This study confirms past results such as [0], which shows that adding some Lactobacillus rhamnosus to the diet of mice modifies the expression of GABA receptors (the ones sensitive to bnezodiazepines, like Xanax), and reduces their anxiety (which can be assessed reliably). The effect is mediated by the vagus nerve. If the nerve is cut, the effect disappears.
In the same vein, [1] found that the pulmonary exposure to Mycobacterium vaccae promotes the growth of serotonin and norepinephin neurons in the brain of mice and makes them "less resigned" [2].
I'm sure there are plenty of similar results.
The sample is relatively small, but not inappropriate for a fMRI study. There are statistical tools adapted for these situations like SPM [3], provided they are used properly (and I wouldn't be able to assess whether or not they were).
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[0] http://www.pnas.org/content/108/38/16050
[1] http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1868963/