The sentence final particles wé and ta in Ikwere

 for African languages spoken outside Nigeria. Similar particles have also been found and massively described in both Mandarin and Cantonese Chinese (see for instance

Introduction

Among the wide class of familiar discourse markers 1 found in many languages of the world, a subset of less familiar particles 2 which mainly (but not exclusively) occurs in the sentence final position is found in some languages including Ikwere, an Igboid language of the Benue-Congo branch. They are referred to as sentence final particles. Sentence final particles have also clearly been identified in other Nigerian languages including Igbo (Williamson 1972;[START_REF] Echeruo | Igbo-English dictionary: a comprehensive dictionary of the Igbo language[END_REF]); Kana (Ikoro 1996: 247); and the Nigerian Pidgin English (Faraclas 1996: 116, 209 and 283). 3 Consistent with their traditional label as discourse markers, sentence final particles seem to express a wide range of meanings both within a given language and across languages. According to Williamson (1972), the particle <o> in Igbo for instance, is used when speaking to someone at a distance; it is also used as a sign of affirmation, or as assent to a salutation. In Kana, these particles may conclude a story, provide the reason for an action, express a speaker's emotions (e.g. total disappointment, deep anger, complete frustration) or indicate an obligation (Ikoro 1996: 247-248). And in the Nigerian Pidgin English, the sentence final particle <o> adds the meaning of empathy, solidarity and realis modality to the assertion (Faraclas 1996: 116).

Aim

This article focuses on two sentence final particles wé and ta in Ikwere. There are two reasons for that: The first is that though Ikwere has several sentence final particles as illustrated in examples (1) to (6) below, wé and ta are among the most common ones encountered in everyday language. The second reason is that so far, I have been able to carry out proper investigation on these two particles alone. This article is an attempt to give a precise account of the specific part each of the two particles plays in sentences and in the construction of the meaning of such sentences, and to work out the invariant meaning element of each of them.

To achieve this aim, I consider their distribution. I also take into account, where necessary, the syntactic, semantic and pragmatic constraints surrounding their use in a given situation. I further consider the way they interact with other elements in sentences in which they occur and check the semantic effects they bring about or the impact their deletion and replacement may have on the well-formedness of sentences. In other words, the way each particle operates 2 in everyday speech is examined in detail and characterized. It is hoped that this work will contribute to a better understanding of discourse markers.

The exact number of sentence final particles (henceforth SFP) in Ikwere remains as yet unknown. However, the most common ones include we , ta , o , Б ѐ , kѐ and nѨ as illustrated below. 4[A cry of pain] (1) m wѨ -nϯ m we 1SG die-PFT ASS SFP5 I am dead (I'm finished).

(2) گ wѐ -nϯ ma ya ta 2SG drink-PFT NON ASS wine SFP By the way, have you had some drink?

(3) m ze -ne -m o 1SG go-PFT ASS AGR SFP I'm gone.

(4) si kܦu ci ke Б ѐ to call.INJ Chike SFP Call Chike (for me) [The speaker is not happy at all].

(5) jϯ kѐ 2SG.EMPH SFP What about you? (Won't you eat?) (6) kwu ru a nѨ carry someone.INJ 3SGO SFP Carry him please.

Theoretical background

This article is realized using the theory of linguistic markers within the framework of enunciative and predicative operations, often referred to as TOPE (French: Théorie des Opérations Prédicatives et Enonciatives), elaborated by Antoine Culioli and his research associates [START_REF] Culioli | Pour une linguistique de l'énonciation: opérations et représentations[END_REF](Culioli , 1995, 1999a&b, 1999a&b; see also [START_REF] Groussier | On Antoine Culioli's theory of enunciative operations[END_REF]Osu 2003a[START_REF] Osu | The invariant property of the Ikwere morpheme mà in its diverse uses[END_REF].

The basic assumption is that there is a finite series of cognitive processes underlying language activity. These processes, called linguistic operations, are represented by words in individual languages. And for that reason words, including the sentence final particles under study, are termed as markers of operations. However, several units can represent the same operation just as one single unit can represent several operations. Therefore, there is no one-to-one relationship between markers and operations. Each marker has its own way of marking a specific operation or operations. And each marker has an invariant property that is flexible enough to enable it to behave the way it does. In this view, the specific role a marker plays while interacting with other markers in the construction of meanings is equivalent to its invariant property. Within Culioli's theoretic model, a notion is a hybrid entity that provides extralinguistic materials which linguistic units are made from. Roughly speaking, a notion is like a mental image, our representation of reality. It is regarded as a complex bundle of physico-cultural properties that has no fixed boundary, meaning that with additional properties the boundary can always be extended. In fact, notions are characterized by two things, namely ramification as a result of inter-notional relationships created by the user (e.g., sheep and dog as opposed to sheep and lion); and abundance as physical, cultural and anthropological properties interrelate in such a way as to refer to a multiplicity of potentialities (see Culioli 1995: 44).

These notional properties are filtered with the help of a certain number of primitive operations that underlie language activity. One such operation is quantifiabilization (also referred to as fragmentation or individuation). It is a complex operation of determination that can be broken down into two components i.e., quantification and qualification. The operation of quantification consists of extracting and singling out an occurrence of a given notion and locating such an occurrence with reference to spatio-temporal parameters (enunciator and time of enunciation) of the referential space while the operation of qualification differentiates the extracted occurrence from all other related occurrences. The result of the operation of quantifiabilization is that a notional domain is constructed. Therefore, a notional domain is an occurrence of a notion. Lexical items, grammatical units and predicative relations (or grammatical sentences) are obtained by filtering notions. For example, with the notion 'being a dog' or 'being red' one can construct a notional domain 'dog' or 'red'; similarly, from the notion of aspect, one can construct aspectual categories; and finally from a complex notion of the type 'lion, trap, escape' one can construct a complex notional domain or predicative relation <lion-escape trap> which utterances can be made from.

It must be underlined that notions are only perceived through words, and that there is no oneto-one relationship between notions and words; a word, a lexical unit, a morpheme or a predicative relation does not equate with a notion, it is all a matter of representation, a matter of regulation in the sense that a lexical unit embodies only some properties of the notion but never all of them. It should then be clear that words are very complex entities. They are a means of representing cognitive activity, real entities, events and states of affairs and at the same time, they are traces of linguistic operations.

A notional domain is organized into three different zones that are intrinsically related. These are, the Interior zone which represents the notional domain by default; the Exterior zone which is the linguistic complement of the Interior; and finally, the Boundary, which is neither the Interior nor the Exterior but compatible with the Interior and the Exterior. Thus the notional domain is inherently composed of two equiponderant values (p and p'). Since p is by definition different from p', the relation between both is characterized as that of alterity (or otherness).

The Interior zone contains a class of occurrences which identify with one another and at the same time differ from each other. The Interior zone also contains an abstract representative occurrence that serves both as its organizing centre (or prototype) and as its attracting centre.

The organizing centre enables one to discern what occurrences do or do not belong to the class associated with the domain. It can be likened to the Platonic archetype. The attracting centre enables one to state that something is identifiable with the representative occurrence to a lesser and lesser degree. And it can be likened this time to the prototype in prototype semantics (see in particular Rosch 1978;[START_REF] Lakoff | Women, fire, and dangerous things: what categories reveal about the mind[END_REF][START_REF] Kleiber | La sémantique du prototype[END_REF][START_REF] Geeraerts | Prototype semantics[END_REF]. The Interior zone represents the notional domain and therefore is associated with a positive value, conventionally noted p while the Exterior zone is associated with the negative value and is noted p'.

The Boundary, also noted p', is assimilated a priori to the Exterior zone. However, it entertains relations which can be qualified as simple difference with the Interior zone. The Exterior zone is regarded as being totally different from the Interior zone. The Exterior zone has therefore nothing to do at all with the Interior zone. For example, if we use the word 'dog' to refer to a notional domain, then anything that has nothing in common with the prototypical representation of 'dog' will belong to the Exterior zone, while a hybrid wolf-dog for instance will be viewed as belonging to the Boundary.

The article is divided into two unequal parts. This can be accounted for by the fact that the SFP wé is more frequent than the SFP ta . The first part deals with the SFP wé. It is argued that this particle marks the operation of asserting.6 But given that there can be different ways of asserting, I underline what kind of assertion the SFP wé brings into play. The second part discusses the SFP ta . I show that it only occurs in questions. I also show that it is used where the need arises to call into question something initially introduced.

The sentence final particle wé

As earlier mentioned, the SFP wé marks assertion. The technical term assertion/asserting here means that the speaker, in his capacity as enunciator, noted S 0 ,7 takes responsibility no matter how little, for the validity of a complex notional domain, in other words, a predicative relation written X-P (where X stands for subject and P for process) in such a way that any other predicative relation (X-Q, X-Q', X-P', Y-Q, Y-Q', Y-P, Y-P', etc.) is automatically excluded. In fact, the operation of asserting implies two complementary processes. First, S 0 (or asserter) indicates that something is the case. Thus, S 0 locates an occurrence of a predicative relation yet unidentified, relative to a referential system also called situation of enunciation (noted Sit 0 ). Second, S 0 constructs an occurrence of a specific predicative relation with two constituent values (recall that p stands for the positive, and p' for the negative). The choice of one value invalidates (rules out) the other. (See in particular Culioli 2002: 280). Culioli further writes:

"To be able to assert, to produce an assertion, I must declare it publicly: an interior assertion is not an assertion; but there also needs be the commitment of someone who takes responsibility, who guarantees your statement or who wishes to assert something despite your position. If it is simply an instance of locution, you do not really have assertion. For there to be responsibility, one must guarantee what one states. " (1995: 92).

