The ability to speak: fromintentions to spoken words

Willem J. M.Levelt*

In recent decades, psychologists have become increasinglyinterested in our ability to speak. This paper sketches thepresent theoretical perspective on this most complex skill ofhomo sapiens. The generation of fluent speech is based on theinteraction of various processing components. Thesemechanisms are highly specialized, dedicated to performingspecific subroutines, such as retrieving appropriate words,generating morpho-syntactic structure, computing thephonological target shape of syllables, words, phrases andwhole utterances, and creating and executing articulatoryprogrammes. As in any complex skill, there is aself-monitoring mechanism that checks the output. Thesecomponent processes are targets of increasingly sophisticatedexperimental research, of which this paper presents a fewsalient examples.

Talking is one of our favourite pastimes. Most ofus spend large parts of the day in conversation, inteaching, in making phone calls, and so on. If weare not talking to others, we are likely to be talkingto ourselves. The ability to speak is a gift ofevolution to mankind. No other animal talks,whereas all healthy members of our species willeventually be able to talk. This skill is universal toour species and it must have played a key role inthe survival of human society in the course ofevolution. Language is the basic tool in co-operative action, in education, in the creation andtransmission of culture and, quite generally, in theregulation of human bondage.

These obvious facts contrast sharply with thetraditional lack of interest that psychology hasshown in the subject of speaking. Take any of theclassical textbooks in psychology or any of themyriad books on the psychology of everyday life,and the chances are that you will find no chapteror even section on speaking or conversation. The

* Max-Planck-Institut fur Psycholinguistik, Wundtlaan 1,NL-6525 XD, Nijmegen, The Netherlands.

sparse exceptions, such as Wundt's textbookof 18961 or Freud's treatise on everyday psycho-pathology of 1904,2 prove the rule. The analysisof how we speak was largely left to linguists,phoneticians and neurologists and they did anexcellent job. When psychologists finally turnedto their forgotten child some three decades ago,they found a wealth of linguistic theory, detailedanalyses of speech errors, phonetic accounts ofarticulation and detailed aphasiological models oflanguage disorders.

This formed an excellent basis for the develop-ment of a psychological perspective on this corehuman skill. More than in any of the sisterdisciplines, the psychological focus is on the processof speaking. How do you get from some communi-cative intention to the overt articulation of speech?What are the processing components involved inthis generation of fluent speech and how do theyinteract in real time? However, to analyse thisprocess, it is essential to know what it is that getsproduced at different levels of processing. One hasto understand semantic, syntactic, lexical andphonetic structure, as well as the breakdown of

CCC 1062-7987/95/13-23© 1995 by John Wiley & Sons, Ltd.

EUROPEAN REVIEW, Vol. 3, No. 1, 13-23 (1995)

14 Willem J. M. Levelt

CONCEPTUALIZER

messagegeneration

-*\ monitorinq|«

f discourse model.(situational & encyclopedic)\ knowledge

parsed speech

preverbal message

FORMULATOR

grammaticalencoding

surfacestructureJphonological& phoneticencoding

SPEECH

COMPREHENSION

SYSTEM

phonetic plan(internal speech)' phonetic string

< AUDITION

- overt speech

Figure 1. A diagram of the processing components involved in thegeneration of speech.

these structures after neurological damage, inorder to design processing theories of any sophisti-cation. When psychologists revived their interestin the process of speaking, the sister disciplineshad already provided a firm knowledge base of thiskind. Here, the new situation—some three decadesago—differed substantially from Wilhelm Wundt'sand Karl Wernicke's over a century ago, whenprocessing theories had to be developed almostfrom scratch.

Analysing the process of speaking requiresaccurate measurement of its time course. How fastdo we retrieve an appropriate word, do we builda phrase around it, do we construct the articulatorygestures for successive syllables in the utterance,etc? To answer such questions it is necessary tomeasure a speaker's reaction times, for instance innaming objects. The generation of speech fromthought is not instantaneous. Mental processestake time, as Donders argued long ago,3 and westill use his methods of mental chronometry to traceand dissect mental events 'on the fly'. In recentyears, mental chronometry has been successfullyapplied to the generation of speech, leading to adeeper understanding of this complex skill.

