Caterina Marino

Judit Gervain
email: judit.gervain@parisdescartes.fr

Marino

The impact of generative linguistics on psychology: Language acquisition, a paradigm example

Keywords: language acquisition, generative linguistics, poverty of the stimulus, logical problem of language acquisition, bootstrapping theories of language acquisition

Noam Chomsky's early work was at the core of the "cognitive revolution" in the 1950s-60s, leading to a paradigm shift from a behavioralist to a mentalistic approach to human psychology. Central to this revolution has been the question of how infants learn language. Here, we provide an overview of how the generative enterprise has shaped research on language acquisition over the last decades. We argue that a large body of empirical knowledge about infants' representation of grammar has accumulated. Many of these questions would most likely not have been investigated empirically without the impetus of such a mentalistic approach.

Introduction

Noam Chomsky's early work and generative linguistic theory more generally were at the core of the "cognitive revolution" in the 1950s-60s, leading to a paradigm shift from a behavioralist to a more mentalistic approach to human psychology. Central to cognitive science, a discipline born out of this initial movement is the question of how human infants learn their native language(s). Here, we provide an overview of how Chomsky's work and the generative enterprise have shaped research on language acquisition over the last decades. We argue that under the influence of generative grammar, several surprising and highly relevant findings about infants' early knowledge of grammar have been revealed, many of which would most likely not even have been investigated empirically without the impetus of such a mentalistic approach. We also highlight that more recently, these advances have led to a new, integrative perspective, going beyond the original theoretical dichotomy of nature (nativism) vs. nurture (empiricism).

Language Acquisition as a Logical Problem, Innateness as a Solution

The problem of accounting for language acquisition appeared very early in Noam Chomsky's work. In his review of Skinner's Verbal Behavior, Chomsky (1959) draws a parallel between the infants' task of learning a language and the linguist's job of describing the grammar of an unknown language. Both the child and the linguist need to derive a rule system from the finite set of linguistic exemplars they encounter. This, Chomsky, argues is a logical impossibility. As the induction problem, a well-known mathematical phenomenon, suggests, for any finite dataset, there is an infinite number of underlying rules that could generate it. It is thus impossible for the learner to know which rule system to select as the "true" grammar of the target language. For language acquisition, this would mean that even if infants settled on a given grammar, there was no guarantee that any two infants settled on the same grammar and would thus end up learning the same language. This, of course, is contrary to empirical fact, as all typically developing infants acquire the language(s) spoken in their linguistic community.

This "poverty of the stimulus" problem is further exacerbated because the linguistic input infants receive is noisy, containing within-and between-speaker variation, errors, omissions, false starts etc. Even more importantly, infants only rarely receive negative evidence [START_REF] Marcus | Negative Evidence in Language Acquisition[END_REF], i.e. information that a sentence or a form is not part of the grammar, i.e. is ungrammatical, because adults usually only correct factual or objective mistakes in children's speech (e.g. This is a tiger when it's actually a lion), but not grammatical errors (e.g. Tom want chocolate) [START_REF] Braine | On two types of models of the internalization of grammars[END_REF]. Further, infants never receive the kind of explicit teaching about the rules of grammar that second language learners often get.

If language cannot be learned from external input alone due to the infinite number of possible grammars, argues Chomsky, an innate system, a "Language Acquisition Device" needs to exist to narrow down the logical space of possible grammars, thereby helping the infant converge on the appropriate grammar. "The fact that all normal children acquire essentially comparable grammars […] suggests that human beings are […] designed to do this, with [a] "hypothesis-formulating" ability of unknown character and complexity" (Chomsky 1959, 49). This mentalistic approach went against the behaviorist account proposed for instance in Skinner's Verbal Behavior [START_REF] Skinner | Century psychology series. Verbal behavior[END_REF], which framed language learning as a stimulus-response phenomenon, the most important learning mechanism of which is imitation.

The innate nature of the language faculty implies that it is grounded in our genetic endowment. This approach moves away from considering language a cultural object that varies infinitely across populations (e.g. Boas 1940) and places it in the domain of human biology, universal across our species.

The poverty of stimulus argument is not without its critiques. [START_REF] Pullum | Empirical Assessment of Stimulus Poverty Arguments[END_REF], for instance, provide empirical arguments suggesting that the infant's input may not be as poor in relevant positive evidence as generative linguists usually claim. One often cited example for poverty is stimulus arguments [START_REF] Crain | Structure Dependence in Grammar Formation[END_REF][START_REF] Crain | Language acquisition in the absence of experience[END_REF] is the structure-sensitivity of question formation when a complex sentence with an embedded relative clause is present. It has been argued that while infants encounter many examples of question formation for simple main clauses (You are there -> Are you there?), instances of complex cases involving embedded relatives are rare (The dog that is in the corner is hungry -> Is the dog that is in the corner _ hungry?, and not *Is the dog that _ in the corner is hungry?). Generativist accounts have argued that sentences of the type Is the dog that is in the corner _ hungry?, constituting positive evidence that question formation involves the main verb (main auxiliary) and not the sequentially first one, are rare in young learners' input, yet children typically for these complex questions correctly from an early age [START_REF] Crain | Structure Dependence in Grammar Formation[END_REF]. By contrast, [START_REF] Pullum | Empirical Assessment of Stimulus Poverty Arguments[END_REF] show that such sentences do appear in the input, citing examples from literature (e.g. William Blake's "Tiger": Did He who made the lamb make thee?), the Wall Street Journal corpus, and most relevantly in the CHILDES corpus. In general, the poverty of stimulus argument has given a strong impetus to empirical studies of the input young learners receive. The CHILDES corpus [START_REF] Macwhinney | The CHILDES Project : Tools for Analyzing Talk[END_REF], one of the largest annotated written and audio(visual) databases of infant-adult and infant-infant interactions, has to a large extent grown out of efforts to better understand the input. Despite this attention and the growing body of available corpus data, no definitive empirical evidence exists to settle the issue of whether the input provides sufficient (positive) evidence for learning. Further work in this regard is thus needed.