In some cases the focus of the assertion or the asserted element is the relationship between the speaker and what he says. Put differently, the focus of assertion is the very act of saying something.

In some other cases the focus of assertion is what the speaker says, otherwise called speech content.

But we can also imagine situations where the object of assertion is the speaker on one hand and what he says on the other. In this case the speaker and what he says are asserted separately, and indicated each time by a different marker.

Alternatively, we can think of situations where none of them is the object of assertion, meaning that there is no assertion at all. See questions and injunctive constructions for example.

Different uses of the SFP wé and analysis

The SFP wé occurs in affirmative, negative and injunctive sentences. It also occurs in questions. I examine each sentence type in turn.

Affirmative sentences

Affirmative sentences are a means by which a speaker positively evaluates events, states of affairs, processes or predicative relations. The speaker considers that a given state of affairs represented by a predicative relation is certain in the sense that its locating relative to time and space is conform to what he expects. Put differently, the speaker states with an affirmative sentence that a state of affairs is the case. Thus if he says 'it is raining', he means that <it is raining> is the case. Now consider (7)-( 9 A sentence like (7) *m wѨ nϯ m 'I'm dead' is not well formed. As a native speaker of Ikwere, one has the impression that it is not a complete sentence, it is not stabilized. But if I add ta 'today' which makes it m wѨ nϯ m ta 'I'm finished/dead today', it becomes stabilized and natural as given in (8). This kind of sentence is produced by someone who has committed an offence and is afraid of the punishment due to him or her. So, (8) can be interpreted as follows: "I foresee a punishment so severe that at the end of it, I will have died". But though the speaker states that the predicative relation <I-be dead>, rewritten X-P, is the case, there is no evidence that he will actually be punished (or dead to use the speaker's metaphor).

Further, if in place of m 'I', we use the third person singular ѐ as in ѐ wѨ nϯ m 'S/he is dead' for instance, we obtain a sentence that is equally natural. With ѐ wѨ nϯ m 'S/he is dead', the speaker tends to answer the question whether or not a certain person is still alive. The question can be implicit or explicit depending on the context. We can therefore deduce that the unnaturalness of ( 7) *m wѨ nϯ m 'I am dead' is due to the fact that one cannot be dead and talk at the same time, at least to our common knowledge. In both m wѨ nϯ m ta 'I'm finished/dead today' and ѐ wѨ nϯ m 'S/he is dead' the speaker states that the relation X-P is the case. This is a way of asserting the validity of X-P. Now, if we replace ta 'today' with the SFP wé as in (9), the sentence is still valid but takes an additional meaning: It is as if the speaker is emphasizing what he says. What specific part does the SFP wé play in the construction of this meaning and how does it do that? First, we find that (9) m wѨ nϯ m we 'I am dead (finished) and I mean it' also typically occurs in a situation where someone is being severely punished. Apart from stabilizing the sentence and rendering it natural, the SFP wé further indicates that there is a shift of the focus of assertion. Thus, instead of the relation X-P the focus is on the act of saying X-P. In other words, this time, the enunciator takes responsibility not for the validity of X-P, but for the very act of saying/producing X-P. And to guarantee his own act of saying something is tantamount to committing himself twice (first for X-P and second for his act of saying). Hence the impression that the speaker emphasizes what he is saying.

But then, why must the speaker want to assert his own act of saying? Is there anything in the situational context that urges him to do that?

In the situation at hand the speaker in (9) tends to say that he is/will be dead. That alone is enough to breed doubts because as I pointed out earlier, a dead person does not talk to our hearing. The speaker's doubts mean that there are two alternative values (p and p') related to two inter-subjective positions, the enunciator's position and the co-enunciator's position. Thus, from the point of view of the enunciator (noted S 0 ), p <I-be dead> is the case. But from the point of view of the co-enunciator (noted S' 0 ), p' <I-not be dead> is the case.

Put differently, there is a relation of alterity (or otherness) 8 between the point of view of S 0 and the point of view of S' 0 . With the SFP wé the point of view of S 0 is presented as the valid one against that of S' 0 i.e., against any other point of view (real or imaginary), and against all odds. Example (10) reveals similar features.

(10) [A cry of pain]

ѐ cگ m we leg 1SGO SFP My leg is paining (me) severely.

Ikwere has several ways of expressing the fact that one is feeling pains. For instance, to explain why you don't want to walk around, you can say ѐ cگ no ցbu m i ցbu ցbu 'I have leg pains (literally, leg is paining me)'. You can equally say, e yi m ѐ cگ kܦe kѐ m mѐ -lѨ ѐ lѨ lѨ 'my dear, my leg(s) do(es)n't let me make any move', etc. Note the use of the reduplicative verb form …mѐ -lѨ ѐ lѨ lѨ here. This form implies that the speaker cannot realize what he refers to as ѐ lѨ lѨ 'the act of shaking one's body' or 'moving one's body' in any way at all. In fact, in some earlier publications [START_REF] Osu | Verb reduplication in Ikwere: beyond form and meaning[END_REF][START_REF] Osu | Entre réduplication et répétition: de l'identification à l'identité[END_REF], I have shown that reduplicative forms generally indicate that an occurrence of a given notional domain is identified relative to the representative occurrence of the domain, with the result that alterity which originally characterizes the domain is eliminated. This domain is therefore said to be a centred domain in the sense that it fully identifies with p, the representative occurrence. Thus, the reduplicative form implies that there is p and just p, meaning that there is no room for anything outside the domain in question or again, nothing outside p.

The sequence ѐ cگ m we in (10) contains no verb. We can assume that the verb is simply elliptical and that the situation in which the sentence is produced can help identify or 8 To be more specific, alterity is a technical term that refers to an inherent property of language activity. It underlines the quantitative and qualitative singularity of a given linguistic object; the property that differentiates such an object from other linguistic objects. With this term, one can account for failures, mistakes, misunderstandings, ambiguity, and polysemy that characterize language activity. This term equally enables one to account for regulation or adjustment between the enunciator and the co-enunciator in the process of meaning construction (cf. Culioli 1990: 103). reintroduce it like this: ѐ cگ m hܦu le m we 'my leg has disappeared'; ѐ cگ ցba ji me ne m mϯ we 'my leg has broken'. Assuming the speaker says ѐ cگ m that is, in the absence of the SFP wé, assuming also that it is used as the short form for ѐ cگ m hܦu le m 'my leg has disappeared', he would simply be stating that the p value of the predicative relation <my leg-disappear> is the case. Consequently, the p' value is presented as 'not being the case'. We find that in the absence of wé the focus is on the predicative relation (i.e. what the speaker says, the content) and not on the act of saying. We must not forget however that the sequence *ѐ cگ m is not very natural in the language.

The use of the SFP wé in ( 10) is linked to the fact that from the speaker's own perception of things, his cry of pain (i.e., what he is saying) is not being taken seriously by someone, real or imaginary. He is then led to oppose two inter-subjective positions, introducing alterity. Thus, according to him, from the point of view of someone else (represented as S' 0 ), perhaps due to the fact that the person is simply not aware of the speaker's condition, the speaker's cry of pain is not serious whereas from the speaker's own point of view (represented as S 0 ), his cry of pain is serious. The use of the SFP wé is intended to call the attention of the person to the speaker's cries, phrased like this: "you don't seem to pay attention to my cries, otherwise you will realize that I have serious pains". Thus, from the alternative point of view, the p' value (i.e., anything other than the act of crying) is valid. But from the point of view of S 0 the p value (i.e., the act of crying) is valid. This is how the focus of assertion shifts from pains to the very act of crying such that any other imaginable act, any alternative point of view introduced is disqualified, and alterity eliminated. The pragmatic effect of this sentence is that the speaker's leg pains can be interpreted as serious pains and therefore, considered for emergency treatment. Let us consider ( 11) and (11a) below: Indeed, the use of the SFP wé in (11) creates the impression that the speaker emphasizes X-P i.e., the fact that he will go away should there be Y-Q i.e. Chike come. In so doing, the speaker seeks to avoid Y-Q. The discussion so far shows that this impression is quite consistent with the way the SFP wé operates in the language. It comes from the fact that the speaker urges the addressee to listen carefully to what he is saying and take it seriously. By urging the addressee, the speaker in his capacity as enunciator, asserts his very act of saying what he is saying. In point of fact, the SFP wé is used here to insist on the possibility of having X-P as the direct effect of Y-Q (Chike-come) like this: If Y-Q takes place then X-P will unfailingly take place.