In the following I will present a global sketchof this skill's organization as we presently see it.In going over its constituent components, I will pre-sent occasional examples of experimental methodsand findings. My aim here is to be informative(about some core issues and research methods)rather than comprehensive. For reviews of the fieldand of recent developments, see References 4, 5and 6.

The diagram in Figure 1 is my summary view ofwhat we presently know theoretically and empiric-ally about the mental mechanism that generatesspeech from communicative intentions. At thesame time, it is a working model for furtherresearch. According to this 'blueprint', the abilityto speak is based on the interaction of a set ofprocessing components that are relatively autono-mous or 'modular' in their functioning. Eachcomponent is comparatively simple; the system'sintelligence derives from the co-operation of thecomponents. The blueprint is a working model inthe sense that it raises three empirical issues. Is thepartitioning of components correct? What opera-tions are performed by each component? How dothe components interact to generate fluent speech?

The ability to speak: from intentions to spoken words 15

These issues are, of course, not independent. Tomodel one component, one must keep an eye onthe system as a whole.

Conceptual preparation

Our present state of knowledge differs for thedifferent components. The situation is worst forthe component labelled 'Conceptualizer'. Talkingis an intentional activity, a way of acting. Aneffective utterance is one that makes the speaker'scommunicative intention recognizable to a partner-in-speech. And intentions are very diverse. Thespeaker may want to inform the interlocutor aboutsomething, to refer to something in the environ-ment, to commit the interlocutor to some immedi-ate or future action, to invite the interlocutor'ssympathy, etc. Conceptualizing is primarily decid-ing on what to express, given the present intention.As speakers we spend most of our attention onthese matters of content. There is much strategyhere and rhetoric, involving politeness, wit, meta-phor or, alternatively impudence, deceit, sarcasmand cynicism. In spite of much, and often highlycreative, research (see for instance Reference 7),these mechanisms are not well-understood. It iscertainly an oversimplification to lump them alltogether and suggest that the choice of content isa modular, coherent process that can be isolatedfrom the rest of human intelligence.

However, there is more to conceptual prepara-tion than the choice of content, and some of it hasbeen successfully studied in recent years. Oneaspect is 'linearization' and another is 'perspectivetaking'. Both of these become relevant when thechoice of content is being completed.

Linearization is the process by which we orderinformation for expression. When somebody asksyou to describe the layout of your home and youdecide to comply, you have two problems to solve.Which features of your home will you mention—that is the content issue—and in what order? Thelatter is called the linearization problem. Your homeis a three-dimensional structure, but speech is alinear medium. You can express just one thing at atime and the relevant items have to be ordered forexpression. Speakers solve this problem by imagin-ing a connected path through their living quartersas if they are taking you on a tour.

(a)

(b)

pwple yellow green black orange gray red

(C)

Figure 2. Patterns used to study how speakers orderinformation for expression. Dots were coloured (herelabelled by colour names).

How is such a path constructed? We studied thisby asking subjects to describe patterns such as inFigure 2. The patterns consist of coloured dots,connected by arcs. Subjects were asked to begintheir description at the dot marked by an arrow,and describe the pattern in such a way that thenext subject would be able to draw the patternfrom the tape-recorded description. How dospeakers linearize such patterns? It turned out thatlinearization followed a few very simple rules.Speakers always follow a connected path throughthe pattern and register the choice nodes they passand to which they have to return for completingtheir description. By the end of a path, they jumpback to the most recent choice node on their' memory stack' and begin a new path from there.This is recursively done until the whole pattern

16 Willem J. M. Levelt

has been covered. Which path do they choosewhen leaving a choice node? They strongly preferto go for the simplest path first, in particular forthe path that has the smallest number of choicenodes. This mechanism is so simple that thecomputed model consists of no more than a fewlines. The mechanism, we could show, minimizesmemory load (and hence attentional effort); it isfast and effective. It is also gullible. It is nothard to design patterns where nodes get skippedby the subject, for instance when they contain non-connected subgraphs. The same basic mechanismturns up in the other complex descriptions, forinstance when you ask your subjects to describetheir kin.

