N E W S A N D V I E W S

NATURE NEUROSCIENCE VOLUME 9 | NUMBER 8 | AUGUST 2006 991

What’s in control of language?Angela D Friederici

Language functions are thought to be controlled largely by cortical areas. A study now finds that the subcortical caudate nucleus is sensitive to language change in bilingual speakers, suggesting a role for this area in control processes.

Anyone who has learned a second language late in life knows how frustrating it is to search for a particular word in still-foreign language, especially when that word is easily remem-bered in one’s native language. Bilinguals, those lucky people who grew up with two lan-guages or learned their second language early in life, do not have this problem. Bilinguals have easy access to words in both their lan-guages, and they can switch between languages without difficulty.

In a recent study in Science1, Crinion and colleagues used functional neuroimaging to investigate how bilinguals accomplish this lin-guistic feat, and found that the left caudate is critical for monitoring and controlling the language in use. The study involved a semantic priming task in which two written nouns were presented sequentially, with a short interval between the first and second words of the pair. Words were from the same language (for example, English trout – SALMON or German forelle – LACHS) or from different languages (for German-English bilinguals, trout – LACHS or forelle – SALMON). Some of these word pairs were related, and others were not. Participants were required to make a semantic decision on the perceptual properties of the object only on the second word of the pair. In monolinguals, the one-language version of this task usually reveals a decrease in reaction time and neuronal activation for semantically related compared to semantically unrelated second words in a pair. Are there similar effects in a cross-lingual setting? To test this, the authors examined three groups: two groups of German-English bilinguals (one in a positron emission tomography (PET) study and one in a functional magnetic resonance imaging (fMRI) study) and a group of Japanese-English bilinguals. All participants had learned English as a second language and had mastered English at different proficiency levels, according to the behavioral tests reported in the online material.

The authors found that a common set of frontal, temporal and parietal regions were activated by semantic decisions in both lan-guages. However, there were large differences among the three groups, and it would have been interesting to learn whether the increased activation in the fMRI German group was correlated with the increased response times observed in this group.

Most relevant for the argument put forward in the present study, however, is the semantic priming effect: the reduction in brain activation to semantically related second words compared to unrelated second words. The study dissoci-ated two such effects in all three groups tested. First, activity in the left anterior temporal pole decreased in proportion to the degree of semantic relatedness, regardless of whether the two words were in the same language or not. This clearly demonstrates that semantic prim-ing of nouns can take place across languages and suggests overlapping neural representa-tions for nouns in different languages.

In contrast, the left caudate showed the same semantic priming effect (reduced activation for the second word of a semantically related word

pair), but only when both words were in the same language. There was no semantic prim-ing across languages for the left caudate. This pattern was independent of whether the second word was in the native or the second language. The left caudate is therefore sensitive to language change across word pairs, suggesting that it is critical for language control.

This is a surprising finding because subcorti-cal structures such as the caudate are not viewed as being primarily involved in conscious cogni-tive control. In contrast, frontal regions located in the lateral prefrontal cortex support conscious cognitive control2,3 and language switching4,5, but were not activated by the priming task used in this study.

This may be because priming is a highly automatic process and may therefore not involve conscious control even when a lan-guage change is involved. This, however, also suggests that the left caudate is involved, first, in automatic semantic priming for words within the same language, as reflected in the activation reduction, and, second, in a percep-tual change between two languages, as reflected in an activation increase. This twofold

The author is at the Max Planck Institute

for Human Cognitive and Brain Sciences,

Stephanstr. 1a, 04103 Leipzig, Germany.

e-mail: angelafr@cbs.mpg.de

Figure 1 The caudate receives input from the prefrontal premotor, temporal and parietal cortex, and connects reciprocally to the cortex via the thalamus. Left, neuroanatomical topography in relation to other brain structures. Right, a principled but simplified view of the connections between the different brain structures.

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N E W S A N D V I E W S

992 VOLUME 9 | NUMBER 8 | AUGUST 2006 NATURE NEUROSCIENCE

observation is condensed in the formula that the left caudate responses are highest when there is a change in meaning (semantically unrelated words in a pair) or a change in lan-guage (different languages in a pair).

