Cognitive Architectures and Language 1 Running Head: COGNITIVE ARCHITECTURES AND LANGUAGE Cognitive Architectures and Language Acquisition: A Case Study in Pronoun Comprehension

In this paper we discuss a computational cognitive model of children’s poor performance on pronoun interpretation (the so-called Delay of Principle B Effect, or DPBE). This cognitive model is based on a theoretical account that attributes the DPBE to children’s inability as hearers to also take into account the speaker’s perspective. The cognitive model predicts that child hearers are unable to do so because their speed of linguistic processing is too limited to perform this second step in interpretation. We tested this hypothesis empirically in a psycholinguistic study, in which we slowed down the speech rate to give children more time for interpretation, and in a computational simulation study. The results of the two studies confirm the predictions of our model. Moreover, these studies show that embedding a theory of linguistic competence in a cognitive architecture allows for the generation of detailed and testable predictions with respect to linguistic performance. Cognitive Architectures and Language 3 Cognitive Architectures and Language Acquisition: A Case Study in Pronoun Comprehension Introduction An influential but also controversial distinction in linguistic research is the distinction between linguistic competence and linguistic performance (Chomsky, 1965). Linguistic competence pertains to the idealized linguistic knowledge a language user has of his or her language, which is often contrasted with linguistic performance, the actual use of this knowledge in concrete situations. This distinction between competence and performance provided a rationale for studying linguistic phenomena separately from cognitive factors. However, this distinction also created the methodological problem that it became impossible to empirically test theories of linguistic competence solely by studying linguistic performance. As a result, linguistic analyses appealing to aspects of linguistic performance such as insufficient working memory capacity, processing limitations or pragmatic skills are difficult to evaluate. Nevertheless, such analyses have been proposed in many areas of language acquisition to explain differences in linguistic performance between children and adults. The aim of this paper is to show that embedding a theory of linguistic competence in a cognitive architecture may allow for the generation of detailed and testable predictions with respect to linguistic performance. A cognitive architecture is a general framework that incorporates built-in and well-tested parameters and constraints on cognitive processes. Within a cognitive architecture, computational models can be built that simulate the cognitive processes involved in performing a task such as interpreting a sentence. The predictions generated by these computational models can be tested on the basis of empirical data, for example the performance Cognitive Architectures and Language 4 results obtained from a psycholinguistic experiment. As a case study, we present an account of the Delay of Principle B Effect in language acquisition (e.g., Chien & Wexler, 1990; Jakubowicz, 1984; Koster, 1993). The Delay of Principle B Effect (DPBE) concerns the observation that children’s comprehension of pronouns is delayed in comparison with their comprehension of reflexives. Initially, children show incorrect performance on pronoun comprehension as well as on reflexive comprehension. However, when they have mastered reflexive comprehension, they still show incorrect performance on pronoun comprehension. This phenomenon in language acquisition is referred to as the DPBE. It can take several years before children show correct performance on both pronoun comprehension and reflexive comprehension. The DPBE has received a variety of explanations, many of which appeal to performance factors to account for children’s errors in comprehending pronouns. One such explanation is formulated within the linguistic framework of Optimality Theory (Hendriks & Spenader, 2005/2006). We show how a cognitive model can be built within the cognitive architecture ACTR (Anderson et al., 2004) that implements an optimality theoretic eplanation of the DPBE. The resulting cognitive model predicts that children will make fewer errors in their interpretation of pronouns but not in their interpretation of reflexives if they are given more time for comprehension, for example by slowing down the speech rate. We tested this prediction empirically in a psycholinguistic study as well as in a computational simulation study. The organization of this paper is as follows. First, we discuss the DPBE and several of the proposed explanations to account for this delay in language acquisition, including a detailed account of the optimality theoretic explanation of the DPBE. Then we present a cognitive model that is based on the optimality theoretic explanation of the DPBE. The hypotheses derived from Cognitive Architectures and Language 5 this cognitive model are first tested in a psycholinguistic experiment involving 75 Dutch children between 4;1 and 6;3 years old. Then a simulation