DO INDIVIDUALS WITH WILLIAMS SYNDROME HAVEBIZARRE SEMANTICS?

EVIDENCE FOR LEXICAL ORGANIZATION USINGAN ON-LINE TASK

Lorraine K. Tyler1, Annette Karmiloff-Smith2, J. Kate Voice1, Tassos Stevens1,Julia Grant2, Orlee Udwin3, Mark Davies4 and Patricia Howlin4

(lCentre for Speech and Language, Birkbeck College, London; 2MRC CognitiveDevelopment Unit, and University College London; 3West Lambeth Community Care

Trust, London; 4St. George’s Hospital Medical School, London)

ABSTRACT

Williams syndrome, a neurodevelopmental disorder, has attracted a great deal of debateconcerning the purported intactness of language in the face of other serious cognitive deficits.As more in-depth studies of specific aspects of WS language have emerged, the notion ofa preserved language module has been seriously challenged. Although WS vocabulary scoresare often impressive, several investigators have claimed that WS semantics are aberrant. Allstudies hitherto have been based on off-line experiments which necessarily involvemetalinguistic processes. This clearly affects the performance of individuals with cognitivedeficits. We report here an on-line study probing the semantic structure of the WS lexicon,using a task – semantic priming – which minimises metalinguistic demands. We show thatWS subjects display the same taxonomic/category and thematic/functional priming effectsas normal controls. The results are discussed in terms of the differences between receptiveand expressive language, as well as the fact that although semantic memory and the automaticaccess to semantic information for individual words is normal in WS, the integration ofsemantic information into sentence comprehension may be abnormal. The importance of on-line tasks to highlight such differences is stressed.

INTRODUCTION

Certain neurodevelopmental disorders are of particular interest to cognitivescience and cognitive neuroscience because of their resulting uneven cognitive-linguistic profiles. This is true of Williams syndrome (WS) (Beuren et al., 1964;Creery, 1953; Fanconi et al., 1952; Lightwood, 1952; Williams et al., 1961),which is due to a micro-deletion on the long arm of chromosome 7 affecting oneof the alleles of the elastin gene (Ewart, Morris, Ensing et al., 1983; Ewart,Morris, Atkinson et al., 1993), as well as other continguous genes such as LIM-Kinasel (Tassabehji et al., 1996; Frangiskakis et al., 1996). WS typically presentswith no evidence of focal lesions. Rather, differing patterns of brain growthduring embryo- and ontogenesis result in different brain volume proportions. WScerebrum is small, but the frontal cortex acquires a near normal volumerelationship to the posterior cortex (Jernigan, Bellugi, Sowell et al., 1993; Wanget al., 1992). Recent work, however, points to cytoarchitectonic anomalies inthe form of exaggerated horizontal organization of neurons within layers,

Cortex, (1997) 33, 515-527

increased cell packing density throughout brain regions, decreased myelination,and abnormally clustered and oriented neurons, particularly in visual cortex(Galaburda, Wang, Bellugi et al., 1994). Despite these abnormalities, WS is ofparticular interest to cognitive neuroscientists because of the resulting unevencognitive-linguistic profile.

WS has attracted a great deal of attention because of its pattern of relativelyspared language, face processing and social cognition, in the face of seriousdeficits in number, spatial cognition, planning and problem solving (Bellugi,Marks, Bihrle et al., 1988; Bihrle, Bellugi, Delis et al. 1989; Karmiloff-Smith,Klima, Bellugi et al., 1995; Mervis and Bertrand, 1997; Mervis, Morris, Bertrandet al., 1997; Udwin, Yule and Martin, 1986, l987; Udwin and Yule, 1991). Theverbal advantage is not always apparent in the early stages of languageacquisition (Udwin, Yule and Martin, 1987; Volterra, Capirci, Pezzini et al.,1996), but does tend to be found in older children and adults. In general, WSmean length of utterance is greater than one would expect for their mental age.Their language is often fluent and peppered with florid and erudite-soundingvocabulary items, not apparent in other mentally retarded groups (Reilly, Klimaand Bellugi, 1990; Volterra et al., 1996).

