The Time Course of the Influence of Implicit Causality Information on Resolving Anaphors

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The Time Course of the Influence of

Implicit Causality Information on

Resolving Anaphors

Andrew James Stewart

Department of Psychology University of Glasgow.

Submitted for the Degree of PhD to the Higher Degrees Committee of the Faculty of Social Sciences, University of Glasgow.

January 1998

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Abstract

T his th esis asks w h eth er in itial an ap h o r p ro ce ssin g proceeds in a re s tric te d m an n er w ith referen ce only to a w ell d efin ed set o f in fo rm a tio n or w h e th er it is the case th a t all facto rs th a t are p o te n tia lly re le v a n t fo r reso lv in g an a n ap h o r e x e rt a p ro c e ssin g in flu en ce at the sam e tim e. In an attem p t to adjudicate betw een these possibilities, we focus on the nature o f the processing influence of implicit causality information on anaphor resolution.

F o llo w in g a sum m ary in C hapter 1 o f issu e s co n cern in g p o ssib le cognitive architectures and a review in C hapter 2 o f previous w ork on a n a p h o r re s o lu tio n , w e p ro p o se a tw o -s ta g e m o d el o f a n a p h o r reso lu tio n . W e p ro p o se th at the first stage in v o lv es co -in d ex atio n b e tw ee n a n ap h o r and a n te ce d en t and is in fo rm e d by lo w -le v e l facto rs. W e claim this stage behaves in a m o d u lar or re stric te d m an n er. T he se co n d stag e in v o lv es in te g ra tiv e p ro c e ssin g and

behaves in a nonm odular or unrestricted m anner. W e suggest that it

is at this second stage o f processing that im p licit causality influences anaphoric processing.

Im p lic it c a u sa lity (G arv ey and C a ram az za , 1974) is a p ro p e rty associated with a p articu lar set o f verbs w hich, in sentence fragm ents such as (1) and (2) below , influences in terp retatio n o f the am biguous pronoun.

(1) John fascinated Bill because he ...

(2) John blamed Bill because he ...

The verb ’fascinate' is classified as an NP1 b iasing verb as it biases tow ards the character occupying the first N oun Phrase as the locus o f cause. Sim ilarly, the verb 'blam e' is an N P2 biasing verb as it biases tow ard the ch aracter occupying the second N oun Phrase as the locus o f cause. R eaders p re fe r to in terp ret the p ro n o u n as co referen tial w ith the c h a ra c te r p re d ic te d by the v erb . P re v io u s w o rk has d em o n strated the in flu e n ce o f im p licit c a u sa lity in both lan g u ag e p ro d u ctio n (e.g. G arvey and C aram azza, 1974) and co m p reh en sio n

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(e.g. C aram azza, Grober, Garvey and Yates, 1977). A reading penalty arises w hen the inform ation in the subordinate clause co n flicts w ith the verb bias, i.e. w hen there is a m ism atch betw een im p licit and explicit causes, as in example (3).

(3) John blamed Bill because he hated Bill.

H o w ev er, se v era l m ajo r m e th o d o lo g ic a l c ritic ism s can be ra is e d against prev io u s w ork exam ining the influence on com prehension o f im p licit c au sality . V ariatio n s in facto rs such as sentence len g th , sentence p lau sib ility and non-hom ogeneity o f strength o f verb biases

may have confounded previous research. E xperim ents l a and lb in

this th esis w ere used to create m aterials co n tro lled for p lau sib ility and strength o f bias. A verage length o f the experim ental sentences was eq u ated across co n d itio n s. From an in itia l set o f 50 verbs ex am in ed in E x p erim en ts l a and lb , we se lec te d 24 th a t w ere strongly biasing and of equivalent plausibility for each cause.

A n i n i t i a l s e lf - p a c e d r e a d in g e x p e r im e n t ( E x p e r im e n t 2) d e m o n s tra te d an im p lic it c a u s a lity c o n g ru e n c y e ffe c t w ith o u r m aterials on w hole sentence reading tim es. E xperim ents (3) and (4) in v o lv ed p resen tin g the ex p erim en tal m aterials in tw o h a lv e s, w ith the split occurring follow ing the anaphor (see (4) and (5) below ). W e added an a d d itio n al b etw een e x p erim e n t fa c to r o f q u e stio n ty p e which encouraged either deep or shallow processing.

(4) John fascinated Bill because he/John was full of interesting stories.

(5) John fascinated Bill because he/Bill was easily entertained.

E ach se n ten c e w as p re s e n te d as tw o frag m e n ts w ith th e sp lit follow ing the anaphor. If im plicit causality exerts an early influence on pro cessin g we w ould expect to find evidence o f the co ngruency effect on reading tim es to the first fragm ent. The only e ffe c t we found on read in g tim e to the first frag m en t w as a re p e a t nam e p en alty resu ltin g from rep e titio n o f the first m en tio n ed c h a ra c te r's nam e. T his d id n o t in te ra c t w ith verb bias su g g estin g im p lic it causality does not influence in terp retatio n o f the anaphor w hen it is

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first encountered. R eading tim es to fragm ent 2 show ed an effect o f im p lic it cau sality . O ur betw een ex p erim en t m an ip u latio n led to a red u ctio n in the strength o f the im p licit cau sality congruency effect u n d er circum stances w here shallow p ro cessin g w as encouraged. In o th er w ords, the relativ e d ifficu lty asso ciated w ith reading sentence co n tin u atio n s going against the bias o f the verb was reduced w hen

readers w ere engaged in shallow er reading. The depth o f processing

m an ip u latio n had no e ffect on the m agnitude o f the rep eat nam e p en alty . T his suggests th at these phenom ena m ay be arising from processing at different stages within the system.

In E xperim ent (5) we attem pted to separate factors arising as a result o f th e in fo rm a tio n p re se n te d to the re a d e r in the ex p erim e n tal sentences from those factors which may have arisen as a result o f the m an n er o f this p resen tatio n . In form ation p rev io u sly p resen ted in a m a in -su b o rd in a te c lau se sen ten ce w as p re s e n te d as tw o sep arate sentences (see examples (6) and (7) below).

(6) John fascin ated B ill. T his w as becau se h e/Jo h n was full o f interesting stories.

(7) Jo h n fascin a te d B ill. T his w as b ecau se h e /B ill w as easily entertained.

W e fo u n d the im p licit cau sality co n g ru en cy e ffe c t in the pronoun conditions but not in the nam e conditions. W e suggest this m ay be due to the reader interpreting the repeat name anaphor as a them atic sh ift signal (cf. V onk, H ustinx and Sim ons, 1992) or perhaps as a resu lt o f increased inform ational load faced by the system . W e did n o t fin d a repeat nam e p en alty asso ciated w ith rep eatin g the first mentioned character's name.

