speaking one language rather than another involves selecting and articulating words that meet the intended meaning . the precise way in which this process is controlled may depend on the language context .
we contrast a context in which a speaker uses just one of their languages to name objects in an experimental session with one where they are required to switch between languages on the basis of a color cue .
for example , blue may signal that the picture is to be named in italian whereas red may signal that it is to be named in german . in principle , only one language need be active in the single language context whereas both languages must be active in the dual language context .
however , research indicates that the languages of bilingual speakers can be jointly active ( e.g. , dijkstra et al . , 1998 ; van hell and dijkstra , 2002 ; von studnitz and green , 2002 , see van heuven and dijkstra , 2010 for a review ) at least to the level of phonology even in a context that requires just a single language ( wu and thierry , 2010b ) .
given joint activation , bilingual speakers may resolve language conflict by suppressing the non - target language at some locus ( green , 1986 , 1998 ; linck et al .
, 2009 ; philipp and koch , 2009 ; macizo et al . , 2010 ; snyder et al .
, 2010 ; hoshino and thierry , 2011 ) or by restricting competition to words within the target language in some other manner ( e.g. , costa and caramazza , 1999 ; elston - gttler , et al . , 2005 ; finkbeiner et al .
does this mean then that the selection process is identical in the single language and in the dual language context ?
we take as our point of departure the idea that anguage control involves a cortical  subcortical circuit ( abutalebi and green , 2007 , 2008 ) and consider whether or not language context ( single or dual ) modulates activity in the circuit .
a major cue is that regions in the left inferior frontal cortex , anterior cingulate cortex ( acc ) and caudate respond differently as a function of the experimental context . in a dual language context , where participants are required to use both languages in naming simple objects , all three regions activate during language switching ( e.g. , abutalebi et al . , 2008 ) .
strikingly , this pattern of activity is absent for the same l1 nouns when participants name them in l1 in a context in which they must either name the object or generate a verb associated with it .
response in the caudate appears particularly sensitive to language switching because it is not observed when the context does not require switching ( e.g. , rodriguez - fornells et al .
we find direct corroboration when , as per the injunction in wu and thierry ( 2010a ) , bilingual speakers use just one of their languages throughout the entire experimental session . in this single language context
, the anterior cingulate and caudate are not significantly activated when participants name pictures and activation in these regions does not differ from that found in monolingual speakers ( green et al . , 2011 ) .
there is however increased activation in left prefrontal regions compared to that shown in monolingual speakers .
we have to distinguish between activation correlated with interference and activation associated with the control of interference .
we found that regional activation in left pars opercularis ( ba44 ) and left pars triangularis ( ba 45 ) was inversely related to the size of the stroop interference shown by monolingual participants outside the scanner .
such an outcome suggests that these regions are involved in the control of interference . regional activation in bilingual speakers relative to monolingual speakers
may also reflect differences in the experienced frequency of words ( as in the  weaker links  hypothesis , gollan et al . , 2008 ) .
however , a frequency - based account is an insufficient explanation because it presupposes that only a single language is selected and is silent on how such selection is achieved .
how then are we to explain the differential response of the control circuit to the different language contexts ?
one possibility is that it reflects the stage at which alternate words are gated from production ( early or late ) .
a single language context may afford early selection of the intended word mediated by the inferior frontal cortex .
alternate words in the other language are blocked at this stage and other structures such as the caudate are not activated . by contrast , in the dual language context , switch trials may necessarily require late selection . on a switch trial ,
the object name , consistent with the previous language , may be released for production because the new language task has yet to dominate .
response of the circuit is similar to that shown when individuals respond on incongruent trials in the stroop task . here
suppressing such interference activates the caudate as well as frontal regions ( ali et al . , 2010 ) .
the experimental context is not the only relevant factor in the response of the control circuit .
two other factors are pertinent : the speaker 's proficiency in their second language and , separately , the behavioral ecology of a proficient bilingual speaker .
caudate activity may be expected in picture naming even in a single language context when speakers are not very proficient in their second language because they will tend to generate the first language name by default .
however , the claim that context affects the locus of selection predicts that the caudate response will be even greater when such speakers perform in a dual language context .
where speakers are proficient in both languages their pattern of language use within their bilingual community may also be important in understanding the response of the control circuit .
we expect the contextual prediction to be met when participants are from communities that use one language at a time and switch to their other language when addressing a different addressee or when there is a change in topic .
