VerbNet Overview Karin Kipper Schuler kipper@verbs.colorado.edu May 31st, 2009
Overview Real world applications need resources with rich syntactic and se- mantic representations. • Most existing broad-coverage resources provide only a shallow semantic representation • Much richer representations are needed • The verbs are key elements in providing this 1
Overview Natural language applications are currently limited to specific do- mains with hand-crafted lexicons. • not available to the whole community • expensive and time-consuming to build Most available broad-coverage resources either focus on syntax or on semantics and do not provide a clear association between the two. 2
Semantic representation must be tied to the syntactic information: • Differences between syntactic frames can help: Eng: John left the room. (exited) Port: John saiu do quarto. Eng: John left the book on the table. (left) Port: John deixou o livro na mesa. • But syntax alone is not sufficient: Eng: John left the room. (exited) Port: John saiu do quarto. Eng: John left a fortune. (gave away) Port: John deixou uma fortuna. 3
Overview Predicate argument relations are of interest for NLP, providing gen- eralizations over data: • Ronaldo scored a goal for the Brazilian team • A goal was scored by Ronaldo for the Brazilian team • Ronaldo wanted to score a goal for the Brazilian team 4
VerbNet connects semantics to syntax Created with these ideas in mind • computational verb lexicon • broad-coverage and domain-independent • clear association between syntax and semantics – lexical semantic information (pred argument structure) – syntactic frames and selectional restrictions – semantic predicates – links to WordNet senses, FrameNet frames, PropBank framesets • refinement of Levin classes to construct the entries 5
Outline • Overview • Building blocks for VerbNet • VerbNet • Parameterized Action Representation (PARs) • Evaluation • Mappings to other Resources • Automatic techniques to extend coverage 6
Levin classes Levin (1993) • Verbs are grouped into classes • Each class is characterized by a set of syntactic patterns John broke the jar / The jar broke / Jars break easily John cut the bread / *The bread cut / Bread cuts easily John hit the wall / *The wall hit / *Walls hit easily • Hypothesis: syntax reflects implicit semantic components contact, directed motion, exertion of force, change of state 7
Example Levin class Break Levin class - Change-of-state break tear chip split crack splinter crash snap crush smash fracture shatter rip 8
Levin classes • classes are not semantically homogeneous { braid, clip, file, powder, etc.. } • classes are not completely syntactically homogeneous • verbs can be in multiple class listings • alternation contradictions – Carry verbs disallow conative but include { push, pull, shove, etc } also in Push/pull class which does take conative 9
Event Structure Verbs refer to events which can be decomposed into a tripartite structure in a manner similar to Moens and Steedman (1988) culmination preparatory consequent process state 10
Verb classes and event structure RUN class BREAK class (bounce, jog, jump, hop, run) (break, chip, crack, tear) preparatory consequent process (activity) state culmination HIT class (batter, kick, hit, slap) 11
Outline • Overview • Building blocks for VerbNet • VerbNet • Parameterized Action Representation (PARs) • Evaluation • Mappings to other Resources • Automatic techniques to extend coverage 12
Characteristics of verbs: Verbs have complex meaning: key components can be made explicit • have participants • space • verbs represent processes/events/states which are located in time • can be subdivided into sub-parts to capture during, end, results 13
Examples of verbs and their components • RUN – express iterative activity, no culmination, or consequent – one participant – motion of participant is a semantic component – path is optional • HIT – express contact between two objects – happens momentarily, has a well defined end, has no consequent – has three participants • BREAK – express a change of state 14
VerbNet class entries Kipper, Dang and Palmer, 2000 • verb classes based on Levin’s classification • classes defined by syntactic properties • capture generalizations about verb behavior • for each verb class – thematic roles – syntactic frames – selectional restrictions for the arguments in each frame – each frame includes semantic predicates with a time function 15