E xperim ents 6 and 7 exam ine the influence o f im plicit causality under co n d itio n s w here there is a gender d ifferen tiatio n betw een the two c h a ra c te rs. E x p erim en t 6 em p lo y ed the sam e se lf-p a ce d read in g m ethodology as was used elsew here in the thesis w hile E xperim ent 7 employed an eye-tracking methodology.

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(8) John fascinated Mary because he was full of interesting stories.

(9) John fascinated Mary because she was easily entertained.

G en d er in fo rm atio n alo n e is su fficien t to id en tify the p ro n o m in al referen t. W e found h o w ev er that gender in fo rm a tio n is not used when a pronoun is first encountered.

E x p erim en t 8 is an a ttem p t to ad ju d icate b etw een two c o n flictin g positions in the literatu re concerning the tim e course o f the influence

of im plicit causality. M cD onald and M acW hinney (1995) propose that

im p lic it c au sality in flu e n ce s p ro cessin g as soon as a p ro n o u n is encountered w hile G arnham , T raxler, O akhill and G ernsbacher (1996) p ro p o se that the in flu en ce occurs during in teg ratio n . A num ber o f d ifferen ces ex ist b etw een the ex p erim en tal stru ctu res o f M cD o n ald and M acW h in n ey and G arn h am et al. W e re -c o n stru c te d o u r m a te ria ls along th e lin e s o f th o se e x am in ed by M cD o n ald and M acW hinney but still found no evidence for an early in flu en ce o f im plicit causality. W e argue that their finding arises as a result o f an experimental confound.

E x p erim en ts 8a and 8b p ro v id e us w ith an o ff-lin e m easu re o f another type o f verb bias we refer to as im plicit consequentiality (see exam ples (10) and (11)). Paralleling Experim ents l a and lb , we used E xperim ents 8a and 8b to create m aterials co n tro lled for p lau sib ility and strength of verb bias.

(10) B ecause H arold dreaded Justin, Justin/he / steadfastly refused to go back to school.

(11) B ecause H arold dreaded Justin, H arold/he / was told to try acting less aggressively.

In E xperim ent 10 we found a sim ilar pattern o f data to E xperim ent 3. V erb sem antics in the form o f im p licit c o n seq u e n tia lity in flu en ces

anaphoric processing during integration. On reading tim e to fragm ent

1 we found a rep eat nam e penalty o f the sam e type as has already been reported.

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W hen a gender contrast is present (cf. E xperim ents 6 and 7), we find e v id e n ce th a t g e n d er in fo rm atio n is u sed im m e d ia te ly if it can unam b ig u o u sly id en tify the pronom inal refe ren t. In line w ith our e x am in atio n s o f im p licit cau sality , we fin d ev id en ce th at im p lic it c o n se q u e n tia lity in flu e n ce s anaphoric p ro c e ssin g d u rin g in te g ra tiv e processing.

So then, verb sem antics influences p rocessing at the second stage o f our proposed m odel. The first stage o f anaphor resolution is inform ed by low lev el facto rs. G ender in fo rm a tio n only ex erts an in itia l p ro cessin g in flu en ce u n der conditions w here the pronoun appears as th e g ra m m a tic a l su b je c t o f a m ain c la u s e (i.e . in im p lic it consequentiality type sentences) and w here it is sufficient to identify the p ro n o m in al a n te ce d en t. V erb se m an tic s in flu e n ce s an ap h o ric processing at no point earlier than integration.

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List of Figures

Chapter 4

Figure 1 - anaphor type x referent 116

Figure 2 - verb bias x referent 117

Figure 3 - verb bias x referent (Name conditions) 118

Figure 4 - verb bias x referent (Pronoun conditions) 119

Figure 5 - m ain effect o f referent (repeat name anaphor) 130

Figure 7 - verb bias x referent (pronoun conditions) 132

Figure 8 - verb bias x referent (nam e conditions) 133

Figure 9 - referent x anaphor 134

Figure 10 - main effect o f referent (repeat nam e anaphor) 137

Figure 11 - verb bias x referent (pronoun conditions) 138

Chapter 5

Chapter 6

Figure 1 -verb bias x referen t interaction 195

Figure 2 - verb bias x referent interaction

(Pronoun co n d itio n s) 196

Chapter 7

Figure 1 - plausibility x NP 211

Figure 2 - referent m ain effect (repeat nam e anaphor) 216

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List of Tables

Chapter 4

T able 1 - E xperim ent l a continuation results 104

T able 2 - E xperim ent lb plausibility results com bined with

E x p erim en t l a co n tin u atio n s results 109

Table 3 - Reading tim es 115

T able 4 - Q uestion response tim es 120

T able 5 - Reading tim es for fragm ent 1 129

Table 6 - Reading tim es for fragm ent 2 130

Table 8 - Reading tim e for fragm ent 1 137

Table 9 - Reading tim e for fragm ent 2 138

Chapter 5

Table 1 - R eading tim es for first sentence 149

Table 2 - R eading tim es for second sentence 150

Table 4 - Reading tim es for first fragm ent 164

Table 5 - R eading tim es for second sentence 165

Table 6 - Q uestion response tim es 166

Table 7 - First-pass reading tim es for region 1 174

Table 8 - First-pass reading tim es for region 3 174

Table 9 - Total reading tim es for region 1 176

Table 10 - Total R eading tim es for region 3 176

Chapter 6

Table 1 - R eading tim es for first fragm ent 194

Table 2 - Reading tim es for second fragm ent 195

Chapter 7

T able 1 - E xperim ent 9a continuation results 205

Table 2 - E xperim ent 9b plausibility results com bined with

E x p erim en t 9a results 210

Table 4 - Reading tim es for second fragm ent 217

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Acknowledgements

I w ish to th an k Dr. M artin P ickering and Prof. Tony Sanford for supervising this thesis.

I also w ish to th an k E ugene D aw ydiak and K irsty D alrym ple fo r assistence in helping to run some of the experim ents. T hanks to the follow ing for discussion, advice and support : A ude O liva, A sifa M ajid, H olly B ranigan, R oger van G om pel, Sim on G arro d , A rlin a Im port, Frederic G osselin, Frances D uffy and everyone else in the Psychology Department Language Group.

A lso thanks to Petrea, Petrana, Jacqui and Joanna. Special thanks to

James Osterberg.

This research was supported by a BBSRC studentship.

Declaration

I declare that this thesis is my own work carried out under normal terms of supervision.

