This study of 22 healthy men found that those who took a probiotic containing Bifidobacterium longum 1714 performed better in a visual memory task. Previously the same team had found that this strain reduced stress, anxiety and depressive symptoms and improved memory in mice. These findings suggest that a lack of beneficial gut bacteria could have a significant effect on human emotional and cognitive functioning.

http://www.ucc.ie/en/media/academic/psychiatry/Allen_Psychobiotic_SfN_2015.pdf