When you describe your apartment, or anyspatial array for that matter, there is still more tobe prepared. Every bit of spatial information thatyou want to express has to be given ' propositionalshape', and this involves perspective taking.Assume your bathroom and kitchen are adjacentstructures. You have a clear image of theirarrangement in your place. You may now decideto express this imagistic information as the bathroomis next to the kitchen. Or do you prefer to say thekitchen is next to the bathroom! In some way oranother you will locate one item with respect tothe other item. There are many ways of doing this,but you will have to make some choice here. Youwill have to map geometrical and topologicalinformation onto logical (locative) relations betweenentities. There are preferences here that followGestalt principles. It is, for instance, more obviousto say there is a chair in front of the cupboard thanto say there is a cupboard behind the chair. This isone aspect of perspective taking, the choice ofreferent and relatum. The preference is always tomake the smaller, foregrounded object the referentthat you locate with respect to the bigger, back-grounded object, the relatum.

But there is more to perspective taking. Figure3 is another pattern that our subjects described.Two thirds of them described the directions withterms such as the ones on the outside of the figure:up, up, right, right, down, left. The pattern was flaton the table; yet still these subjects used terms suchas up and down. What they are doing is making agaze tour. They scan the pattern and tell you howtheir gaze moves: up, up, etc. We call this 'deictic'perspective, because the gaze indicates where youare in the pattern. If you turn the pattern by 90°

right right

up

right straight

straight right

right

down

up

left

straight

Figure 3. The use of direction terms depends on thespeaker's perspective. On the outside, terms from adeictic perspective; on the inside, terms from intrinsicperspective.

or 180°, all direction terms will be different. Onethird of our subjects used the terms depicted on theinside of the pattern: straight, straight, right, straight,right and right. They make something like a bodytour, as if they are walking or driving through thepattern, telling you what turns they take. Thesedirections and turns only depend on the intrinsicshape of the pattern, not on the pattern's orienta-tion with respect to the speaker; the terms will bethe same when you turn the pattern by 90° or 180°.We call this 'intrinsic' perspective. But notice'howdifferent the terms are between the perspectives.The final move, for instance, is left in the deicticperspective and right in the intrinsic perspective.Does left mean right? Of course not. But theproposition that node X is left of node Y cansimultaneously be true from one perspective andfalse from another perspective. What we see hereis that the same spatial relation is captured by adifferent propositional relation, LEFT(X, Y) orRIGHT(X, Y), dependent on the chosen perspect-ive. LEFT and RIGHT are lexical concepts,because we have words for them in the language,and to express some spatial relation in language wemust translate it into lexical concepts. Perspectivetaking mediates here, and this is by no meanslimited to talking about spatial relations. To referto any entity we must capture it by some lexicalconcept. I will refer to the same person as my

The ability to speak: from intentions to spoken words 17

friend, colleague, brother, neighbour, or what haveyou, or to the same planet as morning star orevening star, dependent on the current chosenperspective in the conversation. Perspective takingis always at the core of a speaker's conceptualpreparation.

The eventual result of conceptual preparation issome propositional structure that consists oflexical concepts. This is technically called thespeaker's message. This conceptual structure iswhat the speaker will formulate, i.e. express inlanguage.

Formulating

The Formulator performs two operations: gram-matical encoding and phonological/phoneticencoding. Let us first attend to grammaticalencoding.

