Left-Corner Parsing and Psychological Plausibility

LEFT-CORNER PARSING AND PSYCHOLOGICAL PLAUSIBILITY Philip l~esnik Department of ColImuter and information Science University of Pennsylvania, Philadelphia, PA 19104, USA resnik©linc.cis.upenn.edu Abstract It is well known that even extremely limited center- embedding causes people to have difficulty ill comprehen- sion, but that left- and right-branching constractions pro- duce no such effect. If the difficulty in comprehension is taken to be a result of processing load, as is widely as- sumed, then measuring the processing load induced by a parsing strategy on these constructions may help determine its plausibility as a psychological model. On this basis, it has been ~rgued [A J91, JL83] that by identifying processing load with space utilization, we can rule out both top-down and bottom-up parsing as viable candidates for the human sentence processing mechanism, attd that left-corner pars- ing represents a plausible Mternative. Examining their arguments in detail, we find difficulties with each presentation. In this paper we revise the argu- ment and validate its central claim. In so doing, we discover that the key distinction between the parsing methods is not the form of prediction (top-down vs. bottom-up vs. left- corner), but rather the ability to iastantiate the operation of composition. 1 Introduction One of our most robust observations about language -- dating back at least to the seminal work of Miller and Chomsky [MC63] -- is that right- and left-branching constructions such as (la) and (lb) seem to cause no particular difficulty in processing, but that multiply center-embedded constructions such as (lc) are difficult to understand. a. [[[John's] brother's] eat] despises rats. b. This is [the dog that chased [the cat that bit [the rat that ate tbe cheese]]]. c. #[The rat that [the cat that lille dog] chased] bit] ate the cheese. The standard explanation for this distinction is a tight bound on space in the human sentence process- ing mechanism: center-embedded constructions require that the head noun phrase of each subject be stored un- til the processing of the embedded clause is complete and the corresponding verb is finally encountered) Alternative accounts have been proposed, most shar- ing the premise that the parser's capacity for recur- sion is limited by bounds on storage. (See, for exmn- pie, [Kim73] and [MI64]; for opposing views and other pointers to the literature see [DJK+82].) The distinction between center-embedding and left/right-branching has important implications for those who wish to construct psychologically plausible models of parsing. Johnson-Laird [JL83] observes that neither the top-down nor the bottom-up methods of constructing a parse tree fit the facts of (1), arid pro- poses instead the lesS-well-known alternative of left- corner parsing. Abney mid Johnson [AJgl] discuss a somewhat more general version of Johnson-Laird's argument, introducing the abstract notion of a pars- ing sf~ntegy in order to characterize what is meant by bottom-up, top-down, and left-corner parsing. In this paper, we examine the argument as pre- sented by Abney and Johnson and by Johnson-Laird, and point out a central problem with each variation. We then present the argument in a form …

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