study is discussed in which the effects of speech rate on the comprehension of sentences with pronouns and reflexives are modeled. In this second study, the performance of a group of children is simulated and compared to the results of the psycholinguistic experiment. The paper concludes with a discussion of the considerations and limitations in using cognitive models to study theories of language acquisition. Delay of Principle B Effect (DPBE) A well-established finding in language acquisition research is the observation that, in languages such as English, French and Dutch, children’s comprehension of pronouns is delayed in comparison with their comprehension of reflexives (e.g., Chien & Wexler, 1990; Jakubowicz, 1984; Koster, 1993; Philip & Coopmans, 1996; Spenader, Smits, & Hendriks, 2009). This phenomenon is called the Delay of Principle B Effect (DPBE). Principle B is one of the two principles of Binding Theory that relate to the adult use and interpretation of reflexives and pronouns (Chomsky, 1981): (1a) Principle A: a reflexive must be bound in its local domain. (1b) Principle B: a pronoun must be free in its local domain. The local domain is defined as the minimal clause containing both the lexical anaphor and a subject. An anaphor is bound when it is co-indexed with and c-commanded by an antecedent. Sentences (2a) and (2b) illustrate the application of Principles A and B: (2a) The penguini is hitting himselfi/*j with a pan. (2b) The penguini is hitting him*i/j with a pan. The reflexive himself in (2a) can only co-refer with the local subject the penguin, in accordance with Principle A, and may not co-refer with another referent. In contrast, Principle B prevents the Cognitive Architectures and Language 6 pronoun him in (2b) from co-referring with the penguin. Therefore, him must co-refer with another referent present in the linguistic or extra-linguistic context. From the age of 3;0 on, children are able to interpret sentences with reflexives, like (2a), correctly, thus displaying knowledge of Principle A. However, up to the age of 6;6, children show difficulties in the interpretation of pronouns in sentences like (2b) (e.g., Chien & Wexler, 1990). They seem to choose freely between a coreferential interpretation, in which the pronoun co-refers with the local subject, and a disjoint interpretation, in which the pronoun co-refers with an antecedent outside its local domain. Thus, in comprehension children act as if they only have access to Principle A. Their acquisition of Principle B seems to be delayed. Explanations of the DPBE To explain the Delay of Principle B Effect (DPBE), several theories have been proposed. In this section, we limit ourselves to two well-accepted theories: the pragmatic account of Thornton and Wexler (1999), and the processing account of Reinhart (2006). Both Thornton and Wexler’s and Reinhart’s account proceed from a nativist view on language. Hence, they assume that children have knowledge of both Principle A and B, and should in principle be able to apply this knowledge. However, the accounts differ in their explanation of why Principle B is delayed. Thornton and Wexler (1999) propose that the DPBE is caused by a deficiency in pragmatic knowledge. The starting point for their theory is the observation that in certain special contexts a pronoun may receive a coreferential interpretation, for example when the event being described is unexpected or uncharacteristic. To indicate that such an exceptional coreferential interpretation is intended, speakers stress the pronoun (‘Mama Bear is washing HER’, see Thornton & Wexler, 1999, p. 94), in addition to providing special pragmatic context. Thornton and Wexler argue that children do not yet have sufficient world knowledge and pragmatic Cognitive Architectures and Language 7 knowledge to determine whether the event described by the sentence reflects a typical or atypical situation, that is, to evaluate whether the context licenses a coreferential interpretation. Furthermore, Thornton and Wexler argue that children do not recognize stress on a pronoun as an indication that the speaker intended to express an atypical interpretation. As a result, children accept a coreferential interpretation of a pronoun sentence such as ‘Mama Bear is washing her’. For adult language users, only a disjoint interpretation is possible for this sentence, because adults do not allow a coreferential interpretation in the absence of stress. So children over-accept coreferential interpretations of pronouns because they are unable to distinguish the contexts that license coreferential interpretations from the contexts that do not license such interpretations. Children will have to acquire the world knowledge and pragmatic knowledge necessary to disallow a coreferential interpretation of a pronoun in non-exceptional contexts. Although their account focuses on the comprehension of pronouns, Thornton and Wexler point out that this lack of pragmatic knowledge has ramifications for children’s production as well (1999, p. 95). However, under their account it remains a mystery why children who show difficulties on pronoun comprehension at the same time show adult-like performance on pronoun production (see De