Segmenting the Signal: A Cross-Age Comparison of Speech and Tone Stimuli

The present study sought to extend findings of an “embeddedness constraint” on statistical learning in the visual domain (Fiser & Aslin, 2005) to the auditory domain. In this set of experiments, we explored the possible manifestations of an embeddedness constraint on speech and tone sequences in studies with adult and infant participants. An embeddedness constraint operant in the linguistic domain could benefit learners by helping them to circumvent the “combinatorial explosion” that arises from large amounts of complex data. This constraint could also prove useful in language acquisition tasks, such as word segmentation, allowing learners to advance more quickly to higher-order tasks, such as semantic and syntactic categorization. Furthermore, findings of an embeddedness constraint on both speech and tone stimuli might suggest that this constraint is broadly domain-general. Present results do not indicate an embeddedness constraint on speech or tone input in adults. However, pending studies with infants may suggest that the existence of the embeddedness constraint is itself constrained by maturation or by experience with language. Segmenting the Signal 3 Segmenting the Signal: A Cross-Age Comparison of Speech and Tone Stimuli One of the most challenging questions researchers must answer is how children acquire language so easily when the input is so complex. Comprehension of any given component of language entails a detailed knowledge of many dimensions. For example, to understand one single word, an infant must be familiar with the speech sounds of her native language, how these sounds generalize across speakers, and the word’s boundaries when the target item is spoken in fluent speech. Despite such intricacy, children elegantly synthesize the disparate parts of language into a functioning whole with greater ease and aplomb than adults (Newport, 1990). However, the complexity of language is not the only problem young learners face when acquiring their first language or languages. Infants must also learn to map between different complex aspects of the input. For any given word, there are an infinite number of possible meanings (Quine, 1960). For example, a child may see a big, red, bouncy ball, and hears the label “ball! ” Yet there is no reason for that child to assume that “ball” refers to the object as a whole. It could just as easily refer to a quality of the ball, such as “red” or “bouncy. ” Even in such a simple interaction the possibilities are endless. This complexity inherent in language, the seemingly infinite possibilities from the input, results in a formidable challenge known as the “curse of dimensionality” or the “combinatorial explosion. ” As it relates to language, the curse of dimensionality describes the difficulty of identifying the correct set of rules from infinite possibilities given only a limited number of exemplars. In an attempt to explain how learners circumvent this problem, researchers have amassed a large corpus of evidence to suggest that for both infants and adults to learn, there must be constraints on either the mechanism by which they learn or constraints on the input they receive (Yang, 2004; Pinker, 1994; Thiessen, 2009; Saffran, et al, 1996b). Although the idea of Segmenting the Signal 4 constraints is almost universally accepted by researchers, the nature of these constraints and the definition of the mechanism are sources of great disagreement. Some theorists regard both the mechanism and constraints on learning as domain-specific, or unique to linguistic stimuli, and possibly as biologically or genetically unique to humans. Others view both the mechanism of and constraints on learning as domain-general, or applicable to many different types of input, and possibly present across species. Broadly speaking, the two primary perspectives on language acquisition, Universal Grammar and Learning Theory, have arisen as a result of differing predictions about the nature of constraints and the type of mechanism that governs language acquisition. Nativists view constraints on language acquisition and the mechanism as domainspecific, whereas learning theorists see constraints on language acquisition and the mechanism as domain-general. In order to understand the sometimes subtle differences between predictions made by advocates of either theory, knowledge of the major developments of both perspectives is necessary. The perspective that the mechanism is a uniquely human endowment with languagespecific characteristics stems from Chomsky’s (1959) unequivocal rejection of behaviorist principles as outlined in A Review of B.F. Skinner’s Verbal Behavior. Most famously instantiated in the “Universal Grammar” theory introduced by Chomsky, the nativist claim rests on the idea that language is too complex of a system to be learned without preexisting, innate knowledge of its general concepts (Smith, 2005). In its early years, the nativist position argued