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CHAPTER 1

1.0 Overview

In this th e s is we set out to e x am in e how d iffe re n t ty p es o f in fo rm a tio n are u se d to re so lv e p ro n o m in a l and re p e a t n am e anaphors. The research is m o tiv ated both by th eo retical w o rk on anaphora and by g en eral p ro cessin g p ersp ec tiv e s w hich have b een

adopted by those w orking on parsing. C h ap ter 1 sets out several

th eo re tic al p o sitio n s w hich have b een c en tral in g u id in g p a rsin g research. W e focus on those that have arisen as a result o f adopting Fodor's M o d u larity th esis. W e sum m arise the m ain points o f this proposal and then set out w hat follow s from considering anaphoric pro cessin g as o p eratin g in a m o d u lar fash io n . W e draw e x p lic it p a ra lle ls b e tw ee n an ap h o ric p ro c e ssin g and acco u n ts o f p a rsin g . Broadly speaking, accounts o f parsing can be classified as those w hich

are m odular and those w hich are non-m odular in nature. W e apply

th is m eth o d o f c a te g o ris a tio n to p o s s ib le m o d els o f a n a p h o r processing.

C hapter 2 co n tain s a general overview o f th eo retical concerns and existing process m odels o f anaphor resolution. T his chapter contains an ex am in atio n o f th eo re tic al p o sitio n s in clu d in g the facto rs th at in flu en ce the choice o f anaphoric form , the role p lay ed by focus in fo rm a tio n on in te r p r e ta tio n o f a n a p h o rs an d how d if f e r e n t an ap h o ric fo rm s in te ra c t d iffe re n tly w ith the re a d e r's d isc o u rse

model. From a processing perspective we outline existing accounts o f

anaphor reso lu tio n w hich are in form ed by both low and high level factors. C hapter 2 concludes w ith the proposal o f a 2-stage m odel o f anaphor resolution.

The central question we are interested in is the tim e course associated w ith the in flu e n ce on anaphoric p ro cessin g o f stru ctu ral and non- stru ctu ral in fo rm a tio n . W e focus on one p a rtic u la r type o f non- structural inform ation : a type o f verb sem antic inform ation know n as im p lic it c a u sa lity . C h a p te r 3 c o n ta in s a sum m ary o f p re v io u s empirical work examining this phenomenon.

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O ur e x p e rim e n ta l ch ap ters (4 -7 ) ex am in e the o n -lin e p ro c e ssin g influence o f verb sem antics. A part from one experim ent (E xperim ent 7) w here we adopt an eye-tracking m ethodology, we m easure reading tim e using a self-paced read in g technique and attem pt to tem porally separate the processing influence of low and high level factors.

In C h a p te r 8 w e su m m arise o u r ex p erim e n tal fin d in g s and re evaluate our 2-stage model of anaphor resolution.

1.1 Introduction

T his th esis is an attem pt at ap p ly in g som e o f the th eo re tic al and e x p erim e n tal p rin cip le s w h ich h av e been p rev a len t in the p arsin g lite ra tu re to those aspects o f the lan g u ag e system asso c iated w ith reso lv in g anaphors. This c h ap ter sets out tw o im p o rtan t th eo retical positions w hich have guided research on sentence processing over the last 20 years. The first, F o d o r’s M odularity thesis (1983), concerns the overall conception of how co g n itiv e structure m ay be organised. T his fram ew ork provides us w ith a way o f em pirically exam ining how certain co g n itiv e processes operate. The second th eo retical p o sitio n outlines possible com putational differences in the way in w hich those

p ro cesses m ig h t operate. R oughly they may behave in a serial or

p a ra lle l fash io n . In itially w e shall focus on the g en eral p o sitio n o u tlin ed by F o d o r before d escrib in g the consequences o f p ro cessin g operating in a serial or parallel manner.

1.2 Theoretical Positions

1.2.1 Processing Architectures

Perhaps the m ost influential th eo retical account concerning its e lf w ith the n atu re o f the cognitive a rc h itectu re is F odor's M odularity thesis (Fodor, 1983). Any account o f cognitive functioning is faced w ith the

w orld know ledge problem . R oughly, the w orld know ledge problem is

that it is not possible to form ally capture the com plex nature o f w orld

know ledge and its influence on processing. M odularity m anages to

get round this by proposing th at initial processing operates only over a restric te d , w ell defined set o f in fo rm atio n and th at only at som e

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later point does w orld know ledge exert an influence. As this is absent during initial processing, if we know what inform ation is used w ithin a m odule it should be possible to explicate processing at the m odular level without having to formalise world knowledge.

M o d u larity is a re s tric te d p ro ce ssin g acco u n t. S ta te d sim ply it proposes th at only a restric te d set o f inform ation is used w ithin a m odule. F or any p ro cess, c o n sid er th at th ere is a larg e set o f inform ation w hich u ltim ately is used by th at process. A restricted account sim ply states that only a well defined subset o f this larger set is in itially used by the system . Only later does the rest o f this inform ation exert a processing influence. If we know precisely w hat type o f in form ation is used by the system at this in itial stage we should be able to form ally cap tu re how that in fo rm atio n in flu en ces processing.

A lte rn a tiv e ly , an u n re s tric te d a cco u n t p ro p o se s th a t all o f th e inform ation that can exert a processing influence does so as soon as it becom es available. In other w ords there isn ’t an initial stage o f the sy stem d u rin g w h ich on ly a p a rtic u la r w ell d e fin e d su b set o f inform ation is used. All o f the inform ation available to the processor at any point in time is utilised by the system.

The follow ing section includes a general outline o f w hat follow s from in terp retin g the co g n itiv e system w ithin the m o d u larity fram ew ork. T he in itia l sum m ary is la te r fo llo w ed by an in sta n tia tio n o f the m odularity thesis with resp ect to sentence processing. W e then draw p a ra lle ls b etw een acc o u n ts o f p arsin g and acc o u n ts o f an ap h o r resolution.

1.2.2 Modularity

A lthough the view th at the co g n itiv e system co u ld be decom posed into separate p ro cessin g dom ains w as im p licitly assum ed for som e tim e by m any research ers w ith in cognitive p sy ch o lo g y (e.g. F o rster 1979), F o d o r's M o d u la rity th e s is (1 9 8 3 ) se t fo rth a p re c is e ch aracterisatio n o f w hat follow ed from view ing the cognitive system as c o n s is tin g o f a d is c re te n u m b er o f s p e c ifia b le p ro c e s s in g

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com ponents. In doing so, it revived an older notion that considered th e d ifferen ce b etw een certain dim ensions o f c o g n itiv e fu n ctio n in g w ith in the faculty p sychology fram ew ork. F o d o r takes the faculty psych o lo g y p o sitio n as 'the view that m any fu n d am en tally d ifferen t kinds o f p sy ch o lo g ical m echanism s m ust be p o stu lated in o rd er to ex p lain the facts o f m ental life' (Fodor 1983, p. 1). One po ssib le dissociation betw een faculties is roughly that som e can be considered vertical while others horizontal. H orizontal faculties are those aspects o f cognitive functioning w hich exert an influence over all aspects o f m en tal life. E xam ples include m em ory and atten tio n . C onversely, vertical faculties are b est characterised with reference to their subject m atter, or th eir dom ain o f operation. A spects o f the language or the v isu a l p ro ce ssin g sy stem s co u ld m o st a cc u ra tely be d escrib e d as v e rtical facu lties as they are only o p eratio n al w ith resp ect to th eir p ro cessin g dom ains. The classificatio n o f v e rticality was e arlie r set out by Gall. The position not only considers that an aptitude such as one for m usic, say, is distinct from an aptitude for m athem atics, but also th at the p sy ch o lo g ical m echanism s u n d erly in g these c ap acities are also distinct. In other w ords, w ith reference to these exam ples G a ll’s position is th at there is a certain d elineable aspect o f m ental fu n ctio n in g th at is concerned w ith m athem atics and another separate aspect concerned with musical performance.

