A recent study, “Selective deficit of second language: a case study of a brain-damaged Arabic-Hebrew bilingual patient” by Raphiq Ibrahim (Behavioral and Brain Functions 2009, 5:17), describes something rather remarkable; in the words of Mo at the neurophilosophy blog:
The study, by Raphiq Ibrahim, a neurologist at the University of Haifa, describes a bilingual Arabic-Hebrew speaker who incurred brain damage following a viral infection. Consequently, the patient experienced severe deficits in one language but not the other. The findings support the view that specific components of a first and second language are represented by different substrates in the brain….
The results support a neurolinguistic model in which the brain of bilinguals contains a semantic system (which represents word meanings) which is common to both languages and which is connected to independent lexical systems (which encode the vocabulary of each language). The findings further suggest that the second language (in this case, Hebrew) is represented by an independent subsystem which does not represent the first language (Arabic) and is more susceptible to brain damage.
Thanks for the link, Trevor!