An Introduction to Spoken Language Processing and its Disorders

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Neurolinguistics

What biological factors make human communication possible? How do we process and understand language? How does brain damage affect these mechanisms, and what can this tell us about how language is organized in the brain? The field of neurolinguistics seeks to answer these questions, which are crucial to linguistics, psychology and speech pathology alike. Drawing on examples from everyday language, this textbook introduces the central topics in neurolinguistics:

speech recognition, word and sentence structure, meaning, and discourse – in both ‘normal’ speakers and those with language disorders. It moves on to provide a balanced dis- cussion of key areas of debate such as modularity and the

‘language areas’ of the brain, ‘connectionist’ versus ‘symbolic’ modelling of language processing, and the nature of linguistic and mental representations. Making accessible over half a century of scientific and linguistic research, and containing extensive study questions, it will be welcomed by all those interested in the relationship between language and the brain.

j o h n c . l . i n g r a mis Senior Lecturer on the Linguistics Program at the University of Queensland. He has published widely on speech and language disorders, sound change in second language acquisition, phonetic variation in Australian English, connected speech processes, acoustic phonetics, for- eign accent phenomena and forensic speaker identification.

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C A M B R I D G E T E X T B O O K S I N L I N G U I S T I C S

General editors:p. a u s t i n, j . b r e s n a n, b. c o m r i e , s . c r a i n, w. d r e s s l e r , c . e w e n, r . l a s s , d . l i g h t f o o t, k . r i c e , i . r o b e r t s , s . r o m a i n e , n. v. s m i t h

Neurolinguistics

An Introduction to Spoken Language Processing and its Disorders

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j . a l lw o o d , l . - g . a n d e r s o nand¨o . d a h lLogic in Linguistics d . b. f r yThe Physics of Speech

r . a . h u d s o nSociolinguisticsSecond edition a . j . e l l i o tChild Language

p. h . m at t h e w sSyntax

a . r e d f o r dTransformational Syntax l . b a u e rEnglish Word-Formation s . c . l e v i n s o nPragmatics

g . b r o w nandg . y u l eDiscourse Analysis

r . h u d d l e s t o nIntroduction to the Grammar of English r . l a s sPhonology

a . c o m r i eTense

w. k l e i nSecond Language Acquisition

a . j . w o o d s , p. f l e t c h e randa . h u g h e sStatistics in Language Studies d . a . c r u s eLexical Semantics

a . r a d f o r dTransformational Grammar m . g a r m a nPsycholinguistics

g . g . c o r b e t tGender

h . j . g i e g e r i c hEnglish Phonology r . c a n nFormal Semantics

j . l av e rPrinciples of Phonetics

f. r . p a l m e rGrammatical Roles and Relations m . a . j o n e sFoundations of French Syntax

a . r a d f o r dSyntactic Theory and the Structure of English: A Minimalist Approach r . d . va n va l i n, j r ,andr . j . l a p o l l aSyntax: Structure, Meaning and Function a . d u r a n t iLinguistic Anthropology

a . c r u t t e n d e nIntonationSecond edition

j . k . c h a m b e r sandp. t r u d g i l lDialectologySecond edition c . ly o n sDefiniteness

r . k a g e rOptimality Theory

j . a . h o l mAn Introduction to Pidgins and Creoles g . g . c o r b e t tNumber

c . j . e w e nandh . va n d e r h u l s tThe Phonological Structure of Words f. r . p a l m e rMood and ModalitySecond edition

b. j . b l a k eCaseSecond edition

e . g u s s m a nPhonology:Analysis and Theory m . y i pTone

w. c r o f tTypology and UniversalsSecond edition

f. c o u l m a sWriting Systems: An Introduction to their Linguistic Analysis p. j . h o p p e rande . c . t r a u g o t tGrammaticalizationSecond edition l . w h i t eSecond Language Acquisition and Universal Grammar i . p l a gWord-Formation in English

w. c r o f tanda . c r u s eCognitive Linguistics a . s i e w i e r s k aPerson

d . r a d f o r dMinimalist Syntax: Exploring the Structure of English d . b ¨u r i n gBinding Theory

n. h o r n s t r i n, j . n u ˜n e sandk . g r o h m a n nUnderstanding Minimalism b. c . l u s tChild Language: Acquisition and Growth

m . b u t tTheories of Case g . g . c o r b e t tAgreement

j . c . l . i n g r a m Neurolinguistics: An Introduction to Spoken Language Processing and its Disorders

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Neurolinguistics

An Introduction to Spoken Language Processing and its Disorders

J O H N C . L . I N G R A M

University of Queensland, Australia

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Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo Cambridge University Press