Formal Theories of Grammar and Language Acquisition

Chomsky has proposed several formal theories of Universal Grammar (UG; e.g. Chomsky 1957;[START_REF] Chomsky | Aspects of the Theory of Syntax[END_REF]Chomsky 1981Chomsky , 1995)), the innate mental system that contains the logical space of all possible grammars and as such underlies all human languages. One formalism, the Principles and Parameters model (P&P; Chomsky 1981) offers a framework in which language acquisition is particularly easy to operationalize. The P&P model argues that UG consists of principles, universal features that characterize all human languages, and principles, variables often conceptualized as (binary) switches that account for cross-linguistic variation. For instance, it is a principle of UG that each sentence must have a subject. A classical example of a parameter is the pro-drop or null subject parameter. In languages, e.g. Italian, Hungarian, in which this parameter takes a positive value, the subject of the sentence can be a phonologically empty, null element (pro), e.g. Italian: Piove rain.3sg 'It rains', whereas in languages with a negative value, e.g. English, French, no empty subjects are allowed (*Rains).

The P&P model readily accounts for language acquisition. Principles raise no learning problem, as they are universal across all languages, and they are innate. Parameters, which account for the observable variation across languages, are the real targets of language acquisition: the learner's task is set the values of the parameters to the ones that characterize the native language. This is a systematic and readily operationalizable view of language acquisition. However, it leaves open one question: what are the triggers for parameter setting? In other words, parameters and the linguistic variables and categories that constitute them, such as pro, V(erb) etc., are abstract mental entities. How does the infant know what corresponds to them in the concrete, physical input she receives? This question was formulated by [START_REF] Pinker | Language Learnability and Language Development[END_REF] as the "linking problem".

Bootstrapping theories offer a possible solution. They hypothesize that infants learn about certain abstract structural properties of their native language using perceptually available surface cues that are correlated with the abstract properties [START_REF] Morgan | Signal to Syntax: Bootstrapping from Speech to Grammar in Early Acquisition[END_REF]. In English, for instance, disyllabic nouns typically have initial stress, while verbs have final stress, e.g. 'record (N) vs. re'cord (V). Thus the stress pattern of a word can help infants determine its lexical category even if the word is unfamiliar and its meaning is unknown. Several types of bootstrapping models have been proposed on the basis of the different surface cues that infants may use, e.g. semantic bootstrapping [START_REF] Pinker | Language Learnability and Language Development[END_REF], syntactic bootstrapping [START_REF] Gleitman | The Acquisition of the Lexicon[END_REF][START_REF] Lidz | Understanding how input matters: Verb learning and the footprint of universal grammar[END_REF], prosodic bootstrapping [START_REF] Morgan | Signal to Syntax: Bootstrapping from Speech to Grammar in Early Acquisition[END_REF]Nespor 1990;[START_REF] Riverton | Different Phrasal Prominence Realization in VO and OV Languages[END_REF]Gervain and Werker 2013) and frequency-based bootstrapping (Gervain, Nespor, et al. 2008;[START_REF] Bernard | Prosodic cues to word order: what level of representation?[END_REF]. The specific contributions of each bootstrapping model will be discussed in greater detail in section 4.2.4 below.

This nativist account of language acquisition is often contrasted in the literature by empiricist accounts arguing that statistically-based, general purpose or other non-linguistic, e.g. social, mechanisms can account for language acquisition without the need to assume innate, language-specific mental contents. It is impossible to give an exhaustive account of these proposals here. We would, nevertheless, like to mention a few influential and particularly relevant examples.

One tradition argues that there is enough statistical information (e.g. frequency of occurrence and co-occurrence of different linguistic units) in the input to provide robust patterns, which young learners can pick up on to learn grammar. Thus, connectionist (Elman et al. 1997) and more recently machine learning (LeCun, Bengio, and Hinton 2015) modelers have built networks and algorithms that can learn structural regularities from different linguistic inputs with high accuracy. It has also been shown experimentally that very young infants are sensitive to statistical regularities in their input, such as frequency and conditional probability cues (Saffran, Aslin, and Newport 1996 and subsequent work). This statistical learning based account has produced remarkable results. Very young infants have been shown to rely on transitional probabilities for word segmentation and other linguistic tasks under a wide variety of circumstances from the earliest ages (e.g. [START_REF] Teinonen | Statistical language learning in neonates revealed by eventrelated brain potentials[END_REF][START_REF] Pelucchi | Learning in reverse: Eightmonth-old infants track backward transitional probabilities[END_REF][START_REF] Lany | From Statistics to Meaning: Infants' Acquisition of Lexical Categories[END_REF]). Today's deep learning algorithms [START_REF] Lecun | Deep Learning[END_REF] are able to recognize and label large quantities of images, perform intelligent semantic searches, and are regularly used by our web browsers and smart phones for a large variety of purposes. The accuracy of these algorithms is often very high, and matches human performance. However, very tellingly, their errors reveal non-human-like performance: an image categorization algorithm labeled a sofa with a leopard fabric as a leopard. In a systematic review, [START_REF] Marcus | Deep Learning: A Critical Appraisal[END_REF] argues that while deep learning and other machine learning algorithms are very powerful, they differ from human cognition in many ways, suggesting that the underlying computational mechanisms are different. For instance, deep learning algorithms need massive amounts of training data, while infants can learn from few instances (e.g. they can learn a word form and its meaning from just a single instance of exposure, a mechanism known as zap-mapping or fast mapping). Deep learning algorithms are taskspecific and close-ended. A network trained on image recognition will do poorly on phoneme categorization, whereas human intelligence is flexible and allows for generalizations. And most importantly for the current discussion, these algorithms cannot handle hierarchical structure like the one found in human syntax. [START_REF] Marcus | Deep Learning: A Critical Appraisal[END_REF], therefore, suggests that the best approach, most similar to human cognition and even more powerful than the current algorithms, would be to augment the current models with symbolic representations allowing them to meet some of the challenges mentioned above.