As we have already seen, the process of asserting his very act of saying implies taking into account two alternative views in relation to a given state of affairs i.e., whether or not the speaker means it when he says X-P <I will go away>. From an alternative point of view (represented as S' 0 ), as perceived by the speaker himself, the latter may not be serious when he says that X-P is the case. Therefore S 0 assigns to S' 0 the responsibility of the p' value (which here implies that it is possible that the p value associated with X-P is not the case). But from the point of view of S 0 the speaker is serious when he says that X-P is the case. Therefore S 0 guarantees p to the detriment of p'.

The discussion so far reveals that this marker consistently occurs in affirmative sentences where the speaker senses that what he says is not taken seriously by another person, real or imaginary. His act of saying something is therefore called into question whether explicitly (ex.11) or implicitly (ex. 9). Calling something into question is analysed here as two different ways of perceiving or representing a predicative relation or state of affairs as illustrated by the following interpretation: "Based on the fact that you are entitled to have a perception of things that is different from mine, you may want to think that another act of saying is possible i.e., that I could say something else or use some other words instead. But given the circumstances surrounding the realization of the process P (e.g. to go away; to pain) by the subject X (e.g. I; leg), I maintain my act of saying".

Faced with two inter-subjective positions, S 0 indicates that his position is the right one, and therefore selects the p value. Consequently, p' is not validated. In sum, the speaker indicates publicly that what he says is conform to what he wants to say; he guarantees or takes responsibility for his act of saying. Here again, we find that the SFP wé marks the assertion of the speaker's act of saying.

Negative sentences

Negative sentences also imply that a predicative relation or a state of affairs is certain. The speaker states that the predicative relation which he introduces using a negative sentence is the case. Thus, if he says "John has not killed a lion", he means that <John has not killed a lion> is the case. Similarly, if he says "It is not raining", he means that <It is not raining> is the case. The speaker uses such sentences in order to negatively evaluate something. Consequently, in a given domain of reference with two constituent values p and p', the negative indicates that p' is the case, or again, that 'p is not the case' is the case.

Examples ( 12) and ( 13) contain a reduplicative verb form. This form implies that p is a centred value and for that reason, there is no room for p'.

(12) o si , ϯ kܦa la a la Osi act of protecting someone go away.F.T. NEG REDUP Osi, the act of protecting a friend has no end (or the act of protecting friends should neveer leave you).

In (12) the reduplicated verb is a la 'to go away'. Here is the context in which we find this example: Osi is not happy with his friend who has failed to show any sign of gratitude after granting him protection. Therefore, Osi wants to know if he could choose not to protect his friend next time. His uncle replies that he is bound to protect his friend no matter what happens.

Recall that the notional domain is inherently composed of two equal weighted values (p and p') and that p is by definition different from p'. If we use X to represent ϯ kܦa 'the act of protecting someone' and P for a la 'to go away', we can then say that X is located at the very centre of the Interior zone of P. If we combine all that with negation, we obtain the opposite i.e., X is located at the very centre of the Exterior zone of P, noted X-P'. Now let us consider (13). The fact that the SFP wé combines with a reduplicative verb form in ( 13) is further evidence that this particle does not mark the assertion of the predicative relation. This sentence is produced in a situation where despite his uncle's answer Osi continues to give reasons why he may not protect his friend next time. It's as if he is calling into question what his uncle has said to him. Therefore, the speaker uses ( 13) to emphasize what he has already told Osi.

(13) o si , ϯ kܦa la a la we Osi act of protecting someone go away.F.T. NEG REDUP SFP Osi, the act of protecting a friend has no end (or the act of protecting friends should never leave you).

We find that the use of the SFP wé in this context is a means of avoiding further arguments with Osi. Here again, the speaker uses this particle to indicate to Osi that he, in his capacity as enunciator, asserts something in particular. What matters therefore is whether or not Osi should be allowed to call the act of producing X-P' into question. We have two alternative points of view with S 0 taking responsibility for p' (i.e. his act of saying that X-P' is the case) while assigning the responsibility of p (i.e., any other possible act of saying e.g. that X-P is the case) to S' 0 . The whole process amounts to validating p' and not validating p. It should be noted here that p' is the right value from the enunciator's point of view because of the negative construction.

(14) ya ցbu -le a 3SG.EMPH.EXCL to kill-PFT NON ASS F.T. NEG 3SGO He9 didn't kill it or It is not him who killed it.

After Osi has come back from the bush with a Grass-cutter he claims to have killed, Chima tries to find out whether or not Osi is the one who killed it. With the negative construction, the speaker states that Osi is not the one who killed it. Thus, whereas Osi in his capacity as coenunciator asserts the predicative relation X-P (i.e. Osi-kill Grass-cutter) and validates the p value of the domain in question, the speaker in his own capacity as enunciator asserts the relation X-P' (i.e. Osi-not kill Grass-cutter) and therefore validates p' or put differently, invalidates p. The SFP wé occurs in (15) because the speaker senses that despite his answer, Chima does not totally believe him, meaning that Chima still has doubts as to whether or not Osi is the one who killed the Grass-cutter. So when the speaker uses the marker wé, he doesn't mean to focus on whether Osi did kill the Grass-cutter or not, but to make Chima believe what he said. Thus, Chima's doubts imply that there is an alternative choice to the speaker's act of saying.

And the consequence is that the p value is brought forward, where p stands for the act of saying anything other than that it is not Osi who killed the Grass-cutter. The additional comment "I tell you" renders the idea that the speaker maintains what he earlier said. Therefore, the asserted value is p'.

The use of the SFP wé in negative sentences requires situations where the negative value of a given predicative relation is introduced by the speaker. This act of introducing the negative value is then called into question. With wé the speaker signifies that he maintains what he says. In other words, the speaker in his capacity as enunciator takes responsibility for what he says irrespective of whether or not what he says contains the negative value. In so doing, he validates p' (what he says) and consequently, p (what he does not say or what someone else might say) is invalidated.

Interrogative sentences

Questions generally introduce a predicative relation while indicating that the speaker is not in a position to determine which of the two constituent values of the domain represented by this relation is valid. He therefore calls on someone else to select a value.

The SFP wé only occurs in questions used for greetings. 10 It is very important to underline the tautological nature of such questions since they are asked in order to find out whether the subject of the process is doing something which he is already doing. However, unlike rhetorical questions which do not require answers, greetings in the form of questions must receive a response otherwise the social norm will be broken. Consider (16) for instance.

[The speaker passes by people sitting down. If he fears they might not answer his greetings, he greets them saying:] (16) a nѨ z-a m we 2PL to be-confirmation SFP You are there?

In ( 16) the element -a m in the verb za m (a contracted form for zگ a m ) 'be located in time and space' is used in order to confirm something. And it can occur with or without the SFP wé. Whenever it combines with the question as in ( 16), we obtain a sentence that can be phrased as follows: "While you comply with the custom which holds that one must answer greetings, could you confirm that you are located in time and space?" I interpret the speaker's fear in this example as meaning that the addressee calls his greeting into question. In that case, the SFP wé indicates that the enunciator asserts the act of greeting (recall that his greeting consists of asking the addressee to confirm that he (the addressee) is actually located in time and space. Therefore, any alternative act (e.g. not greeting them the way he did) is excluded. But if, for instance, his act of greeting is not called into question then the use of the SFP wé will be uncalled for. Let us further compare ( 17) and ( 18 And let us take a typical situation where someone returning from the market is drenched in rain. If the speaker greets the person without using the SFP wé, as in ( 17), it will be understood that he does not mean to show any concern about the state which the person is in. In other words, the speaker simply greets the person no more, no less. Alternatively, if he uses the SFP wé as in ( 18), he means to show concern or to sympathize with the person. The question is: How does this particle enable him to achieve that? My analysis is that the speaker assumes that for one reason or another, the addressee may not pay attention to or hear his greetings and therefore respond accordingly. Such an assumption is a way of introducing a different perception of his act of greeting, phrased like this: "To me, you are in such a state I'm afraid you may not pay attention to my act of greeting you, and therefore, you may not reciprocate my greeting. So I am asking you to pay attention and respond accordingly". This phrasing brings forward the fact that the speaker takes the addressee's condition into account. Hence the meaning of "sympathy" or "concern" as previously mentioned. Here again, we have two alternative viewpoints: the point of view of S (the speaker's greeting is not just an ordinary one, it is intended to show you that he feels concerned about your condition) and the point of view of S' 0 speaker's greeting may be an ordinary one). Under this condition, the enunciator guarantees the speaker's position (i.e., the act of seriously greeting the addressee), and therefore, invalidates any other position.

In this kind of use, the speaker introduces X-P e.g. (in 16) <you-be located in time and space>; and (in 17) <you-return from the market>. With the question, marked here by a combination of the non-assertive form of the verb, tone inclusive, and the intonation, he states that he is not in a position to say which of the two constituent values of the notional domain represented by X-P is the right value and therefore asks the addressee to select and validate one of the two values.

Further, we have seen that the circumstances surrounding X-P are such that the speaker's greeting is likely to be called into question. He is led to indicate publicly that in his capacity as enunciator he takes responsibility for his act of greeting the addressee. Consequently, any other conceivable act of saying, any other position in relation to his act is excluded. In sum, the SFP wé indicates that the enunciator takes charge of the speaker's act of greeting.