The Edinburgh Building, Cambridge CB2 8RU, UK

First published in print format

ISBN-13 978-0-521-79190-8

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Figures

1.1 The cerebral cortex: the language areas and major

anatomical landmarks page11

1.2 Phrenology diagram: frontispiece to Spurzheim’s

Outlines of phrenology, 1827 13

2.1 Components of the linguistic model 37

3.1 Lobes of cerebral cortex 43

3.2 Somatosensory cortex 44

3.3 Flat projections of human and macaque cerebral cortex 47

3.4 The Wernicke-Lichtheim model 52

3.5 Disturbances in phoneme perception 57

3.6 The single word processing model 59

4.1 Neural network for printed word recognition 81

5.1 Spectrogram:sheep like soft grass 94

5.2 Speaking style and alternative pronunciations ofI’m

going to leave 99

5.3 Spectrogram:I should have thought spectrograms were

unreadable 99

5.4 Transcription accuracy of the nonce phrases 104

5.5 Levels of prosodic structure 109

6.1 Stop consonant+vowel syllables produced by the

pattern playback synthesizer 116

6.2 Discrimination and identification functions for /b-d-g/

for three listeners 119

6.3 Morphing visual images to create a ‘Clinton-Kennedy’

continuum 121

6.4 Duplex stimulus construction 124

6.5 Prototype (P) and non-prototype (NP) [i] vowels and

perceptual magnet effects 128

6.6 The gating paradigm 133

6.7 Gating experiment: Bengali listeners’ response to

nasalized vowels 136

7.1 How coarticulation effects are simulated in TRACE 146

7.2 Simple recurrent network (SRN) 150

8.1 Hickok and Poeppel’s dorsal and ventral stream model 170

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9.1 PET activation for regular, irregular and nonce

past-tense forms 192

9.2 MEG differences to regular and irregular verbs 196 10.1 Three planes representing the meaning ofPlantin

Quillian’s TLC model 202

10.2 Conceptual dependency diagram forJohn ate a frog 208 10.3 Augmented conceptual dependency diagram forJohn ate

a frog 209

10.4 The areas activated in the verbs–nouns contrast 218 11.1 Types and relative incidence of category-specific

semantic disorders 233

12.1 Partial parsing ofA cat is on the couch 246 12.2 Minimalist derivation ofA cat is on the couch 247

12.3 Surface structure syntax 257

12.4 Contrasting ‘underlying’ structures for sentence 258 13.1 Reading times for relative clauses: Ferreira and

Clifton (1986) 272

13.2 Interaction of verbal working memory capacity with

syntactic and pragmatic cues 273

13.3 Three NPs awaiting case assignment 277

13.4 ERPs to well-formed, semantically anomalous and

syntactically anomalous verbs 287

13.5 Differences in ERPs under ellipsis and discourse model

interpretive (MI) anaphora 293

14.1 Dendrograms forThe baby cries 302

14.2 Dendrograms for sentences 2–3 303

15.1 Hypothetical process of construal of mini-discourse 3 337

16.1 Sample of syntactic and error coding 352

16.2 The Tower of London Test 357

16.3 Temporal lobe activation differences and relation to

prefrontal activation in schizophrenia 364

17.1 Hierarchical clustering of hidden-unit vectors 370

17.2 Relative clause mini-grammar 371

17.3 Elman: state space trajectories 373

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Tables

2.1 Distributional properties of nouns and verbs (in English) page18

2.2 Compositionality of form and meaning 21

2.3 Basic levels and components of linguistic representation

in human languages 23

2.4 Semantic components and syntactic exponents 31 2.5 Grammatical case, thematic role and grammatical

function 35

3.1 Typical phonological errors in Wernicke’s aphasic speech 50 3.2 Complementary symptoms of Broca’s and Wernicke’s

aphasia 51

3.3 Components of the ERP response 61

4.1 Fodor’s criteria for modularity 71

4.2 Production plant states of operation 75

4.3 Competing approaches to language modelling 84 5.1 Properties distinguishing phonetic and phonological

representations 107

6.1 Tendencies towards right-ear advantage in dichotic

listening 116

6.2 Results of three gating experiments: percentage of

responses up to vowel offset 137

8.1 Disorders of auditory processing and word recognition 160 9.1 Form–frequency relations in English past tense 183 9.2 Test conditions and morphological priming effects 185 9.3 Morphological and semantic relatedness priming effects 187

9.4 Morphological type and priming effect 187

10.1 Some meanings ofshowand (scrambled) contexts

of usage 200

10.2 Searching semantic space for commonalities of word

meaning 204

10.3 TLC’s responses to word-pair meaning comparisons 205 10.4 Prime–probe relations used by Mosset al.(1995) 213 10.5 Triplet stimuli used in semantic judgement task (Tyler

et al.,2004) 218

11.1 Semantic feature specification 222

11.2 Semantic feature assignment 223

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11.3 Semantic similarity scores 223 11.4 Types of semantic relation between word pairs 224 13.1 ERP effects of phrase structure and argument structure

violations 289

13.2 ERP effects of Jabberwocky sentences 291

14.1 Sentence types used in Zurifet al.’s (1972) study 301

14.2 Sentences from Linebargeret al. (1983) 305

14.3 Theories of receptive agrammatism 311

14.4 Trace violations and ease of detectability 314 14.5 Sentence types used in Caplan and Waters (2003) 321 15.1 Sentence-level discourse (focusing) devices 340

