stimuli consisted of phonologically valid French syllables and, with the exception of the first member of the triplets, of valid Japanese morae. Four additional triplets with the same phoneme range constraints as for , , , and were used in the training set.

The materials consisting of the twenty triplets were recorded twice: once by a male Japanese speaker and once by a female Japanese speaker. The recordings were made in a sound attenuated room, and digitized at 16kHz/16 bits on an OROS AU22 D/A board. Each stimulus was store in a separate file using a waveform editor. It transpired that although our two Japanese speakers were fluent in French and had some training in phonetics, they could not be prevented from inserting a very short vowel [u] within the consonant clusters in some of the ebzo stimuli. These ebzo stimuli were therefore edited with a waveform editor, and the vocalic part was progressively removed, until a French listener found that he/she could no longer hear the [u] vowel. Four judges were then asked to decide whether the consonants and the vowels making up the stimuli sounded natural in their respective languages. The procedure was repeated until the stimuli were deemed fully satisfactory. The three classes of stimuli had a mean duration of 409ms for ebzo, 498ms for ebuzo and 593ms for ebuuzo, respectively.

One hundred and twenty eight experimental trials were constructed using the 16 experimental triplets. Each experimental trial consisted of three stimuli: A, B, and X, where the first two were spoken in a female voice, and the last in a male voice. A and B were taken from the same triplet but differed in the vowel duration. There was an Epenthesis contrast (ebzo-ebuzo), and a Vowel Length contrast (ebuzo-ebuuzo). Each contrast could appear in 2 different possible orders resulting in 4 A-B combinations for each triplet. The X stimulus was identical either to A or B. The overall design was: 2 & 2 & 2: Contrast & Order & X-identity. By partial counterbalancing, 16 training trials using the four training triplets were obtained. These contained the same conditions as in the experimental trials.

The 128 experimental trials were split into two blocks, with each condition and item equally represented in each block.

Procedure Each experimental trial consisted of the presentation of the three stimuli (A, B and X), separated by a delay of 500ms. Subjects were told that the stimuli were words from a foreign language and that the purpose of the experiment was to test their intuitions about the sounds of foreign words. They were told that the third word (X) was identical to one of the first two (A or B). Their task was to press a button on their left or on their right to indicate whether X was identical to A or to B, respectively. Subjects were given 4000ms to respond. The next trial started 1000ms after each response (or after the 4000ms).

In the ten training trials, subjects received feedback as to whether their response was correct or not. Feedback consisted of the word "Correct" or "Incorrect", or the string "The response is A" (or B) when subjects failed to respond before the deadline. Feedback was displayed for 1000ms, and then was erased from the screen. For incorrect responses, the same trial was presented again immediately until the response was correct. In the two experimental blocks of 64 trials, no feedback was presented. The blocks were randomized separately for each individual subject. A short pause was introduced between the two experimental blocks. Responses were recorded and reaction times measured from the onset of the X stimuli with the EXPE software package (see [START_REF] Pallier | Expe5: an expandable programming language for on-line psychological experiments[END_REF].

Subjects Ten Japanese and ten French subjects participated in the experiment. All were recruited in Paris. The age of the Japanese subjects varied from 20 to 48 years of age (median 36). Two had no knowledge of French. All knew some English. All had begun the study of foreign languages after 12 years of age. There were 4 men and 6 women in the group. The age of the French subjects varied from 20 to 50 years of age (median 24). None spoke Japanese. All had studied English at school. Like the Japanese subjects, the French subjects had started studying a foreign language after the age of 12. There were 9 men and 1 woman in the French group. All subjects were right handed. All volunteered for the experiment and no one was paid for his or her participation.

Results

Four ANOVAs were performed on the entire dataset: two on RT data by subject and by item and two on Error data, again by subject and by item. The ANOVAs had a 2 ' 2 design: Language (French or Japanese) ' Contrast (epenthesis or vowel length contrast).

The means, standard error and error rates are displayed for each condition in Table 1.