(grammatical encoding

In order to encode a message linguistically, a firststep must be to retrieve appropriate words for itslexical concepts (the word left for the conceptLEFT, the word right for the concept RIGHT, etc.).As speakers we are equipped with a mental lexiconthat contains tens of thousands of words. Innormal conversation we retrieve words from thislexicon at a rate of 2 to 3 per second. That retrievalprocess is usually fully automatic; we do not haveto spend much attention on it. I will shortly returnto this process.

Each word we retrieve from the lexicon has aspecification for the syntactic environment itrequires. These environments are, for instance,different for verbs, nouns, adjectives, prepositionsetcetera. But also within such classes there is greatsyntactic variety. During grammatical encoding theretrieved words are ordered and morphologicallyshaped in such a way that all these words' syntacticrequirements are simultaneously met. This is a bitlike solving a set of simultaneous equations, andthere are sophisticated theories about how thisprocess of syntactic unification proceeds.8 Syntacticunification is fast and efficient, but also unintelligentin the sense that it ignores semantics. It only

CONCEPTUALLEVEL

I MENTAL LEXICON

Figure 4. Fragment of lexical network.

bothers about syntax. Hence, it can happen thatwe make speech errors such as this one:

Seymour sliced the knife with a salami

where the two nouns knife and salami exchangedpositions, making the utterance very strange interms of meaning. But the syntax is perfect. Thegrammatical encoder is a syntactic idiot savant.

How do we retrieve appropriate words to startwith? In normal conversation we hardly ever makean error in selecting intended words. Selectionalerrors are below one in a thousand, on average.But they do occur, as in this example:

Don't burn your toes

Here the speaker intended to say Don't bum yourfingers. Most selectional errors are semantic innature, such as here where toe is selected insteadof finger. How does this arise? Ardi Roelofs, ofmy laboratory, has developed a network model oflexical retrieval that can account for such semanticerrors; Figure 4 shows a fragment of it. The nodesin the top layer represent lexical concepts. Asdiscussed, these are the smallest or terminal

18 Willem J. M. Levelt

elements of a speaker's message. There is, forinstance, a lexical concept for SHEEP and anotherone for GOAT. That these are semanticallyrelated is represented by their network of relationsto other concepts. For instance, both are animals,both produce milk, etc. All this is encoded in theconceptual part of the network. When we ask asubject to name a picture and we show them oneof a sheep, the lexical concept SHEEP will receivesome activation.

How is the corresponding word retrieved fromthe lexicon? The theory says that an active conceptspreads its activation to the lexicon. In the firstinstance, the activation spreads to the so-calledlemma level of the mental lexicon. Nodes at thislevel, called lemmas, represent the syntactic prop-erties of words. If SHEEP is the active concept, thelemma sheep and its syntactic properties will beactivated. The lemma's activation spreads, forinstance, to a node noun. In a language such asFrench that marks gender, the equivalent lemmamouton will spread activation to the gender nodemale, etc.

But if SHEEP is the active concept, some of itsactivation will spread (via ANIMAL, MILK, etc)to the semantically related concept GOAT. Andif GOAT is active, it will activate its lemma goat.In the theory developed by Roelofs9 the probabilitythat at any given instant a particular lemma getsselected equals the lemma's activation divided bythe sum activation of all lemmas at that moment.Since sheep receives much more activation that goat(via the big detour just described) the chances arethat sheep gets selected, not goat. However, becausethe rule is probabilistic, there is always a minimalchance that there will be an error of selection. Itis then likely to be a semantic error, because ofthe activation spreading in the meaning-basedconceptual network.