15.2 Examples of discourse connectives 341

16.1 Discriminant function analysis 353

16.2 Categories of communication failure (Docherty

et al., 1996) 355

16.3 Tests of executive control and semantics (Barrera

et al.,2005) 359

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Preface and acknowledgements

This book is intended as a self-contained introduction to the study of the language–brain relationship for students of cognitive science, linguistics and speech pathology. The essentially interdisciplinary nature of the subject matter posed considerable difficulties for the author and will likely do so also for the reader. So please be warned. Despite my considerable efforts to keep the path- ways open between the villages of the cognate disciplines concerned, the jungle is everywhere and its capacity for re-growth is relentless.

As appropriate for an introductory text, the book is accessible to a wide reader- ship. Foundational concepts and issues on the nature of language, language processing and brain language disorders (aphasiology) are presented in the first four chapters. This section of the book should be complementary with many stand- alone introductory courses in linguistics, psychology or neuroanatomy. Subse- quent sections deal with successively ‘higher’ levels of language processing and their respective manifestations in brain damage: speech perception (chapters 5–8);

word structure and meaning (lexical processing and its disorders; chapters 9–11);

syntax and syntactic disorder (agrammatism; chapters 12–14); discourse and the language of thought disorder (chapters 15–16), followed by a brief final chapter, speculating on unsolved problems and possible ways forward. Each major section of the book begins by posing the principal questions at an intuitive level which is hopefully accessible to all. The often quite specialized research methods by which answers to these questions have been sought are then introduced, in a selective review of the literature.

The field of relevant studies was broad to begin with and has grown vastly since the pioneering studies in psycholinguistics, neurolinguistics and computational models of language processing were undertaken in the 1970s and surveyed with such flair and scholarship in Caplan’sNeurolinguistics and linguistic aphasiology:

An introduction(1987). It would be an impossible task to update Caplan’s seminal text in a single volume. Yet that was one of the quixotic goals that originally motivated the writing of this book. So, in each of the major topics that are taken up, the aim is to bring the reader to a view of the problems and issues that animate contemporary research. In this sense, the book is intended as an ‘introduction’ to the field and as such may serve as a resource for an advanced undergraduate or first-year graduate seminar.

It is difficult to date precisely the origins of this book and therefore to duly acknowledge the many people who have contributed towards it. But officially it

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began life as a collaboration with Helen Chenery, under the enthusiastic men- torship of Christine Bartels of Cambridge University Press. Helen’s ghost-like presence can be detected in the persistence of the authorial ‘we’, a writing habit that I evidently found hard to break and a device that I may be guilty of deploying at times, to persuade the reluctant reader to my point of view on matters of deep uncertainty. I am grateful to both of them for their support and wise editorial counsel, especially through the difficult early stages, where something is taking shape, but God knows what the outcome will be and the enormity of the task ahead is beginning to sink in.

Neil Smith read the entire manuscript – not once, but twice – and offered many invaluable and always tactfully put suggestions. I am greatly indebted also to Lucy Carolan, whose impeccable stylistic judgement greatly improved the readability of the text. Max Coltheart and Stephen Crain read selected chapters and offered cogent feedback. Thanks particularly to the students who read drafts of these chapters and in some cases showed in their term essays how the story could be better told. Teaching can be a humbling experience and nothing motivates hard thinking like the blank stares that can accompany the presentation of one’s latest pearls of wisdom. Most importantly, I would like to thank my wife, whose name appears in the dedication, for putting up with a distracted fool for several years of late nights and the squeaking chair in the wee hours.

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Note on the text

Words in bold type are explained in the Glossary (pp. 380–6 below).

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PA RT I

Foundational concepts and issues

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1 Introduction and overview

Introduction

This book is about language processing in the human brain and, more specifically, what happens to spoken language when certain areas of the brain are damaged. Language processing is what takes place whenever we understand or produce speech; a mundane task, but one of extraordinary complexity, whose mysteries have baffled some of the greatest minds across the centuries.

Neurolinguistics is the technical term for this field, introduced into academic usage by Harry Whitaker (1971), who founded the leading journal that bears this title. As Whitaker noted at the time, it is a key assumption of neurolinguistics that ‘a proper and adequate understanding of language depends upon correlating information from a variety of fields concerned with the structure and function of both language and brain, minimally neurology and linguistics’. Today, some thirty years later, it seems necessary to add ‘cognition’ or cognitive science to the list of minimally necessary disciplines. A well-articulated cognitive science is needed to provide the hoped for integration of two otherwise very different fields of study: language and neurobiology.