Note that the answer to his greeting in ( 17) can be o we 'yes' or in ( 16) and ( 18) o we we 'yes of course'. With the 'yes' answer, the addressee simply confirms the state he is in, therefore validating p and stabilizing X-P. But with 'yes of course', he not only confirms the state he is in but also validates p stabilizing X-P and confirms his act of greeting.

Injunctive sentences

The speaker uses injunctive sentences to introduce an event or a process so that it is realized by someone else. Or to put it in Culioli's words:

"In an assertion one states that a certain situation is is not; in the injunctive one says: 'may a certain situation be or not be'. […]. Injunctive constructions range from orders to pleas including even wishes as well as requests and suggestions. […]. Roughly speaking, an order is: A says to B: 'whether you like it or not, do a certain thing'. In the request, one is more polite, one says 'one hopes that you like it'; in the suggestion 'do you like what I like'; in the plea 'I, myself, like [it], I hope you like [it], but the wish does not necessarily itself to one person, as does the plea. It can be addressed to fate, to a divinity, or to all and sundry". (Culioli 1995: 116).

Put differently, in his capacity as enunciator the speaker presents the p value of the domain represented by X-P as the right one and refers its validation to S' 0 . Therefore, unless S' p, the alternative value cannot be eliminated. We can summarize the process involved in injunctive sentences as follows: From the point of view of S 0 , p is the right value, but from the point of view of S' 0 , p and p' are both possible values; so until he validates one value or the other (for example, p where the addressee chooses to effectively realize the process or p' if he chooses not to realize the process) they remain equiponderant, meaning that none of them is privileged by S 0 , or again, both possibilities co-exist.

In a situation where a child has been showing signs that he wants his brother to carry him in his arms and the latter has refused, a neighbour can intervene with either (19) or (20). In the absence of any SFP as in ( 19), the sentence is just a simple request. It can also be a command. But should the neighbour use (20), he will create the impression that he is confirming his request or command irrespective of whether he is explicitly making such a for the first time or not. How is that possible?

We must recall that the use of the SFP wé implies that an alternative point of view is introduced in relation to the speaker's act of saying something. This is about making a request in (20). The alternative point of view is introduced in this example by the fact that the speaker thinks the addressee may not want to carry the baby despite his request. Note however that it is the speaker himself who introduces the alternative point of view. But he does introduce it with the ultimate aim of blocking it. In so doing, he presents p as the good or necessary value so that p' is ruled out. The semantic effect is that the addressee is politely led to comply.

If we substitute nѨ for wé in ( 21), we find that 1) there is need to redefine a given state of affairs; 2) an element is introduced so that it is taken into account in order to redefine such a state of affairs.

In fact, in ( 21), the speaker makes his request so that it will serve as a reference point for redefining the state of affairs in question, like this: take what I am telling you i.e., my point of view as the reference point in order to determine whether or not you carry the child. However, and this is a major feature of nѨ , it is not explicitly indicated whether or not other possible reference points are taken into account and/or eliminated.11

A quite similar analysis of the SFP wé can be made of ( 23). But first, a short comment on ( 22) is in order. This is a simple request for the addressee to come in, no more, no less. Hence one does not have the impression of emphasis. Example (23) however, suits a situation where a person does not come in despite several attempts to make him do so. The SFP wé signifies that the speaker's invitation holds, no going back, no calling it into question. The speaker can further make use of vowel lengthening to double the intensity of his assertion as in (24).

In sum, the SFP wé intrinsically contains a degree of emphasis, and vowel lengthening adds another degree. Example ( 25) is quite similar to ( 9) and (10) seen previously with the difference that (25) contains a verb. Recall from examples ( 9) and ( 10) that the speaker finds himself in an uncomfortable situation. His cry of pains is nothing but a call for help, an emergency call. If we delete the wé particle in (25), we have a sequence that is not stabilized, meaning that we need to introduce some elements to stabilize it. Such stabilizing elements may include o ցܦu 'celebration' as in a nѨ bya nѨ m o ցܦu 'Come and celebrate with me' and o ti bi 'a kind of prenuptial retreat' as in a nѨ bya nѨ m ѐ ra o ti bi 'Come for my prenuptial retreat' (if it is a young lady in the process of marriage).

The injunctive construction alone (in 25) implies that the speaker invites the addressee to realize the process <come and celebrate with me>, <come for my prenuptial retreat>. And the situational context alone is to indicate that one is longer dealing with an ordinary invitation. use of the SFP wé in (25) implies the following: a) Despite the fact that the speaker has been crying out he has not (yet) been rescued. It is as if his cry is not heard or that it is not taking place, in short, it is not recognized by the addressee. And the p' value is introduced. b) The speaker therefore signifies that his cry does exist. In this way, his act of crying is asserted.

Injunctive constructions introduce two alternative values with p as the privileged one from the point of view of S , and p and p' as equiponderant for S' 0 pending the selection of p or p' by S' 0 through effective realization (or not) of the process by a subject. In this case, the SFP wé implies that the speaker's request or order should be considered as something serious.

Summary of the way the SFP wé functions

In affirmative sentences, the speaker affirms X-P, a given predicative relation or a state of affairs. He declares that X-P is the case. But the addressee in his capacity as co-enunciator has a different point of view on this act of affirming the relation. The SFP wé is consistently used in order to assert the act of affirming the relation in question or the act of declaring that X-P is the case. In negative sentences, the speaker negates a predicative relation. And it is this act of negating that is asserted by the enunciator. In interrogative sentences used for greetings, the speaker's act of greeting is likely not to receive a response, which is a way of calling his act into question. But the enunciator guarantees the act. And finally, in injunctive sentences, the speaker makes a request but he is afraid his request may not be granted or listened to. Here again, the enunciator takes responsibility for the act of producing such injunctive sentences.

All this points to the fact that the SFP wé does not focus on the affirmation, negation, question or request. Its focus is the speaker's act of saying (i.e., affirming, negating, greeting, and making a request). Thus the use of the SFP wé implies that a speaker's act of saying is (tentatively) called into question maybe because this act is not considered the right one, from the point of view of S' 0 , an alternative point of view, serving as the source of alterity. And then, S 0 reassesses the relation and presents his act as the right one, with the result that the alternative point of view is ruled out.

The fact that someone says something and his act of saying asserted explains why sentences with the SFP wé sometimes give the impression that the speaker emphasizes what he says.

The sentence final particle ta

The SFP ta only occurs in the final position of interrogative sentences. Let us recall that interrogative sentences generally imply that the speaker is not in a position to state whether or not a given process has taken place. And this may be because he is not able or willing to do so. The effect of this is that the relation is not stabilized.

The question in (26) carries the verb suffix -nϯ (the nasalized form of -lϯ , with the vowel 'e' harmonizing for [-ATR]). This suffix implies that the predicative relation <you-have some drinks> (rewritten X-P) is subject to two different constructions: a construction relative to time and space, meaning that the process represented by the relation actually takes place; and a subjective representation, implying that the process may have been expected or planned.

[Chima has come to his uncle's family. At some point, he goes to meet his uncle. And the latter asks him:] (26) گ wѐ -nϯ ma ya 2SG to drink-PFT NON ASS wine, alcohol Did you have some drinks?

The aim of the question in ( 26) is to know if X-P located relative to time and space corresponds to X-P as expected by the speaker (that is, as represented by a subject). The interesting thing here is that the speaker in his capacity as enunciator does not favour (or give preference to) any answer. Therefore, the answer can be 'yes' represented as the p value or 'no' represented as the p′ value. Both values have equal weight (they are equiponderant).

Given such an open question, it is understood that whichever value the co-enunciator validates, the enunciator will adopt it. And consequently, the two different points of view are reduced to one. that Chima answers 'yes', and begins after a while, to show signs of uneasiness. The uncle may wonder whether the process which has been regarded as a given fact has actually taken place. In that case, the SFP ta becomes compulsory as in ( 27) and the question aims therefore to reassess the situation.

(27) گ wѐ -nϯ ma ya ta 2SG to drink-PFT NON ASS wine, alcohol SFP By the way, did you have some drinks? (I suddenly have some doubts as to whether you have had some drinks).

Thus, from a position where the p value is introduced without any consideration of p′, we move to a position where the p′ value is introduced through the speaker's doubts as an alternative value. It is up to the addressee in his capacity as the co-enunciator, to assert one value or the other. It turns out that the SFP ta emphasizes the relation of alterity between p and p′.

[In a telephone conversation, A asks B:] (28) i n-e me kگ nگ 2SG PROG-to do what What are you doing?

In (28) the progressive marker n-implies that the predicative relation <B-be doing something> (noted X-P) is located relative to time while the question word kگ nگ 'what' indicates that the object of the process 'do something' has to be specified. This can be phrased as follows: Is it the occurrence O i, O j , O k , O y or O z of the class O that is valid? And unless an occurrence is selected, the relation <B-be doing something> cannot be stabilized.