Although the theory can explain this kind oferror, it was tested and time and again confirmedin reaction time experiments. Here, we used thenaming-interference method, introduced in ourlaboratory by Schriefers and Meyer.10 The subjectis shown a picture to be named as fast as possible.We measure the naming latency, i.e. the timefrom us presenting the picture to the subjectinitiating articulation. But we complicate this taskby presenting a distractor word, acousticallyor visually, that the subject is instructed toignore. The distractor word can be presented

INHIBITIONfin ms)

FACILITATION -100-(in ms)

• Real data• Roelofs' model

-400 -300 -200 -100 0 100 200 300 400

SOA (ms)

Figure 5. Reaction times from a picture-word inter-ference experiment by Glaser and Diingelhoff and theirfit by Roelofs's model.

simultaneously with the picture or a bit earlier orlater, i.e. at different 'stimulus onset ^synchronies',or SOAs. When the distractor word is semanticallyrelated to the target, for instance when we presentgoat as distractor when the picture is one of asheep, the naming latency typically increases. Itincreases more than when we present an unrelateddistractor (such as house). Figure 5 shows theclassical results that Glaser and Diingelhoff11

obtained with this paradigm. The naming latencyis maximally affected when picture and distractorcoincide, and it decreases for both longer andshorter SOAs. The figure also shows the (statistic-ally perfect) fit of Roelofs's model to these data.Meanwhile, the model has been reconfirmed timeand again in experiments from our own laboratory(see for instance References 9 and 12).

Phonological encoding

As soon as a lemma has been selected, its activationspreads to the next, lexeme level in the lexicon. Atthis level, a word's phonological properties arestored. For instance, when sheep is selected, itsactivation spreads to the corresponding lexeme/Jip/, and from there to its individual segments /J/,/i/ and /p/. These, in turn, are used to plan thearticulatory shape of syllables, and I will shortlyreturn to that process.

A preliminary point to consider, however, is

The ability to speak: from intentions to spoken words 19

how we access the lexeme to start with. Here, animportant phenomenon is the so-called wordfrequency effect. It is long known that we arerelatively slow at naming objects that have namesinfrequent in everyday language use. For instance,it takes about 200 ms longer to name a picture ofa moth than to name a picture of a mouth. In apicture recognition experiment, Joerg Jescheniakand I showed that this is not due to a difference inthe ease of recognizing the objects. It is really dueto accessing their names. The question then is this:does the word frequency effect arise in selectingthe lemma or rather to accessing its lexeme, itsword form information?

To study this we made use of homophones.Homophones are different words that are pro-nounced the same. An example is we and wee. Inthe network model of Figure 4 these words havedifferent nodes at the lemma level; they differ insyntactic category (we is a pronoun, wee is anadjective), but they share a node at the lexemelevel, because they are alike phonologically:

lemma level:

lexeme level:

we wee

/wi/

If the word frequency effect is caused at the lemmalevel, then it should differentially affect the twohomophones of a pair. The high-frequency homo-phone, i.e. the pronoun we in the example, shouldbe accessed faster than the low-frequency homo-phone wee in the example. That is the most likelyevent. However, if the effect is caused at the lexemelevel, then one should observe something unexpec-ted. The two homophones should be equallyaccessible, in spite of their frequency difference.More importantly, the low-frequency homophone(wee in the example) should behave as if it were ahigh-frequency word; it should be accessed just asfast as its high-frequency partner (we in theexample). In our experiment13 we had subjectsproduce low-frequency homophones (such as wee)as well as two types of control words: non-homophones that were just as low-frequency (forinstance vile) and non-homophones that were justas high-frequency as the high-frequency homo-phone (for instance me, which is of about the samefrequency as we). We wanted to know whetherthe low frequency homophone (like wee) behavesas if it is a low-frequency word (such as vile) or asif it is a high-frequency word (such as me).

The results of the experiment were unequivocal.Low-frequency homophones (like wee) are accessedjust as fast as the high-frequency controls (like me),and they are a lot faster than the low-frequencycontrol words (like vile). This proves that thefrequency effect arises at the lexeme level. It isaccess to the word's phonological shape that isrelatively slow for low-frequency words.

