Have you ever imagined what it would be like to lose language? To not be able to express it or understand it? Well, for people suffering from aphasia, this tends to be a sad reality.

Aphasia is a growing field of study, especially in bilingual patients, who may lose one or both languages. Scientists have been wondering how this works and if it's possible to recover one or both languages. Some believe that recovery of one language over another may have to do with problems in language control, not an actual loss of lexical information in the brain. Using this logic, some scientists did a case study on one man, TD, who suffers from differential aphasia. Differential aphasia means that TD shows different symptoms of aphasia in each language, in his case, one language is more severely impaired than the other.

So you might be wondering...who is TD?

Figure 1. MRI scans showing the damage to TD's brain suffered in the accident. 

TD is a Belgian man whose first language (L1) is French. He grew up in Belgium and studied in Europe until he pursued his PhD, which he did in the United States. While here, he became extremely proficient in English (his L2), ad even used it more than French--he became L2 dominant. He developed aphasia after a traffic accident, and has since had syntactic and semantic comprehension difficulty and anomia in French, specifically. He has syntactic comprehension difficulty in both languages but made more errors and expressed more difficulty in French. Scientists took an interest in his case and designed a study around it.

This study compared TD with two control groups. One was made up of 10 native French speakers which had similar proficiency levels in English--these participants were given lexical decision (LD) tasks to test linguistic cognitive control and flanker tasks to test cognitive control not linked to linguistics. The other control group consisted of 26 neurotypical bilingual participants--these participants were used to compare their functional connectivities within language production, language comprehension and language control areas with those of TD.

For the 10 participants, they had four tasks: three for linguistic cognitive control and one for nonlinguistic cognitive control. The first three tasks consisted of one generalized LD task questioning if the word they were shown existed in any language. The other two LD tasks were specified to ask if the word existed in either English or French. The specified LD tasks also contained words in the non-target language to increase the language control demands. The nonlinguistic cognitive control task was a flanker task which asked the participants to pay attention to and indicate which direction the center arrow was pointing, all while ignoring the 6 surround arrows (Figure 2).

Figure 2. Taken directly from Van der Linden et al. (2018). Described the lexical decision tasks both the first control group and TD has to participate in.

There were three main results in this experiment:

(1) Cognate facilitation (the ability of bilingual speakers to recognize and produce cognates a lot faster than non-cognates) seemed significant for TD in the French selective LD task but not on the English selective LD task. This shows that TD was able to inhibit French so that it wouldn't affect English word recognition, but couldn't inhibit English correctly. However, TD showed similar cognate facilitation effects to the control group in the general task, which tells us that TD didn't lose his representation of French words, they were still active and able to facilitate cognate recognition in English. All together, this suggests that differential aphasia is not just a loss of language representation, but it may be more directly attributed to a loss of language control.

(2) TD showed decreased connectivity between the left angular gyrus and the head of the caudate (HC). The left angular gyrus is important for integration of lexical and semantic information while the head of caudate is important for translation and language selection/switching. Decreased connectivity between these areas shows a decrease in functionality, meaning probable functional deficits. Considering how TD has trouble with semantic and lexical information comprehension and seems to have trouble with language control, this decreased connectivity is supported by his behavioral results.

(3) TD showed increased connectivity with the inferior temporal cortices. These results may be caused by the brain's compensating or reorganization after the loss of language abilities caused by the aphasia.

Why is knowing about TD important? Well, TD has allowed Van der Linden et al. to widen the scope of aphasia--expanding it from a neurolinguistic issue to one that seems to affect other networks as well, especially the cognitive control network. TD places more emphasis on the influence that language control has on the way aphasia presents itself. Looking into these ideas allows us to continue thinking of the brain more holistically, which may be beneficial in further analyzing how to treat aphasia. So, next time you think about what it would be like to lose language, think about TD and how he might have just changed the way that question is viewed.

Sources:

Abutalebi, J., & Green, D. (2007). Bilingual language production: The neurocognition of language representation and control. Journal of neurolinguistics, 20(3), 242-275.

Abutalebi, J., Della Rosa, P. A., Tettamanti, M., Green, D. W., & Cappa, S. F. (2009). Bilingual aphasia and language control: a follow-up fMRI and intrinsic connectivity study. Brain and language, 109(2-3), 141-156.

Van der Linden, L., Dricot, L., De Letter, M., Duyck, W., de Partz, M. P., Ivanoiu, A., & Szmalec, A. (2018). A case study about the interplay between language control and cognitive abilities in bilingual differential aphasia: Behavioral and brain correlates. Journal of Neurolinguistics, 46, 37-68.