Fodor takes G all to task over an extreme vertical faculty position. Gall further proposed that there is no such thing as acuity per se, but only acuity w ith resp ect to som e aspect o f co gnition (e.g. visual acuity, auditory acuity etc). This stance is extended to apply to other aspects o f cognition such as m em ory. H ow ever, the fact th at an in dividual d isp lay s b e tte r m em ory for m athem atical p h en o m en a than lin g u istic p h e n o m e n a does n o t n e ce ssa rily w a rran t the c o n clu sio n th a t the in d iv id u a l p o ss e s se s se p a ra te m em ory sy ste m s fo r the d iffe re n t a sp e c ts o f c o g n itio n a sso c ia te d w ith m a th e m a tic a l o r lin g u is tic

perform ance. It may sim ply be the case that the processes concerned

w ith unravelling the m athem atical input do a b etter jo b o f producing an output o f a form easily represented by the m em ory system than do the analogous p ro cesses asso ciated w ith an aly sin g the lin g u istic input.

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H orizontal faculties such as m em ory or attention seem b est used as d escrip tio n s o f the o v erall m an n er by w hich the system o p erates. They ch arac terise som e n atu re o f the reso u rces av ailab le to the system as a w hole and a re n ’t in sta n tia te d w ith re sp e c t to any particular type of processing task.

Fodor recasts G all's p osition and distinguishes five central p ro p erties a sso ciated w ith c o n stru in g a m en tal fac u lty as v e rtic a l. T h ese p ro p erties d efin e a v ertical facu lty as dom ain sp ecific, g e n etic ally d e te rm in e d , a s s o c ia te d w ith d i s tin c t n e u ra l s tr u c tu r e s an d c o m p u tatio n ally autonom ous. F o d o r ex p lain s this last p ro p erty as follow ing from the position that the v ertical faculties do not com pete for h o rizo n tal reso u rces; in o th er w ords they do not com pete for reso u rces such as a tte n tio n and m em ory. T he k n o w led g e po o l accessible by a given vertical faculty is not the pool o f know ledge about the w orld, but rather know ledge pertinent to the functioning o f that faculty. A reform ulation o f the diagnostic criteria associated w ith vertical faculties form s the basis o f Fodor's proposal that the m odular p e rsp ec tiv e m ore acc u ra tely c ap tu res the n atu re o f the c o g n itiv e architecture. T hose parts o f cognition w hich F odor considers to be m odular are the input system s: aspects o f the cognitive system w hich form the link to the environm ent external to the organism , including

the language system . He proposes th at central processes act on the

output o f th ese m odules and it is only at this p o in t th at w o rld know ledge in flu en ces pro cessin g . T he in p u t system s are co n cern ed w ith p ro c e s s in g th e e x o g e n o u s in f o r m a tio n in to so m e fo rm in te rp re ta b le by th e c e n tra l p ro c e ss in g a sp e c ts o f th e sy stem . M odules can be view ed as determ inistic transform ation functions. In program m ing term s, functions take an input and produce an output. A function responsible for addition, say, w ill alw ays produce the sam e output given the same input. W e know that if we input the num bers 5 and 6, the output w ill be 11. W e know how addition works and we can form alise this. Sim ilarly, a p articu la r m odule receives in p u t A and p ro d u ces o u tp u t B. It can be c o n sid ere d as o p eratin g in a precisely d efin ed w ay, alw ays producing the sam e o u tput given the same input in a manner analogous to our addition function.

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In sum m ary, m odules transform an input in a d eterm in istic fashion; w ith the d eterm in ism d efin ed solely w ith re sp e c t to the in tern al w o rk in g s o f the m odule. T his n o tio n o f e n c a p su la tio n , o r o f com p u tatio n w ithout reco u rse to in form ation o utw ith the m odule is considered by Fodor to be one o f the central defining features o f w hat it constitutes for a processing com ponent to be m odular. T here are five such defining characteristics to which we’ll now briefly turn.

1. In p u t system s are dom ain specific. Each system operates over a p recisely d efin ed type o f input. T hey are tu n ed to p ro cess only information o f a certain type (such as linguistic information).

2. The operation o f input system s is m andatory. W hen an individual hears an u tterance it can't be perceiv ed as anything o th er than an u tteran ce, i.e. it c a n ’t be p erceiv ed as an u n in terp retab le stream of noise. T his autom aticity has been described by M arslen -W ilso n and T yler (1981) with respect to w ord recognition. Subjects cou ld n ’t help but id en tify w ords in the auditory stream even w hen ex p licitly told not to focus on that aspect of the stream.

3. T here is only lim ited central access to the m ental rep resen tatio n s that in p u t system s com pute. There are some levels o f representation within a module that are not available for conscious reflection.

4. In p u t system s are fast. This notion is intrinsically related to, and perhaps follow s from , characteristic (2) which states th at the m odules operate autom atically. Indeed, this type o f b eh av io u r w as taken by Posner and Snyder (1975) as a central defining characteristic o f w hat

they term 'autom atic' processing. T his speed m ay be a consequence

o f th e sm all, w ell d efin ed set o f in fo rm a tio n th a t n eed s to be co n sid ere d by the m odule b efo re an o u tp u t is reach ed . S im ply because th ere is less in fo rm atio n p ertin en t to p ro cessin g w ith in the m odule (see follow ing point), the solution is arriv ed at in a rapid manner.

5. In p u t system s are in fo rm atio n ally en cap su lated . The com putations carried out w ithin a m odule occur w ithout recourse to inform ation not re p re se n te d w ith in th at m odule. O nly the in fo rm a tio n w ith in a

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m odule in flu en ces p ro cessin g w ith in th at m odule. In form ation not contained w ithin the m odule can only influence p ro cessin g at some later point.

The above sum m arises w hat it m eans for p a rticu la r processes to be view ed as m o dular in n atu re. A fu rth er q u estio n can be asked

concerning the precise com putational nature o f these processes. They

may operate in either a serial or a parallel manner.