Considerable progress and a vast body of research have accumulated since then. Yet leading advocates of the cognitive science perspective on language as a biologically grounded human ability (such as Chomsky, Pinker and Deacon, to mention just three) disagree on some fundamental questions. To what extent are our language learning capabilities ‘hard-wired’ into the human brain and unique to the species? How is ‘innate linguistic competence’ actually deployed in language learning? Is it closely bound to specific stages of neurological maturation or can it be re-invoked in maturity for second language acquisition or recovery of language competence after neurological damage? To what extent are the component skills activated in language processing separable from one another in function or in actual locus of operation in the brain? To what extent are language abilities separable from thinking or other mental activities?

Assuming at least some ‘modularity’ of language and its supporting cognitive, perceptual and motor competencies, a number of highly practical questions arise.

Can recovery of language following brain injury be facilitated by therapy inter- vention strategies targeted at specific retained abilities in order to work around lost competencies, or can those lost competencies themselves be recovered?

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Despite the controversies and profound uncertainties concerning the best way forward, there are good reasons for believing that a special relationship exists between human language on the one hand, and what makes human brains different from those of other mammals or our close primate relatives on the other. In this chapter we offer some arguments intended to establish a direct link between the brain and language, through an appeal to the concept of co-evolution of brain and language (Deacon,1997a): the idea that language abilities arose as both a consequence and a cause of recent and rapid evolutionary brain changes, resulting in the emergence of homo sapiens. In chapters2and3we invite you to evaluate the language–brain relationship for yourself, as we describe the language faculty in broad outline from the separate perspectives of the linguist (chapter2) and the aphasiologist (chapter 3). Linguists are trained to analyse patterns of language production and usage, with the aim of unravelling the complex code which enables speakers and listeners to map between sound and meaning. Aphasi- ologists observe the great variety of communication disorders that can arise as a consequence of damage to the language areas of the brain by strokes, tumours or traumatic injury. By and large, the classical studies of aphasia were conducted by neurologists and neuropsychologists who had no specialized linguistic train- ing. Similarly, linguists formulated their theories of human language indepen- dently of any serious considerations of language loss in aphasia. Thus, Whitaker’s (1971) assertion that progress in the study of language depends on some suc- cessful synergy between linguistics and neurology has always been controversial, and so the introductory chapters of this book should be regarded as a first approximation at defining a ‘problem space’ – the language–brain interface. In subsequent chapters, we explore in detail the various stages of language processing, from the decoding of phonological targets in the perception of speech, to word recognition, morphological analysis, syntactic parsing, semantic inter- pretation and understanding discourse. We consider the production of language and production disorders in aphasia only insofar as they throw light upon the nature of the brain’s language processing mechanisms. At the ‘higher’ levels of language processing, a clear distinction between the mechanisms underlying language comprehension and language production is difficult to maintain, despite the fact that the task demands imposed upon listeners and speakers are very different. Speakers and listeners clearly must share a common linguistic knowledge base – a grammar in the broadest sense of the term – but just how that tacit knowledge is deployed in comprehending and speaking is a moot point.

Our concern is primarily with language comprehension and its disorders. How- ever, the neural mechanisms that the brain has evolved for language processing are based, at least in part, upon novel synergies that have evolved between the motor control and the auditory perceptual systems. These synergies are needed for imitation learning of rapid gestural sequences for speech production and perception. Consider, for example, the utility of a vocal communication system that required 20-plus seconds to say: ‘Look out, you are about to step on a snake!’

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Co-evolution of language and the brain 5 We shall consider the evidence for the neural synergy between speech production

and perception in subsequent chapters.

Language is used not only to convey our thoughts and feelings to others, but also to represent them to ourselves. But thinking is not equivalent to talking to oneself, and the linguistic expressions with which we clothe our thoughts are merely signposts to meaning, not explicit representations of those meanings.

Linguistic expressions are under-determined with respect to the message the speaker intends to convey.

Trying to understand how the brain processes language may always lie just beyond the realm of scientific feasibility. But for the sake of thousands of people every year who suffer the traumatic effects of language loss through aphasia we are obliged to make our best effort. Cognitive neurolinguistics has its origins about as far back as one chooses to trace them, from Aristotelean speculations in the third century BC on the nature of words and ideas, or from Broca’s (1861) famous observation that ‘the seat of articulate language lies in the left posterior frontal convolution’, or from Chomsky’s programmatic reformulation of the goals of linguistics as a branch of cognitive science in the 1960s. But the most significant developments in the field have occurred in only the past three decades. Psycholin- guists and neuroscientists have devised behavioural and neuroimaging techniques to fractionate the different stages of language processing: from the instant the auditory system reacts to the acoustic signal of speech, to the few hundred milliseconds that it takes to complete linguistic decoding of the speaker’s message. Most recently, powerful neuroimaging techniques have potentially greatly enhanced our powers of observing ‘real-time’ language processing. The extent to which this potential will be realized in the near future largely depends upon how well the new imaging techniques can be harnessed to the ‘on-line’¹ methods and theories of language processing developed by psycholinguistics over the preceding three decades. There is cause for cautious optimism that we may be on the threshold of new insights into language and the human brain–language relationship, which enables us to communicate with one another a range of ideas, worries, conjectures, desires or demands, unknown to other species, regardless of whether we believe them capable of entertaining such things.

