New insights in the development of positive allosteric modulators of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors belonging to 3, 4-dihydro-2H-1, 2, 4-benzothiadiazine 1, 1-dioxides: Introduction of (mono/difluoro)methyl groups at the 2-position of the thiadiazine ring