A review of the literature on WS language abilities reveals divergent fìndings.Morphosyntactic scores, as measured for example on the TROG (Test ofReception of Grammar, Bishop, 1983), were first reported to be excellent, butthis was on the basis of a very small population and on selected sub-items of thetest (Bellugi, Bihrle, Jernigan et al., 1990). Recently, however, a study offullscale TROG scores on a large WS population (Karmiloff-Smith, Grant,Berthoud et al., 1996) show them to be close to the same subjects’ non-verbalmental age on the Raven Coloured Matrices (Raven, 1986). By contrast, allstudies concur on the advantage that WS subjects show on vocabulary scores,which outstrip their non-verbal scores. None the less, their lexical abilities donot seem entirely normal. For example, although it is undeniable that typical WSadolescents and adults display a very rich vocabulary and fluent speech innormal conversational exchanges, their use of vocabulary items is often odd andrather pedantic (Rossen et al., 1996; Udwin, Yule and Martin, 1987). In fact,recent experimental work has suggested that WS subjects display abnormallexical semantics (Bellugi et al., l988; Rossen et al., 1996; Volterra et al., 1996)and deficits in lexical access (Bromberg, Ullman, Marcus et al., 1994; Pinker1994).

What is the nature of the lexico-semantic deficit? First, a number of studieshave pinpointed difficulties experienced by WS subjects in defining words theyfrequently use (Bellugi et al., 1988; Rossen et al., 1996). Second, Bromberg etal. (1994) have claimed that they have a word-finding problem. In a study ofpast tense and plural inflectional morphology in English, they found that WSsubjects performed better on regular plurals and past tense verbs than onirregular ones. By contrast, a study of word fluency in a group of WS subjects(Rossen et al., 1996) suggested that they had no difficulties in word findingbecause quantitatively they produced an equivalent number of words as controls.However, a qualitative analysis of the results showed that although early intrials WS subjects performed like normal controls, later word lists contained

516 Lorraine K. Tyler and Others

significantly more unusual, low frequency items (e.g., for the “animal” category:yak, pteranodon, ibex, vulture, etc.; for the “food” category: teriyaki, pumpkinpie, etc.; for the “bird” category: kite, cockatoo, goden eagle, spear hawk, etc.).

Further work on the purported semantic deficit has focused on homonymsand lexical ambiguity. A study by Rossen et al. (1997) used three ways ofassessing WS understanding of homonyms. The first was a simple associationtask, subjects having to produce the first word that came to their minds afterhearing each probe homonym. In a second condition, subjects heard three words(e.g., BANK RIVER MONEY) and were asked which two of them went togetherbest. The third was a definition task in which subjects were asked to telleverything they knew about the meaning of the target homonym. In every case,one of the meanings of the homonym was more frequent than the other meaning(see Perfetti, Lindsey and Garson, 1971, for norms). For the simple associationtask, WS subjects performed much like controls. By contrast, the results of theother two tasks showed that the WS subjects used the secondary meaning ofthe homonyms more frequently than either the normal 10-year-old controls orthe Down’s syndrome IQ-matched controls. While both the latter groups showeda preference for primary meanings, the WS subjects demonstrated a nearly equalpreference for both the primary and secondary senses of the homonyms. Thissuggests that WS subjects are less sensitive to frequency than normal controls, afinding corroborated by other studies (Karmiloff-Smith et al., 1996).

A final clue to the possible unusual semantic processing in Williamssyndrome comes from the work of Neville and collaborators (Neville, Mills andBellugi, 1994; Neville, Holcomb and Mills, 1989). In measuring neurophysiologicalcorrelates to semantic processing using Event Related Potentials, theyfound different temporal patterns in the brain activity of WS subjects ascompared to normal controls. Subjects listened to sentences that ended eitherappropriately semantically (e.g., I take my coffee with cream and sugar) or tosentences which were semantically inappropriate (e.g., I take my coffee withcream and radiator). The WS group displayed unusually large positivity at200-300 msec which is not seen in controls at any age. In addition, the WSsubjects showed an N400 response over the left temporal lobe that wasunusually large compared to normal controls. While this points to anabnormality of semantic processing, it says nothing about whether semanticinformation is structured abnormally, although Rossen et al. (1997) argue that iflexical processing shows malfunction, this is probably due to abnormal lexicalorganisation.