Villiers, Cahillane, & Altreuter, 2006; Spenader et al., 2009). In contrast to Thornton and Wexler, Reinhart (2006) argues that children possess all knowledge required for the interpretation of pronouns. The crucial difference between children and adults is that children fail to complete the operation of reference-set computation. Reference-set computation is an operation that is performed by the parser to choose between multiple interpretations generated by the grammar. The operation is required for determining whether a coreferential interpretation is permitted for a pronoun. For a sentence such as (3), for example, the grammar generates two different derivations: one giving rise to a bound variable Cognitive Architectures and Language 8 interpretation (3a), and one giving rise to a coreferential (3b) or disjoint (3c) interpretation. A coreferential interpretation arises if the two variables x and y both happen to be resolved to the same referent, in this case Lili, whereas a disjoint interpretation arises if x and y are resolved to different referents. (3) Only Lili thinks she’s got the flu. (adapted from Reinhart, 2006, p. 167) a. Bound variable interpretation: Only Lili (λx (x thinks x has got the flu)) b. Coreferential interpretation: Only Lili (λx (x thinks y has got the flu) & y = Lili) c. Disjoint interpretation: Only Lili (λx (x thinks y has got the flu) & y ≠ Lili) The grammar allows the bound variable interpretation (3a) for sentence (3), because the pronoun she is not bound within its local domain (cf. Principle B). The grammar also allows the pronoun to be interpreted as a free variable, giving rise to the disjoint interpretation (3c). Whether coreferential interpretation (3b) is allowed, however, must be determined through reference set computation. Reference-set computation involves the comparison of pairs of derivations and their corresponding interpretations. A coreferential interpretation is allowed for (3) only if this interpretation is different from the bound variable interpretation. If these interpretations are indistinguishable, a coreferential interpretation is not allowed because it is inefficient to revert back to an interpretation that is ruled out by the grammar through the discourse option of coreference. With respect to sentence (3), the coreferential interpretation is allowed, because (3a) and (3b) have slightly different meanings. Interpretation (3a) entails that other people do not think that they have got the flu, whereas interpretation (3b) entails that other people do not think that Lili has got the flu. The situation is slightly different for the sentence in (4). (4) Mama Bear is washing her. a. Bound variable interpretation: Mama Bear (λx (x is washing x)) Cognitive Architectures and Language 9 b. Coreferential interpretation: Mama Bear (λx (x is washing y)) & y = Mama Bear c. Disjoint interpretation: Mama Bear (λx (x is washing y)) & y ≠ Mama Bear For this sentence, the grammar (Principle B) disallows the bound variable interpretation (4a), because the pronoun her would be bound within its local domain. Although (4a) is disallowed by the grammar, reference-set computation nevertheless requires that a bound variable derivation is constructed and its interpretation is compared with the coreferential interpretation (4b). Because the two interpretations are indistinguishable, the coreferential interpretation is not allowed for sentence (4). Consequently, only the disjoint interpretation (4c) is possible for this sentence. Reinhart argues that children may be unable to perform this operation of reference-set computation because of working memory limitations. If children fail to complete the operation of reference-set computation, they resort to a guessing strategy and arbitrarily choose between a coreferential and a disjoint interpretation. Other strategies are conceivable as well and are used with other marked forms requiring reference-set computation, such as contrastive stress. Only when children have developed sufficient working memory capacity will they be able to complete the operation of reference-set computation and disallow the coreferential interpretation for pronouns. Because the grammar generates two derivations for pronoun sentences but not for reflexive sentences, reference-set computation is not involved in the interpretation of reflexives. With respect to the production of pronouns, as speakers know which meaning they intend for the utterance, reference-set computation is not involved in production either. This would explain why children are able to produce pronouns correctly from a young age on while still having difficulties with the comprehension of pronouns (De Villiers et al., 2006; Spenader et al., 2009). Cognitive Architectures and Language 10 In this section, we discussed Thornton and Wexler’s (1999) pragmatic account of the DPBE and Reinhart’s (2006) processing account of this puzzling delay in language acquisition. In the next section, we contrast these theories with an alternative theory: the optimality theoretic account of Hendriks and Spenader , which assumes that only Principle A is part of grammar and Principle B is a derived effect. Optimality Theory explanation of the DPBE A third type of explanation of