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Introduction
Separate perceptual and motoric systems provide speech-sign or "bimodal" bilinguals with the unique opportunity to produce and perceive their two languages at the same time. In contrast, speech-speech or "unimodal" bilinguals cannot simultaneously produce two spoken words or phrases because they have only a single output channel available (the vocal tract). In addition, for unimodal bilinguals both languages are perceived by the same sensory system (audition), whereas for bimodal bilinguals one language is perceived auditorily and the other is perceived visually. We investigated the implications of bimodal bilingualism for models of language production by examining the nature of bimodal language mixing in adults who are native users of American Sign Language (ASL) and who are also native English speakers. Below we briefly present some important distinctions between ASL and English, describe some of the sociolinguistic characteristics of this bimodal bilingual population, and then highlight some of the implications that this population has for understanding bilingualism and language production in general.
A brief comparison of American Sign Language and English
American Sign Language has a grammar that is independent of and quite distinct from English (see Emmorey, 2002, for a review). For example, ASL allows much freer word order compared to English. English marks tense morphologically on verbs, whereas ASL expresses tense lexically via temporal adverbs (like many Southeast Asian languages). ASL contains several verbal aspect markers (expressed as distinct movement patterns superimposed on a verb root) that are not found in English, but are found in many other spoken languages (e.g., habitual, punctual, and durational aspect). ASL, unlike English, is a pro-drop language (like many Romance languages) and allows null pronouns in tensed clauses (Lillo-Martin, 1986). Obviously, ASL and English also differ in structure at the level of phonology. Signed languages, like spoken languages, exhibit a level of sublexical structure that involves segments and combinatorial rules, but phonological features are manual rather than oral (see Corina and Sandler, 1993, and Brentari, 1998, for reviews). Finally, English and ASL differ quite dramatically with respect to how spatial information is encoded. English, like many spoken languages, expresses locative information with prepositions, such as in, on, or under. In contrast, ASL encodes locative and motion information with verbal classifier predicates, somewhat akin to verbal...