Co-evolution of language and the brain

It is uncontroversial, in scientific circles at least, that the human brain has undergone very rapid growth in recent evolution. The brain has doubled in size in less than one million years. The cause of this ‘runaway’ growth (Wills,1993)

1 ‘On-line’ refers to observational methods that are intended to capture sentence processing as it takes place in ‘real time’, as distinct from ‘off-line’ observational methods, which are not time- sensitive, that tap into comprehension or production processes after the fact, or after they have taken place. Grammaticality judgements or judgements of semantic well-formedness are examples of ‘off-line’ tasks.

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is a matter of conjecture and endless debate. A strong case can be made that the expansion of the brain was a consequence of the development of spoken language and the survival advantage that possessing a language confers. The areas of the brain that underwent greatest development appear to be specifically associated with language: the frontal lobes and the junction of the parietal, occipital and temporal lobes (the POT junction – more of this later).

It is easy, perhaps all too easy, to reconstruct plausible scenarios illustrating the survival advantages that possession of a hands-free auditory/vocal means of communication with the symbolic power to represent almost any imaginable situation would confer on a social group. Perhaps it was the superior linguistic abilities of homo sapiens, with brains and vocal tracts better adapted for speech and language, that led to the rapid displacement and extinction of the Neanderthals in Europe, some 40,000 years ago (Mellars,1996). Language is of such importance in our daily lives and culture that it is almost impossible to imagine how our species could survive without it.

But perhaps the most surprising thing about the evolution of language and the brain structures required to support it is – as indicated earlier – how rapidly they were acquired by our species. It is well known that quite dramatic phenotypical changes can take place under adaptation pressures in relatively short periods of evolutionary time. However, there appears to be no parallel in other species to the rapid increase in cranial capacity accompanied by the signs of an evolving material culture that one finds in the human archaeological record. What drove this massive yet selective increase in brain tissue, confined mainly to the cerebral cortex and to some regions more than others? According to the co-evolution hypothesis, it was the voracious computational requirements of a symbolic representational system, i.e. of a language. It is not difficult to appreciate this point. Just look up from the book and cast an eye around the myriad of recognizably distinct objects in your immediate field of view. A large proportion of them have names. All the others can effectively be provided with names by verbal constructions such as: ‘low radiation energy sticker’ for the object fixed to the screen monitor casing of my PC. Language, as every language user knows, involves a kind of doubling of our perceptual universe. For every object of experience, there is at least a name or a naming construction to represent that object. Once the germ of a representational system has implanted itself in the mind/brain, there is no quarantining its spread to the whole realm of imaginable experience. This is evident from the period of explosive vocabulary growth that occurs in normal human infants around two to three years of age, for which there is no parallel in even the most loquacious of the signing chimps that have been studied (Savage-Rumbaugh and Levin,1994).

The voracious growth of a representational system is also movingly illustrated in the diary of Helen Keller, the remarkable woman, rendered blind and deaf in infancy, who suddenly discovered the representational function of tactile signs at an age when she was old enough to consciously appreciate their communicative significance. Everything suddenly required a name.

While the origins of language remain obscure, the co-evolution hypothesis claims that once the seeds of a symbolic representational system were sown, the

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An alternative view of co-evolution 7 brain responded with a vigorous and unprecedented increase in its processing

and storage capacity. According to the co-evolution hypothesis, the brain as a system which supports representational computation cannot remain ‘a little bit pregnant’ with language. ‘Representational computation’ is perhaps an awkward way of saying ‘thinking with language’. Representational computation conveys the idea that thinking supported by linguistic expressions involves a second- order level of manipulation, not just of objects, events or states of affairs, as perceived or imagined in ‘the mind’s eye’, but also the manipulation ofsymbolic representationsof those objects, events or states of affairs. Thus, perception and episodic memory provide a first-order ‘internal’ representation of the ‘external’

world. But language users have access to a second-order and publicly shareable level of symbolic representation, whereby objects of perception are coded as linguistic expressions.

In addition to linking the evolution of language to symbolic reasoning – an idea which has a respectable philosophical pedigree in European philosophy (von Humboldt,1999(1836); Cassirer,1953,1962; Werner and Caplan,1963) though not widespread acceptance in contemporary cognitive science – the co-evolution hypothesis asserts that a quantal increase in the brain’s processing capacity was required to accommodate this second-order representational system. Also, that although the evolutionary adaptation of the brain took place in incremental steps, the pace of change was such as to produce a qualitative new step in speciation.