The analysis of the RT data showed a highly significant interaction between Language and Contrast (MinF'(1,29)=14, p( .001). This interaction was due to the fact that for the French subjects, the vowel length contrast yielded longer RTs than the epenthesis contrast (171ms, MinF'(1,19)=12, p( .002), whereas, for the Japanese subjects, there was a trend in the other direction (-105ms, F1(1,9)=4.5, p=.06; F2(1,15)=7.8, p( .02). There was no main effect of Language (F1(1,18)( 1, p) .1; F2(1,15)=3.4, .05( p( .1), and no main effect of Contrast (F1(1,18)0 1, p1 .1; F2(1,15)=3.5, .050 p0 .1).

The analysis of the error data showed the same pattern of results. There was a highly significant interaction between Language and Contrast (MinF'(1,26)=56, p0 .001). This interaction was due to the fact that for French subjects, the vowel length contrast was more difficult than the epenthesis contrast (MinF'(1,16)=18, p0 .001), whereas the length contrast was easier for the Japanese (MinF'(1,13)=35, p0 .001). Overall, Japanese subjects tended to make more errors than the French subjects, although this was only significant in the items analysis (F1(1,18)=3.7, p=.07; F2(1,15)=20.1, p0 .001). Similarly, the epenthesis contrast tended to provoke more errors than the vowel length contrast, but again this was only significant in the items analysis (F1(1,18)=4.1, p=.058; F2(1,15)=13, p0 .002).

Discussion

In this experiment, French and Japanese subjects had to perform an ABX discrimination task on two contrasts: the epenthesis contrast (ebzo-ebuzo) and the vowel length contrast (ebuzo-ebuuzo). We found a cross-over interaction: the Japanese subjects had difficulty with the Epenthesis contrast, whereas the French had trouble with the Vowel Length contrast.

Interestingly, the difficulties of Japanese subjects with consonant clusters is mostly apparent in the Error data (32% of errors, and a 'trend' of 105ms in the latencies) whereas the Vowel Length contrast appears as difficult for the French, most apparently with the RT data (a significant 170ms slowing) and marginally in errors, (21% of errors). Whether such asymmetries reflect some kind of speed-accuracy tradeoff, or a deeper difference in the processing of these contrasts remains to be explored in further studies.

These results demonstrate that the phonotactics of a language influence speech perception with clear stimuli. That is, not only do Japanese subjects report "illusory" vowels in order to conform to their native phonology (Experiment 1), but this affects the perceived similarity (and hence the ability to discriminate) of two stimuli.

Note that in this experiment, we introduced a change in talker between stimulus X and the two preceding A and B stimuli. This was done to induce subjects to disregard low level acoustic characteristics and rely on a more abstract phonological representation. However, most studies of the perception of nonnative contrasts have used a more conventional ABX paradigm with no such change in talker. Would our results still hold without a talker change, that is, in a situation in which subjects can use purely acoustic information? The next experiment addresses this issue.

Experiment 3:

The present experiment was designed to evaluate the effect of a change in talker on the robustness of the language-specific pattern of previously obtained results. In this experiment, we replicate the conditions of Experiment 2 and add a new set of conditions with no change in talker. In this condition, one of the two stimuli, A or B, is acoustically identical to the X stimulus. This should strongly induce subjects to use a rather low level of representation, since in principle it is possible to accomplish this task on a purely acoustic basis. If the epenthesis effect is still present in the same-talker condition, this consolidate the claim that, at a certain level, Japanese subjects are "deaf" to the difference between ebuzo and ebzo.

In this experiment, we will also perform analyses of two other variables in order to further characterize the robustness of the effects: the effect of training and the language background of the subjects.

The first variable we examine is the potential effect of training. Experiment 2 was rather short (15 minutes). It could be that the observed effects were due to subjects not being very familiar with the stimuli and the task. Does the effect disappear or diminish with more extensive exposure to the contrasts? The present experiment contains 266 trials, twice as many as Experiment 2. Furthermore, the lists are randomized and the blocks counterbalanced in such a way that potential sequential effects could be evaluated. If the epenthesis effect is labile, we should find a negative correlation between effect size and sequential position. In addition, Japanese and French subjects should have similar results in the final part of the experiment.