[But to a person refusing to take advice, the speaker A can say:] ( 29 The difference in (29) as compared to (28), is that when speaker A declares using the question that he is not in a position to say whether it is O i, O j , O k , O y , or O z that B is doing, he does not ask B to select an occurrence of the class of O in order to stabilize or validate the relation. In fact if we pay closer attention to the context here (B refuses to take advice) we observe that we are not dealing with a real question; it is a question used to disapprove of B's attitude. More precisely, the question word kگ nگ 'what' implies quite alright that the object of the process 'do something' has to be specified. But with the SFP ta speaker additionally indicates that he is wondering whether what B is doing is worth being recognized.

We can now see how we move from a position where (in 28) the speaker recognizes X-P (he simply notices that the subject is realizing the process and asks him to specify the object or complement of the process) to a position (in 29) where he calls X-P into question, implying that the subject can as well do something else. In other words, we go from a position (in 28) where the p value assigned to X-P by default12 raises no question to a position (in 29) where upon re-evaluation, p and p' are both considered possible values.

[Osi has been treated badly and threatens to retaliate by doing something very bad. But his threat begins to irritate his friend who doesn't believe Osi can do anything tangible. Hence his friend asks him:] (30) ma ka گ z-o ցbu ba dѨ ta 1SG say 2SG FUT-kill human being SFP I am saying (i.e., come on), will you kill somebody?

We notice that the speaker (in 30) simultaneously introduces the predicative relation X-P <you-kill somebody> (with the default value p) and the SFP ta . The introduction of X-P here has to do with the fact that killing somebody is culturally regarded as the worst thing Osi or anyone else could do at least in such circumstances. The z-prefix indicates that the predicative relation <you-kill somebody> is assigned a distinguished value p, meaning that though p' has a lesser weight than p, it still remains a possible value. With the SFP ta the The correct forms are ( 35) and ( 36):

(35) a nѨ z-a m 2PL to be-confirmation Confirm that you are there (see16 already discussed).

(36) گ bѐ -lϯ 2SG to pull apart-PFT NON ASS Good morning or You have come through the night?

As we can see, examples ( 35) and ( 36) are two different ways of greeting somebody in Ikwere. The speaker introduces each time the value p as the privileged value and asks someone else to confirm it. He does not call it into question. Hence SFP ta does not sound natural here.

The SFP ta does not occur if there is no doubt about the p value of the domain X-P, meaning that p has already been asserted. This is illustrated in (37) to (38). (39) and ( 40) contain reduplicative verb forms (ze ri e ri ; za bya a bya ). Since reduplicative forms generally indicate that p is centred such that there is no room for p', any attempt to call p into question is bound to fail. Hence (37) and ( 38) are not natural.

Summary of the way SFP ta functions

From the analysis so far, we can say that the SFP ta marks a reassessment of the validation of p initially introduced without any reference to p', by considering p' as an alternative value so that they both become possible values.

Let's recapitulate. Open questions generally imply that the speaker is not in a position to distinguish between p and p'. And therefore his question signifies that from the beginning p and p' have equal weight. The then has recourse to someone else and asks him or her to distinguish between the two values by selecting one of them. This mechanism can further be represented as follows:

Open questions generally: From the point of view of S 0 : p, p' (p AND p')

From the point of view of S' 0 : p OR p'

But with the addition of the SFP ta the speaker begins by assuming validity for p alone. However, upon re-evaluation, he calls p into question and thus introduces p' so that they both become possible values. The speaker then invites someone else to validate one value or the other. The whole process can further be represented as follows:

Question + ta : From the point of view of S 0 : p p, p' (p AND p') But from the point of view of S' 0 : p OR p'

Conclusion

This article has focused on we and ta , two of the numerous sentence final particles in Ikwere. The aim has been to find out the specific feature of each of them. My analysis has shown that the two sentence final particles are used in order to reassess a process or a predicative relation and therefore to state the speaker's point of view relative to a given state of affairs. I have underlined how each of them contributes to this reassessment, indicating the speaker's point of view.

The SFP wé implies that a speaker's act of saying is (tentatively) called into question from the point of view of S' 0 , urging S 0 to reassess it as the right act, with the result that the alternative point of view is ruled out.

Similarly, the SFP ta marks a reassessment of the validation of p (or p') initially introduced without any reference to p' (or p), by considering p' (or p) as an alternative value so that the two are regarded as possible values.

I have shown that these particles make a significant contribution to the meaning of sentences in which they occur. Thus wé can lead to the meaning of emphasis while ta implies that one is not sure of something in a given situation. And finally, this article has been able to highlight the specific ways in which each particle brings into play alterity, a fundamental property of the language activity.

In the case of the SFP wé, alterity is brought into play by the fact that the point of view of S 0 differs from that of S' 0 relative to an act of saying whereas with the SFP ta , alterity is brought

  PFT ASS today I am dead (finished) today. (9) [A cry of pain] m wѨ -nϯ m we 1SG die-PFT ASS SFP I am dead (finished) and I mean it.

  m we Chike come-PFT NON ASS 1SG return.CTF-PFT ASS AGR SFP If Chike comes, I will go away and I mean it. (11a) ci ke bya -lϯ m la -lϯ m Chike come-PFT NON ASS 1SG return.CTF-PFT ASS AGR If Chike comes, I will go away. If we delete the SFP wé in (11), we have a valid inter-clausal sentence, as illustrated in (11a), in which the speaker states that the relation introduced by the subordinate clause m la lϯ m 'I will go away' is the result of the relation introduced by the main clause ci ke bya lϯ 'If Chike comes'. This kind of relation (11a) does not warrant the additional comment 'and I mean it'.

  .EXCL to kill-PFT NON ASS F.T. NEG 3SGO SFP He didn't kill it or It is not him who killed it. (I tell you).

  ) below: [The speaker greets someone returning from the market:] (17) گ ya -lϯ 2PL return-PFT NON ASS You have returned? (18) گ ya -lϯ we 2PL return-PFT NON ASS SFP You have returned? (I'm greeting you.)

  DAT.INJ 3SGO SFP Carry him, yes you should.

  a nѨ bya -nѨ -m we 2PL come-DAT-1SGO SFP Come to my rescue. (Please).

  come through the night. (A way of saying 'good morning'.)

Note however that kѐ and nѨ can also occur in other positions which are not examined here.

Abbreviations 1SG = 1 st person singular pronoun; 1SGO = 1st person singular pronoun object; 2SG.EMPH = 2 nd person singular pronoun emphatic with contrastive effect; 2PL = 2nd person plural pronoun; 3SG = 3 rd person singular pronoun; 3SGO = 3 rd person singular pronoun object; 3SG.EMPH.EXCL = 3 rd person singular pronoun emphatic with exclusion; ASS = assertive; AGR = 1 st person singular agreement; CTF = centrifugal movement; CTP = centripetal movement; DAT = dative of interest (quite similar to ethical dative in French, used here for lack of a better label); F.T. NEG = falling tone negation marker; INJ = injunctive; LOG = logophoric pronoun; PFT = perfect marker; PFT NON ASS = nonassertive perfect marker; SFP = sentence final particle. Ikwere tones are indicated as follows: a for High, a for Low, a for Rising, a for Falling and ⍺a for Down-step High. The tilde under a vowel indicates nasalization. IPA symbols are used in Ikwere examples except that dЋ is replaced with j; tЀ with c, and j with y. The symbol gb is used here for nonexplosive, bilabial stop with no velar occlusion. (Interested readers may see[START_REF] Clements | Explosives, implosives, and nonexplosives: the linguistic function of air pressure differences in stops[END_REF] for further details).

Within the Culiolian approach, assertion is a technical term and should not be confused with the English 'Assertion'. The latter word in English corresponds to what Culioli describes as the positive assertion. I discuss this technical term further below.

The speaker refers to a real physical entity whereas the enunciator is not just the speaker. The enunciator is neither the psychological subject nor the psychoanalytic ego. The enunciator is an abstract metalinguistic construct derived from the speaker. The enunciator serves as the parameter by which the dynamic process involved in the production of well-formed utterances are calculated and, consequently, understood. In sum, the enunciator is a SIMULATED SUBJECT. For ease of reading, I use the personal pronoun 'he' for enunciator.(cf. Osu 2003a: 543)

I use italics here to mean emphasis.

In fact, greetings in Ikwere are always in the form of a question.

See Osu (2003b) for details.

Recall that the p value is the representative or the default value of a given notional domain.

Acknowledgements

This paper has benefitted from discussion with colleagues at LLACAN-CNRS, France, where a preliminary version was presented in February 2008. I wish to thank every participant for their valuable questions and suggestions. I owe many thanks to the two WOCAL referees whose comments and suggestions have helped me to improve this article. I alone remain responsible for any shortcomings.

speaker indicates that upon re-evaluation he is wondering whether the p value really has more weight than p' <you-will not kill somebody>, in other words, both values are assigned equal weight. Example (30) can be interpreted as follows: "you threaten to do something very bad, now if I take the bad thing you are talking about to mean killing somebody, I am wondering whether you would be able to do it. If you won't be able then why disturb my peace?" [Ekwe will be passing through the speaker's place to the farm. And the speaker is waiting in order to have a talk with him. At a point in time, he asks his neighbor who has been sitting outside:] (31) e kܦe o hܦe le n h-a Ekwe 3SG pass-PFT NON ASS here Has Ekwe passed by?