The present study addresses the question of the organisation of the mentallexicon in WS, and in doing so avoids certain problems inherent in most studieswith WS. To avoid the metalinguistic demands that obtain in many tasks used inthe past with special populations which may place additional cognitive demandson subjects (Tyler, 1992), we used a method which could specifically tap theautomatic activation of semantic information, thereby providing clues to the wayin which a subject’s lexicon is semantically organised. We examined two aspectsof semantic organisation: category structure and functional attributes. Categorystructure tells us whether the WS subjects have normally structured categories;that is, whether desk and chair, for example, are represented as belonging to the

Semantic organization in Williams syndrome 517

same taxonomic category. We focussed on functional attributes as a way ofexamining the semantic properties of concepts, chosing these attributes in viewof our recent research suggesting that they are especially salient and rapidlyactivated in the normal semantic system (Moss, McCormick and Tyler, 1997;Tyler and Moss, 1997), and relatively resistant to the effects of progressive braindisease (Moss, Tyler, Patterson et al., 1995; Tyler and Moss, 1997). Thus, theyprovide an important test case of the extent to which semantic representationsare normally represented. Several possible outcomes arise: (i) that WS semanticorganisation is normal for both category structure and semantic attributes; (ii)that it is predominantly organised around semantic attributes with little categorystructure; (iii) that it is predominantly organised taxonomically; (iv) that it is anunorganised list of lexical entries.

To address the question of how the WS lexicon is structured, we report datafrom a semantic priming task, comparing Williams syndrome and unimpairedsubjects. We use the semantic priming task to determine whether the semanticrepresentations of WS are intact because it primarily taps the automatic, asopposed to controlled, access to semantic representations (Moss and Tyler,1995). Most studies exploring semantic representations in brain-damaged ordevelopmentally disordered subjects use explicit tasks, where the subiect has toaccess semantic representations in a controlled, explicit manner. Such tasks,including sorting, word-to-picture matching, word definitions, synonyms,metaphor understanding, etc., can over-estimate the extent to which semanticrepresentations are impaired (Tyler, 1992). In contrast, tasks like semanticpriming, which probe knowledge representations implicitly, often reveal sparingof semantic knowledge which, on the basis of explicit tasks, is assumed to beinaccessible or lost (Tyler, 1988; Moss, Tyler, Hodges et al., l995b).

Semantic priming occurs when the recognition of a word is facilitated bythe prior presentation of a semantically related prime word (e.g., hat-glove;Fischlers, 1977; Moss, Ostrin, Tyler et al., 1995a). This facilitation is assumedto stem from the semantic overlap between prime and target. An important aspectof the priming task is that it is implicit in the sense that subjects are not requiredto explicitly retrieve the meanings of the words they encounter, facilitation isassumed to derive from semantic information automatically accessed whensubjects hear (or read) the prime word, to which they make no overt response.

However, the presence of a significant priming effect per se does notnecessarily mean that semantic representations are completely intact. Partiallydegraded semantic representations (Chertkow, Bub and Seidenberg, 1989) canalso produce priming, but this is usually indicated by a pattern of hyper-primingin which latencies are especially slow in the control condition leading to extralarge control-prime differences. To be reasonably confident that subjects’ primingreflects intact semantic representations, the size of the priming effect needs to bewithin the normal range. In addition, if subjects show priming for a range ofdifferent types of semantic relations, we are in a stronger position to infer thatthe underlying semantic representation is intact. For these reasons we testedthe WS subjects on a semantic priming task in which we compared priming fortwo types of semantic relationship: category coordinates (taxonomic relations)[e.g. lettuce-cabbage; coat-hat] and functional properties (thematic relations) [e.g.

518 Lorraine K. Tyler and Others

broom-floor]. A case study of a brain-damaged adult and 20 normal controlsusing the same paradigm has been described in detail in Moss et al. (1995a).

MATERIALS AND METHOD

Subjects

Williams Syndrome

Twelve WS subjects were tested, 5 male and 7 female, ranging in age from 14,3 to 30,5years (mean: 21.9 years). The mean IQ on the WISC-R or the WAIS (Wechsler, 1974, 1981,1992) was 63.1 (range 45-87). The mean mental age on the British Picture Vocabulary Scale(Dunn, Dunn and Whetton, 1982) was 10.6 (range 7.1 to 16.4). Test age on the Test forReception of Grammar (Bishop, 1983) was 6.9 (range: 5.3 to 11.0). Finally, on a spatialintelligence test (Raven Coloured Matrices, Raven, 1986), their mean non-verbal mentalage was 6.4 (range: 3.6 to 9.0). This range of scores is normal for a WS group (Udwin,Yule and Martin, 1987.