Furthermore, the co-evolution hypothesis asserts, controversially, that thinking- with-language is a unique facility of human brains. Deacon’s (1997a) book-length exposition of the co-evolution hypothesis is a bold and controversial idea. It has met with a very mixed reception from linguists, depending on their theoretical orientation (Hudson,2001; Hurford,1998; Poeppel,1997). As a scientific hypothesis, it is rather too difficult to prove or to refute. We offer it here primarily to set you thinking along the paths we wish to explore in this book. Norman Geschwind in the 1960s (see chapter3) was the first to offer a clear account of how recently evolved cortical structures that distinguish humans from primates enabled the formation of extensive networks of cross-modal associations, which in his view provided the neural-computational basis for vocabulary formation, and hence the evolution of a natural system of symbolic representation.

An alternative view of co-evolution

Another reason for believing that the joint study of brain–language relationships will be productive derives from the study of language itself and how it is acquired. Language, as we shall presently discover (if you have not done so already), is the most complex of human artefacts,² re-invented by each suc- cessive generation of language learners, who are quite unaware of the enormity

2 artefact: tool or human construction. Language is a cognitive rather than a physical artefact; a vessel for containing or carrying meanings.

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of their accomplishment. Linguists like Noam Chomsky have long argued that young children can only accomplish the remarkable feat of learning their native language by virtue of inheriting some specialized neural machinery specifically designed for that task. The reference here is to Chomsky’sprinciples and param- eters(P&P) model of grammar (Haegeman,1991; Radford,1997). The principles are structural properties to which all languages supposedly conform, constituting auniversal grammar(UG). The parameters define the ways languages can vary from one another. The idea is that if a large part of the structural complexity of human language is pre-programmed into structural principles, then language learners have only to discover the parameter settings appropriate for their language community. Thus, the ‘principles’ set limits on how human languages may vary, confining natural languages to a restrictive set of possible types, thereby narrowing the ‘search space’ of the language learner. Furthermore, if a special

‘parameter setting’ mechanism for language learning can be invoked, then it is easier to see how first language acquisition could be under the control of ‘instinc- tive’ maturational mechanisms, by analogy to such behaviours as nest building in birds or ‘learning to walk’ in mammals. In this way, a language faculty can be con- ceived as a special-purpose module of the mind/brain, dedicated to the demands of spoken language communication and acquired through special learning mechanisms linked to the maturation of perceptual, motor and cognitive systems of the infant brain.

Clearly a great deal of investigative groundwork is needed to isolate the principles and parameters that underlie natural languages and to then show how such principles and parameters may be incorporated into a model of first language acquisition.³But this is precisely what linguists and psycholinguists in the Chomskian paradigm seek to do. We cannot evaluate the P&P theory until we have elaborated at least a first approximation model of language structure, which we will begin to do in chapter3, and elaborate with respect to a specific theory of agrammatism in chapter14. The P&P theory of language is in fundamental respects antithetical to the idea, advanced in theprevious section, that language is an undifferentiated ‘symbolic system’. Nevertheless, P&P theory also provides an alternative formulation of the co-evolution hypothesis that the emergence of natural language drove the most recent ‘runaway’ stage of evolution of the human brain, albeit a formulation with a very different conceptual foundation as a modular ‘faculty of language’.

We briefly sketch here in somewhat stark terms some differences in perspective between language as a symbolic system (as expounded by Deacon,1997a) and the P&P theory of language, which represents, if any one position can, the textbook orthodoxy of linguistic theory. We will elaborate the major theoretical issues currently in dispute in chapter 4, in the attempt to build a biologically grounded theory of language processing. Deacon’s model of language has been

3 Second language learning appears to be different in fundamental respects from first language acquisition. Parameter setting may only be available as a window of opportunity during the critical period of first language learning; or once set, parameters may not be re-set.

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An alternative view of co-evolution 9 described as linguistically naive. This may be true, but in adopting positions

opposed to the prevailing orthodoxy in linguistic theory, he finds allies in alternative models of language and language learning which have psycholinguistic credibility.

By enumerating the differences between the two perspectives we will generate some clear expectations as to where to look for significant theoretical alternatives, for contending hypotheses about how the brain might organize itself for language.

Firstly, there is the issue ofmodularity which expresses itself at the level of both broad and fine-grained mental architecture. At the level of broad mental architecture, Deacon’s view of language as a symbolic system draws no clear distinction between cognition and language processing. By contrast, while the P&P model is not very explicit on how the distinction should be drawn, it is recognized that there is a necessity to do so, if language is to be consistently viewed as a modular component in an integrated cognitive system. The existence of mental disorders specific to language processing (aphasia and aphasic disorders) would seem to argue in favour of modularity in the broad. But as we shall see, the history of aphasia is a battlefield littered with fallen standards of both houses in this unresolved dispute.

