ÿWPCà ¦7|;$UVx(S5œ3™¡FQjüŒÏ^ M@X¥ÓvZ2˜(›cå=ìLÓý{¯áÉeÈé À˜á²]§Ž\G·%2Ž  Ðis€contiguous€to,€is€juxtaposed,€and€is€close€to"€andÏòòsurroundsóó€as€including€"limits,€bounds,€confines,Ð À  Ðencloses,€and€circumscribes")€suggests€that€lexicalÏentries€(that€is,€nodes)€in€the€verb€hierarchy€andÏadjective€network€can€themselves€be€used€as€relations.ÌÌWe€were€required€to€include€lexical€relations€and€rulesÏin€the€semantic€network.€€ñ\ñ(Flickinger,€1987)ñ\ñ€firstÏovertly€included€inflectional€relations€as€distinct€nodesÏin€a€lexical€hierarchy.€€There€are€several€types€ofÏderivational€rules€that€operate€in€the€lexicon.€€At€theÏmost€basic,€we€have€morphological€relations€betweenÏlexical€entries€(e.g.,€see€ñ^ñ(Pentheroudakis€&ÏVanderwende,€1993)€and€ñ`ñ(Quirk,€et€al.,€1985:€AppendixÏI)ñ`ñ€on€word€formation).€€These€rules€operate€at€theÏlexical€entry€level,€rather€than€at€the€sense€level.€€TheseÏrules€have€not€been€seen€as€distinct€lexical€entries,€butÏthat€they€can€and€perhaps€should€be€is€based€onÏconsideration€of€sense„extension€or€lexical€rules.€€TheseÏrules€operate€primarily€on€what€have€been€identified€asÏindividual€senses€of€lexical€entries€(e.g.,€the€òògrindingóóÐ c# Ðrule€which€converts€an€animal€sense€of€a€lexical€itemÏinto€a€meat€sense,€for€which€see€ñbñ(Copestake€&€Briscoe,Ï1991)ñbñ,€where€the€rule€itself€is€placed€among€the€lexicalÏentries€and€is€viewed€as€providing€the€conditions€for€theÏtransformation€of€one€sense€into€another).ÌÌWe€also€were€required€to€consider€semantic€featuresÏassociated€with€lexical€items€(Katz€&€Fodor,€1963).€Ïñfñ(Copestake€&€Briscoe,€1991)ñfñ€showed€that€such€featuresÏcould€be€identified€as€distinct€lexical€items€(hereafterÏtermed€òòpseudoentriesóó)€that€can€be€inherited€by€regularÐ ^'®". Ðlexical€entries.€€We€also€used€the€qualia€structures€ofÏñhñ(Pustejovsky,€1995)ñhñ.€€The€formal€(that€is,€what€is€this)Ïaspect€of€a€qualia€structure€corresponds€to€the€ISAÏbackbone€of€WordNet,€while€the€constitutive€aspectÏcorresponds€to€its€meronymic€links.€€The€agentive€andÏtelic€(the€purpose€associated€with€the€noun)€qualia€areÏnot€currently€included€in€WordNet.€€We€may€view€theseÏaspects€as€relations€in€the€UMLS€hierarchy€and€henceÐ f-¶(6 Ðgive€them€independent€status€as€lexical€entries€withÏselectional€restrictions€identifying€which€nodes€of€theÏnetwork€may€satisfy€the€restrictions.ÌÌThe€final€relation€is€the€semantic€role.€€For€lexical€itemsÏhaving€subcategorization€patterns,€such€as€verbsÏidentifying€cases,€these€relations€provide€links€to€otherÏentries.€€This€is€done€by€identifying€selectionalÏrestrictions,€which€are€nothing€more€than€the€restrictionÏof,€say,€the€òòagentóó€or€òòinstrumentóó€to€a€subset€of€theÐ y É? Ðsemantic€network.ÌÌIn€general,€then,€nodes€of€the€semantic€network€may€beÏviewed€as€lexical€entries€or€pseudoentries€representingÏconcepts.€€Semantic€relations€may€be€viewed€as€labelsÏon€the€links€between€nodes.€€However,€these€relationsÏare€better€viewed€as€distinct€nodes€in€the€network,€withÏthe€property€that€they€act€as€a€linking€mechanismÏbetween€other€nodes;€semantic€relations€may€also€existÏwithin€their€own€hierarchy.€€(These€are€similar€to€roleÏnodes€in€ñjñ(Brachman€&€Schmolze,€1985)ñjñ.)