Classes of Parsing Models

2.3.2 Delayed Attachment Parsers

  The second permissible behaviour for the parser is to ‘wait and see’. No commitment is made until disambiguating information is encountered. The ambiguity is still parsed, but as a lower-level fragment (e.g. Frazier Rayner, 1982). Most accounts hold that the fragments are reconstituted at the point of disambiguation.

2.3.3 Parallel Parsers

  The third behaviour is to deal with an ambiguity by computing many, even all, solutions. Typically, these are maintained until disambiguating information is encountered. Some models rank the solutions (e.g., Gibson, 1991; Gorrell, 1987; Kurtzman, 1985; Spivey Tanenhaus, 1998). Thus, not all alternatives are pursued. One notable model (MacDonald, Pearlmutter Seidenberg, 1994) assigns a weight to each parallel representation. These weights are founded upon the interaction of non- encapsulated sources of information: lexical, phonological, morphological, syntactic, text or discourse. One important implication of this model is its susceptibility to prior context. The authors posit that the interaction of information may be affected by working memory constraints (see MacDonald, Just Carpenter, 1992).

2.3.4 Predictions

  Serial Models When the point of ambiguity is reached, on-line processing should continue without a

  loss in speed because there is no cost in pursuing one analysis. (The case is less clear for annotated serial models; the cost of annotation might be undetectable.) Two scenarios are possible at the point of disambiguation. In the first, the parser will have chosen the correct analysis. Thus, on-line reading times will not differ between temporarily ambiguous (TA) sentences and unambiguous controls. In the second, the parser will choose the wrong path. The parse tree must be dismantled and reconstructed to incorporate the new (disambiguating) material. The time it takes to complete this operation represents the classic ‘garden path’ effect (Frazier, 1979). For a TA sentence, the effect may be quantified as the difference between reading times at or after the disambiguating region in comparison with an unambiguous control.

  Less straightforward accounts, such as probabilistic serial models, differ in their predictions. In this case, parsing solutions are selected on the basis of their relative probability. This account has rather more ‘degrees of freedom’ in accounting for garden-path effects because judgement of probability and access to probabilistic information sources may vary between individuals. Where one individual selects a parsing solution that is subsequently disproved, another may not. However, one would expect that, in general, average disambiguation times will be longer for TA sentences than unambiguous controls.

  Delayed Attachment Models There are many models of delayed attachment, and the manner in which they delay, and

  why, will influence empirical predictions. Rather like probabilistic serial models, delayed attachment models may accommodate any strength of garden-path effect with hindsight. This is because there may be an interaction between the cost associated with

  a point of ambiguity – the parser notes its presence and enters a fragment-processing mode – and reuniting the structure once disambiguating material is encountered. The cost of mode change may be traded against the benefit of a simpler analysis (the fragment being less complex).

  Parallel Models Unweighted parallel models predict no processing cost at either the point of ambiguity

  or point of disambiguation. However, constrained parallel models hold that, upon entering the ambiguous region, parallel structures are limited by working memory limitations. There should, therefore, be a processing cost at the point of ambiguity as this ceiling of processing is reached.

  Weighted parallel models, on the other hand, maintain weighted parallel representations. At the point of ambiguity, the weights are updated so that the greatest activation is given to the most likely parse tree (this likelihood might be determined by multiple constraint satisfaction, see MacDonald, Pearlmutter Seidenberg, 1994). There is a cost associated with the update of the weights from a preferred solution to a less-preferred one. This cost would manifest itself as a garden-path effect. However, this cost is not strictly a garden-path effect because the parser is not serial. As a final empirical note, weighted parallel models tend to carry the assumption that probabilistic information and its accessibility will vary between individuals and sentences (MacDonald, Just Carpenter, 1992).

2.3.5 Evidence

  Here we will examine key evidence for each class. Constrained parallel models are relatively simple to evaluate: they predict a cost upon entering the ambiguous region and this is not generally the case (e.g. Frazier Rayner, 1982; Ferreira Henderson, 1990; Kennedy Murray, 1984; Kennedy, Murrary, Jennings Reid, 1989; Mitchell, Corley Garnham, 1992). Unweighted parallel models are similarly discountable Here we will examine key evidence for each class. Constrained parallel models are relatively simple to evaluate: they predict a cost upon entering the ambiguous region and this is not generally the case (e.g. Frazier Rayner, 1982; Ferreira Henderson, 1990; Kennedy Murray, 1984; Kennedy, Murrary, Jennings Reid, 1989; Mitchell, Corley Garnham, 1992). Unweighted parallel models are similarly discountable

  Evidence also speaks against ranked parallel models (e.g. Gibson, 1991). For such models, alternatives at the point of disambiguation are ranked. This rank determines accessibility. The nature of the disambiguating material should not influence this accessibility (Frazier, 1998). However, there is evidence that this may be the case. Fodor and Inoue (1994) presented participants with the following TA sentences:

  (11) Have the soldiers marched to the barracks tomorrow.