the DPBE is provided by Hendriks and Spenader (2005/2006). They argue that the DPBE is the result of a direction-sensitive grammar, that is, a grammar that has different effects in production and comprehension. Their account is formulated within the framework of Optimality Theory (OT), a linguistic framework that models the relationship between a surface form and its underlying structure by means of optimization from a particular input to the optimal output for that input (Prince & Smolensky, 2004). In the domain of semantics, OT describes the relation between an input form and the optimal meaning for that form (e.g., Hendriks & De Hoop, 2001). Applied to syntax, OT describes the relation between an input meaning and the optimal form for expressing that meaning. OT thus provides an account of linguistic competence with respect to language production (i.e., OT syntax) as well as language comprehension (i.e., OT semantics). In OT, the grammar consists of a set of violable constraints, rather than inviolable rules. For every input, which can be either a form or a meaning, a set of potential outputs, or candidates, is generated. These candidates are evaluated on the basis of the constraints of the grammar. In OT, constraints are as general as possible and hence may conflict. OT resolves conflicts among constraints by ranking the constraints in a language specific hierarchy on the basis of their strength. One violation of a stronger (i.e., higher ranked) constraint is more important than many violations of a weaker (i.e., lower ranked) constraint. The Cognitive Architectures and Language 11 optimal candidate is the candidate that commits the least severe constraint violations. Only the optimal candidate is realized. Direction-sensitive grammar For their explanation of the DPBE, Hendriks and Spenader (2005/2006) exploit the fact that an OT grammar is inherently direction-sensitive: The form-meaning relations defined by the OT grammar are not necessarily the same from the speaker’s perspective (involving optimization from meaning to form) as from the hearer’s perspective (involving optimization from form to meaning) (Smolensky, 1996). This property of OT is a result of the output orientation of the markedness constraints in OT. OT assumes two kinds of constraints. Faithfulness constraints evaluate the similarity between input and output. Because faithfulness constraints pertain to the mapping between input and output, these constraints are direction-insensitive and also apply in the reverse direction of optimization. An example is the constraint PRINCIPLE A (5), which prohibits reflexives from being locally free. This constraint induces hearers to assign a locally bound interpretation to reflexives and at the same time prohibits speakers to express a disjoint interpretation by using a reflexive. (5) PRINCIPLE A: A reflexive must be bound in its local domain. Markedness constraints on forms, on the other hand, reflect a preference for unmarked forms, irrespective of their meaning. Because they pertain to the output only, markedness constraints on form only have an effect when a form must be selected from a set of candidate forms. That is, they only have an effect from the speaker’s perspective. An example is the constraint AVOID PRONOUNS. For hearers, this constraint does not have any effect, because for hearers the form is already given as the input. The hearer’s task is to select the optimal meaning for this form. Since the constraint AVOID PRONOUNS does not distinguish between potential meanings, it does not Cognitive Architectures and Language 12 have any effect from the hearer’s perspective. The constraint AVOID PRONOUNS is part of the constraint hierarchy REFERENTIAL ECONOMY (6). This constraint hierarchy consists of several markedness constraints, of which AVOID REFLEXIVES is the lowest ranked. The hierarchy reflects a preference for less referential content: Reflexives are preferred over pronouns, and pronouns over full NPs. (6) REFERENTIAL ECONOMY: Avoid full NPs » Avoid pronouns » Avoid reflexives 2 In this discussion we limit ourselves to the choice between pronouns and reflexives and hence only consider the constraint AVOID PRONOUNS. This constraint is violated by any pronoun in the output, and is satisfied by any reflexive in the output. The presence of markedness constraints such as AVOID PRONOUNS can lead to an asymmetry between production and comprehension, as is shown below. The evaluation of candidates on the basis of the constraints of the grammar can be illustrated with an OT tableau. Figure 1 displays the two comprehension tableaux representing the comprehension of a reflexive and the comprehension of a pronoun, respectively. The input to a comprehension tableau is a form and the output is the optimal meaning for this form. The constraints are presented in columns in order of descending strength, from left to right. PRINCIPLE A must be ranked higher than AVOID PRONOUNS because otherwise pronouns would never be selected. The relevant candidate outputs (in this case, potential meanings for the input form) are listed in the first column. A violation of a constraint is marked with a ‘*’, and a fatal violation with a ‘!’. The optimal output is marked by ‘☞’.