Decades of research have shown that, from an early age, proficient bilinguals can
speak each of their two languages separately (similar to monolinguals) or rapidly
switch between them (dissimilar to monolinguals). Thus we ask, do monolingual and
bilingual brains process language similarly or dissimilarly, and is this affected
by the language context? Using an innovative brain imaging technology, functional
Near Infrared Spectroscopy (fNIRS), we investigated how adult bilinguals process semantic
information, both in speech and in print, in a monolingual language context (one language
at a time) or in a bilingual language context (two languages in rapid alternation).
While undergoing fNIRS recording, ten early exposed, highly proficient Spanish-English
bilinguals completed a Semantic Judgment task in monolingual and bilingual contexts
and were compared to ten English monolingual controls. Two hypotheses were tested:
the Signature Hypothesis predicts that early, highly proficient bilinguals will recruit
neural tissue to process language differently from monolinguals across all language
contexts. The Switching Hypothesis predicts that bilinguals will recruit neural tissue
to process language similarly to monolinguals, when using one language at a time.
Supporting the Signature Hypothesis, in the monolingual context, bilinguals and monolinguals
showed differences in both hemispheres in the recruitment of DLPFC (BA 46/9) and IFC
(BA 47/11), but similar recruitment of Broca's area (BA 44/45). In particular, in
the monolingual context, bilinguals showed greater signal intensity in channels maximally
overlaying DLPFC and IFC regions as compared to monolinguals. In the bilingual context,
bilinguals demonstrated a more robust recruitment of right DLPFC and right IFC. These
findings reveal how extensive early bilingual exposure modifies language organization
in the brain-thus imparting a possible "bilingual signature." They further shed fascinating
new light on how the bilingual brain may reveal the biological extent of the neural
architecture underlying all human language and the language processing potential not
fully recruited in the monolingual brain.