BDNF and Keratoconjunctivitis sicca: The cognitive function, working memory, dry eye, AChE, MAO, MAO-A, MAO-B, and GABA-T activities, BDNF, HAC, HDAC, and DNMT activities, pH, and amount of lactic acid-producing bacteria, particularly Lactobacillus and Bifidobacterium spp.