Sunday, November 26, 2006

She's Found Control Over Zwei Sprachen: Left Caudate in Bilinguals

Previously, The Neurocritic speculated on the significance of reduced rCBF (regional cerebral blood flow) in the left caudate nucleus in a SPECT study of women speaking in tongues (Newberg et al., 2006). This speculation was based on a recent fMRI study in bilinguals:
Crinion J, Turner R, Grogan A, Hanakawa T, Noppeney U, Devlin JT, Aso T, Urayama S, Fukuyama H, Stockton K, Usui K, Green DW, Price CJ. (2006). Language control in the bilingual brain. Science 312:1537-4.

How does the bilingual brain distinguish and control which language is in use? Previous functional imaging experiments have not been able to answer this question because proficient bilinguals activate the same brain regions irrespective of the language being tested. Here, we reveal that neuronal responses within the left caudate are sensitive to changes in the language or the meaning of words. By demonstrating this effect in populations of German-English and Japanese-English bilinguals, we suggest that the left caudate plays a universal role in monitoring and controlling the language in use.
As usual, The Neurocritic feels obligated to investigate (and possibly criticize) the notion that "the left caudate plays a universal role in monitoring and controlling the language in use." Why the caudate? Why not somewhere in the left frontal lobe??

In their fMRI experiment, Crinion et al. took advantage of the neuronal adaptation technique (Grill-Spector & Malach, 2001), or repetition suppression (Henson, 2003), a form of priming in which a relative reduction in neural activity is observed when a stimulus is repeated (repetition priming - see Henson & Rugg, 2003), or a semantically related stimulus is presented (semantic priming - see Wible et al, 2006).

What was the experimental design?
The influence of the prime on the target was identified by comparing the response to prime-target pairs that were either semantically related (bathtub-SHOWER) or unrelated (spoon-SHOWER). We then identified language-dependent semantic responses by comparing the effect of semantic priming when prime and target were in the same language (trout-SALMON in English or forelle-LACHS in German) or different languages (e.g., trout-LACHS or forelle-SALMON).
The task was to make a semantic decision on the second word of the pair (e.g., multi-coloured or plain? WASP vs. WORM). And the goal was to find brain regions sensitive to semantic priming OR to switches between languages (or to an interaction between the two). Contrary to popular belief, neuroimaging evidence indicates that the two different languages of fluent bilinguals are represented in the same set of brain regions (Perani & Abutalebi, 2005). The major question here, then, is
how the brain determines or controls the language in use.
What they found is illustrated in the figure to the left.

Language-dependent neuronal adaptation in the left caudate. Activation for unrelated minus semantically related word pairs in the same language only. The average of all fMRI data indicated that the only significant effect across the whole brain was in the left caudate.
(A) German-English fMRI; (B) Japanese-English fMRI. [from Crinion et al. 2006]


What does this mean? There was a reduction in activity for semantically related vs. unrelated pairs when presented in the same language, but not when the pairs were presented in different languages. This finding in the left caudate contrasts with what was observed in the left anterior temporal lobe, which was a priming-related reduction regardless of whether the word pairs appeared in the same or different languages.

The authors favor the following explanation:
The second interpretation is that the same neurons respond to semantic input in both languages with increased neuronal firing when there is a change in language. Neuronal populations that respond to a change in language would indicate a possible mechanism for language control that regulates output whenever a change in input is detected.
This view is somewhat different to the explanation put forth in an earlier paper by these authors (Price et al., 1999):
Price CJ, Green DW, von Studnitz R. (1999). A functional imaging study of translation and language switching. Brain 122: 2221-2235.

The neural systems underlying translation and language switching were investigated using PET. Proficient German-English adult bilinguals were scanned whilst either translating or reading visually presented words in German (L1), English (L2) or alternating L1/L2. We refer to alternating L1/L2 as 'switching'. The results revealed contrasting patterns of activation for translation and switching, suggesting at least partially independent mechanisms. Translation, but not switching, increased activity in the anterior cingulate and subcortical structures whilst decreasing activation in several other temporal and parietal language areas associated with the meaning of words. Translation also increased activation in regions associated with articulation (the anterior insula, cerebellum and supplementary motor area) arguably because the reading response to the stimulus must be inhibited whilst a response in a different language is activated. In contrast, switching the input language resulted in activation of Broca's area and the supramarginal gyri, areas associated with phonological recoding. The results are discussed in terms of the cognitive control of language processes.
Note that language switching was NOT associated with increased subcortical activity in that study. Furthermore,
...we suppose no unitary switch mechanism specific to changing language. Indeed, neuropsychological case reports provide no warrant for it. Evidence that the supramarginal gyri are critical is countered by the patients with lesions in such regions without switching problems, yet the supramarginal gyri, we will suggest, are nevertheless involved in switching. On the other hand, neuropsychological data do suggest the relevance of systems (e.g. the frontal lobes) involved in the general control of action.
I'm always facinated with how researchers reconcile conflicting results from their own experiments. I've seen a pair of papers from the same lab, published in the same year, where neither paper cites the other one because of inconsistencies between them. I'm all about incorporating contradictions into a larger picture, albeit a fragmented and non-parsimonious one. That's reality, not some neat simplistic explantion of your own narrow findings, divorced from all other research on the topic. It's truly amazing that some science articles (and Science articles) can be so non-scholarly.

In the next post, The Neurocritic will examine how language control and the left caudate nucleus meshes with other neuroimaging studies of this particular brain region.

References

Grill-Spector K, Malach R. (2001). fMR-adaptation: a tool for studying the functional properties of human cortical neurons. Acta Psychol 107:293-321.

Henson RN. (2003). Neuroimaging studies of priming. Prog Neurobiol. 70:53-81.

Henson RN & Rugg MD. (2003). Neural response suppression, haemodynamic repetition effects, and behavioural priming. Neuropsychologia 41: 263-270.

Perani D & Abutalebi J. (2005). The neural basis of first and second language processing. Curr Opin Neurobiol. 15: 202-206.

Price CJ, Green DW, von Studnitz R. (1999). A functional imaging study of translation and language switching. Brain 122: 2221-2235.

Wible CG, Han SD, Spencer MH, Kubicki M, Niznikiewicz MH, Jolesz FA, McCarley RW, Nestor P. (2006). Connectivity among semantic associates: an fMRI study of semantic priming. Brain Lang. 97:294-305.

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