Controls

We compare the data from the WS subjects with that from the group of 20 subjectsdescribed in Moss et al. (1995a). These subjects were slightly older than the WS subjects(age range: 18-40 years), but Moss et al. (1995a, l995b) have shown that the same patternof priming is obtained across a wide age-range.

Materials

Moss et al. (1995a) chose 48 prime-target pairs to test priming for category coordinatesand functional relations. Twenty-four of the pairs were category coordinates; this is the typeof semantic relation which has been most widely used in the study of semantic representationsin normal subjects and patients with semantic deficits. Half of the coordinate pairs camefrom artefact categories and half from categories of natural kinds (e.g. lettuce-cabbage; coat-hat), all taken from familiar superordinate categories (Battig and Montague, 1969). Theother 24 pairs were nouns that were not members of the same category but shared afunctional semantic relation (e.g. theatre-play; umbrella-rain).

Primes and targets were closely matched across the two sets for the main variables knownto interact with priming-length, frequency, familiarity and concreteness. The coordinatesand functional pairs were also matched for strength of semantic relation between the primesand targets. This was achieved by means of a pre-test in which 17 subjects were asked torate the degree of relatedness between the pairs of words on a scale of 0 (very unrelated) to9 (very related). Table I shows the statistics for the two types of semantic relation.

Finally, degree of normative association between the prime and target was also controlledfor. Since normative association may be due to spread of activation between frequently co-occurring words at a pre-semantic level (Moss et al., 1995a; Glosser and Friedman, 1991),it is necessary to control for this variable so that we can separate priming due to semanticand associative relations. Thus, half of the pairs in each condition were strongly associated(mean = 40%) and half were not (mean = 1%). Association data were taken either fromPostman and Keppel (1970) or Moss and Older (1996). In sum, there were 12 items in eachcell of the design: related associated category; unrelated associated category; related associatedfunctional; unrelated associated functional; related non-associated category; unrelated non-associated category; related non-associated functional; unrelated non-associated functional.

Each target was paired with its related prime word and with an unrelated control word.Control words were selected by re-pairing targets with semantically and phonologicallyunrelated primes within the test set. In order to avoid repetition of targets within a test list,the materials were divided into two versions; in one the target was paired with a relatedprime and in the other it was paired with an unrelated control. Materials from all conditions

Semantic organization in Williams syndrome 519

were fully counterbalanced across the two versions, such that the WS subjects encounteredall 48 targets when tested on both versions.

Task

We established in pilot studies that the WS subjects could not reliably perform the twotasks that are widely used in semantic priming studies: lexical decision and pronunciation.However, they could perform the word monitoring task quickly and accurately (Marslen-Wilson and Tyler, 1980). We therefore used a primed monitoring task in which subjectspress a response key when they hear a pre-specified target word (see Moss et al., 1995a,l995b). Targets occurred as the penultimate item in a short word-list (varying in length from5-10 words) and was preceded by either a prime or control word. Table II provides examplesof the test word strings and catch trials. Timing pulses, which were placed at the onset ofthe target, started a timer which was stopped when the subject pressed the response key.

The filler words preceding the prime-target pair in the lists were all semantically unrelatedto the prime and target, and none started with the same phonological onset as the target toavoid anticipation responses. The length of the filler words was balanced across trials. Aset of catch trials in which there were several intervening items between prime and targetwere also included to discourage strategic responding (see Table II).

We established in pre-tests that the appropriate ISI for the WS subjects was 1500 ms.Previous data reported in Moss et al. (1995a) collected from normal controls showed thatthe same pattern of priming effects is obtained both with an ISI of 1500 ms and a muchshorter ISI of 200 ms.

Procedure

The materials were recorded by a female native speaker of British English and digitisedat a sampling rate of 20 kHz onto computer hard disk. The stored speech files were playedout onto DAT tapes.

Controls

Ten control subjects were tested on each of the two versions (20 subjects in total). Targetwords were presented on cards, one at a time, which stayed in front of the subject until theend of the trial. The spoken word lists were played out over headphones with 1500 msbetween each word. Subjects used their dominant hand to press a response key, situated infront of they, when they heard the target word. Monitoring reaction times were measuredfrom the onset of the target word.