Another empiricist approach, inspired by usage-based theories of language processing (Bybee, Hopper, and Hopper 2000), argues that statistical information in the input complemented with social learning mechanisms such as joint attention between an infant and a caregiver in a given communicative situation best explain language acquisition [START_REF] Tomasello | Do young children have adult syntactic competence?[END_REF]. According to this approach, the infant picks up common sequences from the input, with at most one open slot in them (e.g. Where is the ___ ?) and can initially only insert a limited set of items in this open slot. She then proceeds by gradually generalizing from these initially limited and item-based templates to construct an increasingly abstract grammar. In so doing, she relies to a large extent on her advanced social abilities, which allow her to understand the intentions and thus the intended meaning of the communicative partner (e.g. infant and caregiver are engaged in jointly attending to a toy, and the caregiver holds it out for the infants to grab, while saying Here, the ball!, allowing the infant to infer that ball must refer to the object being offered). While successful at explaining some aspects of language learning, e.g. word learning, this approach has not been able to offer an account of how (complex) syntactic structure might be learned.

More generally, while the empiricist approaches have indeed offered valuable insight into the mechanisms underlying learning tasks such as lexical acquisition or the development of semantics, they remain relatively uninformative about how syntactic structure may be learned.

Empirical Findings Inspired by Generative Linguistics

This mentalist and biologically based account of language acquisition proposed by generativists gave rise to a new research agenda, drawing attention to some previously unstudied phenomena and revealing links between observations hitherto believed to be unconnected. First, assuming the existence of innate mental contents pushed researchers to investigate very young infants, including newborns and even fetuses. While the late emergence of an ability is not necessarily an argument against its innateness (i.e. teeth or sexual maturation), its early presence, i.e. before experience and opportunities for learning occur, is strong evidence in favor of it. Prelinguistic infants' cognitive and linguistic abilities thus became a major focus of interest. Second, and relatedly, since observable behavior was no longer the only admissible source of data, studies on perception multiplied. This was particularly relevant for studying prelinguistic infants, as their overt behavioral repertoire is strongly limited and linguistic production doesn't begin before the first birthday. The realization that prelinguistic infants nevertheless possess considerable knowledge about speech and language would not have been possible without investigating perception. Third, the idea that language is a biological phenomenon brought about an evolutionary perspective, with an increasing number of studies comparing the abilities of humans and animals. Several types of non-human animals provide relevant comparisons. Primates are interesting as they are our closest relatives and thus share a large part of our evolutionary history and genetic endowment. Birds are often studied because they are vocal communicators and as such have sophisticated vocalizations, perceptual and vocal learning abilities. Dogs have also been increasingly compared to humans, as they share our cultural history through domestication, and "accept" humans in their social structure. Non-human animals are often compared to human adults as well as to human infants. This latter comparison is all the more relevant, as both animals and human infants are non-linguistic, providing a particularly close match in terms of linguistic experience and knowledge, as well as in the experimental methods applicable to them. Fourth, as another consequence of this biological perspective, neurological and brain research has also been brought to bear on speech and language processing and even more abstractly on formal linguistic theories, especially with the recent advent of brain imaging methods (which, of course, in itself is not brought about my the generativist stance).

The empirical data available on language development has thus radically increased and diversified in the last decades, partly due to the cognitive and biologically-based perspective of the generative enterprise. Below, we review some of the most important empirical findings, asking whether and if yes how they confirmed or infirmed the original generativist assumptions about language.

Evidence for the biological basis of language from language acquisition

Research supporting the idea that language is part of our biology, and thus obeys principles that other biological systems also show started very early after the cognitive revolution. One of the most important findings, proposed first by [START_REF] Lenneberg | The Biological Foundations of Language[END_REF], has been that language is a critical period phenomenon: during the first years of life, language input allows a child to become a native speaker of the language(s) she is exposed to. After the closure of this window of opportunity, learners may still develop high proficiency, but will most typically not achieve full native competence.

Early evidence for this proposal came from different sources. First, several cases of linguistic isolates or feral children were found, who practically received no language input due to parental neglect. Among these children Genie and Chelsea, discovered at ages 13 and 31 years, respectively, never developed native competence after they started receiving language input, while Isabelle, who was found at age 6, rapidly caught up with her peers and learned English natively [START_REF] Curtiss | The linguistic development of Genie[END_REF][START_REF] Snow | Relevance of the Notion of a Critical Period to Language Acquisition[END_REF]. One crucial difference that may explain the different outcomes in these three cases is the age at which these individuals started receiving typical language input. Only Isabelle, the child who started learning language before the onset of puberty, could acquire language natively. Similar observations have been made about second language learners (Snow and Hoefnagel-Hohle 1978;[START_REF] Johnson | Critical period effects in second language learning: The influence of maturational state on the acquisition of English as a second language[END_REF]. Immigrants to the USA who arrived between ages 3-7 years developed fluency in English that was undistinguishable from that of infants born and raised in English-speaking American families, while immigrants who arrived after this age showed competence that decreased gradually with increasing age of arrival. These observations confirm that language is a critical or sensitive period phenomenon, similarly to other well-documented biological phenomena such as the development of ocular dominance in the visual cortex or imprinting in young animals.

The discovery of a critical period for language placed even more emphasis on the importance of acquisition for the understanding of human language. Studies on early development, especially on children's early production in different languages began, documenting the fact that across languages, young children go through approximately the same language developmental milestones. Babbling starts around 4-6 months, the production of the first words at 12 months, of the first two-word combinations at 2 years, and 'telegraphic speech', i.e. combinations of several content words with some of the functors omitted, at 3 years. The fact that young children from different cultures follow the same developmental trajectory, despite important differences between the grammatical and vocabulary structures of the target languages, was also taken as evidence for the biological roots of language.

An even more striking demonstration came from Nicaraguan sign language [START_REF] Senghas | Children creating language: How Nicaraguan Sign Language acquired a spatial grammar[END_REF][START_REF] Kegl | Language Emergence in a Language-Ready Brain[END_REF]. Deaf children born to hearing/speaking families typically do not receive language input as they don't hear the speech around them, and they develop simple signs to communicate, called home signs. In the 70s-80s in Nicaragua, such home signer children were brought together in special schools. In the span of a few decades, generations of these children developed a new, full-blown language, known as the Nicaraguan Sign Language. Remarkably, this language emerged without any input from an existing natural language, spoken or signed, as the children initially did not have full language competence, only using home sign. The fast and effortless emergence of this new sign language is considered strong evidence that the human brain is born "language-ready" [START_REF] Kegl | Language Emergence in a Language-Ready Brain[END_REF], biologically prepared to process language, and is able to create it, even in the face of absent or fragmentary input.