The second variable is language background. We had subjects fill out a detailed biographical questionnaire concerning their language experience. We were particularly interested in the degree of fluency of our Japanese subjects in a language that includes consonant clusters (such as English or French). It could be that with exposure to such languages, speakers of Japanese learn to overcome the epenthesis effect. If so, we should find that the more proficient bilinguals show less effect (or no effect) compared to less proficient bilinguals or monolinguals.

Method

Materials The same materials as in Experiment 2 were used. We used the same 128 ABX experimental trials of Experiment 2 (A and B stimuli spoken by the female talker, and X stimuli by the male talker) and created another 128 trials with the stimuli A, B and X all spoken by the same male talker. In these last trials, X was acoustically identical to either A or B. The overall design was: 2 x 2 x 2 x 2 : Contrast x Order x X-identity x Talker.

The 256 experimental trials were split into four blocks of 64 trials, with each condition and each item equally represented in each block.

Procedure The same procedure as in Experiment 2 was used.

Subjects Twenty Japanese subjects were recruited (10 in Paris, 8 in New York and 2 in Nagoya), and tested individually in a quiet room. None of them had participated in the previous experiments. Their ages ranged from 22 to 40 (median 29). There were 14 women and 6 men in the group.

Twenty French subjects recruited in Paris were tested on the same materials. None of them had participated in the previous experiments. Their ages ranged from 19 to 50 (median 21.5). There were 4 women and 16 men in the group.

Subjects filled out a detailed biographical questionnaire about their experience with foreign languages. They also rated their own fluency and pronunciation in these languages on a 10 point scale.

Side

Results

As in Experiment 2, we ran four ANOVAs, two by subjects and two by items, on reaction times and error rates, respectively, with Language, Talker, and Contrast as experimental factors. In Table 2 the means, standard error and error rates are displayed for each condition.

The analysis of the RT data showed that there was a highly significant interaction between Language and Contrast (MinF'(1,53)=15, p2 .001). This interaction was due to the fact that for French subjects, the vowel length contrast yielded significantly slower reaction times than the epenthesis contrast (117ms, MinF'(1,34)=14, p2 .001), whereas for Japanese subjects, there was a nonsignificant trend in the other direction (-27ms, all ps3 .1). No other interaction was significant, except the interaction between Language and Talker, which was only significant in the items analysis (F12 1; F2(1,15)=16, p2 .001).

There was a main effect of Talker, with the same talker yielding faster RTs than the different talker (85ms, MinF'(1,52)=12, p2 .001). There was also a main effect of Contrast, with the vowel length contrast on average yielding slower RTs than the epenthesis contrast (45ms, F1(1,38)=7.8, p2 .01; F2(1,15)=5.2, p2 .04). Finally, Japanese talkers tended to have longer RTs than French subjects, but this was only significant in the items analysis (55ms, F14 1; F2(1,15)=18, p4 .001).

The analysis of the error data showed similar results. There was a highly significant interaction between Language and Contrast (MinF'(1,40)=34, p4 .001). This interaction was due to the fact that for Japanese subjects, the epenthesis contrast yielded significantly more errors than the vowel length contrast (MinF'(1,31)=22, p4 .001), whereas for French subjects, there was a significant effect in the other direction (MinF'(1,33)=8.6, p4 .006). No other interaction reached significance.

There was a main effect of Talker, with a different talker yielding more errors than the same talker (MinF'(1,36)=6.5, p4 .02). There was also a main effect of Contrast, with the epenthesis contrast on average yielding more errors than the vowel length contrast, (MinF'(1,35)=4.8, p4 .04). Finally, Japanese talkers tended to make more errors than French subjects, but this was only significant in the items analysis (F1(1,38)=3.4, .054 p4 .1, F2(1,15)=9.5, p4 .01).

