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I was surprised to find a literature review about probiotics which suggested they may have significant CNS effects. The tl;dr of the review seems to be: 1) You want doses of at least or CFU, and 2) You want, in particular, the strains B. longum, B. breve, B. infantis, L. helveticus, L. rhamnosus, L. plantarum, and L. casei.

I then sorted the top 15 results on Amazon for "probiotic" by these desiderata, and found that this one seems to be best.

Some points of uncertainty:

  • Probiotic manufacturers generally don't disclose the strain proportions of their products, so there's some chance they mostly include e.g. whatever's cheapest, plus a smattering of other stuff.
  • One of the reviewed studies suggests L. casei may impair memory. I couldn't find a product that didn't have L. casei but did have at least CFU of each other recommended strain, so if you take the L. casei/memory concern seriously your best option might be combining this and this.
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For convenience, here's a slightly edited-for-clarity version of the abstract:

38 studies (all randomized controlled trials) were included: 25 in animals and 15 in humans (2 studies were conducted in both). Most studies used Bifidobacterium (eg, B. longum, B. breve, and B. infantis) and Lactobacillus (eg, L. helveticus, and L. rhamnosus), with doses between and 10^10 colony-forming units for 2 weeks in animals and 4 weeks in humans.
These probiotics showed efficacy in improving psychiatric disorder-related behaviors including anxiety, depression, autism spectrum disorder (ASD), obsessive-compulsive disorder, and memory abilities, including spatial and non-spatial memory.
Because many of the basic science studies showed some efficacy of probiotics on central nervous system function, this background may guide and promote further preclinical and clinical studies. Translating animal studies to human studies has obvious limitations but also suggests possibilities. Here, we provide several suggestions for the translation of animal studies. More experimental designs with both behavioral and neuroimaging measures in healthy volunteers and patients are needed in the future.

Possibly another good example of scientists failing to use More Dakka. The mice studies all showed solid effects, but then the human studies used the same dose range (10^9 or 10^10 CFU) and only about half showed effects! Googled for negative side effects of probiotics and the healthline result really had to stretch for anything bad. Wondering if, as much larger organisms, we should just be jacking up the dosage quite a bit.

On the other hand: half of mouse studies working in humans is an extremely good success rate. We should be quite suspicious of file-drawer effects and p-hacking.

I agree the effect is consistent enough that we should be suspicious of file drawer/p-hacking—although that's also what you'd expect to see if the effect were in fact large—but note that they were different studies, i.e. the human studies mostly weren't based on the non-human ones.

I was initially very concerned about this but then noticed that almost all the tested secondary endpoints were positive in the mice studies too. The human studies could plausibly still be meaningless though.

Has anyone (esp you Jim) looked into fecal transplants for this instead, in case our much longer digestive system is a problem?

In the early 1900s the Smithsonian Institution published a book each year, which mostly just described their organizational and budget updates. But they each also contained a General Appendix at the end, which seems to have served a function analogous to the modern "Edge" essays—reflections by scientists of the time on key questions of interest. For example, the 1929 book includes essays speculating about what "life" and "light" are, how insects fly, etc.

I made Twitter lists of DeepMind and OpenAI researchers, and find them useful for tracking team zeitgeists.

Apparently Otzi the Iceman still has a significant amount of brain tissue. Conceivably memories are preserved?

Another (unlikely, but more likely than almost all other ancient people) candidate for partial future revival: During the 79 AD eruption of Vesuvius, part of this man's brain was vitrified.

I found LinkedIn's background breakdown of DeepMind employees interesting; fewer neuroscience backgrounds than I would have expected.