Research on the language development of blind children led to similar conclusions. While these children are deprived of a very important source of perceptual input about the world, and about the possible meaning of linguistic communication around them, their language development is largely unperturbed [START_REF] Landau | Language and experience: Evidence from the blind child[END_REF]. This suggests again that language development is to a large extent independent of perceptual experience and is driven by internal biological constraints.

Research on early speech perception and language development

The above-discussed general biological principles underlying language have highlighted the importance of early language development. Infants' and young children's speech perception and language learning abilities have started receiving much scrutiny, especially once experimental methods to test infants under laboratory circumstances were developed in the 70s-80s. We discuss some of the most interesting and relevant empirical findings below. It needs to be noted that this is far from being an exhaustive review.

Becoming a native listener: perceptual narrowing and phonological development

The critical period for language is best documented and closes earliest in the phonological domain [START_REF] Werker | Critical Periods in Speech Perception: New Directions[END_REF]. Newborns and young infants are able to make almost all phonological discriminations that exist in the world's languages, including those that are not found in the language(s) they are learning [START_REF] Eimas | Speech perception in infants[END_REF]. After several months of experience with the native language, sensitivity to non-native phonological distinctions, including phoneme, tone and lexical stress contrasts, decreases, and by the end of the second half of the first year of life, infants become better at discriminating the contrasts found in their native language, while unable to distinguish most, albeit not all, non-native contrasts [START_REF] Werker | Cross-language speech perception: Evidence for perceptual reorganization during the first year of life[END_REF][START_REF] Kuhl | Innate predispositions and the effects of experience in speech perception: The native language magnet theory[END_REF][START_REF] Best | The emergence of native-language phonological influences in infants: A perceptual assimilation model. The development of speech perception: The transition from speech sounds to spoken words[END_REF][START_REF] Mattock | The developmental course of lexical tone perception in the first year of life[END_REF]Narayan, Werker, and Beddor in press). Importantly, during this process, the initial perceptual boundaries in phonological space are suppressed, maintained or sharpened as a function of experience, but entirely new boundaries are not created.

Perceptual reorganization is accompanied by an increased brain specialization for the native language, i.e. an increasing left-lateralization, and a gradual decrease in the neural plasticity of the brain areas involved in speech perception [START_REF] Sato | Development of Hemispheric Specialization for Lexical Pitch-Accent in Japanese Infants[END_REF][START_REF] Minagawa-Kawai | Neural attunement processes in infants during the acquisition of a language-specific phonemic contrast[END_REF][START_REF] Friederici | Brain responses in 4-month-old infants are already language specific[END_REF][START_REF] Friederici | The Brain Basis of Language Processing: From Structure to Function[END_REF].

Similar narrowing is also observed in other perceptual domains, such as face perception, anchoring speech perception development in more general neural maturational processes [START_REF] Maurer | Perceptual Narrowing during Infancy: A Comparison of Language and Faces[END_REF][START_REF] Slater | The Shaping of the Face Space in Early Infancy: Becoming a Native Face Processor[END_REF][START_REF] Watson | Infant Perceptual Development for Faces and Spoken Words: An Integrated Approach[END_REF].

Children go beyond the input in their production: evidence for abstract structural representations

The hypothesis of an innate universal grammar predicts that children should have abstract grammatical representations, which help them learn grammar. Considerable empirical evidence has accumulated suggesting that young children are indeed able to go beyond imitating the input they receive and organize their linguistic knowledge into structuresensitive, abstract rules from their earliest productions.

One classical demonstration comes from an experimental paradigm known as the wugtest [START_REF] Berko | The child's learning of English morphology[END_REF]. Preschool children were shown picture cards in which a cartoon-like animal or an action performed by a person was named using a nonsense word, obeying English phonotactic regularities. The children were then prompted to generate the plural or the past tense of the nonsense "nouns" and "verbs" (e.g. This is a wug. Now there is another one. […] There are two _.) Children successfully created the expected forms (wugs) despite the fact that these were nonsense words, which they never heard before. This confirms that young children have a representation of morphological rules that is sufficiently abstract to allow generalization to a new item.

Similar results were obtained for the existence of structure-sensitive rules in syntax (Stephen [START_REF] Crain | Structure Dependence in Grammar Formation[END_REF]. Preschoolers, who were familiar with the formation of yes/no questions by subject/auxiliary inversion in English (The man is tall. -> Is the man _ tall?), were given a sentence elicitation task in which they had to form yes/no question from complex sentences with a subject relative (The man who is tall is in the other room.), a construction they do not yet produce and which occurs infrequently in the input. There are infinitely many possible generalizations that are compatible with the simple yes/no question (e.g. front the leftmost auxiliary, front the main clause auxiliary etc.). However, young children seem to only entertain the structure-sensitive (and correct) one (front the main clause auxiliary: Is the man who is in the other room _ tall?), not the others, as they do not make mistakes of the type Is the man who _ in the other room is tall.

While even very young infants readily generalize beyond the input, their early production is not error-free. Interestingly, many of the errors they make also provide evidence for the application of abstract rules: children often overgeneralize. Forms such as *goed (instead of went) are not uncommon. Since these forms are not produced by adults, and are thus absent form the input, they provide clear evidence for the existence of abstract rules in children's early grammars. Young children's multiword speech thus provides evidence for abstract representations in production from the toddler years onwards. Can we find even earlier evidence? Since there is no considerable production before this age, we need to turn to perception and consider whether infants' speech perception and language learning abilities provide any cues. The artificial grammar learning paradigm was adapted to infants with this purpose.