Influence of training

We began studying the effect of training by using a correlation analysis. For each subject, the sequence of reaction times on experimental trials was cut into 16 successive bins of 16 datapoints. We found a significant negative correlation between sequential position and mean reaction time (R5 =.67, F(1,14)=28, p4 .001). We also found a significant negative correlation between sequential position and error rate (R5 =.67, F(1,14)=28, p4 .001). These effects show that training does have an impact, and that subjects improve their performance with time. We then computed the mean interaction between language and contrast for each sequential position. There was no significant correlation between sequential position and interaction size either in the reaction time (R5 =.16, F(1,14)=2.6, p6 .1) or in the error analysis (R5 =.17, F(1,14)=2.9, p6 .1).

In a second step, we ran ANOVAs similar to the ones reported above, but restricted the analysis to the final block of 64 trials (after 202 trials). In this analysis, the interaction between Language and Contrast was still significant, both for the reaction times (MinF'(1,51)=4.65, p4 .04) and the error data (MinF'(1,36)=17.4, p4 .001).

The effects on the last block were very similar for same talker and different talker conditions both for the reaction times and the errors, although there was a nonsignificant trend towards a smaller magnitude of the effect for the same talker condition (10ms in the reaction times, and one percent on the error data).

We ran a new set of analyses including Response Type (A-or B-responses) as a within subject and within item factor. A-responses yielded longer RTs (127ms, MinF'(1,52)=74, p4 .001) and more errors (12% vs. 5%, MinF'(1,52)=32, p4 .001) than B-responses. Bresponses showed a trend for less Language by Contrast interaction than A-responses, a trend which was significant for the error data (triple interaction between language, condition and Response Type: MinF'(1,46)=22, p4 .001).

However, even with B-responses there was still a significant interaction between Language and Condition (MinF'(1,50)=8.68, p4 .005 for the reaction times, MinF'(1,31)=11.4, p4 .002 for the errors). This interaction was significant even for the same talker condition (MinF'(1,52)=6.9, p4 .015 for the reaction times; MinF'(1,29)=10.4, p4 .003, for the errors).

Influence of language background Inspection of the questionnaire reveals that the Japanese subjects mostly had experience with French or English (one reported having studied some Italian, and one some Russian). They had all begun to study these foreign languages in school after the age of 12. Four Japanese subjects did not fill out the questionnaire. We separated the subjects into two groups, one labeled 'low proficiency' (7 subjects), the other labeled 'high proficiency' (9 subjects) based on the means of their own evaluation of fluency and pronunciation. 'High proficiency' subjects could all understand spoken English or French and sustain a conversation in these languages with good fluency and a moderate foreign accent, as assessed by the experimenters. 'Low proficiency' subjects had trouble both understanding and being understood in English or French; some of them could not express themselves in either of these languages.

We found that the Proficiency factor introduced no significant effect nor any interaction in the analysis of errors (p7 .1). In fact, the 'high proficiency' group displayed roughly the same pattern of errors as the 'low proficiency' group (both showed 16% of errors in the epenthesis contrast).

In a further analysis, we selected the four best Japanese subjects with the greatest proficiency in English or French (both self-rated, and as evaluated by an external judge). The selected subjects had all lived in France or the US for more than 4 years (one is an English teacher, another a student of phonetics, and two others are university students in the US), and were very fluent in French or English. For these subjects, the percent error on the epenthesis contrast was in the same range as that of the other Japanese listeners (15.9% on average vs. 4.7% for the vowel length contrast).

We also analyzed the linguistic background of the French subjects. They all knew English (all had learned it after age 6). Some also knew German, Italian, Spanish, or Arabic. Note that none of these languages use vowel length contrastively. However, English, Spanish, and Italian use stress contrastively, and vowel length is used as a cue for stress. We then tentatively separated these subjects into two groups (high and low proficiency) according to their evaluation of proficiency in these languages. We found no effect of fluency on the error data or on the reaction times (p7 .1) which showed the same effects in the two groups.