  (12) Have the soldiers marched to the barracks already?

  (13) Have the soldiers marched to the barracks, would you.

  Sentence (11) elicited a strong garden-path effect in comparison with (12). The preferred analysis has an interrogative structure until, with tomorrow, the structure is disambiguated as imperative. However, if the structure is disambiguated – again, as an imperative – with the adverbial would you (13), the garden-path effect is reduced or eliminated. Clearly, then, ranked parallel models must be modified to incorporate the characteristics of the disambiguating material. This would render them weighted parallel models. The Fodor and Inoue study is entirely consistent with serial parsers because reanalysis can incorporate the characteristics of the disambiguating material.

  So far, the evidence speaks against constrained, unweighted and ranked parallel

  models 1 . Weighted parallel and serial models remain. Though the predictions of such models overlap considerably, the key to their behaviour lies in their system state. During the ambiguous region, the parallel parser should entertain more than one active parse tree (or its equivalent) whereas the serial model should have only one. Thus, within this region, the parallel parser should have already partially computed the dispreferred structure; the serial parser should not (Hickok, 1993; MacDonald, Just, Carpenter, 1992; Nicol Pickering, 1993).

  There is some evidence for this partial computation, though it is not conclusive. For example, Pearlmutter and Mendelsohn (1998) found an effect of plausibility for the dispreferred analysis in this TA sentence:

  1 Annotated serial models will considered under the umbrella term ‘serial’ because there is little to distinguish them empirically.

  (14) The report that the dictator describedbombed the country

  seemed to be false.

  This is a complementnominal structure. The verb bombed elicited a greater processing cost than described (as compared with baselines The report showing that the dictator describedbombed the country seemed to be false). The authors ascribe this cost to the relative implausibility of bombed as a host for the dispreferred RC interpretation – it is less likely that a dictator would bomb a report than describe it. If this is true, then the parser is influenced by the plausibility of alternative structures. For a deterministic serial parser, the disprefered analysis should have no effect because it is not processed at this point. Pearlmutter Mendelsohn (1998) point out that a serial parser may

  accommodate the effect if it is probabilistic in nature 1 . However, Pearlmutter and Mendelsohn go on to provide evidence that purports to be problematic for a probabilistic serial parser. They noted that the noun’s preference for a sentential complement (SC) reading of the above sentence was negatively correlated with the processing cost at the disambiguating point (which disambiguated as a SC structure). They suggest that this serves as the probabilistic information for the serial parser. If this is the case, then as the noun’s preference for an SC reading decreases, the parser’s preference for an RC reading should increase. This negative correlation was not reliable; the authors claim that a serial probabilistic parser cannot account for the finding. Lewis (2000), however, proposes an alternative explanation for the unreliable correlation. Simply, it might be the case that, with a noun that favours the SC interpretation, analysis is accelerated in the disambiguating region. So far, then, the evidence against a serial probabilistic parser is inconclusive.

  Finally, we should examine a finding that is problematic for delayed attachment models. With such models, the current parse tree is abandoned at the point of ambiguity. The remainder of the sentence is parsed separately until disambiguating information is encountered. At this time, the two structures are reunited. This account does not permit any interaction between the two structures. Mitchell (1994) tested this property with the following TA sentence:

  1 They go on to criticise a serial parsing account, though their argument centres around a null finding and is still subject to debate (Lewis, 2000).

  (15) Just as the guard shouted the intruder escaped…

  (16) Just as the guard shouted the command an intruder escaped…

  (17) Just as the guard shouted the command was given…

  The delayed attachment parser should begin a separate parse tree when the is encountered after shouted. If the tree is indeed separate, there should be no matching of the properties of the next word (intruder or command in (15) and (16) or (17)) against shouted to check its legality. In this example, shouted is intransitive. Intruder must therefore be the subject of the matrix clause. It becomes the point of disambiguation. A processing cost at this point was found. But how did the parser ‘know’ that the combination of shouted and intruder disambiguated the sentence if the two structures are handled separately? Without a substantial modification to the model, it cannot.

  Summary

  To date, several classes of parser have been proposed: serial parsers that may proceed deterministically or probabilistically; delayed attachment parsers; parallel parsers that develop unweighted, ranked or weighted representations. On balance, the evidence marginally favours a weighted parallel parser, though attempts to falsify both deterministic and probabilistic serial parsers are not conclusive.