Artificial grammars: a central paradigm

Originally created to study implicit sequence learning [START_REF] Reber | Implicit learning of artificial grammars[END_REF](Reber , 1969)), the artificial grammar learning paradigm was adapted to investigate language learning in adults and infants. In this paradigm, a set of rules are defined over a vocabulary of typically nonsense (or a mix of nonsense and existent) words. Learners are exposed to grammatical strings generated by (some of) the rules of the artificial grammar, and are tested on the learned strings, new grammatical strings (generalization) and/or new ungrammatical strings. This paradigm allows researchers to have more control over the stimulus material than if they used stimuli from a natural language, and they can, to a large extent, reduce the effects of experience. This paradigm is thus suitable to emulate language acquisition in adult learners and it is also attractive to investigate early language acquisition, because the stimulus material can be simplified to make up for infants' attentional and memory limitations.

This paradigm has been widely used (for reveiws, see Gomez and Gerken 2000; Gervain, de la Cruz Pavia, and Gerken in press) to study statistical learning, lexical acquisition, as well as the acquisition of morphological, syntactic and semantic regularities in infants and young children (e.g. [START_REF] Saffran | Statistical learning by 8-month-old infants[END_REF][START_REF] Marcus | Rule learning by seven-month-old infants[END_REF][START_REF] Dawson | From Domain-Generality to Domain-Sensitivity: 4-Month-Olds Learn an Abstract Repetition Rule in Music That 7-Month-Olds Do Not[END_REF][START_REF] Gomez | Artificial grammar learning by 1-year-olds leads to specific and abstract knowledge[END_REF][START_REF] Marchetto | Finding Words and Word Structure in Artificial Speech: The Development of Infants' Sensitivity to Morphosyntactic Regularities[END_REF].

A seminal artificial grammar learning study [START_REF] Marcus | Rule learning by seven-month-old infants[END_REF]) aimed to show that very young infants are able to learn abstract, 'algebraic' rules, i.e. rules containing variables. The identity relation was chosen as the target rule as it is the simplest abstract relation possible. Infants were trained on a grammar generating three-word strings with two identical and one different item (ABB, AAB or ABA), and they were tested on their ability to discriminate the trained grammar from the other grammars. For instance, infants familiarized with AAB sequences were tested with new AAB sequences, i.e., sequences consistent with the grammar of familiarization, as well as on ABA sequences, inconsistent with the familiarization grammar. Infants discriminated the inconsistent sequences from the consistent ones. Importantly, infants showed discrimination for sequences made up of words that were not presented during familiarization (e.g., if familiarization items were "ba po ba", "ko ga ko", "ba ba po", "ko ko ga", test items were "wo fe fe" etc.). This way, it could be showed that infants generalize the underlying abstract rule, rather than relying on item-based information, e.g., statistics (frequency of occurrence or co-occurrence, etc.) or the specific position of a given string element.

These initial results gave rise to a large body of literature further investigating how infants process, learn and represent identity-based structures, whether this processing is specific to the language domain and to what extent it can be considered abstract (Marcus, Fernandes, and Johnson 2007;[START_REF] Dawson | From Domain-Generality to Domain-Sensitivity: 4-Month-Olds Learn an Abstract Repetition Rule in Music That 7-Month-Olds Do Not[END_REF][START_REF] Saffran | Dog Is a Dog Is a Dog: Infant Rule Learning Is Not Specific to Language[END_REF][START_REF] Frank | Information from multiple modalities helps 5-month-olds learn abstract rules[END_REF][START_REF] Johnson | Abstract rule learning for visual sequences in 8-and 11month-olds[END_REF]. Most importantly for our purposes, the interpretation that these simple repetition-based structures are represented as algebraic rules was questioned. It has been argued that adjacent repetitions are salient, Gestalt-like primitives, which may be detected automatically by the perceptual system without recourse to symbols or abstract representations [START_REF] Endress | Perceptual and memory constraints on language acquisition[END_REF]. However, even if the detection of identity/repetition is a primitive, explaining the ability to discriminate adjacent repetitions (ABB) from non-adjacent ones (ABA, as in [START_REF] Marcus | Rule learning by seven-month-old infants[END_REF] or random sequences (ABC, as in Gervain, Macagno, et al. 2008;[START_REF] Gervain | Binding at birth: The newborn brain detects identity relations and sequential position in speech[END_REF], this account cannot explain how infants discriminate between two adjacent repetition based structures, such as AAB vs. ABB, an ability observed in 7-month-olds [START_REF] Marcus | Rule learning by seven-month-old infants[END_REF] and newborns [START_REF] Gervain | Binding at birth: The newborn brain detects identity relations and sequential position in speech[END_REF]. This is because discriminating AAB and ABB structures requires the combination of repetition detection with sequential ordering (initial vs. final position of the repetition).

Artificial grammar learning is thus a paradigm that has proven particularly useful in exploring the nature of infants' knowledge and representations of language structure.

Bootstrapping approaches to the acquisition of grammar

The above-presented empirical results probe the acquisition of specific aspects of grammar and vocabulary. But how do they scale up to explain the complex process of language acquisition. As discussed above, the P&P paradigm provides a general approach to explain language acquisition universally across all languages. However, the linking problem arises [START_REF] Pinker | Language Learnability and Language Development[END_REF]: how do infants know what cues in the input to pay attention to in order to set a given parameter? Bootstrapping mechanisms rely on the assumption that infants can organize their linguistic knowledge by analysing the perceptually available cues present in the input that, in turn, correlate with the underlying linguistic structure. [START_REF] Pinker | Language Learnability and Language Development[END_REF] was one of the first to introduce the notion of "bootstrapping". He argued for an innate correspondence between semantics and syntax. According to his semantic bootstrapping hypothesis, the basic semantic notions present in our every-day life (e.g. "actions" or "concrete objects") can be linked to syntactic or lexical categories (e.g. "verbs" and "nouns"). Infants may rely on this innately specified knowledge to expect nouns to refer to objects, and verbs to refer to actions. By observing the contingency between specific words and their meanings, infants might be able to link semantic information to the syntactic structure of a sentence (linking rules).