More generally, every single Japanese subject that we tested in this experiment showed the epenthesis effect, that is, each subject showed more errors on the epenthesis contrast than on the vowel length contrast. Such regularity is also true of Experiment 2. In contrast, 18 out of 20 French subjects showed either no difference, or the opposite pattern (and 9 out of 10 in Experiment 2). In other words, the observed cross-over interaction of language and contrast in the error data is highly robust and reproducible from subject to subject, at least in the sample we tested.

Discussion

In this experiment, we studied the effect of a talker change on the size of the languagespecific effects reported in Experiment 2. We found that even though the same talker condition elicited significantly shorter reaction times and fewer errors than the different talker condition, this variable had a very small effect on the previously reported interaction between language and contrast. We found that Japanese subjects had more difficulty with the epenthesis contrast than with the vowel length contrast, and the French vice-versa, regardless of whether the ABX task involved tokens produced by the same talker or not. This is all the more remarkable since in the same talker condition, a judgment of acoustic identity alone was sufficient to perform the task.

In addition, we found that after more than 200 trials such cross-linguistic effects still obtained. Although training has a very powerful effect on both reaction time and error rate, it does not significantly modulate the size of the effects.

Moreover, we found that the interaction was still significant on B-responses, even in the same talker condition. This is noteworthy because subjects only need to judge the acoustic identity of two adjacent stimuli (B and X) in order to respond. Indeed, Response Type had a powerful effect on performance: A-responses generated both more errors and longer reaction times than B-responses. However, that variable did not interact with the cross-linguistic effect.

Finally, a post-hoc analysis in terms of linguistic background revealed no clear effect of fluency in a consonant cluster language such as English or French. That is, both fluent and nonfluent Japanese speakers showed an epenthesis effect of about the same size. Of course, although we used Japanese subjects in France or the US, we did not use extremely fluent bilinguals. Even our "high proficiency" subjects had learned English or French after age 12 and had a noticeable Japanese accent in these languages.

It remains an open question as to whether extremely proficient bilinguals or more early bilinguals might have a reduced epenthesis effect.

Finally, we have to address a minor caveat. When we compare Table 1 and Table 2, the percentage of errors for the epenthesis contrast in Japanese subjects is smaller in Experiment 3 than in Experiment 2 (16% instead of 32%, a significant difference, p8 .05). Given that, individually, neither training nor nature of talker significantly reduce the epenthesis effect, why should such a difference obtain?

This apparent discrepancy may be due to the fact that two weak variables can nonetheless conjointly have a significant effect. Indeed, if we look at the first experimental block in the present experiment, we find that the epenthesis contrast yielded 28% errors for the different talker condition, which is not significantly different from the 34% score of the equivalent first block in Experiment 2. At the very onset of both experiments, comparable effect sizes were thus found for the different talker conditions. In the next trials, however, divergences appear, as the score stays at 31% in block 2 of Experiment 2, but drops to a value centered around 16% in Experiment 3. Such a drop is not found for the same talker condition which yields an initial score of 12% and stays around this value throughout Experiment 3.

In other words, there is an initial difference between same and different talker conditions (p8 .05), but after the first block, the different talker condition drops to the same value as the same talker condition. This suggests that it is only training in a same talker condition that reduces the size of the epenthesis effect in the different talker condition. One might think that the same talker condition should allow the subject to focus his/her attention on the right acoustic/phonetic cues, a strategy that can be used on subsequent trials. But we have not demonstrated that, with more extensive training, even better performance cannot be achieved. So, more research would be needed to explore this point.

General Discussion

The present series of experiments has shown that Japanese listeners, in contrast to French listeners tend to perceive illusory epenthetic [u] vowels within consonant clusters. Indeed, Japanese subjects have difficulty discriminating between a stimulus that does not include a vowel (ebzo), and one that does (ebuzo). However, Japanese subjects, unlike French, easily discriminate stimuli that contain one versus two successive [u] vowels. The epenthesis effect we have established is robust. It was present in each of the Japanese volunteers that we tested and was still significant even when the experimental setting was designed to help subjects discriminate (Experiment 3). Moreover, we found very little evidence that proficiency in English or French changes the pattern of data. Needless to say, no tendency toward epenthesis was present in our French volunteers.