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TABLE IStatistics of the Materials

Prime TargetCategory coordinates SPADE RAKE

CAT DOG

Frequencyl 44 40Familiarity2 5.5 5.3Concreteness2 5.9 5.8Semantic relatedness 5.8

Functional relations BROOM FLOORTHEATRE PLAY

Frequencyl 56 58Familiarity2 5.3 5.5Concreteness2 5.7 5.8Semantic relatedness 6.01 Mean frequency per million words (Hofland and Johansson, 1982). 2 Familiarity and concreteness, on a rating scale of 1-7 (Coltheart, 1981).

TABLE IIExamples of the Materials

Functional pairhen-farmrelated: larder unrelated: larder

sun sun health health essay essayhen clam FARM FARM chisel chisel

sweater-warm related: ghost unrelated: ghost

racket racket sweater pliersWARM WARM punch punch

Category pairmouse-hamsterrelated: bed unrelated: bed

sugar sugarbrick brickvivid vividhalf halfproceed proceedmouse clownHAMSTER HAMSTERfoot foot

Catch trialhouse-cottage

turnip house frostwaste sauce COTTAGE pencil

Williams Syndrome

Each subject was tested individually on both versions of the materialsl. Each target wordwas read aloud to the subject a couple of times by the experimenter and the subject repeatedthe word back before the list began. Subjects were instructed to keep their “favourite” fingeron the response key at all times. Eleven of the 12 subjects were tested on both versions ofthe materials with at least one week separating each test session. The remaining subjectwas only available for two days and completed the second version one day after the first.

RESULTS

Controls

The data are those reported in Moss et al. (1995b). There were 0.64%anticipations (responses made before hearing the target) and time-outs (responses

1 The WS subjects were tested on both versions of the materials while the controls were each tested on one version.In previous studies we have shown that this procedural variation does not affect the results.

Semantic organization in Williams syndrome 521

over 3 seconds) which, together with 3.7% extreme values (condition mean ± 2SDs), were replaced with the mean. Means were then calculated over subjectsand items and entered into two ANOVAs. There was a large priming effect forall conditions (F1 = 52.0; d.f. = 1, 18; p<.01; F2 = 94.0; d.f. = 1,39; p<.01) withmonitoring RTs to targets following related primes (366 ms) significantly fasterthan when following unrelated control primes (436 ms). The amount of primingfor associated pairs (70 ms) was similar to that for nonassociated pairs (73 ms;F1 and F2 < 1). There was also no difference in the amount of priming forcategory coordinates (67 ms) and functionally related items (75 ms; F1 and F2< 1).

Williams Syndrome

Most of the subjects were able to carry out the word monitoring task withoutdifficulty. Only two had abnormally high rates of anticipations and time-outs(36% and 23%). Since we considered it to be inappropriate to replace so muchmissing data, we did not include these subjects’ data in the analysis. The other10 subjects managed to perform the task satisfactorily. They made a total of4.3% anticipations and time-outs (range: 1-8%) and their RTs, although slightlyslower than those of the controls, were reasonably fast (mean: 547 ms; range:343-773 ms). Anticipations, time-outs and extreme values (mean ± 2 SDs) werereplaced by the cutoff value (mean + 2 SDs) for each subject. Just as for thecontrols, the WS subjects’ monitoring latencies were significantly faster in therelated (513 ms) compared to the unrelated (582 ms) condition (F1 = 19.19;d.f. = l, 9; p = 0.001; F2 = 37.04; d.f. = l, 40; p<.001). Moreover, there were nosignificant interactions (all Fs < 1), indicating that there was no difference in thepriming effects for associated vs nonassociated items (associated: 71 mspriming; nonassociated: 68 ms priming) and for functional (65 ms priming) andcategory relations (73 ms priming). The mean RTs in the experimental conditionsfor the controls and the WS subjects are shown in Table III.

These large and robust priming effects for the WS subjects were similar tothose of the controls. Both functionally related word-pairs and categorycoordinates primed strongly, indicating that a range of different kinds of semanticinformation is rapidly activated when a word is heard, and this information servesto prime subsequent words which are related in different ways. There are nosigns in these data that the organisation of the lexicon is significantly impairedin Williams syndrome.