After accessing the lexeme, word form informa-tion becomes available, but not as a whole, as acomplete word template. This has long beenknown from phonological speech errors, such asthis one:

With this wing I thee red

Here the poor minister exchanged /w/ and /r/ indelivering this important formula. Psycholinguistshave collected huge numbers of such errors, andthe analyses show that a word's phonologicalsegments are not fixated in their position, but haveto be inserted in the right metrical slot as we speak.More specifically, a word's (or lexeme's) phono-logical information is of two kinds, the word'smeter and the word's segments or phonemes. Weknow this intuitively from tip-of-the-tongue situa-tions. When we can momentarily not recall aperson's name, we often do know the name'smeter or accent patterns (e.g. it is a three-syllableword with main stress on the first syllable) and canproduce metrically similar names. Hence, theword's meter is independently retrieved. This alsoholds for normal, undisturbed retrieval. PaulMeyer of our institute showed this in a picturenaming experiment. Subjects had to name pictures,such as one of a cigar. But when the picture waspresented they also heard a distractor word, thatthey were instructed to ignore. The distractorword could have the same meter as the target ora different one. For cigar, for instance, distractorscould be saloon (same iambic meter) or salmon(different trochaic meter). We had predicted thatnaming would be faster in the former case than inthe latter, and that is what Meyer found.

This metrical information is an importantingredient in preparing connected speech. We donot speak in isolated words, but in larger metricalunits. An important unit here is the phonologicalword. Let us consider an example. Somebody saysPolice demand it. Here demand it is a phonologicalword. How do we know? Word boundary informa-tion is lost in a phonological word, and that is

20 Willem J. M. Levelt

segmentalspell-out

activationof lexemedemand, it

metricalspell-out

d, e, m, a.n.d, i, t\

phonologicalword formation

/N

association andsyllabification

(0

o c c

A A Ad e man d i t

Figure 6. The encoding of a phonological word.

especially apparent in the word's syllabification. Itis not the original lexical word (for instance demandor it) that is the domain of syllabification, but thephonological word. The eventual syllabification ofour example will be de-man-dit; the last syllabledit straddles the lost boundary between the twocomposing lexical words.

We presently believe that phonological wordssuch as demandit are generated as sketched inFigure 6. As soon as the metrical frames of demandand it have become available, they are blended tocreate a new three-syllable metrical frame. Then,the independently retrieved segments of demandand it are one-by-one associated to this newmetrical frame, following rules that are more orless known.14 As the association proceeds 'fromleft to right', syllables are created 'on the fly', first/de/, then /man/ and then /dit/. Antje Meyer andHerbert Schriefers obtained convincing experi-mental support for the theory that segments areassociated with the metrical frame one afteranother, 'from left to right'.15 These were picture-word interference experiments. Here I will mentionanother result that Linda Wheeldon and I recentlyobtained.16 We gave our Dutch subjects a targetphoneme to monitor, for instance the /f/ of felix.

1400

1350

1300

1250

1200

1150

1100

ihoneme monitoring latency in nis.

example: l i f t e r

/

onset 1 coda 1 onset 2 coda 2

POSITION OF PHONEME IN WORD

Figure 7. Detecting latencies for phonemes in internallygenerated translation words.

We then presented them with an English wordwhose Dutch translation they knew very well (mostof our Dutch subjects have a good basic knowledgeof English). For instance they heard the Englishword hitch-hiker. Its Dutch translation is lifter. Thesubject's task was to push a button if the Dutchtranslation contains the target phoneme. That isindeed the case for the translation equivalent ofhitch-hiker (Dutch lifter has /f/ as its third segment).So, subjects never said lifter; they only pushed thebutton as soon as they became aware of the segment/f/ in the Dutch translation of hitch-hiker.

The results of this experiment, demonstratedfrom the word lifter, but based on a wide varietyof target words, are presented in Figure 7. Thefigure shows the reaction time for /{/ as a targetphoneme, but also for other target consonants inthe word, /I/, /t/, and /r/. The reaction timesincrease significantly from left to right, supportingthe notion that phonemes are indeed, one afteranother, attached to the metrical frame. Severalcontrol experiments excluded obvious alternativeinterpretations.

The main output, then, of phonological encodingis a string of phonological syllables, like /de/'/man/'/dit/. But how do we generate the articulatorygestures for each of these syllables? This requireswhat we have called 'phonetic encoding'.