1.2.3 Serial and Parallel Processing _

A m b ig u ity in lan g u ag e is w id e sp re a d , c o v e rin g le v e ls o f w ord m ean in g , sy n ta ctic a n a ly sis and re fe re n c e . F ro m a p ro c e ssin g perspective some basic questions are com m on at each o f these levels. W hen an am biguity is encountered, how is it treated ? Is one possible in te rp retatio n selected rath e r than an o th er or is every in terp retatio n selected, eith er to the sam e or d ifferin g deg rees ? C o n sid er the follow ing exam ple w here an am biguous w ord is en co u n tered before its disambiguating context.

(1) The bug was found by the insect hunter/ security team.

(adapted from Swinney, 1979)

The word 'bug' is am biguous betw een an 'insect' reading and a 'spying

d evice' reading. W hen it is encountered the read er does not know

w hich interpretation w ill be correct. There are two possible ways in which processing of the word may proceed at this point. One m eaning o f the w ord could be selected, perhaps determ ined by the individual's p rio r experience w ith the w ord. Serial pro cessin g involves a single solution being proposed at any one tim e. In the case o f the above exam ple, if the 'insect' reading is initially proposed but turns out to be incorrect, the alternative interpretation is adopted.

A different account w hich we can term a p arallel processing account w ould po sit that both m eanings are accessed and held in m ind until d isa m b ig u a tin g in fo rm a tio n is ab le to s e le c t b e tw e e n th e tw o. Multiple solutions can co-exist.

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In the same way that w ords can be am biguous, so too can sentences. C onsider Exam ples (2) and (3) below (taken from T yler and M arslen- Wilson, 1977).

(2) Flying planes are dangerous.

(3) Flying planes is dangerous.

In Exam ple (2) the phrase ‘flying p lan es’ should be interpreted as a c o m p lex N oun P h ra se . In E x am p le (3) h o w e v e r it sh o u ld be interpreted as a V erb Phrase. W hen syntactic am biguities o f this sort

are encountered, how are they treated ? How does the parser decide

w hich in terp retatio n to select ? Is one solution in itially adopted or are m ultiple solutions h eld in m ind ? W hat inform ation is used to d ecid e b etw een a lte rn a tiv e s ? Som e stro n g p re d ic tio n s can be g e n e ra te d on th e b a sis o f w h e th er we c o n s id e r the p a rs e r as operating in a modular or nonmodular fashion.

1.2.4 Modularity applied to language processing

B roadly speaking, theories o f parsing fall into one o f two categories :

m odular and nonm odular accounts. M odular accounts propose th at an

in itial stage o f the p a rs e r behaves in a restric te d m anner, in itially sen sitiv e to only som e o f the in fo rm atio n p o te n tia lly relev an t for sentence processing. One o f the m ost influential parsing accounts, the G arden Path theory (F razier, 1979), proposes th at this inform ation is p urely syntactic. Stages follow ing this level o f processing m ay take ad v an tag e o f n o n -sy n ta c tic in fo rm atio n to ev alu ate w hat has been o u tp u t by th e sy n ta c tic a lly d e te rm in e d p re c e d in g stag e (F ra z ie r,

1987; Rayner, Carlson and Frazier, 1983).

W ithin m odular accounts o f parsing, at points o f syntactic am biguity in the input, the p a rs e r can m ake a co m m itm en t to one o f the p o ten tially av ailab le sy n ta ctic analyses or m ay p ro p o se analyses in p arallel (Gibson, 19??). D ecisions at this level can be m ade by the system using only syntactic know ledge (perhaps by parsing prin cip les re fle c tin g the n atu re o f the u n d erly in g sy n ta ctic stru ctu re o f the

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sen ten ce). C o n sid e r ex am p le (4) b elo w (tak en from T ru esw ell, Tanenhaus and Gamsey, 1994):

(4) The evidence examined by the lawyer turned out to be unreliable.

The verb 'exam ined' is am biguous betw een a past tense and a passive p articip al in terp retatio n (i.e. 'that was ex am in ed '). The anim acy o f th e p rec ed in g N oun P h ra se , 'th e e v id e n c e ’, p ro v id es in fo rm a tio n w hich ru les out the p a st ten se in te rp re ta tio n . M odular acco u n ts p ro p o se th at such in fo rm a tio n is n 't a v a ila b le in itia lly w ith in the system to help resolve am biguity (F erreira and C lifton, 1986). It is only at some point following that information of this type is used.

A lte rn a tiv e ly , n o n m o d u la r acco u n ts p lac e no re s tric tio n on w hen inform ation can exert an influence and p ropose that all in fo rm atio n th a t is a v aila b le to the p a rse r is a v a ila b le at the sam e (early ) processing stage. D ifferent sorts o f inform ation are considered to act as c o n stra in ts w h ich r e s tric t p o ss ib le sy n ta c tic a n aly se s. T he constraints do not qualitativ ely differ from each other although there m ay be quantitative differences in th eir relativ e contributing w eights. In the case o f E xam ple (4), those arguing fo r a co n strain t based p o sitio n claim th at in fo rm atio n ab o u t anim acy is available to the p arser to influence initial parsing. In other w ords the reader w ill not be g ard en p a th e d and w ill c o rre c tly in itia lly in te rp re t the verb

‘examined’ as a passive participal.

W e can also interpret the behaviour o f the p arser w ith respect to the serial and p a ra lle l p ro c e ssin g p o sitio n s o u tlin e d above. P a ra lle l m odular accounts allow for the p arser to co n stru ct m ultiple analyses w hich are then decided betw een using no n -sy n tactic inform ation at a

subsequent stage. In the case o f a serial p arser, w here only one

analysis is pursued at any one tim e, at points o f am biguity a decision m ust be m ade as to w hich analysis to adopt. M odular serial accounts p e rm it this d e c isio n to be m ade th ro u g h refe ren c e to o n ly the

syntactic structure o f the possible analyses. T hese accounts propose

th at w hen an am b ig u ity is e n c o u n te re d th e p a rse r em p lo y s the principles o f M inim al A ttachm ent and Late C losure in order to select

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reference to the p h rase-stru ctu re o f the analysis being co n stru cted . M inim al A ttachm ent proposes that the parser will p refer to co n stru ct the in terp retatio n that requires po stu latio n o f the m inim al n u m ber o f tree nodes w hile Late C losure proposes that the parser will p refer to incorporate a new phrase w ith the one currently being p ro cessed if gram m atically p erm issib le. T hese p rin cip le s co v er in itial p a rsin g . Only later does non-syntactic information exert an influence.

C o n strain t b ased accounts are p arallel p ro cessin g accounts as they propose th at a num ber o f analyses are pursued on the basis o f the available inform ation, although ultim ately only one is selected. B oth syntactic and non-syntactic inform ation is used by the system at the same time.

