jurafsky&martin_3rdEd_17 (1).pdf

Finally it has proved difficult to formally define

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Finally, it has proved difficult to formally define the thematic roles. Consider the AGENT role; most cases of AGENTS are animate, volitional, sentient, causal, but any individual noun phrase might not exhibit all of these properties. These problems have led to alternative semantic role models that use either semantic role many fewer or many more roles. The first of these options is to define generalized semantic roles that abstract over the specific thematic roles. For example, PROTO - AGENT and PROTO - PATIENT proto-agent proto-patient are generalized roles that express roughly agent-like and roughly patient-like mean- ings. These roles are defined, not by necessary and sufficient conditions, but rather by a set of heuristic features that accompany more agent-like or more patient-like meanings. Thus, the more an argument displays agent-like properties (being voli- tionally involved in the event, causing an event or a change of state in another par- ticipant, being sentient or intentionally involved, moving) the greater the likelihood
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22.4 T HE P ROPOSITION B ANK 381 that the argument can be labeled a PROTO - AGENT . The more patient-like the proper- ties (undergoing change of state, causally affected by another participant, stationary relative to other participants, etc.), the greater the likelihood that the argument can be labeled a PROTO - PATIENT . The second direction is instead to define semantic roles that are specific to a particular verb or a particular group of semantically related verbs or nouns. In the next two sections we describe two commonly used lexical resources that make use of these alternative versions of semantic roles. PropBank uses both proto- roles and verb-specific semantic roles. FrameNet uses semantic roles that are spe- cific to a general semantic idea called a frame . 22.4 The Proposition Bank The Proposition Bank , generally referred to as PropBank , is a resource of sen- PropBank tences annotated with semantic roles. The English PropBank labels all the sentences in the Penn TreeBank; the Chinese PropBank labels sentences in the Penn Chinese TreeBank. Because of the difficulty of defining a universal set of thematic roles, the semantic roles in PropBank are defined with respect to an individual verb sense. Each sense of each verb thus has a specific set of roles, which are given only numbers rather than names: Arg0 , Arg1 , Arg2 , and so on. In general, Arg0 represents the PROTO - AGENT , and Arg1 , the PROTO - PATIENT . The semantics of the other roles are less consistent, often being defined specifically for each verb. Nonetheless there are some generalization; the Arg2 is often the benefactive, instrument, attribute, or end state, the Arg3 the start point, benefactive, instrument, or attribute, and the Arg4 the end point.
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