Others have argued that this semantic bootstrapping is not always possible, as infants would need to be able to process sentence structure to begin with in order to be able to establish the correspondence with semantics. The syntactic bootstrapping hypothesis (Gleitman, 1990) proposes that learning goes the other way: it is by observing the syntactic structure that infants may deduce knowledge of the meaning. Specifically, this procedure "deduces the word meanings from the semantically relevant syntactic structures associated with a verb in input utterances" (Gleitman, 1990, 30). For instance, a transitive verb is likely interpreted as an action performed by an agent on an object (e.g. he sent a letter), whereas an intransitive verb is more typically an action performed by an agent with no object involved (e.g. she was laughing). Using this regularity, infants may categorize a verb as transitive if it appears in the sentence with two nominal arguments, but as intransitive if only one noun phrase is present. This is a plausible learning mechanism to argue verbs, because infants learn nouns earlier than verbs. Indeed, infants' first 50-100 words mainly consist of nouns.

Importantly, infants are also sensitive to the acoustic information carried by the speech stream from very early on. It has therefore been suggested that prosodic and phonological cues contained in speech help infants extract information about grammatical structure. The central argument of the prosodic/phonological bootstrapping hypothesis (Gleitman & Wanner, 1982;[START_REF] Riverton | Different Phrasal Prominence Realization in VO and OV Languages[END_REF][START_REF] Morgan | Signal to Syntax: Bootstrapping from Speech to Grammar in Early Acquisition[END_REF] suggests that certain acoustic/phonological properties of speech cue structural properties of syntax. Variations in duration, intensity and pitch in the speech signal are systematically related to the prosodic hierarchy, which in turn correlates with the syntactic hierarchy of the sentence [START_REF] Nespor | Prosodic Phonology[END_REF]Selkirk, 1984;[START_REF] Riverton | Different Phrasal Prominence Realization in VO and OV Languages[END_REF]. Therefore, these acoustic cues might help infants parse speech into smaller, syntactically relevant units [START_REF] Morgan | Signal to Syntax: Bootstrapping from Speech to Grammar in Early Acquisition[END_REF]Christophe et al., 1997;[START_REF] Christophe | Prosodic structure and syntactic acquisition: the case of the head-direction parameter[END_REF].

One domain in which prosodic bootstrapping has been well established is the acquisition of word order. Languages of the world vary systematically in the relative order their principal syntactic components [START_REF] Greenberg | Universals of Human Language[END_REF][START_REF] Dryer | The Greenbergian word order correlations[END_REF], such as the Verb (V) and its Object (O) and more generally functors and their corresponding content words. Importantly from the point of view of language acquisition, the two possible word orders, VO/functor-initial and OV/functor-final, have different prosodic and acoustic correlates [START_REF] Nespor | Prosodic Phonology[END_REF][START_REF] Riverton | Different Phrasal Prominence Realization in VO and OV Languages[END_REF][START_REF] Christophe | Prosodic structure and syntactic acquisition: the case of the head-direction parameter[END_REF]Gervain and Werker 2013). The phonological phrase in VO languages (e.g. French, English and Italian) is characterized by final prominence, marked by increased duration on the prominent item as compared to the non-prominent item (e.g. to Ro:me). OV languages (e.g. Turkish, Japanese and Basque), by contrast, are characterized by initial prominence, marked by increased pitch or intensity on the prominent as compared to the non-prominent element (e.g. Japanese: ^Tokyo kara Tokyo from 'from Tokyo'). Languages with different word orders thus use different acoustic cues to mark prominence. Acoustic cues are readily available in the speech signal, and a well established auditory bias, known as the Iambic-Trochaic Law [START_REF] Hayes | Metrical Stress Theory : Principles and Case Studies[END_REF], suggests that sound sequences in which elements contrast in duration are naturally perceived as forming units with an iambic, prominence-final pattern, while sequences in which elements contrast in pitch or intensity are perceived trochaically, i.e. with initial prominence. This correlates perfectly with the underlying prosodic and syntactic structures, and thus offers infants a readily available, perceptual, language-independent cue to word order, one of the most basic properties of grammar. Experimental evidence has indeed demonstrated that infants readily rely on this acoustic/prosodic cue. Newborn infants show a preference for the acoustic pattern (iambic for durational contrasts and trochaic for pitch/intensity contrasts) found in the language(s) heard prenatally [START_REF] Abboub | Prosodic grouping at birth[END_REF] and they are also sensitive to the phonological differences between functors and content words [START_REF] Shi | Newborn infants' sensitivity to perceptual cues to lexical and grammatical words[END_REF]. Infants between 6 and 12 weeks of age are able to distinguish between the prosodic patterns of sentences in French, a VO language, and Turkish, an OV language, on the basis of these acoustic differences alone, i.e. even when sentences are delexicalized by replacing all vowels with a schwa and all consonants with a pre-defined member of their respective manner of articulation category. Furthermore, 8-month-old OV-VO bilinguals can use these acoustic cues to guide their choice of word order in an artificial grammar task (Gervain and Werker 2013).

Another well-established cue that can be used in combination with phrasal prosody to learn word order is word frequency, because prosodic and word frequency information are aligned at the phrasal level. In VO languages, prosodic prominence falls on the final constituent of the phrase, which is typically a content word, whereas its functors, which precede it, are non-prominent. In OV languages, prominence also falls on the content word, but in these languages content words tend to be phrase-initial. The frequency-based bootstrapping hypothesis (Gervain, Nespor, et al. 2008;[START_REF] Bernard | Prosodic cues to word order: what level of representation?[END_REF] relies on the language universal division between functors and content words (Chomsky 1995;[START_REF] Fukui | A theory of category projection and its applications[END_REF][START_REF] Abney | The English noun phrase in its sentential aspect[END_REF]. The two lexical classes differ in their frequency of occurrence. Individual functors have a much higher frequency of occurrence than individual content words [START_REF] Cutler | The predominance of strong initial syllables in the English vocabulary[END_REF][START_REF] Kucera | Computational Analysis of Present-Day American English[END_REF]Gervain, Nespor, et al. 2008). Sensitivity to this difference in word frequency distribution is detectable pre-lexically. Eight-month-olds exposed to languages with opposite word orders, e.g. functor-initial Italian and functor-final Japanese, showed opposite preferences for word order in an artificial grammar task. Italian infants preferred sequences starting with a frequent word, while Japanese infants preferred sequences starting with an infrequent word, mirroring the word orders of these two languages (Gervain et al., 2008).