In an attempt to further specify the role of the left caudate, Crinion et al. refer to two other imaging studies on word processing in monolinguals, comparing semantic decisions to a phonological task6,7, which also found activa-tion in the left caudate. On the combined basis of these studies, they conclude that the left caudate responds in general when the pattern of neural input changes. This conclusion, however, is not entirely clear, as these two last studies differ from the present study in that identical words were presented during semantic and phonological task blocks, thus eliminating any direct percep-tual changes within a short time window.

Moreover, this functional description of the left caudate cannot easily be generalized to a number of other fMRI studies that also found activation in the left caudate. One study compared monolingual reading of late-learned words to those learned early in life and observed increased activation in the left cau-date8. Another word processing study found an increase in the left caudate activation for words compared to pseudowords in monolin-guals9 and for translating words from native to second language compared to reading aloud words in the native language5. At the sentence level, increased activation of left caudate has been observed for processing the second lan-guage as compared to the native language during reading and auditory comprehension for correct, semantically incorrect and syn-tactically incorrect sentences10–12. Thus, these studies together suggest that the left caudate is part of a language network, be it monolingual or multilingual. At this stage, it is still unclear whether its specific function is that of control-ling the language in use.

Instead, the reviewed studies suggest that the left caudate activates when the language processing system cannot rely entirely on automatic mechanisms but has to recruit controlled processes as well. This notion can

explain the results in the study by Crinion et al. as well, as more conscious processes are likely to be recruited across languages com-pared to processing within the same language. It is compatible with the finding that, com-pared to real words, it takes longer to decide whether a pseudoword (a nonsense letter string similar to a real word) is a meaning-ful word or not8. Such lexical decision times are also slower for words learned later com-pared to those learned early on9. Moreover, it is in line with the observation that second language processing is more demanding than native language processing10–12.

This more general functional description of the left caudate fits with the suggestion that it is crucial in controlling and selecting motor sequences necessary for articulation1,12. This argument is supported by neuroanatomical data showing that the left caudate receives pro-jections from prefrontal, temporal and parietal cortex, and connects reciprocally with the cortex via the thalamus (Fig. 1). The anterior portion of the body of the left caudate, the area activated in the study by Crinion et al., receives dense projections from the lateral and medial premo-tor cortex, which is functionally related to the sequencing and scheduling of compiled motor routines13. For their own findings, Crinion et al. suggest that the left caudate is active because of the differing motor patterns across languages. A more general account could be that the motor-related patterns necessary for articulatory processes are activated during language com-prehension whenever comprehension requires less automatic and more controlled processes. A number of event-related brain potential studies support this interpretation by demonstrating a selective deficit of controlled syntactic processes as a result of focal lesions of the basal ganglia, including the left caudate nucleus14,15. The extent to which controlled processes at the word and sentence level depend on the articulatory aspects of language use needs to be tested using articulatory suppression tasks.

The new data open an exciting view of the neural basis of language processing by highlighting the crucial role of subcortical

structures and their possible interplay with well-known cortical language regions. Though the data on this topic are still sparse, available studies suggest that the left caudate’s function is not to control the language in use, but rather to recruit controlled processes when language processing cannot rely primarily on auto-matic processes. Such a description of the left caudate’s function, which stresses the system’s adaptation to the processing demands rather than its control over processes, is in accordance with the functional neuroanatomical view that control processes are primarily supported by cortical regions. If this more general functional description of the left caudate is valid, the degree to which the language system recruits controlled rather than automatic processes will determine the degree to which the left caudate is activated during language process-ing. This should hold not only for monolingual and multilingual adults, but also for children during language development. Thus the left caudate may be more involved during language acquisition compared to adult processing.

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15. Kotz, S.A., Frisch, S., von Cramon, D.Y. & Friederici, A.D. J. Int. Neuropsychol. Soc. 9, 1053–1060 (2003).

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