Subject-Object Asymmetries in Korean Sentence Comprehension

Subject-Object Asymmetries in Korean Sentence Comprehension Jiwon Yun (jy249@cornell.edu) John Whitman (jbw2@cornell.edu) John Hale (jthale@cornell.edu) Department of Linguistics, 203 Morrill Hall, Cornell University Ithaca, NY 14850 USA Theories of the Subject-Object Asymmetry Abstract The Entropy Reduction Hypothesis (Hale, 2006) derives the subject-object asymmetry in Korean relative clauses. This asymmetry has been observed by Kwon, Polinsky, and Kluender (2006), among others. Agreement between the En- tropy Reduction predictions and the available empirical data suggests that the heightened comprehension difficulty attested in object-extracted relatives is due to distinctive incremental parser states associated with comparatively greater temporary ambiguity. Keywords: sentence comprehension, relative clauses, Korean, probabilistic grammar, Entropy Reduction, syntax Introduction Relative clauses (RCs) have long been objects of fascination for cognitive scientists interested in language comprehen- sion (Kaplan, 1974). In the well-known “subject-extracted” (SRC) and “object-extracted” (ORC) cases, a large literature exists. In languages such as English and French, a process- ing advantage for SRCs has been confirmed in a wide variety of measures including phoneme-monitoring (Frauenfelder, Segui, & Mehler, 1980), eye-fixations (Holmes & O’Regan, 1981), reading times (King & Just, 1991), PET (Stromswold, Caplan, Alpert, & Rauch, 1996) and fMRI (Just, Carpen- ter, Keller, Eddy, & Thulborn, 1996). It has been suggested that the SRC advantage may be a processing universal (Lin, 2008). If ORCs are harder than SRCs in all languages, then what is it about human sentence comprehension that makes this so? The Korean language is a key test for any uni- versal processing theory because it is syntactically different from English and French. These differences include verb- final clauses and prenominal RCs. In this paper, we offer an account of the SRC/ORC asym- metry in terms of the information-processing difficulty of incremental parsing in general. This proposal relates the hardness of parsing to syntactic facts about Korean. A lan- guage independent complexity metric known as Entropy Re- duction (Wilson & Carroll, 1954; Hale, 2003, 2006) cor- rectly derives the SRC advantage when applied with a Korean grammar. This demonstration supports the claim that human comprehension difficulty reflects the kind of information- processing work that Entropy Reduction quantifies. 1 1 A longer companion paper, Hale (under review), develops an automaton model of the sentence comprehension process. It presents a generalized left-corner parser that operates in accordance with the Entropy Reduction Hypothesis when its decisions about how to re- solve nondeterminism are guided by experience. As an empirical phenomenon, the SRC/ORC processing asymmetry is well-established. However, its implications for the architecture or mechanisms of human language compre- hension remain controversial. Three broad classes of theory have been advanced. L INEAR D ISTANCE theories, illustrated in Figure 1, point to a greater number of intervening elements between the relativized position and the headnoun to which it is meaningfully related. The boxed e notation stands for an “empty” element. Particular theories of L INEAR D ISTANCE offer alternative ways of measuring the separation between this omitted position and the headnoun (Wanner & Maratsos, 1978; Gibson, 2000; Lewis & Vasishth, 2005). These theories all provide an adequate account of the English pattern, and in some cases relate this prediction to plausible mechanisms of human sentence comprehension. They are thwarted, however by data that confirm an SRC-over-ORC processing advantage in Korean (O’Grady, Lee, & Choo, 2003; Kwon et al., 2006; Lee, 2007). Figure 1(b) shows how theories of this type de- rive the wrong prediction for Korean. The second broad class includes S TRUCTURAL D ISTANCE theories. The simplest theory of this kind maintains that ORCs are harder because the relativized element is more deeply embedded when it is an object. If ORCs are formed by a movement rule, then this movement would “cross” both a VP node and an S node to arrive at its surface posi- tion (O’Grady, 1997, 179). Hawkins (2004, 175) singles-out “a connected path that must be accessed for gap identifica- tion and processing. ” Hawkins’ path is shown using dotted branches in Figure 2. This path is shorter for SRCs in both Korean and English. This general account is thus adequate but not very precise. It leaves open, for instance, the ques- tion of where exactly greater difficulty should start to accrue during incremental processing. The third broad class contains the I NFORMATION - THEORETICAL approaches. The Entropy Reduction Hypoth- esis (ERH) fits into this class. It holds that a person’s dif- ficulty at a word reflects the amount by which that word helped him or her to ascertain which construction the speaker intends. The ERH uses the concept of entropy to quantify the average uncertainty about derivations consistent with an observed initial string. This entropy is high when there are many equiprobable continuations and low when there are just a few continuations or the probability distribution on them is sharply concentrated. This quantity standsin for the degree of confusion in the comprehender’s mind. When it is reduced in the transition from one word to the next, the comprehender