Research that has studied bimodal bilinguals has offered scientists more insight and evidence to answer one of the most prominent research questions in psycholinguistics and cognitive neuroscience – how do healthy bilinguals use their two languages without confusion and in a highly linguistically principled manner, and which brain mechanisms are involved?

Supports the Differentiated Language System Hypothesis

Children who simultaneously acquire two languages from birth are faced with the challenge of processing two different sets of input and creating two distinct language systems, developing separate lexicons and grammatical rules for each. One of the underlying questions scientists have addressed is that of when these two language systems are created. There are two conflicting hypotheses regarding this question:

  1. Unitary Language System Hypothesis: bilingual babies begin language acquisition with one, fused language system that only gradually differentiates over the first three years of life
  2. Differentiated Language System Hypothesis: bilingual simultaneous language learners develop two separate lexicons and grammar systems from the start

Research has been conducted to find evidence supporting both views, but modality differences between the hands and the tongue offer a unique test of these existing hypotheses; once the constraints of the mouth are removed, asynchronous language development and unsystematic language mixing could occur if early bilingual language exposure first begins with single and fused linguistic representation that only gradually differentiates over the first three years of life (as the unitary language system hypothesis states). However, research conducted by Petito et al. concluded that bimodal bilingual children were not delayed in reaching linguistic milestones for both of their languages, nor were any asynchronies observed in the timing of the linguistic milestones across the spoken and signed modalities.

In addition, Petito observed the amount of translation equivalents - different words in each language that refer to the same concept - seen in bimodal bilingual children. Compared with translation equivalents of unimodal bilinguals, bimodal bilinguals use of such words was comparable. In other words, speakers of both sign and spoken language would use words from both languages interchangeably - a sign that they know both words, but would replace one with the other when needed. Such results provide powerful cross-modal support for the hypothesis that young bilinguals can differentiate their two lexicons from their very first words.

Contradicts Bilingual Paradox

Many parents are concerned that bilingual language exposure is somehow `bad' for very young children, interfering with typical language development and delaying the acquisition of both languages. This is commonly knows as the bilingual paradox. However, studies involving bimodal bilinguals indicate the opposite:

  • Unimodal and bimodal bilingual children achieve the classic early linguistic milestones on a similar time table in each of their two languages and on a time table that was fundamentally similar to monolinguals
  • Both unimodal and bimodal bilingual children demonstrate normal vocabulary growth in each of their language
  • Bimodal bilingual children who “mix” languages do so systematically; not a sign of confusion
    • Signing-speaking bilinguals simultaneously mix their signs and speech, but they do so in semantically principled and highly constrained ways. Signs and words used simultaneously are nearly always from two different grammatical categories. However, each part adds relevant information to form a cohesive whole, as the mixed elements add to the meaning of the utterances in semantically appropriate ways.

Ultimately, scientists have found that bimodal bilingualism helps negate the idea that bilingualism has negative effect on language development and learning in young children.

Bilingualism as Language-Specific

One of the main questions surrounding bilingualism is whether or not code-switching, or using two languages simultaneously, is a general-cognitive process or a linguistic process. Brain imaging studies with unimodal bilinguals support the idea that cognitive-general mechanisms are heavily involved in dual language use in Bilingual mode. Some of the implicated cognitive-general mechanisms are:

  • Prefrontal cortex
    • has been shown to participate in bilingual language use during both language production and comprehension
    • typically participates in other tasks that require complex task monitoring and response selection as well
  • Anterior cingulate cortex (ACC)
    • participates in selective attention, error monitoring and interference resolution

However, bimodal bilinguals provide a unique opportunity to study the brain. Unimodal bilinguals' use of two languages within one modality might activate cognitive-general regions, which might result in part from high attention-sensory/motor costs of integrating and differentiating two languages within one modality MORE than language-specific regions. Therefore, in 2010, Kouvelman et al. studied bimodal bilinguals in order to focus more on the linguistic-specific regions used in bilingualism. Near-Infrared Spectroscopy (fNIRS) brain imagine was used to monitor blood oxygen levels in the brain during bilingual mode. The results revealed that in bilingual mode, as compared to monolingual mode, participants showed greater recruitment of left posterior temporal regions such as Wernickes area. These regions are language-specific, implicated in semantic and phonological processing in native sign and spoken language users. Overall, this study supports language-specific processing involvement when bilinguals must use both of their languages in the same context.

Sources

Kouvelman, I. et al. (2010) Dual language use in sign-speech bimodal bilinguals: fNIRS brain-imaging evidence. Brain & Language, 109, 112-123

Petito, L. et al. (2001) Bilingual signed and spoken language acquisition from birth: implications for the mechanisms underlying early bilingual language acquisition. Journal of Child Language, 28, 453-496.