ÌÌò òòò6€€Ô% € ÔComparison€of€Extended€SemanticÐ F–L ÐNetworks€with€Previous€Workóóó óÐ /M ÐÌThe€sublexicons€identified€in€the€analysis€of€a€cohesiveÏtext€used€as€a€test€item,€a€cohesive€set€of€responses€toÏthat€test€item,€and€a€pre„defined€category€system€clearlyÏcarve€out€portions€of€semantic€networks.€€While€someÏparts€of€these€sublexicons€correspond€to€subtrees€rootedÏat€particular€nodes€in€these€networks,€several€additionalÏuseful€"cuts"€through€the€network€have€been€identified.ÌÌ€ñmñ(Nida,€1975)ñmñ€indicates€that€a€semantic€domain€may€beÏdefined€based€on€any€semantic€features€associated€withÏlexical€items.€€He€used€this€observation€to€assert€thatÏany€attempt€to€identify€a€single€hierarchy€or€ontologyÏwas€somewhat€arbitrary€and€dependent€on€a€user's€need.€ÏThe€work€done€here€supports€that€observation,€but€doesÏso€in€the€context€of€a€much€larger€conception€ofÏsemantic€relations€among€nodes€in€a€network.ÌÌThe€identification€of€sublexicons€that€cut€across€parts€ofÏspeech€corresponds€to€the€presence€of€semanticÏcomponents€in€identifying€categories.€€This€notion€isÏsupported€in€ñoñ(Burstein,€et€al.,€1996)ñoñ,€where€conceptÏgrammars€are€based€on€collapsing€phrasal€andÏconstituent€nodes€into€a€generalized€XP€representation,€ÏThis€notion€of€using€semantic€components€as€the€basisÏfor€category€development€has€been€invoked€as€well€in€ñqñÏñrñ(Laffal,€1995)ñrñ.ÌÌThe€semantic€network€carved€out€by€adhering€toÏprinciples€of€lexical€cohesion€is€the€most€interesting.€€InÏthe€first€place,€the€principal€nodes€of€the€network€areÐ ¶-)l Ðnot€in€themselves€naturally€related,€thus€following€NidaÏin€allowing€the€use€of€"on€the€fly"€criteria€for€definingÏa€sublexicon.€€In€the€second€place,€the€need€to€invokeÏsemantic€roles€(and€their€selectional€restrictions)€inÏanalyzing€the€discourse€structure€clearly€develops€"onÏthe€fly"€conceptual€clusters,€similar€to€those€describedÏin€ñtñ(McRoy,€1992)ñtñ,€particularly€situational€clusters.€€TheÏrelationship€between€the€sublexicon€formed€from€theÏETS€response€set€and€the€ETS€test€item€suggests€furtherÏthat€the€identified€sub„semantic€network€has€affinitiesÏto€scripts€as€described€by€ñvñ(Schank€&€Abelson,€1977)ñvñ.€ÏThe€procedures€described€here€are€consistent€with€thoseÏdescribed€in€ñxñ(Barrið/ðre€&€Popowich,€1996)ñxñ€and€indeedÏwould€benefit€from€those€procedures€in€furtherÏautomating€the€identification€of€situational€clusters.ÌÌò òòò7€€Conclusions:€Use€of€Extended€SemanticÐ À  ÐNetworks€for€Characterization,€Definition,Ïand€Naming€of€Categoriesóóó óÐ ’â  ÐÌCategory€development€is€important€in€many€fields€ofÏresearch€and€is€usually€approached€with€statisticalÏtechniques€of€classification€based€on€assignment€ofÏcharacteristics€to€units€of€analysis.€€In€fields€associatedÏwith€linguistics,€category€development€has€been€mostÏoften€used€for€content€analysis€and€informationÏretrieval.€€In€these€fields,€after€the€development€of€aÏcategory€system€and€identification€of€lexemes€judged€toÏlie€within€each€category,€the€analysis€of€text€is€thenÏbased€on€the€frequency€with€which€the€lexemes€of€aÏcategory€occur€within€units€of€analysis.€€The€field€ofÏinformation€retrieval€performs€this€last€step,€usuallyÏwith€only€a€rudimentary€category€system€based€onÏidentifying€the€root€words€of€lexemes.€€However,€withinÏinformation€retrieval,€considerable€promise€ñzñ(Liddy,€etÏal.,€1993)ñzñ€has€been€obtained€by€using€a€category€systemÏbased€on€the€subject€codes€in€Longman's€Dictionary€ofÏContemporary€English€(LDOCE)€ñ|ñ(Longman€DictionaryÏof€Contemporary€English,€1978)ñ|ñ.