DISCUSSION

Despite previous claims to the contrary, our data argue for normalorganization of the semantic system in people with Williams syndrome. Theydisplay the same pattern of priming for a range of semantic relations as shownby normal controls. Their reaction times were only slightly slower than normals,and there was no evidence of hyper-priming, i.e., very large priming effectsaccompanied by large RTs in the unrelated condition. Our on-line task enabled

522 Lorraine K. Tyler and Others

TABLE IIIMean Monitoring Latencies (ms) for the Controls and WS Subjects

Related Unrelated Difference

ControlsOverall 365 437 72Functional 370 445 75Category 361 428 67

WSOverall 513 582 69Functional 500 564 65Category 527 600 73

us to show that the automatic access to semantic representations is also normal,contrary to claims made by other researchers on the basis of off-line tasks(Bromberg et al., 1994; Pinker, 1994). However, Bromberg et al.’s (1994)conclusions were based on the finding that WS subjects only had difficultyaccessing irregularly inflected forms in production; their ability to accessregularly inflected words was normal. Therefore, although we cannot definitivelyconclude from our study that the automatic access of all word meanings is intactin WS, we can conclude that the structure of semantic memory is unimpaired inWilliams syndrome, contrary to claims by Rossen et al. (1996).

This does not mean, however, that there are no semantic impairments inWilliams syndrome, or that differences can be explained away by the differingdemands of on-line and off-line tasks alone. Two additional lines of researchsuggest that individuals with WS go about language acquisition and languageprocessing in ways that differ from the normal path. For example, Mervis,Morris, Bertrand et al. (1997) compared the abilities of WS and normal childrenon four related phenomena: pointing, exhaustive sorting, spontaneous fastmapping, and the onset of the vocabulary spurt. Exhaustive sorting involves theseparation of a set of objects from various categories into groups of objects of asimilar basic-level category. The fast mapping constraint stipulates that novelwords map onto objects for which the child does not already have a name(Golinkoff et al., 1994; Mervis and Bertran, 1997). The vocabulary spurt usuallyoccurs in normal development after a period of slow learning of new words untilthe child’s lexicon reaches about 100 to 150 words, at which point the rate ofproducing new words increases suddenly and exponentially. The relationbetween pointing and naming, cross-sectional and longitudinal data fromnormally developing toddlers and from children with Down’s syndrome indicatethat pointing precedes naming, i.e. naming builds on previously establishedreference. By contrast, WS children did not show this pattern: in Williamssyndrome naming preceded pointing by several months, suggesting that newwords may initially be little more than rote learned phonological sequences withshallow referential meaning. Further, in normal children and in those withDown’s syndrome, spontaneous exhaustive sorting coincides with the appearanceof fast mapping and with the vocabulary spurt, suggesting that word learning isrelated to conceptual development. Again, these capacities did not coincide inWilliams syndrome. WS children underwent some form of vocabulary spurt at a

Semantic organization in Williams syndrome 523

time when none of them yet showed signs of exhaustive sorting. Finally,although exhaustive sorting coincided with fast mapping in Williams syndrome,it was not until the children’s vocabularies were well beyond the vocabularyspurt and had reached around 500 words.

Subsequent studies of older WS children and adults show that they, too, goabout learning new words in ways that differ from normal controls. In a seriesof studies on lexical constraints, Stevens (1996; see also Stevens and Karmiloff-Smith, 1996), demonstrated that whereas normal children abide by four principleswhich narrow their hypotheses about new word meanings, WS subjects onlyabide by two of these. Like normals they show fast mapping and the mutualexclusivity constraint (the mutual exclusivity constraint stipulates that an objectcannot have more than one name, so if the child already knows one name for anobject, he or she will presume that the new word refers to another object present).By contrast, the WS subjects did not abide by the whole object constraint (whichstipulates that a novel label heard in the presence of a novel object refers to thewhole object rather than to its component parts), nor by the taxonomic constraint(which stipulates that in the presence of a novel word for an object, childrenwill, when requested for another X, choose an X from the same taxonomiccategory, and not one simply with the same colour, texture or shape fromanother taxonomic category). So although ultimately WS children’s lexicon maybe in part organised taxonomically, unlike normal children, they do not go aboutlearning new words constrained by taxonomic considerations.

