A stable sense of self is rooted in the lungs, heart and gut (Alessandro Monti, Psyche, Dec 06 2021)

“Think back to a recent episode in which you felt as if you were being true to yourself. How would you describe what you did?

Perhaps you would say that you ‘trusted your guts’ or ‘followed your heart’, rather than ‘thinking with your head’.

You might also assume that these idioms involving the guts or the heart belong to an outdated folklore – that they are a poetic rather than a scientific expression of what’s happening when we tap into our sense of self.

Yet, emerging scientific evidence increasingly suggests that being aware of who you are – being self-conscious – really does depend, not just on processes in your brain, but also on what’s happening deep in your viscera. 

Consider how, right now, you could be in a very different place, mood or situation than 20 seconds or 20 years ago, yet you feel that in a fundamental sense you are the same person.

This is partly because, as William James put it in The Principles of Psychology (1890), you are aware that ‘the same old body’ is always with you, exuding warmth and intimacy.

With the exception of dreaming and altered states of mind, all conscious experiences entail this subtle but pervasive feeling of bodily self-consciousness.

But where does it come from? (…)

Indeed, a remarkable feature of visceral organs is the fact that they go through steady, predictable physiological cycles. 

Heartbeats, breaths and gut contractions repeat themselves with regularity, keeping the body warm and fed – a physiological equilibrium known as homeostasis. 

Moreover, each of these cycles involves peripheral nerves – especially the vagus nerve – sending chemical and electrical signals to the central nervous system.

As a result, the activity of specific regions of the central nervous system synchronises with cardiac, respiratory and gastric fluctuations.

While sensory impressions coming from the external environment vary from moment to moment and fade away, this coupling between brain and viscera is a permanent feature of your physiology.

You can close your eyes, cover your ears, hold your nose or seal your mouth, but you cannot cut yourself off from your bowels. (…)

Given that gut signals seem both to regulate food intake and to buttress one’s sense of self, poor processing of these signals could explain why individuals living with anorexia or bulimia nervosa display both abnormal eating habits and a fuzzy sense of self, sometimes morphing into self-disgust.

If gut signals are exceedingly high, they might anchor the self too heavily to what happens in their stomach and intestine. 

Conversely, if they are exceedingly low, they might make the self too reliant on external feedback – or just too unstable.

The latter case could apply also to people with depersonalisation, whose feeling of being detached from their body and self might be due to the fact that their selfhood is not rooted enough in their viscera (although there is evidence both for and against this claim).

Such an impairment could be even more pronounced in Cotard’s delusion, a rare syndrome in which people feel they are ‘empty’, ‘rotting inside’, ‘without internal organs’, or even ‘non-existent’ or ‘dead’.

Smart pills tracking the activity of the gut, combined with questionnaires and manipulations of bodily self-consciousness, such as bodily illusions, could thus deepen the current knowledge of these psychological and psychiatric conditions. (…)

An important limitation of contemporary psychology and neuroscience is that scholars replaced the old Cartesian dualism – mind versus body ­– with a new dualism: brain versus body.

The new dichotomy was even cruder than the old one, and certainly no less rigid.

Le Journal des sciences: Dépression : le microbiote intestinal communique avec le cerveau par le nerf vague

"De plus en plus d’indices pointent vers un rôle du microbiote dans les troubles psychiatriques, l’anxiété et la dépression, mais pour l’instant chez l’humain il s’agit de corrélations.

On sait qu’une dysbiose, donc un dérèglement du microbiote est associé aux troubles dépressifs mais cela ne dit rien du lien de causalité, qui est la cause et qui est la conséquence.

En revanche chez la souris, on en sait davantage, notamment parce qu’on peut les étudier dans des conditions stériles.

Une modification de la composition du microbiote murin entraîne des changements comportementaux et cognitifs : des troubles de l'attention, de la perception, de la mémoire, d’interaction avec les autres congénères et des symptômes dépressifs.

Et oui, les symptômes dépressifs s’observent aussi chez la souris.

Pour explorer un lien causal entre microbiote et dépression, ces scientifiques ont induit un état dépressif chez ces animaux par de petits stress chroniques, avant de transférer le microbiote de ces souris dépressives à des souris saines.

Résultat, le transfert de microbiote suffit à déclencher des troubles dépressifs et cette information passe par un lien physique qui est le nerf vague.

Entretien avec Eleni Siopi maîtresse de conférence à l’Université Paris cité.

Elle est l’autrice principale de ces travaux menés lors de son post-doctorat à l’Institut Pasteur et parus dans Molecular Psychiatry."

Eleni Siopi:

"Nous avons vu qu'il existe une voie anatomique directe qui lie les intestins et le cerveau, qui s'appelle le nerf vague.

Le microbiote intestinal a accès à ce nerf, il peut influencer l'activité de ce nerf vague.

