BDNF and major depressive disorder: ID: Intradimensional; ED: Extradimensional; CANTAB: Cambridge Neuropsychological Test Automated Battery; LTP: Long-term potentiation; EEG: Electroencephalography; MMN: Mismatch negativity; ANOVA: Analysis of variance; Y-BOCS: Yale-Brown Obsessive Compulsive Scale; MADRS: Montgomery-Asberg Depression Rating Scale; BDNF: Brain-derived neurotrophic factor