These results buttress the hypothesis that subjects actively use phonotactic knowledge in speech perception. This complements and extends the work by Massaro and Cohen (1993). Indeed, not only does phonotactic knowledge influence the perception of individual segments, but it can induce the perception of "illusory" segments. Moreover, it does so in perfectly clear stimuli. This shows that the way in which the continuous speech stream is segmented into discrete phonemes is not universal, but depends on what the typical pattern of alternation between consonants and vowels is in the language in question. How could such effects be accounted for? Different models provide possible answers. [START_REF] Church | Phonological parsing and lexical retrieval[END_REF] has proposed that, in the course of speech perception, a hierarchical phonological representation is elaborated. This representation is made possible by relying on the language specific rules internalized during childhood. In order to accommodate our data, such models would have to stipulate that incorrect or deviant phonological forms are automatically regularized by the parsing device. The exact nature of the regularization routines, however, needs to be further specified. [START_REF] Norris | Shortlist: a connectionist model of continuous speech recognition[END_REF] has proposed a model of lexical recognition that uses phonemic units as input, lexical units as output, the two being linked by a recurrent network with hidden units. Such hidden units encode sequential statistical regularities of the segments. This model may have the potential to account for effects like the ones we reported in this paper. However, more studies are needed to explore the adequacy of the fit between the model and the data.

Finally, [START_REF] Mehler | Constraining models of lexical access: The onset of word recognition[END_REF] have proposed SARAH, a model based on an array of syllable detectors. In this model, speech sounds are chunked and categorized into syllable-sized units. The repertoire of syllables includes the totality of the syllables used in the language. In this view, an account of the epenthesis effect is quite straightforward. Indeed, Japanese subjects do not have syllable detectors that contain consonant clusters or coda consonants. Upon hearing a word like ebzo, they use the nearest syllabic candidate available, namely, e+bu+zo.

Obviously, more studies are necessary to choose among these specific models. However, our findings already allow us to pinpoint shortcomings in another class of models, namely, those that represent phonemes [START_REF] Mcclelland | The TRACE model of speech perception[END_REF][START_REF] Norris | Shortlist: a connectionist model of continuous speech recognition[END_REF], or subphonemic elements [START_REF] Klatt | Review of the ARPA speech understanding project[END_REF][START_REF] Klatt | Review of selected models in speech perception[END_REF][START_REF] Marslen-Wilson | Levels of perceptual representations and process in lexical access: words, phonemes and features[END_REF] without any mention of higher order structures. In such models, no direct effect of the phonotactic organization of the language being used is expected in processing. Rather, such effects have to arise indirectly from another information source, such as the statistical properties of the lexicon.

For instance, [START_REF] Mcclelland | The TRACE model of speech perception[END_REF] modeled the phonotactic effects found by [START_REF] Massaro | Phonological constrainst in speech perception[END_REF] by allowing lexical feedback to bias the low-level representation of an ambiguous sound (but see [START_REF] Massaro | Integration versus interactive activation: The joint influence of stimulus and context in perception[END_REF]. In this topdown account, illegal sound patterns have to fight against the collective action of similar words that have a more frequent sound pattern. It is unlikely that the effects we report here can be modeled in a similar way. First, in TRACE, the sequence of phonemes is represented spatially, and thus, although lexical feedback could replace a segment by another one, it could not simply insert a segment. Second, even if this were possible, it would require that the majority of lexical items activated by the nonword ebzo contain a [u] sound between [b] and [z]. And this would have to be the case for each of the 16 items we used in the experiments since epenthesis was found to happen for each of these items. This is an unlikely situation.