At the fine-grained level, within language itself, Deacon’s ‘symbolic system’ of language also draws no hard and fast distinctions between components of linguistic competence, such as the computational aspects (syntax) and the encyclopaedic (lexical) aspects of the speaker’s internal grammar. But in the P&P model, as indicated previously, the principles that govern structure building operations in the syntactic component of the grammar are quite distinct from the constraints that apply in the lexicon in word formation. We might expect, therefore, that language disorders in aphasia might fractionate along fault lines between modular components of the language faculty as described by P&P theory. Again, the modular view appears to be supported on superficial inspection of the syndrome of ‘agrammatism’ in Broca’s aphasia compared with the pattern of lexico-semantic impairment observed in Wernicke’s aphasia. But on closer inspection the association between the linguistic competence model and patterns of aphasic performance turn out to be deeply problematical, as we shall see. The P&P theory has been productive of a great deal of research into aphasia in recent years, but so too have non-modular language processing theories based on neural network models explicitly framed in opposition to the perceived prevailing linguistic orthodoxy.

Related but distinct from the question of modularity are issues oflearnability and how language abilities are embedded in the biological makeup of brain’s capacity for language. Deacon’s theory postulates a somewhat elusive propensity for ‘symbolic processing’ underlying our unique linguistic capabilities. He is at pains to demonstrate that some apes, like the celebrated Kanzi (Savage-Rumbaugh and Levin,1994), have this capacity also, to a limited extent. But this is quite a different standard of proof from showing that human infants (or the infants of some other species) have the capacity to spontaneously acquire languages whose syntax conforms to specific properties specified by a theory of UG, and, equally

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critically, aninabilityto learn the grammars of artificial languages whose syntactic rules violate principles of UG (Smith, Tsimpli and Ouhalla,1993).

Or to consider an example much closer to the themes of this book: a theory of UG might be expected to predict that at least some acquired language disorders in aphasia should reflect specific patterns of language impairment that are more or less isomorphic with the specific components of a grammar competence model. Grodzinsky’s (2000) trace deletion (or chain disruption) hypothesis of agrammatism is such a case in point, which we will consider in detail in chapter14.

Chomsky and the generative grammarians may be correct. On the other hand, we should also consider the possibility that human languages are just too complex, too diverse, or too contrary, to be reducible to a core set of principles and parameters. However, if the co-evolution hypothesis (in either of its competing versions) is correct, then we might hope to find independent confirmation of its validity by studying what Eric Lenneberg (1967), another pioneer in the field, called the biological foundations of language. This involves examining neurological structures that underpin language comprehension and production, correlating language acquisition with brain maturation in infancy, investigating loss of language caused by damage to various brain regions, and correlating the evolution of different brain structures across the species with the evolution of language. Some progress has been made in these endeavours. In this book we will focus primarily on two sources of evidence: (a) what can be learned about language and the brain from psycholinguistic and neurolinguistic studies of language processing in ‘normal’

language users (such as, dear reader, you and I), and (b) from clinical and experi- mental studies of those who have suffered neurological disorders or diseases which have impaired some or all aspects of their spoken language.

Language areas in the brain

Language is predominantly lateralized to the left hemisphere in the vast majority of people, even the majority of left-handers. While the functional asymmetries of the left and right hemispheres are well known and have been much debated in the popular and technical literature (Hellige,1993; Chiarello, 1998), anatomically, the structures of the brain appear to be quite symmetrical.

But the one known region where a structural asymmetry has been found occurs in theplanum temporale, which is part ofWernicke’sarea, the second language area, known after its discoverer Karl Wernicke in1874. The planum temporale of the left temporal lobe was found to be larger than its right hemisphere counterpart in 84 per cent of cases (Galaburda, Lemay, Kemper and Geschwind,1978). The reason why this rather unique asymmetry was not observed by previous generations of anatomists, though it is quite visible to the naked eye, is that the planum temporale is located within the fold of the sylvian fissure, out of sight from surface inspection of the temporal lobe.

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Aphasia as evidence of the brain’s representation of language 11

Figure 1.1 The cerebral cortex: the language areas and major anatomical landmarks

The functional significance of this long-overlooked cerebral asymmetry is no doubt related to the fact that the planum temporale overlaps with Wernicke’s area.

Aphasia as evidence of the brain’s representation of language

The study of aphasia, or the loss of language functions caused by damage to the ‘language areas’ of the brain, has been our major historical source of evidence for the study of brain–language relationships. We can trace the clinical study of brain–language relationships to Paul Broca’s (1861) famous discovery of the language area that bears his name, located in the posterior region of the left frontal lobe of the cerebral cortex. The precise role of Broca’s area in normal language functioning remains controversial to this day (see chapter9).

