A Theory of Neurolinguistic Development
This set of neural and cognitive supports constitutes a specialization in social cognition (Brothers, 1990; Locke, 1992, 1993b)—an important component of a pluralistic system of social specializations (Karmiloff-Smith, Klima, Bellugi, Grant, & Baron-Cohen, 1995)—and henceforth I will refer to socially cognitive mechanisms and socially cognitive operations. At the most general level, this specialization supports an affectively oriented developmental growth path that channels infants in the direction of spoken language.
More specifically, socially cognitive mechanisms orient infants to linguistic displays—the physical activity of people who are talking—thus providing information about language (Locke, 1993b, 1995). These mechanisms, in turn, regulate social and referential activity that occurs in the context of linguistic displays.
Research in the past several decades has identified many of the socially cognitive operations that seemingly facilitate vocal learning and early word production. These include the infant's disposition to (1) take vocal turns with a partner (Ginsburg & Kilbourne, 1988; Papousek & Papousek, 1989), (2) orient to and mimic variegated prosody (Masataka, 1992), (3) gesture communicatively (Bates, Bretherton, Beeghly, & McNew, 1982; Morissette, Ricard, & Decarie, 1994), (4) assimilate ambient phonetic patterns (Boysson-Bardies, Vihman, Roug-Hellichius, Durand, Landberg, & Arao, 1992), and (5) as they develop a ''theory of other minds,'' seek to interpret and alter the mental activity of interlocutors (cf. Baron-Cohen, Tager-Flusberg, & Cohen, 1993). With relevant perceptual experience and motor development, these operations allow infants to ''get by'' in their native language, to pass as speakers when their linguistic capacity is still immature. The infant's socially cognitive operations therefore contribute to the first few essential phases in the development of linguistic capacity.
While many more studies need to be done, there is some evidence for facilitative effects by these so-called ''precursors'' to language. Osterling and Dawson (1994) found a strong inverse relationship between the amount of time infants spent looking at their mother's face in the first year of life and the probability of being diagnosed autistic several years later. Tomasello and his colleagues (Tomasello, Mannle, & Kruger, 1986; Tomasello & Farrar, 1986) observed a positive relationship between the amount of time infants participated in joint attention episodes with their mothers at 15 months and extent of expressive vocabulary at 21 months. Snow (1989) found that vocal imitation at 14 months was related to the number of nouns and verbs produced, the total productive vocabulary, and the ratio of words produced to words comprehended at 20 months. Developments such as these occur inconspicuously—even Lenneberg (1967) missed them, thinking that lan270 guage learning begins at 2 years—but they are a vital part of early language learning.
Our species preadaptation in social cognition is present in varying (usually lesser) degrees in other primates. For example, gorillas appear to use eye movements instrumentally (Gomez, 1990), but seem not to engage in referential pointing (Hewes, 1981). There is some evidence of transient vocal accommodation during dyadic exchanges (Maurus, Barclay, & Streit, 1988), but little or no uncontested evidence of lasting vocal learning (Masataka & Fujita, 1989; Owren, Dieter, Seyfarth, & Cheney, 1992). In the vocal exchanges between monkeys, there is support for a conversation-like temporal structure (Biben, Symmes, & Masataka, 1986; Hauser, 1992), although the evidence is currently limited to mature animals. Studies indicate that some species of monkeys recognize offspring from the voice (Cheney & Seyfarth, 1990; Waser, 1977), and that others use several distinctive call types only or primarily in certain caregiving contexts (e.g., while inspecting or retrieving their infant; Biben, Symmes, & Bernhards, 1989). There is anecdotal evidence of deception (Whiten & Byrne, 1988), which has been challenged (Heyes, 1993). After a largely negative history (Gallup, 1982), there is now some evidence for a concept of self (Hauser, Kralik, Bott-Mahan, Garrett, & Oser, 1995) but there is still little evidence of other minds (Premack & Woodruff, 1978). With appropriate experience (Tomasello, Savage-Rumbaugh, & Kruger, 1993), certain of the apes may spontaneously display an ability to comprehend utterances that equals that of a 2-year-old child, and some disposition to use symbols creatively (Savage-Rumbaugh, Murphy, Sevcik, Brakke, Williams, & Rumbaugh, 1993).