Hit class Class hit-18.1 Parent — Members bang (1,3), bash(1), batter(1,2,3), beat(2,5), ..., hit(2,4,7,10), kick(3), ... Themroles Agent Patient Instrument Selrestr Agent[+int control] Patient[+concrete] Instrument[+concrete] Frames Name Syntax Semantic Predicates Transitive Agent V Patient cause(Agent, E) ∧ “Paula hit the ball” manner(during(E),directedmotion,Agent) ∧ !contact(during(E), Agent, Patient) ∧ manner(end(E),forceful, Agent) ∧ contact(end(E), Agent, Patient) Transitive Agent V Patient cause(Agent, E) ∧ with Prep(with) Instrument manner(during(E),directedmotion,Agent) ∧ Instrument “Paula hit the ball with a !contact(during(E),Instrument,Patient) ∧ stick” manner(end(E),forceful, Agent) ∧ contact(end(E), Instrument,Patient) 16
Thematic roles • use roles to provide as much information as possible for classes • thematic roles vs. generic arguments • specification of roles supplies part of the semantic description for the class Build-26.1 “The artist (Agent) carved a toy (Product) out of a piece of wood (Material)” • roles also help differentiate classes Admire-31.2 : Experiencer and Theme Hit-18.1 : Agent and Patient 17
Thematic roles • small set of roles (Agent, Theme, Location, ...) • roles used across classes • certain roles need specific characteristics to be present (Patient → undergoes change) • thematic roles used are generally accepted 18
Selectional Restrictions • semantic restrictions on the thematic roles – based on EuroWordNet concepts (Vossen 2003) – associate VerbNet with an ontology publicly available, widely used – IS-A hierarchy with multiple inheritance and no cycles • syntactic restrictions for the syntactic frames (e.g., sentential, plural ) 19
Selectional Restrictions force int-control machine vehicle human animate animal natural plant body-part comestible machine phys-obj concrete artifact tool garment rigid solid non-rigid pointed shape elongated substance idea abstract sound SelRestr communication regionPP location place time object state scalar currency organization 20
Syntactic Frames Describe possible surface realizations for verbs in a class • constructions such as transitive, intransitive, resultative, and a large set of Levin’s alternations • Examples: 1. Agent V Patient (John hit the ball) 2. Agent V at Patient (John hit at the window) 3. Agent V Patient[+plural] together (John hit the sticks together) 21
Semantic Predicates Semantics of a syntactic frame captured through a conjunction of semantic predicates • each semantic predicate includes a time function showing at what stage in the event the predicate holds start(E), during(E), end(E), result(E) • similar to Moens and Steedman’s event decomposition • choice influenced because it was suitable for PARs (pre-conditions, post-conditions, and results) • semantic predicates can be: General (e.g., motion and cause ), Specific (e.g., suffocate ), or Variable (Prep) 22
Semantic Predicates • relations between verbs (or verb classes) captured implicitly by the predicates for the class • aspect captured by the temporal function present in the predi- cates: – activities (e.g., run ) have during(E) – bounded activities (e.g., hit ) have during(E) and end(E) – accomplishments (e.g., break ) have result(E) 23
Hit class Class hit-18.1 Parent — Members bang (1,3), bash(1), batter(1,2,3), beat(2,5), ..., hit(2,4,7,10), kick(3), ... Themroles Agent Patient Instrument Selrestr Agent[+int control] Patient[+concrete] Instrument[+concrete] Frames Name Syntax Semantic Predicates Transitive Agent V Patient cause(Agent, E) ∧ “Paula hit the ball” manner(during(E),directedmotion,Agent) ∧ !contact(during(E), Agent, Patient) ∧ manner(end(E),forceful, Agent) ∧ contact(end(E), Agent, Patient) Transitive Agent V Patient cause(Agent, E) ∧ with Prep(with) Instrument manner(during(E),directedmotion,Agent) ∧ Instrument “Paula hit the ball with a !contact(during(E),Instrument,Patient) ∧ stick” manner(end(E),forceful, Agent) ∧ contact(end(E), Instrument,Patient) 24
Hierarchical organization Refinement of Levin classes • verb classes are hierarchically organized – the original set of Levin classes has been further subdivided into additional subclasses which are more syntactic and semantically coherent – members have common semantic predicates, thematic roles, syntactic frames – a particular verb or subclass inherit from parent and may add more infor- mation 25
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