GDNF and Mental deterioration: In terms of the effects on the neural system, many neural factors, such as nerve growth factor (NGF), glial-derived neurotrophic factor (GDNF), vascular endothelial growth factor (VEGF), and brain-derived neurotropic factor (BDNF), are hypothesized as being involved in the protection afforded by bilingualism against cognitive decline (69) (Figure 2).