The speaker does not know whether or not Ekwe has passed by. The two values p <Ekwe-has passed by> and p' <Ekwe-has not passed by> are assigned equal weight, and it is only the neighbour's answer that will specify which of them is valid.

(32) e kܦe o hܦe le n h-a ta Ekwe 3SG pass-PFT NON ASS here SFP Has Ekwe passed by? (How am I sure he has not passed by at this time of the day).

The specific thing with ( 32) is that the speaker first thinks that p' i.e., <Ekwe has not passed> is the case. But when he realizes what time of the day it is, he begins to wonder if it is still the case that Ekwe has not passed by. Through the act of wondering, p i.e., <Ekwe has passed> is introduced with the effect that both p and p' become possible values.

In sum, the functioning of ta involves the following: 1) The p value (or in some cases p' as in 32) of a given notional domain represented by a predicative relation is initially introduced and regarded as valid without any reference to p' (or p); thus, no reference is made to any relation of alterity between the two values.

2) But for some reason (e.g. in (27) Chima is not happy), the speaker has doubts over p (or p') bringing about a reassessment of the value thus introduced.

3) The speaker then introduces p' (or p) alongside p (or p') so that they are considered possible values. Where both values are pre-constructed with one of them assigned a more or lesser weight than the other, the use of ta means that the two values are assigned equal weight. This is exactly the case in (30).

Some further arguments in support of the above analysis

The overall analysis is supported by the fact that the SFP ta does not occur in questions in which the validation of p is certain from the point of view of the enunciator (i.e., it is real, in conformity with the way the enunciator represents reality).

Thus, the SFP ta does not occur if the aim of the question is to confirm something, in other words, if the enunciator favours one of the two values. This is illustrated in (33) and (34): into play through shifting from a position where p exists alone to a position where p coexists with p'.


Benjamin Elie
email: elie@limsi.fr

Jodie Gauvain
email: gauvain@limsi.fr

Jean-Luc Gauvain

Lori Lamel
email: lamel@limsi.fr

Modeling the effect of military oxygen masks on speech characteristics

Keywords: speech variation, articulatory perturbation, oxygen mask, data augmentation

Wearing an oxygen mask changes the speech production of speakers. It indeed modifies the vocal apparatus and perturbs the articulatory movements of the speaker. This paper studies the impact of the oxygen mask of military aircraft pilots on formant trajectories, both dynamically (variations of the formants at a utterance level) and globally (mean value at the utterance level) for 12 speakers.

A comparative analysis of speech collected with and without an oxygen mask shows that the mask has a significant impact on the formant trajectories, both on the mean values and on the formant variations at the utterance level. This impact is strongly dependent on the speaker and also on the mask model. These observations suggest that the articulatory movements of the speaker are modified by the presence of the mask.

These observations are validated via a preliminary ASR experiment that uses a data augmentation technique based on articulatory perturbations that are driven by our experimental observations.

Introduction

The speech production of military aircraft pilots is affected by various environmental factors, such as loud noise and its subsequential effect, the Lombard speech [START_REF] Lombard | Le signe de l'elevation de la voix[END_REF][START_REF] Junqua | The Lombard reflex and its role on human listeners and automatic speech recognizers[END_REF], strong acceleration (g-force) [START_REF] Bond | The Effects of High Sustained Acceleration on the Acoustic Phonetic Structure of Speech[END_REF][START_REF] Gulli | G-load effects and efficient acoustic parameters for robust speaker recognition[END_REF][START_REF] South | Some characteristics of speech produced under high G-force and pressure breathing[END_REF], the wearing of an oxygen mask [START_REF] Bond | Acoustic-phonetic characteristics of speech produced in noise and while wearing an oxygen mask[END_REF][START_REF] Vojnović | The influence of the oxygen mask on long-time spectra of continuous speech[END_REF], and intense psychological workloads [START_REF] Keränen | Prosodic features of speech produced by military pilots during demanding tasks[END_REF][START_REF] Huttunen | Effect of cognitive load on articulation rate and formant frequencies during simulator flights[END_REF]. Among these factors, the effect of wearing an oxygen mask has received attention since the 80's. Wearing an oxygen mask affects the speech production, since it modifies the vocal apparatus and perturbs the articulatory movements of the speaker. The vocal tract configuration is modified as it is closed by a chamber between the mask wall and the speaker's mouth. Speakers have reported [START_REF] Bond | The Effects of High Sustained Acceleration on the Acoustic Phonetic Structure of Speech[END_REF] that the presence of the mask hinders their lips and jaw articulatory movements, which could result in a smaller mouth opening, or a limited lip protrusion [START_REF] South | Some characteristics of speech produced under high G-force and pressure breathing[END_REF]. It also increases the breath noise and adds various acoustical noises (e.g. respiratory valves, microphone artifacts). Bond et al. [START_REF] Bond | Acoustic-phonetic characteristics of speech produced in noise and while wearing an oxygen mask[END_REF] analyzed acoustic-phonetic characteristics of speech wearing a mask and found that the overall effect of the oxygen mask is to compress the F 1 -F 2 vowel space. For other phonetic characteristics, such as fundamental frequency, and phoneme duration, wearing a mask had only a marginal influence. Other studies have investigated the impact of cognitive workloads on aircraft pilots and, consequently, analyzed speech of speakers wearing an oxygen mask. Keränen et al. [START_REF] Keränen | Prosodic features of speech produced by military pilots during demanding tasks[END_REF] reported that the average fundamental frequency is raised by about 20% relative in a noisy condition. Huttunen et al. [START_REF] Huttunen | Effect of cognitive load on articulation rate and formant frequencies during simulator flights[END_REF] found additional formants at around 300 Hz and between 2500 and 3000 Hz. Using nu-merical simulations of the transfer functions of the vocal tract to which an additional cavity is connected downstream, Vojnović et al. [START_REF] Vojnović | Transfer characteristics of vocal tract closed by mask cavity[END_REF] associated the additional formant at 3000 Hz to the acoustic resonance of the mask cavity. They also observed a slight rise of the vocal tract resonances by at most 5%.

All of these speech modifications drastically degrade the performance of automatic speech recognition (ASR) systems when the speaker wears an oxygen mask [START_REF] Steeneken | The effect of an oxygen mask on automatic speech recognition[END_REF]. Using recent speech recognition systems trained with normal speech [START_REF] Gauvain | Conversational telephone speech recognition[END_REF][START_REF] Laurent | Investigating techniques for low resource conversational speech recognition[END_REF][START_REF] Gauvain | Challenges in audio processing of terrorist-related data[END_REF], the Word Error Rate (WER) obtained for speech with the oxygen mask doubles in comparison to that of normal speech from the same speaker. In order to build accurate ASR models for military aircraft pilots, the speech variations needs to be clearly identified and quantified. One difficulty of the automatic recognition of military pilot's speech is the lack of available data (e.g. because of confidentiality issues). A possible solution could consist in artificially generating data by transforming normal speech so that it reflects the acoustic parameters of speech of military aircraft pilots. This approach can complement data augmentation techniques [START_REF] Ramirez | A survey of the effects of data augmentation for automatic speech recognition systems[END_REF], such as the Vocal Tract Length Perturbation (VTLP) [START_REF] Jaitly | Vocal tract length perturbation (VTLP) improves speech recognition[END_REF][START_REF] Kim | Improved vocal tract length perturbation for a state-of-the-art end-to-end speech recognition system[END_REF], which is commonly applied to generate new data by virtually modifying the length of the vocal tract. Although this technique works well for generic tasks, we believe that it needs to be adapted to generate new data that fits the speech corresponding to that of the recognition task considered in this work.

For that purpose, this paper focuses on the analysis of the dynamic speech variations with an oxygen mask, with the aim of building transformation operators that can be applied to normal speech so that the transformed speech signal resembles that of speech with an oxygen mask. Acoustic variations of speech are analyzed at the utterance level with a focus on the articulatory perturbations due to the presence of the mask by comparing the same sentences pronounced by the same speakers both with and without the mask. The experimental study analyzes the global and local variations of formant trajectories in speech from 12 speakers. In comparison to previous studies (e.g. [START_REF] Bond | The Effects of High Sustained Acceleration on the Acoustic Phonetic Structure of Speech[END_REF]), variations of formant values are analyzed both dynamically (variations of the formants at a utterance level) and globally (mean value at the utterance level). Additionally, individual variations of speech across speakers are analyzed. In order to validate our model, preliminary ASR experiments are carried out using an articulatory-based perturbation technique for data augmentation, where the articulatory perturbations are driven by our experimental observations. The organization of this paper is as follows. Section 2 describes the experimental observation of the speech variations with an oxygen mask, including the subjects, the analysis methods, and the presentation of the results. Section 4 presents the ASR experiments using an articulatory-based data augmentation technique to improve the performance of ASR tasks on speech with an oxygen mask.