Nous avons constaté que le microbiote perturbé par le stress chronique induit une augmentation de l'activité du nerf vague chronique.

Cette suractivation du nerf vague est suffisante pour complètement perturber la neurotransmission au niveau du cerveau.

Pour vraiment prouver ce lien entre microbiote, nerf vague, et cerveau, vous avons effectué une vagotomie.

Nous avons constaté qu'en dissociant les intestins et le cerveau via la coupure du nerf vague, les souris ne sont plus déprimées suite à une inoculation avec le microbiote perturbé.

Cette étude permet d'envisager des stratégies thérapeutiques alternatives pour soulager la dépression.

C'est important de souligner qu'il n'y a que 30% des personnes traitées avec les antidépresseurs qui ressentent un effet bénéfique, donc l'efficacité n'est pas du tout satisfaisante.

Bien sûr, on ne peut pas couper le nerf vague chez l'humain, mais on peut montrer que le bien-être du nerf vague est important pour notre sommeil et respiration.

Sufferers of chronic pain have long been told it’s all in their head. We now know that’s wrong (Linda Geddes, The Guardian, June 28 2021)

“Traditionally, doctors have divided pain into two categories: nociceptive pain, triggered by injuries such as bruises, burns, fractures, sprains or inflammatory conditions such as arthritis, and detected by pain receptors in our skin, bones and other tissues;

and neuropathic pain triggered by damage to the nerves that carry sensory signals from these tissues to the brain and spinal cord.

But a few years ago, the IASP introduced a third category: nociplastic pain – pain arising from the altered processing of these sensory signals, without any evidence of actual or threatened tissue damage.

One thing that can happen is that the pain signals carried by peripheral nerves from the skin, muscles, joints or internal organs to the spinal cord can become amplified, worsening the pain. (…)

For one thing, nerves in the brain and spinal cord – the central nervous system – can become hyper-excitable, meaning that minor bumps and grazes become agonising, and even non-painful stimuli such as brushing your hair or touching your skin can trigger pain.

“It is like an allergic reaction in the pain system,” Arendt-Nielsen says. This “central sensitisation” can also spread.

“One of the features that we see in many patients with chronic pain is that they have this generalised pain hypersensitivity.

They might have osteoarthritis and pain in their knee, but if you quantitatively assess the sensitivity of the pain system, they have a lower lower pain threshold throughout their bodies,” Arendt-Nielsen says. (…)

“In many people with conditions like low back pain, endometriosis, irritable bowel, headache, fibromyalgia, you’re not going to find a problem in the area of the body where the person is experiencing pain.

There’s more and more evidence that these are central nervous system, systemic conditions, where the pain can present in different areas of the body at different points in time over that person’s life,” says Daniel Clauw, a professor of anesthesiology, medicine and psychiatry at the University of Michigan.

He likens the experience of pain to the noise produced by an electric guitar; to make it louder, you can either strum the strings harder or turn up the amplifier. 

In this analogy, the strings are represented by the peripheral nerves carrying sensory information from our organs and tissues, and the amplifier by the brain and spinal cord.

A patient with third-degree burns is having their strings strummed extremely hard, whereas in someone with fibromyalgia – a long-term condition that causes pain all over the body – their amplifier has been set too loud. 

“They can have pain, without the strings even having to be strummed,” Clauw says. (…)

Not everyone who develops a chronic pain condition will develop more of them but they seem to be particularly prevalent among women, and those affected also often suffer from non-pain conditions such as sleep and mood disorders, and fatigue.

Some of this variation between men and women may be due to immune system differences, with women also more prone to autoimmune disease.

Immune cells trigger inflammation, which can trigger pain. 

Sex hormones such as oestrogen and testosterone can further exacerbate or dull pain, depending where they are released and in what quantities.

Laid on top of those basic amplifier settings are events that happen to you as you go through life.

Chronic pain is more common among people who experienced lots of surgery as children, or emotional or physical abuse.

There is also a strong link between chronic pain and depression. Of course, living with chronic pain can be depressing, but depression can also amplify pain processing in and of itself.

This is not the same thing as suggesting that someone’s mental attitude can affect their pain.

“Rather, people often don’t realise that the brain pathways that drive depression are intrinsically linked to the ones that drive chronic pain,” says Dr Kirsty Bannister, a senior lecturer at King’s College London, who researches these pain pathways. (…)

Although opioids are effective at dampening short-term severe pain or cancer pain, they are highly addictive and accidental overdose kills about 50,000 people in the US alone each year.

They can also make chronic pain worse.

This is because opioids provide relief by blocking pain receptors but your body responds by increasing the number of pain receptors, meaning you need higher doses to get the same relief.

In chronic pain conditions such as fibromyalgia, people have already been producing natural opioids called endorphins, which bind to the same receptors.