Disease or injury to the recently evolved regions of the cerebral cortex may be revealing of how language is organized in the brain. We can have various types of injury. Focal damage to a limited region may occur as a consequence of a

‘stroke’, when a blood vessel bursts or an artery is blocked and there is oxygen deprivation to some local region of the brain. Alternatively, damage may be more

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diffuse, such as occurs in Alzheimer’s disease. Here the disease process affects the inter-connectivity of nerve cells over a wide area of the cortex, usually starting in the medial regions of the temporal lobe (affecting word retrieval) and later spreading to other areas of the cortex and affecting language more profoundly (particularly semantic processing). Focal brain damage is more likely to affect specific language functions. The classical nineteenth-century discoveries of the language centres were made as a result of observing the effects of focal brain lesions.

The language faculty (localization and modularity) Are different aspects of language located in different regions of the brain (thelocalizationhypothesis) or are language abilities distributed throughout the whole brain (the holistic view)? The localization hypothesis, you may know, goes back to the pre-scientific theories ofphrenology(Gall,1809), the study of skull shapes, which tried to associate mental abilities with bumps on the skull.

Phrenology has long been discredited. Bumps on the outer surface of the skull are not a reliable guide to the relative size or shape of the underlying brain regions.

Nor is there solid evidence that local variations in size and shape of the cerebral cortex can be associated with mental abilities or psychological traits.⁴ This is aneurolinguisticquestion. It bears upon the relationship between the brain and language functions. Can different components of linguistic competence be separated out from one another and be seen to function more or less autonomously (the modularity hypothesis)? Or is it the case that different aspects of language and language-related cognitive functions interact with one another to such an extent that they cannot be separated (the interactionist position)? This is apsy- cholinguisticquestion. It bears on the relationship between language and mind (or language and cognition).

We shall explore both of these issues in some depth in subsequent chapters as we investigate different aspects of the complex chain of events that underlie spoken communication and the disruptions to it that can occur when the brain machinery is damaged. There are many unsolved mysteries involving the relationship between physiological processes of the brain and psychological processes of the mind which constantly threaten to derail our inquiry. Most of what we calllanguage processingtakes place beyond the accessibility of conscious awareness. We fancy that we are aware of what we are trying to say as we speak, and we are at least partially aware of what others are saying to us, because we can frequently

4 On the other hand, convolutions of cerebral tissue and blood vessels may leave indentations on the insideof the skull that may be used as archaeological evidence of brain evolution (Jerison,1990).

Also, the search to correlate regional brain size with mental abilities continues, illustrated by the widely publicized recent report that Einstein’s brain, while unremarkable in other respects, was found to be unusually convoluted and 15 per cent larger than 100 control cases in both left and right inferior parietal lobes, an area ‘associated with spatial and mathematical reasoning ability’

(Witelson, Kigar and Harvey,1999).

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The language faculty (localization and modularity) 13

Figure 1.2 Phrenology model, according to L. N. Fowler. (1811–1896)

anticipate what they will say next. But for reasons of time-pressure if no other, we cannot possibly consciously monitor more than a small fraction of the highest levels of decision making and control that are involved in spoken communication.

For a start, consider that something of the order of a hundred individual muscles must be coordinated, within an order of hundredths of a second precision, when we speak; or consider that individual words from a mental lexicon of tens of thousands of items must be accessiblein real timewhen we listen to speech, and that complex syntactic structures must be constructed on the fly as words are put together – again, in real time, as sentence meanings are extracted from word strings.

It is not hard to establish convincing links between the evolution of the brain and language. Classical studies of aphasia have provided suggestive models

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of how language abilities may be organized in the brain, though the evidence is by no means conclusive or internally consistent. It is very hard to declare with any degree of confidence that such and such a theory has been shown to be correct or incorrect about how the brain processes language. Faced with the complexity of spoken language processing and the more tangible and therefore more astonishing complexity of the brain, and given the still feeble investigative tools at our disposal (though great strides seem to have been made in the last two decades), any theory of language processing that we can currently envis- age is almost certain to be demonstrably wrong beyond the very short horizons of the foreseeable future. If current trends of technical and scientific advances continue, all currently competitive theories of language processing will probably seem ridiculously simplistic from a vantage point not far into the new millennium.

Faced with this dilemma, we have not tried too hard to present the reader with a theoretically coherent account of language processing, or to resolve conflicts and contradictions between contending theories. We will be well satisfied simply to expose them clearly for your scrutiny.

Study questions for chapter 1

1. What is ‘language processing’ about? What does on-line language processing entail?

2. What does it mean to say that language (or language processing) is a ‘modular’ ability? How would you establish that a given ability is modular?

3. What does the ‘co-evolution of brain and language’ hypothesis claim?

4. Explain what is meant by a ‘second-order’ representation of experience and how it relates to the co-evolution hypothesis.

5. How could one test the coevolution of brain and language hypothesis?

6. Is the co-evolution hypothesis falsifiable?

7. Is falsifiability a good or a bad thing in a scientific theory? Justify your position.

8. How does the linguistic concept of ‘universal grammar’ speak to the concept of a ‘modular’ faculty of language?