Recently, researchers have put effort into overcoming this gap between rodents and humans by relying on the brain-derived neurotrophic factor (BDNF) in quantifying environmental affordance for physical activity for hippocampal neurogenesis sustainability in healthy subjects [25] and determining the relationship between the level of environmental enrichment and the brain-derived neurotrophic factor (BDNF) in subjects with major depressive disorder [26]. This evidence concerns the gene BDNF and major depressive disorder.