Speech corpora

This section describes the corpora used for the two parts of the paper: the experimental analysis of the effect of the oxygen mask on speech characteristics and the preliminary ASR experiment. The corpus used for the analysis of speech variations is designed to be easy to utter: a dozen of syllables per sentence and all sentences are in the native language of the speakers (i.e. French) with a close to standard phraseology. The corpus for the ASR experiments was designed to target pilot's speech, namely with the specific Air Traffic Control and military aviation grammar phraseology.

Corpus used for the analysis of speech variations

Data were collected in two recording sessions. The first one includes 8 subjects, 7 male speakers (BE, CB, JD, MCB, MV, PYL, and WB), and 1 female speaker (JG). The second one includes 6 subjects, 2 male speakers (BE and MS), and 4 female speakers (JG, FSA, FSB, and FSC). Speakers BE and JG participated in both sessions. All 12 subjects are French native adult speakers with no known speech disorder or hearing impairment.

The text corpus used for both recording sessions consists of 50 sentences taken from the Combescure corpus [START_REF] Combescure | 20 listes de dix phrases phonétiquement équilibrées[END_REF]. This corpus is a well know reference for linguistic studies and was specifically designed to reproduce all French phonemes with individual recurrence rates similar to those encountered in spoken French. The choice of this corpus was also motivated by the fact that the sentences are short enough to limit pronunciation errors and respiratory pauses when using the oxygen mask, while being long enough to observe the acoustic effects of the articulatory gestures. The shortest sentence has 8 syllables and the longest 18.

Recordings were made in a quiet environment. The nomask recordings were acquired with a dynamic headset microphone and using the capsule of a dynamic microphone mounted in the oxygen mask for the mask recordings. Data were sampled at the rate of 8 kHz for the post-processing. Two kinds of oxygen masks were used for the experiment. The first one (denoted M1) is a Ulmer 82AB and the second one (denoted M2) is a Ulmer UA21S.

Corpus used for the ASR experiment

Since the aim of the prelimary ASR experiment is to validate the proposed approach, a small, task-specific system was constructed. The training data come from two corpora in English totaling 15.7 hours of speech: (1) Air Traffic Control (ATC) data (2) recordings of read speech using the official military aircraft phraseology. The ATC corpus contains 7000 utterances with a total duration of 8.1 hours and the phraseology corpus contains 7628 utterances with a total duration of 7.6 hours.

The test corpus is a set of 385 sentences with a mix of official military aircraft phraseology and sentences inspired from real flight missions. All sentences were uttered by 4 different speakers with an oxygen mask in quiet conditions, for at total duration of 18 minutes.

Analysis of speech variations

Formant trajectories were extracted from audio recordings using the Parselmouth library [START_REF] Jadoul | Introducing parselmouth: A python interface to praat[END_REF], which is a Python interface to access Praat functionalities [START_REF] Boersma | Praat: doing phonetics by computer[END_REF]. From the trajectories of the first three formants at the utterance level, two parameters were computed for each individual trajectory: the median formant F i and the formant span Si.

The formant span computation is an adaptation of the method initially proposed by Jany-Luig [START_REF] Jany-Luig | Prosodic and Paralinguistic Speech Parameters for the Identification of Emotions and Stress[END_REF] for pitch contours, but applied to formant trajectories. It is based on previous work by Patterson and Ladd [START_REF] Patterson | Pitch range modelling: linguistic dimensions of variation[END_REF], in which the authors proposed to use clear pitch marks (maximums and minimums of the pitch contour) to compute the pitch span. Then, the authors defined the pitch span as the difference between the average distance between the maximums and the local mean contour and the average distance between the minimums and the local mean contour. By applying this method to the formant trajectories, it is possible to get a feature that measures the range of articulatory movements produced by the subject: for a given utterance, the lower the formant span, the smaller the articulatory gesture. The median formant is defined as the median value of an individual formant across the whole utterance.

For each parameter, we computed a normalized value, named the R-index, defined as the value for a given sentence uttered with the oxygen mask normalized by the value of the same sentence uttered by the same speaker without the mask. Hence a value larger than 1 means that this parameter is raised in the mask condition, and a value smaller than 1 means that it has been reduced. We also performed a one-way ANOVA on the two groups of data (without and with a mask) to assess the significance of the effect of the mask for each parameter. Another one-way ANOVA is performed for the relative data considering each speaker-session as a separate group.

Results are presented for each subject and experimental session. The label indicates the following information: subject's initials, session number (1 or 2), gender (M or F), mask number (1 or 2). For instance, BE1M1 means that the data corresponds to subject BE, recorded in the first experimental session, the subject is male and wearing the mask M1. i.e. the relative modifications of the median formant frequency in the mask condition compared to the no-mask condition, for the first three formants F 1, F 2, and F 3. It shows a large cross-speaker variability, with no clear general tendency across speakers. For instance, some speakers raise the first formant F 1 (BE with mask M1, CB, FSA, FSB and FSC), while it is lowered for others (BE with mask M2, JG with mask M2, and WB1). The modification may be large : values range from -15% upto +26%. Grouping the data from all speakers leads to a median variation of -0.25% (p = 0.66). The results for BE and JG show that there is a combined effect of the type of mask and its placement on the speaker: BE raises F 1 with M1 but lowers it with M 2, and the lowering of F 2 for JG is much larger with M2 (-15%) than with M1 (-4%).

Median formant

The second formant F 2 shows less variation than F 1 across speakers. Globally, speakers tend to lower F 2, with a median variation of -6% across all speakers. The decrease is almost -20% for JG with mask M2. However, the presence of the oxygen mask has no effect on the F 2 for some speakers (BE for mask M1, CB, FSA, FSB, and JD), and for one speaker, FSC, F 2 raises with a median value of +9%.

The results for the third formant F 3 are similar to those observed for F 2: there is a global tendency for speakers to lower F 3. The median variation across speakers is -6.2%. The median value of RF 3 of individual speakers is generally between -5% and -15%.

Overall it can be seen that the formant values are highly impacted by the presence of the oxygen mask. While there is no clear tendency for F 1, which can be either raised or lowered, depending on the speaker, F 2 and F 3 are usually lowered by usually less than 20%.

Formant span

The relative modifications of the formant span, analyzed through RSi are displayed in Table 2. It shows that the median value of RS1 is less than 1 for most speakers. The median ratio across all speakers is 0.73, which corresponds to a lowering of about 27%. The minimal median R-index for the S1 is -55% for WB. Only speaker MS has an RS1 mostly larger than 1. Since F 1 is commonly associated with the jaw and lip movements, this suggests that the speakers tend to decrease their jaw movements when wearing an oxygen mask. This is in agreement with remarks made by pilots that have been reported in previous studies [START_REF] Bond | The Effects of High Sustained Acceleration on the Acoustic Phonetic Structure of Speech[END_REF][START_REF] South | Some characteristics of speech produced under high G-force and pressure breathing[END_REF].

The median value of RS1 across all speakers is -4%, with p = 0.54. However, a large cross-speaker variability is observed. Indeed S2 is raised significantly for CB, JG with mask M1, MCB, MV, and WB. On the contrary, it is significantly reduced for speakers BE and JG, both with mask M2, FSA, FSB, and FSC. There is no significant impact for other speakers (p > 0.1). Since, the second formant frequency is commonly associated with the tongue position, one hypothesis could be an articulatory compensation, but further experiments should be carried out to validate it.

Finally, RS3 also varies significantly according to the speaker. Across all speakers, the median value is +0.99% (p = 0.001), but it is significantly increased for speakers BE, MCB, MS, MV, and PYL, with median values going from +18.7% to +49.0% (p < 0.007), and decreased for speakers FSA, FSB, FSC, and WB, with median values going from -68.4% to -27.0%.

Articulatory-based perturbations for data augmentation

This section presents some preliminary speech recognition experiments using an articulatory-based data augmentation technique. These experiments make use of task-specific data for model training and an independent test corpus. The data augmentation technique is based on articulatory perturbations that model the effect of the oxygen mask on the formant trajectories that were presented in Section 3.

Articulatory perturbations

From the data presented in Section 3, it is possible to extract (non-parametric) probability density functions for each formant parameter and each subject. Then, for any utterance in the initial dataset, a new value of the formant parameters is randomly chosen from the associated probability density function, and data augmentation is performed by applying the appropriate transformations to the formant trajectories. Changing the formant median value Fi by a random value αi is associated with a translation, namely F i = αiFi. Scaling the formant span by a random value βi, requires applying a detrending operator L(Fi), which consists of subtracting the falling or rising line of declination of the formant trajectory Fi and subsequently centering it around zero by subtracting the mean. The inverse operation is then denoted L -1 (Fi). The new formant trajectory is then

F i = L -1 [βiL(Fi)].
Finally, each windowed signal frame xw(t) of the audio speech signal is transformed using a piecewise linear frequency warping function on the spectral envelope. The transformation acts such that the peaks of the intial spectral envelope corresponding to the initial formant frequencies Fi match the target formant frequencies F i . The new signal frame x w (t) is then

x w (t) = F -1 Xw(f ) |Cx(f )| |Cx(f )| , (1)
where

Xw(f ) = F [xw(t)]
is the Fourier transform of the windowed signal frame xw(t), the operator F -1 is the inverse Fourier transform, Cx(f ) is the cepstrum-based spectral enve-lope of xw(t), and Cx(f ) is the frequency-warped version of Cx(f ). Figure 1 illustrates the transformation on the formant trajectories and the piecewise linear frequency warping function applied to the spectral envelope. Audio transformations were performed using the articulatory perturbations application of the SpeechHiker Python library 1 .

