jurafsky&martin_3rdEd_17 (1).pdf

We call this shallow semantic representation level

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We call this shallow semantic representation level semantic roles . Semantic roles are representations that express the abstract role that arguments of a predicate can take in the event; these can be very specific, like the BUYER , abstract like the AGENT , or super-abstract (the PROTO - AGENT ). These roles can both represent gen- eral semantic properties of the arguments and also express their likely relationship to the syntactic role of the argument in the sentence. A GENTS tend to be the subject of an active sentence, THEMES the direct object, and so on. These relations are codified in databases like PropBank and FrameNet. We’ll introduce semantic role labeling , the task of assigning roles to the constituents or phrases in sentences. We’ll also discuss selectional restrictions , the semantic sortal restrictions or preferences that each individual predicate can express about its potential arguments, such as the fact that the theme of the verb eat is generally something edible. Along the way, we’ll describe the various ways these representations can help in language understanding tasks like question answering and machine translation. 22.1 Semantic Roles Consider how in Chapter 19 we represented the meaning of arguments for sentences like these:
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378 C HAPTER 22 S EMANTIC R OLE L ABELING Thematic Role Definition AGENT The volitional causer of an event EXPERIENCER The experiencer of an event FORCE The non-volitional causer of the event THEME The participant most directly affected by an event RESULT The end product of an event CONTENT The proposition or content of a propositional event INSTRUMENT An instrument used in an event BENEFICIARY The beneficiary of an event SOURCE The origin of the object of a transfer event GOAL The destination of an object of a transfer event Figure 22.1 Some commonly used thematic roles with their definitions. (22.1) Sasha broke the window. (22.2) Pat opened the door. A neo-Davidsonian event representation of these two sentences would be 9 e , x , y Breaking ( e ) ^ Breaker ( e , Sasha ) ^ BrokenThing ( e , y ) ^ Window ( y ) 9 e , x , y Opening ( e ) ^ Opener ( e , Pat ) ^ OpenedThing ( e , y ) ^ Door ( y ) In this representation, the roles of the subjects of the verbs break and open are Breaker and Opener respectively. These deep roles are specific to each event; Break- deep roles ing events have Breakers , Opening events have Openers , and so on. If we are going to be able to answer questions, perform inferences, or do any further kinds of natural language understanding of these events, we’ll need to know a little more about the semantics of these arguments. Breakers and Openers have something in common. They are both volitional actors, often animate, and they have direct causal responsibility for their events. Thematic roles are a way to capture this semantic commonality between Break- Thematic roles ers and Eaters .
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