Brain differences between bilinguals of differing proficiency: an empirical look at an emotional issue.

Difficulty with acquiring a second language is not by any stretch of the imagination an illness. Yet in some communities where social and political pressures to achieve proficiency in the mother tongue exist, those who are not facile in language acquisition experience suffering not dissimilar to what persons with a chronic illness experience: emotional pain, a sense of deficiency, restriction of access to educational and sometimes, career opportunities. Politicians and parents alike may argue the case for and against expecting children to be bilingual but in many instances, emotive arguments have been raised without the support of empirical evidence. If it is truly a more thoughtful society we wish to build and one where reason serves to guide action more than emotion, it behooves us to study the brain, the source of our ability to acquire language. My laboratory has been involved in the characterisation of the bilingual brain for several years (1-5). One of our goals was to seek clues as to why, despite being immersed in the same environment, some individuals have difficulty acquiring a second language even though they have an excellent command of their first language. Several lines of evidence suggest that phonological working memory (PWM) plays a crucial role in determining language acquisition ability (6). Working memory is a psychological construct based on observations on short-term memory. It refers to a short-term capacity limited system whose contents are organised into phonological and visuo-spatial sections. While there are many criticisms of this construct, it has motivated memory research in a resounding manner over the years and a credible alternative platform to study short-term memory that has broad-based appeal has yet to emerge. Previous theoretical and behavioural work points to the existence of two dissociable components in PWM: a subvocal rehearsal system and a short-term phonological store. Functional imaging results suggest that the subvocal rehearsal system is located in Broca's area (Brodmann's area (BA) 44), left premotor cortex (BA 6) and supplementary motor area (BA 6) whereas the phonological store resides in the left inferior parietal cortex (BA 40) (7,8). The obligatory role of the frontal opercular areas in phonological processing has been recently demonstrated using repetitive transcranial magnetic stimulation (rTMS) (9). PWM is thought to temporarily store unfamiliar sound patterns, while more permanent representations are being constructed for long-term memory storage (6). In support of these postulates, a number of behavioural studies have shown that measures …