such speakers are practiced at avoiding language conflict . in terms of the abutalebi and green ( 2007,2008 ) model
they recruit the acc and left frontal cortex to monitor and to control their response ( see kerns et al . , 2004 ) .
in contrast , in communities where individuals code - switch , language control is different ( green , 1998 ) . speakers have no need to avoid switching rather they freely exploit the activation of both languages .
their skill lies less in avoiding language conflict than in utilizing the joint activation of both languages and adapting their utterances appropriately .
speakers adapt french verbs through the addition of a german particle ( -ieren ) as in  choisieren  from the french
. for speakers from code - switching communities , speaking in just one language without code - switching is likely to be effortful .
we might then expect them to show increased caudate response in the single language context compared to that shown by speakers from a non - code - switching community .
fluent code - switching is likely to recruit other structures implicated in language control and so studying it is important .
we do not know the neural bases of code - switching but we know that those who code - switch are adept at adapting words to the current syntactic context though of course there will be individual differences in the inventiveness of such code - switches .
neuropsychological data implicate cerebellar structures in the control of morphosyntax ( fabbro et al . , 2000 ) .
( 1994 ) found that damage to the right cerebellum led to morphological deficits including inappropriate substitutions of bound grammatical morphemes .
the fundamental idea is that right cerebellum acts together with left frontal structures as part of a language control circuit ( fabbro et al . , 2000 ) .
critical support for this notion comes from data showing that damage to the right cerebellum suppresses activation in left frontal cortex and elicits aphasic symptoms .
reperfusion of the cerebellum reduces such symptoms ( marien et al . , 2001 ) .
code - switching involves adapting morphosyntax . given the neuropsychological data , we can predict that such adaptation will involve the right cerebellum . from a functional point of view , since code - switching involves the on - line adaptation of the morphosyntactic resources of each language , fluency in code - switching requires timing and synchronization  a role for which the cerebellum is also well suited ( see kotz and schwartze , 2010 for an integrative framework ) . a plausible prediction from existing data
is that code - switching is mediated by the co - activation of right cerebellar regions and left frontal cortex .
research on code - switching is therefore important for extending our understanding of language control in bilingual speakers and building on the model of abutalebi and green ( 2007 , 2008 ) .
if functional demand shapes neural circuits then there may be structural correlates in the regions linked in control . in communities where speakers must switch on demand between their two languages , anterior cingulate and/or left inferior frontal regions
may show greater gray matter density compared to monolingual speakers of just one of those languages .
in contrast , in communities where bilinguals code - switch , right cerebellar structures may show enhanced gray matter density .
recent research points to a bilingual advantage in controlling non - verbal interference ( see bialystok et al . , 2009 , for a review ) attributable to the use of cognitive control processes in resolving language conflict .
if code - switching utilizes the joint activation of two languages rather than suppresses one in favor of the other then speakers from code - switching communities may not show such an advantage ( see green , 2011 ) .
they may be adept more generally at seeing how one verbal form can be transformed into another to yield a novel meaning ( e.g. , turning a noun such as  piano  into a verb as in  she pianoed him  ) . alternatively , and perhaps more plausibly , cerebellar circuits may be specialized and so skills in adapting words will dissociate from the ability to adapt non - verbal objects ( e.g. , seeing how a pen might be used as a clothes peg ) such specialization indicates a difference in the correlates of frontal versus cerebellar control processes .
we suggest , in conclusion , that differences in the control demands on bilinguals from different behavioral ecologies are a further source of individual variance in executive control .
the contexts of language use may then temper the cognitive advantages of bilingualism and be relevant to understanding the effects of stroke on speech recovery in bilinguals .
the author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest .
 this paper proposes that different experimental contexts ( single or dual language contexts ) permit different neural loci at which words in the target language can be selected . 
 however , in order to develop a fuller understanding of the neural circuit mediating language control we need to consider the community context in which bilingual speakers typically use their two languages ( the behavioral ecology of bilingual speakers ) . 
 the contrast between speakers from code - switching and non - code - switching communities offers a way to increase our understanding of the cortical , subcortical and , in particular , cerebellar structures involved in language control . 
 it will also help us identify the non - verbal behavioral correlates associated with these control processes 
Language Control in Dual and Single Language Contexts
Language Control and Code-Switching
Conclusions and Perspectives: The Behavioral Ecology of the Bilingual
Conflict of Interest Statement