Johan Lehrer’s Wall Street Journal column for September 17-18, 2011 reports a fascinating pair of facts – and then makes a puzzling application of them.
The first fact concerns probiotic bacteria, which are often found in yogurt and other dairy products. Researchers provided mice with either a normal diet, or a diet rich in probiotic bacteria, and then subjected them to stressful situations. The mice with the probiotic-enriched diet showed less anxiety and had lower levels of stress hormones.
By itself, this result is not so interesting. After all, it could be that the probiotic bacteria affect digestion, then blood chemistry, and finally hormone levels. But the second fact shows that a different mechanism is at work.
The second fact is that when neural connections between gut and brain were severed, the probiotic-enriched diet no longer produced the effect of reducing symptoms of stress. This fact suggests that the effect of the difference in diet works directly through the gut-brain neural connection, rather than through a less direct blood chemistry path.
It’s as if we have a sense organ in our gut that feeds into an evaluative system. It doesn’t give us any sensations, but it tells our brains how things are in our digestive systems. If things are going well down there, we’re less prone to anxiety when stressful situations arise.
That’s a surprise that contributes to a sense of wonder at how deliciously complex unconscious processes can be. Lest one think that this has nothing to do with us, Lehrer also reports a study that showed an analogous result in human subjects who received large doses of probiotics for a month. (No cutting of nerves in that case, of course.)
Now for the puzzling conclusion. These and other studies are taken by Lehrer to show that “the immateriality of mind is a deep illusion. Although we feel like a disembodied soul, many feelings and choices are actually shaped by the microbes in our gut . . . . ” And, although he concedes that “This doesn’t mean, of course, that the mind-body problem has been solved”, he goes on to declare that “it’s now abundantly clear that the mind is not separate from the body . . . . Rather, we emerge from the very same stuff that digests our lunch.”
But “shaped” is one of the many words that mean “caused”, with the addition of something about the manner of causing (as in “burned” or “built”), or degree of causal contribution (as in “influenced” or “forced”). What the cited research shows is that causes of anxious behavior and hormone levels include the presence of probiotic bacteria in the gut, and that the means of that causal contribution works through a neural connection. That is surprising and fascinating, but it offers no evidence whatsoever that feelings of anxiety are the same things as any material events.
In general, causes and effects are different. From “How anxious you feel depends in part on what kind of bacteria you have in your gut” it does not follow that feelings are material – only that feelings, whatever they are, can be caused in a very surprising way.
Similar remarks apply to “emerge”. Different people use this word in different ways, so it’s not a very helpful term. But one of its meanings is “causes”. Yes, it is indeed fascinating that what’s in our gut can cause how we feel, and do so through a direct, neural pathway. But no, that does not show that feelings are material events. It does not show that immateriality of feelings is a deep illusion.
For some purposes, the point I’m making may not matter. It’s an important fact that what goes on in our consciousness is brought about by events in our neural systems, and the studies Lehrer cites in this article do help drive that point home. But when the mind-body problem is introduced into the discussion, it becomes important to distinguish between the views (1) that neural events cause mental events such as feelings and (2) feelings are the same things as neural events. The evidence Lehrer cites in his article support (1), but are silent as regards (2).
[Jonah Lehrer, “The Yogurt Made Me Do It” The Wall Street Journal, September 17-18, 2011, p. C12.]
Your Brain and You 
I conferred with my colleague… They mentioned that several other nerves send signals from the stomach to the brain, so there is bi-directional communication, just not over the one Vagus nerve. It would be plausible that severing the Vagus nerve further disrupts downstream pathways – including these other nerves communicating back to the brain?
Matthew,
This is getting really interesting! The issue seems to be about the mechanism of the connection in the direction of gut-to-brain (and not whether there is a connection in that direction). The mechanism you suggest seems possible, and, in general, I’d agree that our mechanisms may very well be more complex than we might at first suspect. But, as Francis Urquart was fond of saying, I couldn’t possibly comment on that — it’s a question I’m certainly going to have to leave to the anatomists.
Update, 9/23/11. I’ve looked into an extremely well regarded text by Kandel and coauthors. On p. 874, it lists a number of neurons by type, where the types are Sensory, Motor, or Mixed. The last means that the nerve in question is a bundle of neurons, some of which are motor (i.e., signaling in the direction brain-to-body) and others of which are sensory (going in the direction body-to-brain).
The vagus nerve is classified as “Mixed”. It is said to mediate sensation from a long list of locations, including abdominal organs.
Also, p. 879 refers to vagus nerve transmission of visceral sensation from gastrointestinal organs.
[Kandel, E. R., Schwartz, J. H., and Jessell, T. M., Principles of Neural Science, 4th edition (New York: McGraw-Hill, 2000).]
Hey Bill,
Great post! I recently read the article you reference about the mice and their probiotic diets. I was recently discussing this subject with a colleague who is well versed in anatomy. This colleague mentioned that the Vagus nerve only sends electrical signals one way…. From the Brain to the Stomach, and not the other way around. I find it curious that the Brain could know so much about the stomach’s state when the communication is unidirectional and in the opposite direction we might be inclined to believe otherwise, given the nature of this discovery. Thoughts?
Matthew,
Thank you for the comment, which certainly raises an important question. I’m not an anatomist myself, but the suggestion that the vagus nerve sends signals only in the direction from brain to gut does not fit with what the authors of the study say. For example, they refer to “the bidirectional communication between the brain and the gut” (4th of six pages). They also explain their motivation for investigating the results of cutting the vagus nerve in this way: “To further understand the role of the vagus nerve in communicating sensory information to the brain . . . .” (3rd of six pages).
[Javier A. Bravo et al, “Ingestion of Lactobacillus strain regulates emotional behavior and central GABA receptor expression in a mouse via the vagus nerve”, PNAS Early Edition, accessed here.]