In sum m ary th en , m o d u lar acco u n ts o f p arsin g p ro p o se th a t only sy n tactic in fo rm a tio n is used im m ed iately . S em an tic in fo rm a tio n exerts an in flu en ce later. N on-m odular or constraint based accounts propose that all inform ation that is relevant exerts an influence at the same time.

So far we have only focused on am biguity at the level o f parsing. There is also am biguity at other levels o f processing language. The following section briefly outlines the level of referential ambiguity.

1.3 Parallels between Accounts of Parsing and Accounts of Anaphor Resolution

A naphoric pronouns refer to some previously m entioned ch aracter in a text. In o rd er to correctly in terp ret a pronoun, the read er m ust correctly identify to w hich character it refers. In the same way that there are constraints as to w hich syntactic analysis is possible in the case o f p a rsin g a m b ig u ities, th ere are also c o n stra in ts on w h ich reference assignment is possible in the case of referential ambiguity.

(5) John fascinated Mary because he was interesting.

The gender co n strain t in the pronoun lim its the pronoun's antecedent to one m atching this characteristic. G ender is a strong constraint and

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cannot easily be violated. U ltim ately it lim its w hich analysis can be adopted. If the anaphor reso lu tio n system b eh av es in a m odular

fashion this inform ation m ay not be used im m ediately how ever. An

in itial in terp retatio n by the system m ay p ro ceed w ith referen ce to solely structural information.

In exam ple (6) the pronoun can p o ten tially refer to eith er character although it is preferentially interpreted as coreferential with 'John'.

(6) John fascinated Bill because he was interesting.

This is because o f a property associated w ith the verb called im plicit c a u sa lity (G arv ey and C a ram az za , 1974). T he v erb 'fa s c in a te ' possesses a bias which, in a sentence such as (6), biases interpretation o f the pronoun as referring to the first N oun Phrase. Verb bias is a w eaker co n strain t than gender as it can be v iolated (as in exam ple (7)).

(7) John fascinated Bill because he was easily interested.

The pronoun is now in terp reted as co referen tial w ith 'B ill', although this goes against the im plicit causality bias o f the verb. W hen is verb semantic information such as implicit causality used by the system ?

W e can ask o u rse lv es the sam e b a sic q u e stio n ab o u t an ap h o r

resolution as has been asked about parsing. Does the system behave

in a restricted or nonrestricted fashion? If it behaves in a restricted fash io n , an in itial stage o f the an ap h o r reso lu tio n m echanism w ill operate only over a p articu lar subset o f the inform ation available in the input. T hat inform ation m ay be solely structural and an initial stage m ay em ploy some p rin cip le such as P arallel Function Strategy (PFS). Stated simply PFS proposes that a pronoun w ill be interpreted as co referen tial w ith the c h aracter o ccu p y in g the sam e gram m atical role in the preceding clause. In other w ords the pronoun in exam ple (8) b elo w w ill be p re fe re n tia lly in te rp re te d as re fe rrin g to the character ‘John’.

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PFS can be co n sid ered as analogous to the strateg ies o f M inim al A ttach m en t or Late C losure in parsing. It is a purely stru ctu rally in fo rm ed heuristic. If the anaphor reso lu tio n system behaves in a m odular or restricted fashion it m ay be the case th at strategies such as PFS initially inform the system . Only later does inform ation such as verb semantics exert a processing influence.

T he inform ation not considered in itially w ill ex ert an influence at a

later point, perhaps to guide reanalysis follow ing m isassignm ent. If

we adopt a n o n restricted , co n strain t based p o sitio n , we propose that all inform ation that ultim ately influences anaphor resolution does so d u rin g the sam e stag e in p ro cessin g and th at no in fo rm a tio n is accorded a privileged status other than in the sense th at a p articu lar type of information may be assigned a greater constraining weight.

S ta te d sim p ly , m o d u la r acc o u n ts o f p a rsin g p ro p o se th at in itia l processing is inform ed by a well defined set o f inform ation. In the case o f the G arden P ath m o d el th is is sy n tax , i.e. low lev el in fo rm atio n . A t the start o f th is ch ap ter we d escrib ed how we w a n ted to draw p a ra lle ls b etw een the p ro ce sses o f p a rsin g and refe ren c e reso lu tio n . A stru c tu ra lly d riv en strateg y for reso lv in g an ap h o rs such as PFS is e q u iv a le n t to the p a rsin g stra te g ie s o f M in im al A ttach m en t and L ate C losure. I f the an aphor reso lu tio n system is m odular, an in itial stage o f processing should proceed with reference only to a w ell defined set o f inform ation. W e claim that if this low level in fo rm atio n is purely structural, as is the case w ith m o d u la r p a rsin g th e o rie s , a strateg y such as PFS w ill in itia lly d e te rm in e how a re fe re n tia l am b ig u ity w ill be tre a te d w ith o u t reference to higher level factors such as verb sem antics. Only at some su b seq u e n t stage o f p ro ce ssin g w ill sem an tic in fo rm atio n e x ert a

pro cessin g influence. If resolving anaphoric referen ce behaves in a

n o n m o d u lar fash io n , all in fo rm a tio n th at is re le v a n t w ill ex ert a pro cessin g influence at the same point (cf. T rusw ell, T anenhaus and Gamsey, 1994).

T h e fo llo w in g c h a p te r p ro v id e s a su m m ary o f th e lite ra tu r e exam ining anaphors in general. A t the end o f that chapter we shall

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set out the p re c ise p re d ic tio n s th a t fo llo w from the re fe re n c e resolution system behaving in a modular or nonmodular fashion.

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CHAPTER 2

2.1 Introduction

T his c h ap ter p ro v id es a sum m ary o f th e issu es a sso c ia te d w ith

anaphora. It begins by sketching the functional differences betw een

anaphors o f d ifferen t form s. It then exam ines the role o f discourse focus on an ap h o ric reso lu tio n and the tim e course o f re so lu tio n . T h ere then fo llo w s a sum m ary o f th o se a cc o u n ts o f re fe re n c e resolution w hich we can term restricted accounts (see C hapter 1 for definition) focusing as they do on the role o f structural inform ation in the process o f reso lu tio n . W e then exam ine how o th er ty p es o f in fo rm a tio n are u se d by the a n a p h o r re s o lu tio n sy stem b e fo re p ro p o sin g a tw o -sta g e m odel o f a n a p h o r re so lu tio n in fo rm e d at different points in time by structural and non-structural factors.