Preliminary speech recognition experiments

Table 1 gives the word error rate (WER) obtained on the test data using a speech recognizer [START_REF] Gauvain | Challenges in audio processing of terrorist-related data[END_REF] relying on a HMM-TDNN [START_REF] Peddinti | A time delay neural network architecture for efficient modeling of long temporal contexts[END_REF] acoustic model and a standard 3-gram language model. The baseline training result is reported in the first row of the table, followed by the result using a classical vocal tract length perturbation (VTLP) data augmentation method, and using the proposed articulatory perturbation method based on the statistical distribution of the articulatory variations taken across all speakers. Since we are making use of a non-deterministic NN training process, all models have been trained five times and the WERs averaged. The models trained with the augmented corpus are seen to outperform the baseline models. The best performance is obtained using our novel articulatory-based augmentation technique. The WER is reduced by over 20% (max is 22%) from the baseline value (20.0% vs. 25.7% ). The articulatory-based augmentation technique also outperforms the VTLP-based augmentation technique, which reduces the WER by 16% from the baseline. 1 https://gitlab.com/benjamin.elie/spychhiker

Discussion and conclusions

This paper has presented a study about the variations of speech production when wearing a military aircraft oxygen mask. It consists of a comparative experimental study that analyzes the impact of the oxygen mask on articulatory features extracted from real speech recordings. Our experiments reported large cross-speaker variability for these features that have not been reported in previous studies [START_REF] Bond | The Effects of High Sustained Acceleration on the Acoustic Phonetic Structure of Speech[END_REF][START_REF] Huttunen | Effect of cognitive load on articulation rate and formant frequencies during simulator flights[END_REF]. Given the hypothesis that the mask perturbs the articulatory movements of the speakers, the cross-speaker variability can be explained by several factors such as the speaker physiology and the fit and degree of mask tightening. Despite the large variability, the experiments highlight global tendencies.

The formant trajectories are significantly impacted by the presence of the oxygen mask. Indeed, compared to normal speech, the speakers' mean formant frequencies are modified. For F 1, speakers either increase or decrease the mean formant frequency, from -15% upto +26%. F 2 and F 3 are generally reduced by 20% and 15%, respectively. The global shape of the formant trajectories is also modified by the presence of the oxygen mask. The most significant variation lies in F 1, for which our experiments show an important reduction of the variations of the trajectory around the mean value. This reduction of the formant span is between 20% and 40% among speakers, compared to the formant span observed in normal speech. These results support the hypothesis of articulatory perturbations due to the presence of the mask, especially for the jaw and the lips, which are commonly associated with F 1 variations. This hypothesis is also supported by the fact that previous studies showed that adding a cavity downstream the vocal tract has only a minor influence on formants [START_REF] South | A model of vowel production under positive pressure breathing[END_REF][START_REF] Vojnović | Transfer characteristics of vocal tract closed by mask cavity[END_REF][START_REF] Vojnović | Influence of overpressure breathing on vowel formant frequencies[END_REF].

This paper also presented a preliminary ASR study which uses an articulatory-based data augmentation technique to train models with data that reflect the aforementioned observed acoustic variations. This data augmentation technique extends the VTLP technique by a piecewise linear frequency warping function applied to the spectral envelope that enables each formant to be modified independently at each time frame. Thus, it is possible to change the formant trajectories of the original speech so that they correspond to typical formant trajectories observed in speech recorded wearing an oxygen mask. These preliminary ASR results validate our proposed method: in comparison with models trained with the original speech data, the WER of the test data is reduced when using models trained with the articulatory-based augmented data. This shows that specific ASR tasks can benefit from adapted data augmentation techniques.

In this paper, we only focused on articulatory perturbations because they are likely the most significant factor explaining the acoustic variations of speech with an oxygen mask. However, other factors that may be taken into account in future work include microphone effects and prosodic variations due to cognitive workloads [START_REF] Keränen | Prosodic features of speech produced by military pilots during demanding tasks[END_REF][START_REF] Huttunen | Effect of cognitive load on articulation rate and formant frequencies during simulator flights[END_REF].

Acknowledgements

This study is partly supported by the Man Machine Teaming project MEVAC and the RAPID project both funded by the Defence Innovation Agency (Agence de l'innovation de défense) in liaison with the French Armement General Directorate (Direction générale de l'armement). The authors would also like to thank Synapse Défense and the CEAM for having participated to the recording sessions.

Figure 1 :

 1 Figure 1: Example transformations applied to the formant trajectories and the piecewise linear frequency warping function applied to the spectral envelope. Left: transformation factors α and β are randomly generated following a probability density function computed from our statistical observations (here RS 1 for all speakers). Center : the initial formant trajectories for F 1, F 2, and F 3, and the trajectories resulting from the transformation factors of α = [1.2, 0.8, 1] and β = [0.4, 1, 1.6]. Right: piecewise linear frequency frequency warping function at the time frame represented by the dashed vertical line in the left plot.

Table 1 :

 1 Median values of RF i for the first three formants for each recording session. Numbers in brackets are the standard variations. Scores in blue font are less than 1. Scores followed by * are statistically significant with p < 10 -5 .

		RF 1	RF 2	RF 3
	BE1M1	1.05 (0.06)	0.98 (0.06)	0.95 (0.05) *
	BE2M2	0.88 (0.04) * 0.92 (0.07) * 0.87 (0.03) *
	CB1M1	1.07 (0.06) * 0.99 (0.06)	0.98 (0.03)
	FSA2F1	1.14 (0.06) * 1.04 (0.08)	0.94 (0.04) *
	FSB2F1	1.27 (0.13) * 1.03 (0.08)	0.94 (0.05) *
	FSC2F1	1.26 (0.11) * 1.09 (0.10) * 0.92 (0.05) *
	JD1M1	1.04 (0.08)	0.97 (0.11)	1.03 (0.06)
	JG1F1	0.96 (0.07)	0.83 (0.08) * 0.94 (0.04) *
	JG2F2	0.85 (0.08) * 0.79 (0.07) * 0.91 (0.03) *
	MCB1M1	0.98 (0.11)	0.96 (0.09)	0.98 (0.03)
	MS2M2	0.94 (0.07)	0.89 (0.07) * 0.90 (0.03) *
	MV1M1	1.00 (0.08)	0.91 (0.11)	0.98 (0.05)
	PYL1M1	1.03 (0.08)	0.90 (0.08) * 0.96 (0.05)
	WB1M1	0.93 (0.07) * 0.78 (0.16) * 1.04 (0.04) *
	all speakers	1.00 (0.15)	0.94 (0.12) * 0.94 (0.06)

* Table 1 displays the values of RF i,

Table 2 :

 2 Median values of RSi for the first three formants for each recording session. Numbers in brackets are the standard variations. Scores in blue font are less than 1. Scores followed by * are statistically significant with p < 10 -5 .

		RS1	RS2	RS3
	BE1M1	0.72 (0.21) * 1.12 (0.43)	1.32 (0.48) *
	BE2M2	0.66 (0.19) * 0.77 (0.27) * 1.19 (0.46)
	CB1M1	0.82 (0.33)	1.25 (0.30)	1.02 (0.27)
	FSA2F1	0.72 (0.19) * 0.73 (0.17) * 0.66 (0.14) *
	FSB2F1	0.94 (0.36)	0.76 (0.16) * 0.44 (0.13) *
	FSC2F1	0.78 (0.26)	0.54 (0.14) * 0.32 (0.11) *
	JD1M1	0.73 (0.22)	1.04 (0.44)	0.84 (0.57)
	JG1F1	0.67 (0.28) * 1.48 (0.51) * 0.97 (0.32)
	JG2F2	0.89 (0.35)	0.71 (0.32) * 1.01 (0.29)
	MCB1M1	0.49 (0.24) * 1.36 (0.54) * 1.25 (0.45)
	MS2M2	1.31 (0.50) * 0.99 (0.32)	1.49 (0.47) *
	MV1M1	0.73 (0.31) * 1.31 (0.41) * 1.27 (0.34)
	PYL1M1	0.70 (0.29) * 1.09 (0.41)	1.37 (0.38) *
	WB1M1	0.45 (0.22) * 1.73 (0.50) * 0.73 (0.21) *
	all speakers	0.73 (0.36) * 0.98 (0.49)	1.01 (0.49)

Table 3 :

 3 WER obtained on the test data with ASR models trained with normal speech (baseline), adding VTLP-based augmented data (+VTLP), and adding the data augmented using our articulatory perturbation technique following the statistical distribution obtained from all speakers (+AP(all speakers)).

	Training	# hours mean WER (%)
	Baseline	15.7	25.7
	+VTLP	31.4	21.6
	+AP(all speakers)	31.4	20.0