jurafsky&martin_3rdEd_17 (1).pdf

Wouldnt be able to distinguish between a parse that

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wouldn’t be able to distinguish between a parse that got most of the parts wrong and one that just got one part wrong. 13.9 Human Parsing Are the kinds of probabilistic parsing models we have been discussing also used by humans when they are parsing? The answer to this question lies in a field called human sentence processing . Recent studies suggest that there are at least two Human sentence processing ways in which humans apply probabilistic parsing algorithms, although there is still disagreement on the details. One family of studies has shown that when humans read, the predictability of a word seems to influence the reading time ; more predictable words are read more Reading time quickly. One way of defining predictability is from simple bigram measures. For example, Scott and Shillcock (2003) used an eye-tracker to monitor the gaze of participants reading sentences. They constructed the sentences so that some would have a verb-noun pair with a high bigram probability (such as ( 13.45 a)) and others a verb-noun pair with a low bigram probability (such as ( 13.45 b)). (13.45) a) HIGH PROB: One way to avoid confusion is to make the changes
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240 C HAPTER 13 S TATISTICAL P ARSING during vacation b) LOW PROB: One way to avoid discovery is to make the changes during vacation They found that the higher the bigram predictability of a word, the shorter the time that participants looked at the word (the initial-fixation duration ). While this result provides evidence only for N -gram probabilities, more recent experiments have suggested that the probability of an upcoming word given the syn- tactic parse of the preceding sentence prefix also predicts word reading time (Hale, 2001; Levy, 2008) . Interestingly, this effect of probability on reading time has also been shown for morphological structure; the time to recognize a word is influenced by entropy of the word and the entropy of the word’s morphological paradigm (Moscoso del Prado Mart´ ın et al., 2004) . The second family of studies has examined how humans disambiguate sentences that have multiple possible parses, suggesting that humans prefer whichever parse is more probable. These studies often rely on a specific class of temporarily am- biguous sentences called garden-path sentences. These sentences, first described Garden-path by Bever (1970) , are sentences that are cleverly constructed to have three properties that combine to make them very difficult for people to parse: 1. They are temporarily ambiguous : The sentence is unambiguous, but its ini- tial portion is ambiguous. 2. One of the two or more parses in the initial portion is somehow preferable to the human parsing mechanism. 3. But the dispreferred parse is the correct one for the sentence. The result of these three properties is that people are “led down the garden path” toward the incorrect parse and then are confused when they realize it’s the wrong one. Sometimes this confusion is quite conscious, as in Bever’s example ( 13.46 ); in fact, this sentence is so hard to parse that readers often need to be shown the correct structure. In the correct structure,
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