Another recent area of such research on bilinguals is how their two languages are stored and used in the brain. For example, if someone learns two languages from birth, are the two languages stored differently in the brain from someone who learns a second language at school or in adult life? The effect of age of learning on language and cognition outcomes, commonly known as the Critical Period Hypothesis, has been a topic of heated debate in psycholinguistics/neurolinguistics research communities (DeKeyser & Larson-Hall, 2009; Huang, 2014). One much publicized piece of neuro linguistics research by Mechelli et al. (2004) suggested that learning a second language increases the density of gray matter. When comparing 25 monolinguals, 25 early bilinguals and 33 late bilinguals, gray matter density was greater in bilinguals than monolinguals, with early bilinguals having increased density compared with late bilinguals. Thus such density of gray matter 'increases with second language proficiency but decreases as the age of acquisition increases' (p. 757) such that 'the structure of the brain is altered by the experience of acquiring a second language' (p. 757). Moreover, the region of the brain where this increase in gray matter has been observed is activated during vocabulary acquisition in both monolinguals and