Not only, word frequency is also used as a cue to establish the lexical categories of function and content words. Content words come in open classes (e.g. iPad, Brexit etc.), whereas functors constitute closed classes, into which no new items can be added without a major language change. At 8 months, infants treat frequent words as belonging to closed classes, accepting no replacement for the frequent words in an artificial grammar, whereas they process infrequent words as belonging to open classes readily accepting substitutions with new items (Marino, Bernard, and Gervain under review). As a confirmation, 17-monthold infants expect infrequent, but not frequent words to have semantic content, serving as possible labels for objects [START_REF] Hochmann | Word Frequency as a Cue for Identifying Function Words in Infancy[END_REF]. This further strengthens the claim that frequency acts as a cue to true lexical categories. Interestingly, non-linguistic animals (rats) are also sensitive to frequency information in a similar artificial grammar paradigm, but do not encode the relative position of frequent and infrequent words [START_REF] Toro | Frequency-Based Organization of Speech Sequences in a Nonhuman Animal[END_REF].

The prosodic and frequency-based bootstrapping hypotheses are particularly attractive, because they provide perceptually based mechanisms to explain how infants might start breaking into their native grammar(s) very early, before and thus independently of the lexicon and implies the existence of a rudimentary representation of word order as early as 8 months of age.

In sum, a substantial amount of empirical evidence has gathered suggesting that young infants, even newborns, exhibit more sophisticated speech perception and language learning abilities, as well as more knowledge about the specific properties of their native language than behaviorist approaches prior to the cognitive turn would have predicted. Specific mechanisms have been proposed

Comparative research

The evolution of language has received considerable attention (e.g. Bickerton 2016;[START_REF] Pinker | Natural language and natural selection[END_REF][START_REF] Hauser | The faculty of language: what is it, who has it, and how did it evolve?[END_REF]Christiansen and Kirby 2003), especially since the biological nature of language came to the forefront. Historical evidence in this domain is scare. In the last 15-20 years, however, cross-species research directly comparing humans' and animals' performance in language learning tasks has provided invaluable insight into the abilities we share with non-human animals, and those that are unique to humans, possibly to human language. This has been particularly productive in the artificial grammar learning and neuroimaging domains, in which adults, infants and animals can easily be compared, as no prior linguistic knowledge is necessary to complete the experimental tasks. In a seminal, but much debated paper, Hauser, Chomsky and Fitch [START_REF] Hauser | The faculty of language: what is it, who has it, and how did it evolve?[END_REF] suggested that the only unique human evolutionary step towards language is the emergence of the combinatorial operation ('merge' in the sense of Chomsky 1995), underlying humans' ability to entertain hierarchically embedded, recursive representations, unavailable to non-human animals. They backed this claim up with comparative artificial grammar learning data from monkeys and humans [START_REF] Fitch | Computational constraints on syntactic processing in a nonhuman primate[END_REF]. Several subsequent publications suggested that some animal species, e.g. songbirds, are able to learn centrally embedded, recursive structures, similarly to humans [START_REF] Gentner | Recursive syntactic pattern learning by songbirds[END_REF], while others argued that in the original tasks, even humans didn't necessarily learn such complex structures [START_REF] Hochmann | Do Humans Really Learn AnBn Artificial Grammars from Exemplars?[END_REF]. A large amount of recent work (for a recent summary, see the special issue Petkov and Marslen-Wilson 2018) has used the same artificial grammar learning paradigms as those used with infants to test whether animals are able to extract identity-/repetition-based regularities (in primates: [START_REF] Rey | Regularity Extraction Across Species: Associative Learning Mechanisms Shared by Human and Non-Human Primates[END_REF]; in song birds: Chen, van Rossum, and ten Cate 2015), non-adjacent dependencies (in primates: [START_REF] Malassis | Non-adjacent Dependencies Processing in Human and Non-human Primates[END_REF][START_REF] Ravignani | Action at a Distance: Dependency Sensitivity in a New World Primate[END_REF][START_REF] Wilson | Auditory artificial grammar learning in macaque and marmoset monkeys[END_REF] or prosodic patterns (in rats: de la Mora, Nespor, and Toro 2012; in song birds: Spierings, Hubert, and ten Cate 2017). The debate is still open as to whether animals have recursive linguistic representations and about the level of the Chomsky hierarchy that best characterizes their competence.

Neuroimaging studies

In the past decades, brain imaging techniques have revolutionized our ability to explore the human brain. How the brain processes speech and language has been investigated in thousands of electroencephalography (EEG), magnetoencephalography (MEG), functional magnetic resonance imaging (fMRI), near-infrared spectroscopy (NIRS) and positron emission tomography (PET) studies from birth to adulthood. The brain network responsible for language processing is now quite well described in adults (e.g. Friederici 2011; Poeppel 2014; Kutas and VanPetten 1988;[START_REF] Bastiaansen | Oscillatory neuronal dynamics during language comprehension[END_REF] and its origins are starting to be understood in infants [START_REF] Dehaene-Lambertz | Functional neuroimaging of speech perception in infants[END_REF][START_REF] Dehaene-Lambertz | How does early brain organization promote language acquisition in humans?[END_REF][START_REF] Gervain | Near-infrared spectroscopy: a report from the McDonnell infant methodology consortium[END_REF][START_REF] Friederici | Brain responses in 4-month-old infants are already language specific[END_REF][START_REF] Pena | Sounds and silence: an optical topography study of language recognition at birth[END_REF].