ÌÌLabeling€texts€with€a€pre„defined€set€of€categories,€evenÏusing€such€categories€for€identifying€the€principalÏtopics€of€texts,€has€shown€a€clear€benefit€of€customizingÏlexicons€to€suit€computational€tasks€ñ~ñ(Hearst€&€SchðGðtze,Ï1996)ñ~ñ.€€This€approach€to€customizing€the€lexicon€builtÏupon€the€semantic€network€embodied€in€WordNet,€butÏmade€use€of€only€its€principal€ISA€backbone,€that€is,€theÏhypernymic€and€hyponymic€relations€between€synsets.ÌÌThe€most€straightforward€categories€are€thoseÏcharacterized€as€lists€of€subtrees€in€an€ISA€hierarchy.€ÏThe€definition€of€such€a€category€is€simply€the€set€ofÏhighest€nodes€that€cover€all€concepts€in€the€category€(asÏdescribed€for€the€MCCA€category€òòDetached€rolesóó).€Ð -m(5 ÐEach€node€should€be€a€concept€in€the€category€or€aÐ Þ-.)6 Ðnode€such€that€any€of€its€children€in€the€category€haveÏthat€node€as€its€direct€parent.€€It€is€legitimate€to€excludeÏsubtrees€rooted€at€the€descendants€of€the€nodes€in€theÏset.€€The€name€for€the€category€is€equivalent€to€the€setÏof€nodes€defining€the€category.€€The€category€systemsÏdescribed€in€ñ€ñ(Nida,€1975;€Hearst€&€SchðGðtze,€1996;ÏLaffal,€1995)ñ€ñ€are€of€this€type.ÌÌAnother€relatively€straightforward€type€of€category€isÏthat€formed€with€lexical€items€that€have€well„definedÏmorphological€or€derivational€relations€between€pairs€ofÏthe€concepts.€€This€group€can€also€contain€other€typesÏof€lexical„rule€relations.€€These€morphological,Ïderivational,€and€lexical„rule€relations€are€subsidiaryÏwithin€the€category.€€There€is€a€set€of€semanticÏcomponents€that€lie€below€them;€these€are€the€principalÏdefining€characteristics€of€the€category,€handled€as€inÏthe€previous€type€of€category.€€The€category€system€in€ñ‚ñÏñƒñ(Burstein,€et€al.,€1996)ñƒñ,€which€generalizes€part€ofÏspeech,€is€of€this€type.ÌÌThe€most€complex€type€of€category€system€is€the€oneÏnecessary€to€describe€the€discourse€structure€of€a€text.€ÏWhen€a€text€has€been€analyzed€as€described€in€theÏprevious€section,€the€result€is€a€set€of€discourseÏsegments€related€to€one€another€(with€many€identifiedÏas€subsidiary),€discourse€entities€and€eventualities,€andÏvarious€(primarily)€role€relations€between€these€entities.€ÏWe€view€the€concepts€and€relations€(including€theÏdiscourse€relations)€as€essentially€present€in€andÏlicensed€by€the€lexicon,€and€then€instantiated€by€theÏgiven€text€to€carve€out€a€subnetwork.€€The€definition€ofÏthis€subnetwork€is€then€constructed€by€identifying€theÏhighest€nodes€in€the€ISA€backbone€and€the€additionalÏrelations€that€operate€on€the€backbone,€along€withÏselectional€restrictions€that€are€used.ÌÌCharacterizing€this€subnetwork€is€again€a€matter€ofÏidentifying€the€topmost€ISA€nodes€(and€perhaps€moreÏimportantly€here,€more€accurately€identifyingÏdescendants€that€are€to€be€excluded).€€Naming€thisÏsubnetwork€is€again€based€on€the€set€of€topmost€nodes,Ïwhich€may€in€this€case€include€relations€(as€from€theÏUMLS€tree€of€semantic€relations).€€This€process€ofÏcharacterizing€a€subnetwork€is€quite€similar€to€theÏdevelopment€of€supercategories€in€(Hearst€&€SchðGðtze,Ï1996).€€Thus,€to€at€least€that€extent,€this€process€may€beÏviewed€as€leading€to€identification€of€the€topic€of€a€text.ÌÌò 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