Our research is consistent with other studies showing that it will be difficult to build a realistic model of speech perception that only relies on linear strings of phonemes. For instance, as [START_REF] Dupoux | A destressing "deafness" in french?[END_REF] already showed, the way in which suprasegmental information is perceived depends on the accentual regularities in the language of the hearer. Spanish listeners have no difficulty in swiftly responding to a difference in accentual pattern (vásuma vs. vasúma), while French listeners are slow and error prone. Such effect arise, we believe, because Spanish uses accent contrastively (bebé vs. bébe), while French does not. More research is needed to understand how models can be modified to take into account such higher-order properties of signals.

Materials used in Experiments 2, 3 and 4

abge-abuge-abuuge, agmi-agumi-aguumi, akmo-akumo-akuumo, a9 mi-a9 umi-a9 uumi, ebza-ebuza-ebuuza, egdo-egudo-eguudo, ek9 i-eku9 i-ekuu9 i, e9 mo-e9 umo-e9 uumo, ibdo-ibudo-ibuudo, igna-iguna-iguuna, ikma-ikuma-ikuuma, i9 to-i9 uto-i9 uuto, obniobuni-obuuni, ogza-oguza-oguuza, okna-okuna-okuuna, o9 ta-o9 uta-o9 uuta.

Figure 1 :

 1 Figure 1: Percent [u] vowel judgments in stimuli like ebuzo in French and Japanese subjects as a function of vowel duration (Experiment 1).

Table 1 :

 1 Mean reaction time (ms), standard error, and error rate in ABX judgments on an epenthesis contrast and a vowel length contrast in French and Japanese subjects (Experiment 2).

	Language	RT	SE Err	RT	SE Err
		Vowel Length Contrast	Epenthesis Contrast
			ebuzo-ebuuzo		ebuzo-ebzo
	Japanese	1082 45 7.5%	1187 75	32%
	French	1173 73 21%	1002 54	5.8%

Table 2 :

 2 Mean reaction time, standard error, and error rate in ABX judgments on an epenthesis contrast and a vowel length contrast in Japanese subjects and French Subjects (Experiment 3).

		RT	SE Err	RT	SE Err
			Vowel Length	Epenthesis
			Contrast	Contrast
			ebuzo-ebuuzo	ebuzo-ebzo
			Japanese Subjects
	Same Talker	1008 41 3.1%	1032 48 13.7%
	Different Talker 1058 45 5.6%	1089 46 19.1%
	Mean	1033 30 4.4%	1060 33 16.4%
			French Subjects
	Same Talker	1095 76 8.9%	991	55 4.1%
	Different Talker 1225 72 10.8%	1095 58 5.4%
	Mean	1160 53 9.8%	1043 40 4.7%

The one exception to this regularity is when an /n/ appears after a vowel and before a consonant, in words like Honda, kanji and tempura.

These are significant, even if we used the Bonferoni correction and adjust the significance level to the more conservative value of .007.

We thank Dianne Bradley, Susana Franck, Takao Fushimi, Peter Golato, and Takashi Otake for useful comments on the paper and discussion. We thank Stanka Fitneva, Olivier Crouzet and Laurent Somek for experiment preparation and running. We are especially grateful to Hideko Yamashki for her invaluable help in recruiting Japanese subjects and Alain Grumbach for providing access to French subjects. We also thank Franck Ramus and Evelyn Hausslein for additional help in recruiting subjects. This work was supported in part by a grant from the Human Frontiers Science Program and by a Human Capital and Mobility Program grant. During this work, C. Pallier was supported by a Lavoisier Grant from the French Ministry of Foreign Affairs and by a post-doctoral grant from the Fyssen foundation.

Mailing address: Emmanuel Dupoux, 54 Bd Raspail, 75006 Paris, France.

Authors Notes

Appendix

Materials used in Experiment 1

abge-abuge, abno-abuno, agmi-agumi, akmo-akumo, ebza-ebuza, egdo-egudo, ibdoibudo, igna-iguna, obni-obuni, ogza-oguza.