Phonetic encoding

The theory we are presently developing is that, asa speaker, you have access to a 'mental syllabary'

The ability to speak: from intentions to spoken words 21

(see Figure 1). The syllabary contains a specifica-tion of the articulatory gesture for each phono-logical syllable you generate (i.e. one for /de/, onefor /man/, one for /dit/, etc). The idea is thatas soon as a syllable has been generated intern-ally, its phonetic gesture is retrieved from thesyllabary to be executed by the articulatorysystem.

How can one demonstrate the existence of sucha syllabary in our minds? Levelt and Wheeldon14

argued as follows. We know that there exists aword frequency effect and that it arises when theword form or lexeme information is retrieved (seeabove). If we then, at a later stage, retrieve syllabicpatterns, another frequency effect may arise. Wemay be slower in retrieving a low-frequencysyllable, one that is not used much, than ahigh-frequency one. Because these two retrievalsteps (lexeme, syllable) are strictly sequential, thetwo frequency effects should be independent andadditive. These are strong, non-trivial predictions.To test them we had subjects produce bisyllabicwords of four kinds. The words could either berelatively high-frequency (such as lady and language)or relatively low-frequency (like litter and lantern).But each word either consisted of two high-frequency syllables (such as lady and fitter) or oftwo low-frequency syllables (such as lantern andlanguage). In other words, we completely crossedthe two frequency variables. We measured thenaming latencies.

The results are given in Figure 8 and they confirmour expectations precisely. Both frequency effectsare statistically significant, but there is no interaction;the two effects are additive, the two curves areparallel. Various additional experiments showedthat the syllable frequency effect is entirely due tothe frequency of a word's last syllable. This is howit should be. The speaker initiates a word'spronunciation only after all of its syllables havebeen retrieved from the syllabary. However, eachsyllable retrieval is time-locked to completion inconstruing the corresponding phonological syllable,not to retrieving the previous syllable program.Hence, successive syllable frequency effects do notadd; it is only the last syllable whose frequencyeffect we can observe in the latency data.

There is much more to phonological encodingthan can be discussed here, such as the generationof intonation, of pauses, of loudness patterns, etc.Still, the preparation of syllables is at the core of

word onset latency in ms.630

610

590

570

low-frequency words

high-frequency words

Ilow high

SYLLABLE FREQUENCY

Figure 8. Naming latencies for words varying in wordfrequency and in syllable frequency.

speech preparation, because syllables are the basicunits of articulation. The eventual output, then, offormulating is a phonetic or articulatory plan,which can phenomenologically present itself to usas internal speech.

Articulation

The phonetic plan, in particular the syllabicgestures, will eventually be executed by thearticulatory apparatus, a highly sophisticatedsystem that controls the movements of lungs,larynx, pharynx and mouth. A whole network ofneural substrates in the cortex, the basic gangliaand the cerebellum are involved in this, our mostcomplex motor behaviour. I will refrain fromdiscussing this module (see Reference 4 for anextensive review), but still remark that this wholeapparatus originally developed for us to breathe, todrink, to chew, to swallow. It is one of evolution'smost impressive achievements that the samestructures gradually developed to acquire a com-pletely different function, the execution of speech.And this happened without abandoning the originalfunctions. Surprisingly, this cohabitation turnedout to be possible. The only concession we hadto make is that we risk choking when we try tocombine speaking and ingesting.

22 Willem J. M. Levelt

Self-monitoring

The person we listen to most is ourselves. We canlisten to our internal speech, as we do when talkingto ourselves. And we always listen to our ownovert speech. That involves the same apparatus aslistening to the speech of others. Just as we candetect errors, disfluencies or other problems ofdelivery in the speech of others, we can detecttrouble in our own speech. If that trouble is seriousenough, we can decide to halt and make acorrection.