that a Language Acquisition Device (LAD) governed primary language acquisition (Chomsky, 1965), but more recently this theory has evolved into a set of universal language principles with language-specific parameters (Culicover, 1997). The principles and parameters approach to language acquisition relies on a set of universal principles found only in human languages. Segmenting the Signal 5 According to this account, language-specific parameters are activated or deactivated depending on which language (or languages) a child is exposed to. The nativist position has found much support from linguists who advocate the “unity in variety” (Kager, 1999) approach very elegantly instantiated in linguistic models such as Optimality Theory (Prince & Smolensky, 2004). Optimality Theory resolves seemingly random nuances in the languages of the world by ranking a set of constraints on language. This model of universal grammar allows for an unlimited set of candidates from which the “optimal” candidate is selected by ranking and evaluating different language-universal constraints. The optimal candidate is the one which violates the fewest high-ranking constraints (see Kager, 1999 for discussion and examples). Central to all nativist theories of language acquisition is the idea that the input alone does not contain enough information for a child to acquire language. Based on Gold’s (1967) proof, which asserts that a language cannot be learned by positive evidence alone, Chomsky’s (1980) “poverty of the stimulus” argument forms the foundation for the “logical problem of language acquisition. ” The logical problem of language acquisition arises from a lack of negative evidence. Negative evidence is provided to the learner through corrections to the learner’s output. This is in contrast to positive evidence, which consists of the set of syntactically correct examples a learner hears in the input. The problem with learning from positive evidence alone (and thereby the alleged need for negative evidence) is that positive evidence can be consistent with multiple different hypotheses about the underlying structure of the input. For example, without negative evidence, a Francophone child could hypothesize that all adjectives follow the nouns which they modify. However, this hypothesis, while supported by much positive evidence, is not correct. Adjectives describing beauty, age, goodness, or size actually precede the nouns which they modify. The nativist solution to this logical problem is a set of innate, Segmenting the Signal 6 domain-specific constraints that assist the learner in acquiring his native language. These language-specific constraints are often referred to as a “universal grammar. ” It is important to note that Universal Grammar refers to both the domain-specific mechanism that enables children to acquire language as well as an actual manifestation of “universal grammar” that is common to all human languages. This universal grammar explores the abstract similarities between human languages (e.g. CV is the universal unmarked syllabic structure (Zamuner, 2003)) and explains them in terms of the biological or genetic mechanism of Universal Grammar (see Comrie, 1989 for a detailed discussion). The mechanism of Universal Grammar is defined as a species-specific, domain-specific, language module that humans are born with but then decays or disappears with time (Chomsky, 1965). This biological or genetic mechanism is thought to constrain the preferred types of cognitive representations that subsequently appear in the data. Both the mechanism and the manifestation of universal grammar can be viewed as constraints on language learning. Indeed, the manifestation of Universal Grammar operates as a constraint on the input (Bickerton, 1981), and both the mechanism and the manifestation constrain the output of the human (Landau, 1998). However, the interaction between the mechanism of Universal Grammar and its manifestation poses a problem for those researchers interested in language evolution. Christiansen and Chater (2008) argue that an insurmountable flaw in the Universal Grammar theory of language acquisition is the “circularity trap (5)” because it poses a classic chicken-and-egg problem: which came first, the mechanism or the manifestation? As the mechanism of Universal Grammar is purported to be highly specific, it seems unlikely that all aspects present in the mechanism could have (or would have) evolved suddenly through one single chance mutation. If the mechanism of Universal Grammar evolved Segmenting the Signal 7 over time, it may have been shaped by demands independent of the original mechanism, such as production limitations or social pressures. Such an explanation is inconsistent with a central claim of Universal Grammar: that the input is constrained by the mechanism and not vice versa. Conversely, a Learning Theory of language acquisition allows for interaction between the input and the mechanism. Rather than viewing language acquisition as the result