A final line of research suggesting differences in semantic processing comesfrom work using Event Related Potentials (ERPs). These studies have shownthat WS people have a different pattern of brain activity in response to semanticanomalies than normal controls (Neville et al., 1994). If we assume that ERPspick up automatic processes involved in language use, then Neville’s resultsappear at first glance to be discrepant with ours. However, they may not be.Our study focused on the semantic organisation of words, and the automaticactivation of individual word meanings. Thus, we were looking at the processingof words in isolation and independent of their sentential contexts. In contrast,the ERP study measured subjects’ processing of words in sentences. Takingthese two studies together suggests that the structure of semantic memory isnormal in WS, and they do not have any difficulty in accessing word meanings.Where things start to look abnormal is when WS people try to integrate wordmeanings into due developing sentential representation as they process utteranceson-line, producing the abnormal pattern of ERPs in response to semanticanomalies. This interpretation is supported by other data showing far bettervocabulary scores than those measuring sentential understanding (Karmiloff-Smith et al., 1996). These various studies could all be related to the differentbrain organisation of WS people mentioned in our introduction. However, whatthe present research shows is that despite brain differences as compared to normalsubjects, the semantic organisation of the WS lexicon is functionally equivalent.

The fact that we find normal priming effects in WS, arguing for the normalorganisation of semantic memory, in contrast to abnormal performance on manyoff-line tests of semantic memory highlights the significance of using on-linetasks with this population. This point is reinforced by some follow-up testing of

524 Lorraine K. Tyler and Others

two of our WS subjects. We asked them for semantic relatedness judgements forthe same word pairs as used in the on-line primed monitoring task. In contrastto their normal priming, we expected the subjects to perform poorly on thesemantic relatedness task since it involves explicit, controlled access to semanticrepresentations. The two subjects were quite accurate on the unrelated pairs,indicating that they did not have an across-the-board “yes” bias (which wouldgenerate lots of errors in the unrelated condition). However, they made a largenumber of errors in the related condition, judging related pairs to be semanticallyunrelated. For unrelated word-pairs, the subjects made 2% and 11% errors,respectively. For the related condition, they made 29% and 25% errors. Overallthey made an average of 17% errors. Age-matched controls make overall between0% and 4% errors, suggesting that WS subjects are especially impaired whenthey have to make explicit judgements about word meanings.

The difference between normal priming effects and poor semantic relatednessjudgements clearly justifies the use of on-line task with this population andsuggests that WS subjects may perform badly to the extent that the languagetask used involves more cognitive demands. Off-line tasks may well over-estimate the extent of a problem, suggesting that impairments are more seriousthan they actually are (Tyler, 1992). Because of their uneven profile, it isimportant to investigate WS language abilities using tasks which minimisecognitive demands. However, we have to treat such a conclusion with somecaution because there are off-line language tasks on which WS subjects performwell; for example, scores on the British Picture Vocabulary Scale (similar to thePeabody in the USA) are often very high. So the fact that a task is off-linedoes not per se explain the differences between our language measures. Itremains to be shown which off-line tasks pose greater cognitive demands thanothers. It could be that the study of abnormal populations with uneven profilessuch as Williams syndrome will help to clarify such a task analysis and beparticularly useful for testing brain-damaged normal adult patients.

In conclusion, we argue that the attraction of on-line tasks lies in theircapacity to reveal normal semantic organization despite aberrant semanticprocessing-two facets of semantics which off-line tasks necessarily confound.

Acknowledgements. We wish to thank the Williams Syndrome Foundation, U.K. for theirsupport in helping us contact families and all the individuals who so generously participatedin our study. Mark Davies, Patricia Howlin and Orlee Udwin are also thanked for kindlyproviding us with the WISC-R/WAIS IQ and Raven data for two of the subjects includedin this study. The WS subjects were tested at the MRC Cognitive Development Unit andthe normal controls at the Birkbeck Centre for Speech and Language. The research waspartially supported by an MRC programme grant to Lorraine K. Tyler and William D.Marslen-Wilson.

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Lorraine K. Tyler, Centre for Speech and Language, Department of Psychology, Birkbeck College, Malet Street, London WC1E 7HX,U.K.e-mail: ubjta39@.ccs.bbk.ac.uk

(Received 20 March 1996; accepted 31 October 1996)

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