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<Paper uid="C90-3021">
  <Title>A Computational Theory of Processing Overload and Garden-Path Effects</Title>
  <Section position="1" start_page="0" end_page="0" type="abstr">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> The limited capacity of working memory is intrinsic to human sentence processing, and therefore must be addressed by any theory of human sentence processing. I assume that the amount of short term memory that is necessary at any stage in the parsing process is determined by the syntactic, semantic and pragmatic properties of the structure(s) that have been built up to that point in the parse. A sentence becomes unacceptable for processing reasons if the combination of these properties produces too great a load for the working memory capacity (cf.</Paragraph>
    <Paragraph position="1"> Fr~ier (1985), Gibson (1987)):  (1) tl ~ Aixi &gt; K  where: K is the maximum allowable processing load (in processing load units or PLUs), x~ is the number of PLUs associated with property i, n is the number of properties, Ai is the number of times property i appears in the structure in question.</Paragraph>
    <Paragraph position="2"> Furthermore, I hypothesize that the human pat'scr prefers one structure over another when the processing load (in PLUs) associated with one structure is markedly lower than the load associated with another. That is, I hypothesize there exists some arithmetic preference quantity P, corresponding to a processing load difference, such that if the processing loads associated with two representations differ by P, then only the representation associated with the smaller of tile two loads survives. Given the existence of a preference quantity P, it is easy to account for garden-path effects and preferred readings of ambiguous sentences. Both effects occur because of a local ambiguity which is resolved in favor of one reading. Given two representations for the same input string that differ in processing load by at least P, only the less expensive structure will be maintained. If that sU'ucture is not comp,qtible with the rest of the sentence and the discarded structure is part of a successful parse of the sentence, a garden-path effect results. If the parse is suceessful, but the discarded structure is compatible with another reading for the sentence, then only a preferred reading for the sentence has been calculated (cf. Gibson (1987), Gibson &amp; Clark (1987), Clark &amp; Gibson (1988)). 1 Thus if we know 1 An alternative to the preference constraint presented here is the serial hypothesis, which allows at most one representation for the input string at any stage in the parse (see, for example, Frazier &amp; Fodor (1978), Frazier (1979), Marcus (1980), Berwick &amp; Weinberg (1984), and Pritchett (1988)). There is a longstanding debate in the psycholinguistic literature as to whether or not more than one representation for an input can be maintained in parallel. It turns out that the parallel view appears to handle some kinds of data more directly than the serial view, where one reading of a (temporarily) ambiguous sentence becomes the strongly preferred reading, we cm~ write ,an inequality associated with this preference: (2)</Paragraph>
    <Paragraph position="4"> where: P is the preference factor (in PLUs), xi is the number of PLUs associated with property i, n is the number of properties, Ai is the number of times property i appeaJs in the unpreferred structure, Bi is the number of times property i appears in the preferred structure.</Paragraph>
    <Paragraph position="5"> In this paper I will concentrate on syntactic properties: z in particular, I present two properties based on the 0-Criterion and Projection Principle from Government and Binding Theory (Chomsky (1981)). 3 Once these properties are ,associated with processing loads, they can predict a large array of garden-path effects. Furthermore, it is demonstratexl that these properties also make desirable cross-linguistic predictions with respect to unacceptability due to memory capacity overload. The organization of this paper is given as follows. Section 2 describes the s~ucture of the underlying parser that is ,assumed. Section 3 includes the proposed syntactic properties. Section 4 examines a number of locally ambiguous sentences, including some garden-paths, with respect to these properties. Section 5 explores a number of acceptable and unacceptable sentences and demonstrates that the properties proposed in Section 3 make the right predictions with respect to processing overdeg load. Furthermore, it is demonstrated in this section that these properties seem to make the right predictions cross-linguistically. Some conclusions ate given in the final section. 4 keeping in mind that the data are often controversial. See, for example, Kurtzman (1985)or Gorrell (1987) for a history of the debate along with evidence in support of the parallel hypothesis. Note in particular that data normally taken to be support for the serial hypothesis include garden-path effects and the existence of preferred readings of ambiguous input. However, as noted above, limiting the number of allowable representations is only one way of constraining parallelism so that these effects are also easily accounted for in a parallel framework.</Paragraph>
    <Paragraph position="6"> /Note that I assume that there also exist semantic and pragmatic properties which are associated with significantproccssing loads.</Paragraph>
    <Paragraph position="7"> Sin another syntactic theory, similar properties may be obtained from the principles that correspond to the 0-Criterion and Projection Principle in that theory. For example, the completeness and coherence conditions of Lexieal Functional Grammar (Bresnan (1982)) would derive properties similar to those derived from the 0-Criterion and Projection Principle. The same empirical effects should result from these two sets of properties.</Paragraph>
  </Section>
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