To study this process of self-monitoring, Icollected and analysed some 1000 spontaneousself-corrections.17 There are essentially threephases in the process of self-monitoring. The firstphase I analysed was the halting process itself.What types of trouble make speakers halt? Theyare largely of two kinds. The first kind is all-outerrors, as in left to pink—er straight to pink, wherethe subject made an error of lexical selection (leftinstead of straight). The second kind is an inappro-priateness of sorts, where the speaker feels thatfurther specification is necessary, as in to the rightis yellow, and to the right-further to the right is blue.I also analysed how fast a speaker interrupts afterthe trouble appeared. The main rule here turnedout to be quite simple. Halting is done right upondetecting the trouble, and this can be in the middleof a word. There is no tendency to safeguard theintegrity of syntax in self-interruption, the breakcan be made anywhere in the sentence. Butdetecting can be slow, and the speaker will thenstop one or more syllables after the trouble spot,as in and from green left to pink-er from blue left topink, where green is the error.

The second phase is the editing phase. Afterhalting speakers often use specific editing terms,such as er- to signal that trouble is on. It turnedout that the editing term depends on the kind oftrouble. Errors are mostly followed by terms suchas no, or and sorry, whereas appropriateness troubleis predominantly signalled by terms such as ratheror that is.

The third phase is the re-start, producing theself-correction proper. Where self-interruptionfully ignores syntax, restarting is syntacticallyhighly principled. Original utterance and repairrelate in the same way as two conjuncts in a syntacticco-ordination. For example, Is the nurse-the doctor

interviewing patients? is a normal well-formedrepair, and so is the corresponding co-ordina-tion Is the nurse or the doctor interviewing patients?But Is the doctor seeing-the doctor interviewingpatients? sounds ill-formed, and so does thecorresponding co-ordination Is the doctor seeingor the doctor interviewing patients? Notice thatthe repairs proper are the same in the two ex-amples (the doctor interviewing patients). The crucialpoint is that the repair should syntactically fitthe interrupted utterance. Apparently, in makinga self-repair the speaker keeps the interruptedsyntax in abeyance and grafts the correction ontoit. This is, no doubt, the reason why it is oftenpossible to 'splice away' self-corrections in recordedspeech, a well-known practice in the broadcastingbusiness.

Conclusions

The present paper briefly considered the 'blue-print of the speaker' in Figure 1. Its aim was togive a sketch of the main processing componentsthat co-operate in the interaction of fluent speechand of the research methodology presently appliedto the analysis of speech production. However, theblueprint was announced as a working model, aswe do not have a comprehensive theory of thismost complex of human skills, and it will be longbefore one is in the offing. What is new, however,is that we now have a theoretical perspective onthis, our ability to speak. There is a satisfying(though of course not complete) agreement amongcolleagues in this field, on what the issues are andhow they are mutually related. This is certainly animportant condition for further progress. Theother encouraging development is that we nowhave a wide range of empirical and, in particular,experimental methods by which the emergence ofan utterance can be studied as a process. Mentalchronometry has now firmly established itself inthis field, together with a range of other methodsthat I did not have the opportunity to discuss.In short, this paper covered a subject of inquirythat will contribute essentially to our self-under-standing as human beings, and that is now on theverge of becoming a coherent and successfulenterprise.

The ability to speak: from intentions to spoken words 23

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Author's biography:Willem J. M. Levelt is director of the Max PlanckInstitute for Psycholinguistics in Nijmegen andProfessor of Psycholinguistics at Nijmegen Uni-versity. He is a graduate of Leiden University andhas held positions at the Institute for Perceptionin Soesterberg, Harvard University, the Universityof Illinois, Groningen University, the Universityof Louvain, and the Institute for Advanced Studyin Princeton. Among his books are On BinocularRivalry (1968), Formal Qrammars in Linguistics andPsycholinguistics (1974) and Speaking (1989). He is amember of the Academia Europaea, the RoyalDutch Academy of Sciences and the DeutscheAkademie der Naturforscher Leopoldina.