While this technological development and knowledge of brain function and anatomy it brought about are independent of the generative tradition, the relevance and interpretation of brain imaging data for language sciences have definitively been influenced by the biological stance of the generative enterprise. If language is an "organ", brain imaging should allow us to investigate it. Indeed, certain imaging studies were directly inspired by linguistic theory and explicitly set out to find evidence for the neural mechanisms encoding formal linguistic operations, such as merge or move, and possible vs. impossible linguistic rules [START_REF] Friederici | The role of left inferior frontal and superior temporal cortex in sentence comprehension: localizing syntactic and semantic processes[END_REF][START_REF] Friederici | Verb argument structure processing: The role of verb-specific and argument-specific information[END_REF][START_REF] Musso | Broca's area and the language instinct[END_REF]. Thus, Musso and colleagues have shown increased activation in Broca's area when German-speakers syntactic rules of Japanese or Italian, but no activation when the same Japanese or Italian words were used in rules that are not allowed by UG, such as counting rules. Importantly for the issue of language development, evidence has been found that the infant language network is already similar to the mature, adult network. In particular, both the temporal, auditory areas and the frontal regions, responsible for higher order structural or sequential processing, are operational from birth [START_REF] Dehaene-Lambertz | Functional neuroimaging of speech perception in infants[END_REF][START_REF] Pena | Sounds and silence: an optical topography study of language recognition at birth[END_REF]Gervain, Macagno, et al. 2008). While many brain imaging studies were unable to identify the neural correlates of different syntactic constructions as posited by generative grammar, and even provided evidence against generativist assumptions [START_REF] Pulvermüller | Words in the brain's language[END_REF][START_REF] Van Turennout | Brain activity during speaking: From syntax to phonology in 40 milliseconds[END_REF][START_REF] Bastiaansen | Syntactic unification operations are reflected in oscillatory dynamics during on-line sentence comprehension[END_REF], the generativist quest for evidence in favor of the mental and biological reality of these theoretical constructs bolstered the idea that neuroimaging can directly inform language theory, and not just investigations of brain function, anatomy or pathology.

The most recent perspective: integrative models

As summarized above, the nativist-empiricist (or 'nature-nurture') debate dominated much of the second half on the 20 th century as competing paradigms in explaining the language faculty. As a result, we now have a much better empirical understanding about how the human mind and brain process and learn language. Very early language acquisition has played a pivotal role in these debates, and much has been learned about how babies break into language. Interestingly, the influence was reciprocal. Generative theory also changed as a result of empirical evidence, as reviewed in Chomsky (2007). Specifically, research on language acquisition, and in particular, the problem of learnability has given a strong push towards simpler syntactic formalism, which Chomsky's Minimalist Program (N. Chomsky 1995) made explicit.

These empirical observations, as well as recent advances in brain imaging, genetics and epigenetics [START_REF] Werker | Critical Periods in Speech Perception: New Directions[END_REF][START_REF] Lewkowicz | The Development of Intersensory Temporal Perception: An Epigenetic Systems/limitations View[END_REF], have led to new theoretical questions and a more integrated view of innate and learned abilities, rendering the strict nature-nurture dichotomy obsolete.

The last decades of biological research have in fact proposed a new model of how gene expression is regulated by inherent biological factors, but also by experience. It has been proposed that DNA expression can be modified during the entire life span of an individual through experience and environmental factors. This up-or downregulation of DNA expression is transmissible (e.g. during genome imprinting or embryonic stage), thus these epigenetic re-arrangements change gene expression without changing the DNA sequences [START_REF] Roth | Regulation of chromatin structure in memory formation[END_REF]. Epigenetics studies these flexible re-arrangements of our genome.

The human genome is no longer considered exclusively as a "committed" blueprint, but rather as a "potentiality".

Epigenetic modulations of gene activity may be induced by environmental factors such as as diet, stress, exposure to certain chemical substances and specific behavioural stimulations, e.g. extensive practice, sensory deprivation etc. (e.g. Franklin & Mansuy, 2009;[START_REF] Gräff | Epigenetic codes in cognition and behaviour[END_REF][START_REF] Jirtle | Environmental epigenomics and disease susceptibility[END_REF]Liu, Van Groen, Kadish, & Tollefsbol, 2009;[START_REF] Roth | Regulation of chromatin structure in memory formation[END_REF][START_REF] Zhang | Epigenetics and the environmental regulation of the genome and its function[END_REF]. These modulations have most often been studied in animals. One study has shown, for instance, that better quality infant-caregiver interaction (e.g. more time spent nursing) produces epigenetic modification that impact the stability of gene expression in the central nervous system, thereby modifying behaviour. [START_REF] Roth | Regulation of chromatin structure in memory formation[END_REF].

In this perspective, dynamic and complex interactions take place between the environment and the genome [START_REF] Waddington | Canalization of development and the inheritance of acquired characters[END_REF]. These interactions trigger epigenetic modulations as a response to the experience, forming epigenetic "memories" of the external world. Since the earliest stages of the development, environmental feedback is necessary to develop optimal functioning. Behavioural outcomes are thus often mediated by the long-term influence of these experiences.

This framework puts to notions of critical or sensitive periods and brain plasticity into a new perspective [START_REF] Werker | Critical Periods in Speech Perception: New Directions[END_REF]. Different aspects of language acquisition have different temporal windows of opportunity. The critical periods of phonetic, lexical and syntactic learning are different (Kuhl, 2010). This series of sensitive periods for language creates a cascading dynamics for language development, where each ontogenetic accomplishment support and interacts with the subsequent ones. This epigenetic cascade "can be altered by sensory deprivation, pharmacological exposure and linguistic experience" (Werker and Hensch, 2015, p.187). For instance, under specific circumstances such as deafness or exposure to drug like serotonin reuptake inhibitors (SRI), the opening and closing of the critical period is shifted (Weikum et al. 2012). In congenitally deaf people, measures of acoustic discrimination indicate that plasticity stays in place for a longer period than in normally hearing people [START_REF] Faulkner | Some observations about cochlear implants: Challenges and future directions[END_REF][START_REF] Kral | Developmental neuroplasticity after cochlear implantation[END_REF]. Also, untreated maternal depression delays the closure of the sensitive period, while exposure to SRI (in infants whose mothers are depressed, but medicated), accelerates it (Weikum et al. 2012).

These advances show that both genetically endowed and environmental factors play an important role, and the most important question is to understand how they interact synergistically to enable human development.

Acknowledgements

This work was supported by the Human Frontiers Science Program Young Investigator Grant nr. RGY 0073/2014, an ERC Consolidator Grant 773202 ERC-2017-COG "BabyRhythm", the Labex EFL grant of the ANR progam "Investissements d'Avenir" (reference: ANR-10-LABX-0083), as well as the Marie Curie ITN grant "PredictAble" to JG.