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There is growing evidence that various experiences have a significant effect on behavioral, neuropsychological, and structural aspects of cognitive performance. For example, video game players have been shown to have enhanced visual selective attention (Green and Bavelier, 2003), skills that can be increased by extensive video game training (Feng, Spence and Pratt, 2007), and architects have demonstrated higher levels of visuo-spatial ability than non-architects (Salthouse and Mitchell, 1990). Neural connections can also be modified: Canadian postal workers who continually interpret codes containing both letters and numbers have enhanced pathways between the letter and number representational systems relative to American postal workers who deal only in numeric codes (Polk and Farah, 1998). Structural changes from experience have been documented as well. London taxi drivers who have extensively engaged in route-finding have been shown to have enlarged regions of the hippocampus responsible for spatial navigation (Maguire et al., 2000). Professional musicians who play string instruments for which the sound pitch and quality emanates from control of the four fingers of the left hand have been shown to have increased cortical representation of those fingers (Elbert et al., 1995). Finally, individuals who speak a second language have been shown to have increased density of grey matter in the left inferior parietal cortex, a change that is more pronounced in early bilinguals and those with greater proficiency in the second language (Mechelli et al., 2004). This region has been shown to be responsive to vocabulary acquisition in monolinguals and bilinguals as well as producing enlargements in slightly different areas depending on the two languages of the bilingual (Green, Crinion and Price, 2007). Furthermore, the accumulated effect of stimulating experience across the lifespan translates into cognitive reserve, a concept describing the protective effects of experience against cognitive decline with aging (Stern, 2002; Fratiglioni, Paillard-Borg and Winblad, 2004; Kramer et al., 2004; Staff, Murray, Deary and Whalley, 2004; Valenzuela and Sachdev, 2006). It is evident, therefore, that experience has a powerful effect on cognitive performance and brain organization and structure. Is bilingualism one such experience that leads to these general cognitive outcomes?
The central aspect of the bilingual experience that may be responsible for generalized effects on cognitive performance comes from the well-documented observation that...