Notes on computational linguistics.pdf - UCLA Department of ...

Stabler - Lx 185/209 2003
(58) Marcus (1980) made two very interesting proposals about the structural ambiguity of sentence prefixes.
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a. First: (at least some) garden paths indicate failed local ambiguity resolution. Marcus proposes
that humans have difficulty with certain local ambiguities (or fail completely), resulting in the familiar
“garden path” effects: 30 The following sentences exhibit extreme difficulty, but other less
extreme variations in difficulty may also evidence the greater or less backtracking involved:
a. The horse raced past the barn fell
b. Horses raced past barns fall
c. The man who hunts ducks out on weekends
d. Fat people eat accumulates
e. The boat floated down the river sank
f. The dealer sold the forgery complained
g. Without her contributions would be impossible
b.
This initially very plausible idea has not been easy to defend. One kind of problem is that some
constructions which should involve backtracking are relatively easy: see for example the discussions
in Pritchett (1992) and Frazier and Clifton (1996).
Second: to reduce backtracking to human level, delay decisions until next constituent is built.
Suppose we agree that some garden paths should be taken as evidence of backtracking: we would
like to explain why sentences like the ones we were considering earlier (repeated here) are not as
difficult as the garden paths just mentioned:
a. i. Have the students take the exam!
ii. Have the students taken the exam?
b. i. Is the block sitting in the box?
ii. Is the block sitting in the box red?
The reason that k symbols of lookahead will not resolve these ambiguities is that the disambiguating
words are on the other side of a noun phrase, and noun phrases can be arbitrarily long. So Marcus
proposes that when confronted with such situations, the human parser delays the decision until
after the next phrase is constructed. In effect, this allows the parser to look some finite number
of constituents ahead, instead of just a finite number of words ahead. 31 This is an appealing idea
which may deserve further consideration in the context of more recent proposals about human
languages.
29 These proposals were developed in various ways by Berwick and Weinberg (1984), Nozohoor-Farshi (1986), and van de Koot (1990).
These basic proposals are critiqued quite carefully by Fodor (1985) and by Pritchett (1992).
30 There are many studies of garden path effects in human language understanding. Some of the prominent early studies are the
following: Bever (1970), Frazier (1978), Frazier and Rayner (1982), Ford, Bresnan, and Kaplan (1982), Crain and Steedman (1985),
Pritchett (1992).
31 Parsing strategies of this kind are sometimes called “non-canonical.” They were noticed by Knuth (1965), and developed further
by Szymanski and Williams (1976). They are briefly discussed in Aho and Ullman (1972, §6.2). A formal study of Marcus’s linguistic
proposals is carefully done by Nozohoor-Farshi (1986).
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