In contrast to research on parsing, research exam ining the processing o f an ap h o rs form s a less h o m o g en o u s body o f w ork. F o d o r's M odularity thesis has played an im p o rtan t part in guiding em p irical w ork on p a rsin g b e h a v io u r b u t has h ad little in flu e n ce on the literatu re exam ining the level o f anaphor resolution. It is certain ly p o ssib le to ex am in e w h e th er th at p a rt o f the c o g n itiv e sy stem responsible for resolving anaphoric reference behaves in a m odular or n o n m o d u la r fash io n . T h at it h a sn 't y et b een done is p e rh a p s indicative o f the large num ber o f o th er issues researchers exam ining the behaviour o f anaphora have deem ed m ore w orthy o f exam ination. T he fo llo w in g se c tio n p ro v id e s a su m m ary o f som e o f th e se th eo retical issues and the em pirical w ork generated as a resu lt. A n u m b er o f d im en sio n s can be e x tra cted from the lite ra tu re along

w hich we can categorise previous research. These dim ensions include

the tim e course o f the anaphor reso lu tio n process, the b eh av io u r o f differen t form s o f anaphora and the influence o f both stru ctu ral and

non-structural factors on anaphor processing. Initially we w ill focus

on the functional role o f anaphora w ithin a text before spending some time summarising the work pertinent to each o f these dimensions.

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2.2 Anaphora

In o rd er to c o n stru ct and m ain tain a m ental m odel stan d in g in relatio n to the text being read, readers m ust in teg rate each unit o f tex t w ith those preceding it. In other w ords, they m ust be aw are of the cohesive relations betw een units. C ohesion m ay be achieved in a num ber o f ways. For our purposes we shall concentrate on referential cohesion. T his is accom plished through the ap p licatio n o f referring expressions.

(1) John saw Mary in the park. He waved at her.

In ex am p le (1) above, the tw o sen ten ces can on ly be p ro p erly co m p reh en d ed if the read er su ccessfu lly in terp rets the p ro n o u n s in th e seco n d sen ten ce. T he re fe re n tia l lin k s m u st be e sta b lish e d b etw een an aphor and an teced en t b efo re in te g ratio n o f the sem antic in fo rm a tio n in the se co n d se n te n c e d e sc rib in g th e re la tio n s h ip between the two characters can be achieved.

In the case o f exam ple (2) below the pronoun in the second sentence c a n n o t be su c c e ssfu lly re so lv e d in itia lly as th e d isa m b ig u a tin g information doesn’t occur until after the pronoun.

(2) John saw Bill in the park. He waved at John.

If we rep lace the pronouns in exam ple (1) w ith rep e titio n s o f the ch aracters nam es (see exam ple (3) below ), the tex t sounds aw kw ard although the referential links are maintained.

(3) John saw Mary in the park. John waved at Mary.

W hat determines the type of anaphoric form that may be used ?

2.2.1 Differences between Anaphors

In th is sectio n we describe w hy p eo p le select one an ap h o ric form over another and the processing b eh av io u r asso ciated w ith p articu lar

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types o f anaphors. S electio n o f form is in trin sically linked to the degree of focus of the intended referent.

2.2.1 What is focus ?

W e can define focus op eratio n ally as the entity w hich readers prefer a text to continue with reference to. In exam ple (4) below , there is a p re fe re n c e fo r the se n te n c e to co n tin u e w ith re fe re n c e to the character 'John'.

(4) John fascinated Bill because ...

A continuation may be o f the form '... he was very interesting.' The pronoun and NP 'John' are co referen tial so for this sentence we can say that the character referred to by 'John' is in focus.

T he notion o f focus is cen tral to several p sy ch o lo g ical th eo ries o f language processing (e.g. Sanford and Garrod, 1981; G ordon, Grosz and G illiom , 1993). R oughly, an entity can be considered to be in focus when it occupies a privileged role in the reader's centre of attention.

2.2.2 What determines focus ?

A num ber o f devices can be used to place an entity in focus including recency o f m ention (C lark and Sengul, 1979) and p rio r topicalisation

(A nderson, G arrod and S anford, 1983). Sanford, M oar and G arrod

(1988) dem onstrate th at the w ay in w hich a ch aracter is in troduced in a tex t influences how stro n g ly focused it w ill be. Specifically, characters introduced by a p ro p er nam e are m ore highly focused than characters introduced through the use of a definite description.

T here is a difference betw een local and global levels o f focus. The global level can be in terp reted as corresponding to entities relevant to the topic o f the text as a w hole w hile the local level can be seen as b e in g com posed o f tem p o rary sh ifts in focus b e tw ee n m ain and tra n sie n t characters (A n d erso n et al, 1983). S ev eral p sy ch o lo g ical th eo ries re stric t th e ir ex am in a tio n o f focus to focus at the local discourse level. C entering (see below ) restricts its account to adjacent

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utterances. There is also a level o f focus at the m ore global level, roughly w hat can be considered the discourse topic. The topic can be co n sid ered to be w hat the discourse as a w hole is about. L ocal discourse focus can shift throughout a section o f text but the global focus will remain more or less constant.

S anford and G arrod (1981) propose their M em ory Focus m odel in an attem p t to account fo r the linkage betw een focus th at occurs over short discourse segm ents and a m ore general global lev el o f focus. T h ey em p h a sise th e ro le th a t b a c k g ro u n d k n o w le d g e p lay s in stru c tu rin g w ithin th is m odel. V ery ro u g h ly , th is M em ory Focus m o d el is p ro p o se d to c o n sist o f tw o d im e n sio n s c o n n e c te d by m appings. The first d im ension, E x p licit F ocus, co n sists o f tokens standing for characters relev an t to a p a rticu la r stretch o f discourse. T he Im p licit Focus d im ension is considered to be th at p art o f the d isco u rse m odel w hich m aps onto p re-ex istin g k n o w led g e structures asso ciated w ith the situ atio n described by the tex t (M insky, 1975). G arrod, Freudenthal and B oyle (1994) suggest that it is the level o f

Im p licit Focus that pronom inal anaphora access. Pronouns provide a

direct route into the conceptual level of the reader's discourse model.

M ore e x p licit form s o f an ap h o r access th eir an teced en ts in a less d irect way. W ithin the M em ory Focus account they co rresp o n d to tokens in the E xplicit Focus dim ension o f the m odel. F or characters central to the discourse, there is a rich set o f m appings betw een the E xplicit and Im plicit Focus aspects o f the discourse m odel. A nderson, G arro d and S anford (1983) d em o n strate th at even w ith sh ifts o f scen ario (i.e. w here Im p licit Focus changes) th ese elem ents rem ain central in the reader's discourse model.

Pronouns map d irectly onto im p licit focus and th erefo re access the conceptual level o f the discourse m odel directly. T he tokens in the E xplicit Focus part o f the m odel are associated w ith a m ore superficial

level o f representation. The m apping betw een these tokens and their

corresp o n d in g roles in Im p licit Focus m ust be u n d ersto o d for N oun P h ra se an ap h o rs to a cc ess th e sam e lev el as th a t a cc essed by pronouns. This is a less direct route to accessing the conceptual level than the use of pronominal anaphora.

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2.2.3 How does focus influence selection and interpretation of anaphoric form ?

A riel (1990) argues that the choice o f anaphor is determ ined by the degree o f focus p ossessed by its intended referent. A highly focused referen t w ill be p referen tially referred to using a pronoun, w hile a full name will be used to refer to an antecedent not in focus. There is a negative c o rrelatio n betw een in fo rm atio n al co n ten t o f an anaphor and degree of focus of its referent.

C entering theory also proposes that reference to a focused entity will be p referen tially rea lised through the use o f a pronoun. P ronouns referring to antecedents not in focus tak e a relativ ely long tim e to read. E hrlich and R ayner (1983) d em onstrated longer fixation tim es, and by ex ten sio n p ro ce ssin g d iffic u lty , on the reg io n fo llo w in g a pronoun when the pronoun's antecedent was at a distant point in the

text. This is supported by Ehrlich (1980), C lark and Sengul (1979)

and Frederikson (1981).

G ern sb ach er (1989) p ro p o ses th at recen cy o f m en tio n is also an im portant determ iner o f level o f explicitness o f anaphor. She states 'the longer the d istan ce betw een an an aphor and its antecedent, the m ore ex p licit the anaphor' (p. 138) w here distance is taken to m ean the physical distance between an anaphor an its antecedent.

The degree to w hich an antecedent is topical or part o f global focus also seems to partly determ ine the level o f explicitness o f a follow ing anaphoric ex p ressio n . A ntecedents w hich have fallen out o f focus n eed ed to be re fe rre d to using a re la tiv e ly in fo rm a tio n a lly rich a n ap h o r (A riel, 1990). A n d erso n , G a rro d and S a n fo rd (1 9 8 3 ) exam ined the role o f p rio r to p ic alisa tio n in determ in in g an teced en t a c c e ss ib ility an d fo u n d a p re fe re n c e fo r p ro n o u n s to re fe r to to p icalised en tities o v er n o n -to p icalised ones. S im ilar effects w ere found by Chafe (1974), G ivon (1983) and M arslen-W ilson, Levy and T yler (1982) who report that the m ore topical the antecedent, the less explicit the anaphor used to refer to it.

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2.2.4 The Influence of Focus on Anaphoric Processing

R egardless o f how we precisely characterise focus, antecedents which no longer occupy a position w ithin the focus o f a reader's discourse m odel m ust be re-in tro d u ced u sin g a refe ren tially sp ecific device. W hen an anaphor is em ployed th at is m ore referentially specific than appropriate (i.e. if its antecedent is in focus), the resu lt is aw kw ard and u n n atu ral sounding text. H ow ever, V onk, H ustinx and Sim ons (1992) report that under conditions w here a text contains a shift o f them e, overspecification acts as an im portant signal to the reader that such a shift is occurring. Consider the following set of sentences :

1. Sally Jones got up early this morning. 2. She wanted to clean the house.

3. Her parents were coming to visit her. 4. She was looking forward to seeing them. 5. She weighs 80 kilograms.

6. She had to lose weight on her doctor's advice. 7. So she planned to cook a nice but sober meal.

A lthough the pronoun 'she' in sentence (5) unam biguously refers to the character 'Sally Jones', V onk et al describe a preference to use an anaphor o f increased specificity, i.e. the repeat nam e 'Sally'. The is b ecau se sen ten ce (5) sh ifts them e from the v isit o f S ally Jo n es' p aren ts to h e r w eight problem . W ith o u t an o v e rt th em atic sh ift signal, it is difficult to integrate the content o f sentence (5) w ith what has b een re a d p rev io u sly u n til the c o n n ec tio n b eco m es c le a r in sentence (7). V onk et al propose that readers interpret anaphors o f a m ore specific form than is necessary as signals o f shifts o f them e. We can rep lace the pro n o m in al anaphors in sentences (1) th ro u g h (4) with the referentially unambiguous repeat name, 'Sally Jones'.

1. Sally Jones got up early this morning. 2. Sally Jones wanted to clean the house.

3. Sally Jones' parents were coming to visit her. 4. Sally Jones was looking forward to seeing them.

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A dopting the V onk et al line o f argum ent, we argue that the difficulty in reading such text is due to the reader interpreting the overspecific anaphor in each sentence as a signal for a them atic shift. As no shift o ccurs, the conditions for using such o v er-sp ecificatio n are v iolated and processing disruption is encountered. The V onk et al position is

supported by both p roduction and com prehension data. They report

th at them atic shifts are produced by subjects w hen they have to use an o v e rs p e c ific a n ap h o r and th a t w hen re q u ire d to p ro d u ce a th em atic sh ift, subjects p ro d u ce o v ersp ecific an ap h o rs. They also rep o rt two com prehension probe experim ents w hich indicate that the p resen c e o f an o v e rsp ec ifie d an ap h o r red u ces the a cc essib ility o f

inform ation contained in the preceding sentence. If the overspecified

an aphor is treated by the read er as a cue th at the next stretch o f d isc o u rse w ill in tro d u c e a new them e ra th e r th an c o n tin u e the existing one, a new foregrounded segm ent of the discourse m odel w ill be introduced and the segm ent containing the content o f the previous theme will fall out of focus, at least temporarily.

The them atic shift signal docum ented by V onk et al is a very specific e x a m p le a s s o c ia te d w ith o v e rs p e c ifie d a n a p h o rs . A s su ch o v e rsp e c ific a tio n is re la tiv e , it is n o t p o ssib le to d eterm in e how p a rtic u la r a n a p h o ric fo rm s p e r se d iffe r in th e ir p ro c e s s in g

consequences. Indeed, the behaviour o f a p articu lar anaphor relative

to other possible anaphors m ay be o f a very d ifferen t form from the absolute behaviour of that anaphor in general.

2.2.5 How anaphors access levels within the Discourse Model

C lo itre and B ever (1988) and G arrod, F reudenthal and B oyle (1994) p ropose a m uch m ore general fram ew ork in w hich they consider the o p e ra tio n a l d istin c tio n s b e tw ee n d iffe re n t an ap h o ric form s. It is p o s s ib le to c o n sid e r a n a p h o ra as p o in te rs w ith in th e re a d e r's discourse m odel. There seems to be evidence to suggest that different anaphoric form s tap in to d ifferen t levels o f rep resen tatio n w ithin this discourse model.

U sin g m aterials such as (1) b elo w , C lo itre and B ev er p resen te d subjects w ith a pair o f sentences follow ed by a probe word w hich had

The Time Course of the Influence of Implicit Causality Information on Resolving Anaphors