Newly detailed nerve links between brain and other organs shape thoughts, memories, and feelings (Emily Underwood, ScienceMag, June 10 2021)

“For more than 100 years, scientists have known that the vagus nerve carries signals between the organs and the brainstem.

As part of the parasympathetic nervous system—active when the body is at ease or recovering from stress—the vagus regulates autonomic functions such as heart rate, breathing, and digestion.

But new studies have shown signals carried by vagal fibers climb beyond the brainstem, revealing a broad interoceptive network in the brain that interprets internal changes, anticipates the body’s needs, and sends commands to fulfill them.

The network includes brain regions involved in more complex cognition, which means the nerves monitoring the body’s basic workings also respond to—and influence—how we remember, process emotion, and even construct our sense of self. (…)

The findings, published last year in the Proceedings of the National Academy of Sciences, might revive the notion that stress causes stomach ulcers.

That idea was largely dismissed in the 1980s after a study that led to a 2005 Nobel Prize traced them to bacterial infections with Helicobacter pylori.

But the new work suggests stress, by disrupting either vagal pathway, might hinder digestion and create a more welcoming environment for ulcer-inducing bacteria in the stomach, Strick says.

Other studies of vagus connections suggest they influence memory and learning.

In a 2018 study in rats, neuroscientist Scott Kanoski at the University of Southern California severed sensory vagal connections between the stomach and the hippocampus, a brain region crucial to forming memories.

The interruption prevented the animals from remembering new objects and locations and slowed the birth of neurons. (…)

Lisa Feldman Barrett, a neuroscientist at Northeastern University, points to evidence that mood disorders can stem from metabolic problems—which in turn can have roots in stressors that affect the brain, such as early childhood trauma and neglect or sleep deprivation. 

Such experiences can also shape how we interpret internal sensations. 

Rather than passively receiving information, the brain is constantly constructing a model of its sensory conditions and guessing what caused them in order to direct the correct response, Barrett says. 

“You feel a tug in your chest, and your brain has to decide if it’s because you ate too much for dinner or if it’s the early sign of a heart attack.” (…)

He hopes the buzzing pill could prove a handy way to test people’s interoceptive acuity and identify links to physical and mental health.

“One of the debates about interoceptive awareness is whether it is good to have more of it, or less,” says Wen Chen, branch chief at the National Center for Complementary and Integrative Health, who recently convened the National Institutes of Health’s first meeting on interoception research. 

People with conditions such as generalized anxiety disorder or irritable bowel syndrome may be “hyperfocused on sensations from their bodies,” adds Vitaly Napadow, a neuroscientist at Harvard University’s Athinoula A. Martinos Center for Biomedical Imaging.

Cognitive training might help them manage overwhelming internal sensations, he says. (…)

To Tallon-Baudry, the research in coma patients challenges a long history of thinking of bodily regulation as separate from “higher” mental processes, such as language, that constitute our sense of self. 

Four centuries ago, René Descartes famously conceptualized the mind as being separate from the body. 

It Takes Guts to Fend Off Loneliness and Achieve Wisdom (Neuroscience News, Mar 25 2021)

“The human gut microbiota is comprised of trillions of microbes — bacteria, viruses, fungi — that reside within the digestive tract.

Researchers have known for a while about the “gut-brain axis,” which is a complex network that links intestinal function to the emotional and cognitive centers of the brain.

This two-way communication system is regulated by neural activity, hormones and the immune system;

alterations can result in disruptions to stress response and behaviors, said the authors, from emotional arousal to higher-order cognitive abilities, such as decision-making.

Past studies have associated gut microbiota with mental health disorders including depression, bipolar disorder and schizophrenia, as well as personality and psychological traits regarded as key, biologically based components of wisdom. (…)

The authors said that the mechanisms that may link loneliness, compassion and wisdom with gut microbial diversity are not known,

but observed that reduced microbial diversity typically represents worse physical and mental health, and is associated with a variety of diseases, including obesity, inflammatory bowel disease and major depressive disorder.

A more diverse gut microbiota may be less susceptible to invasion by outside pathogens, which could contribute to and help promote better resilience and stability of the community.

“It is possible that loneliness may result in decreased stability of the gut microbiome and,

consequently, reduced resistance and resilience to stress-related disruptions, leading to downstream physiological effects, such as systemic inflammation,” the authors wrote. (…)

The relationship between loneliness and microbial diversity was particularly strong in older adults, suggesting that older adults may be especially vulnerable to health-related consequences of loneliness, which is consistent with prior research.

Gut Bacteria Connected to How Babies Experience Fear (Neuroscience News, June 04 2021)

“The human digestive system is home to a vast community of microorganisms known as the gut microbiome. 

The MSU-UNC research team discovered that the gut microbiome was different in infants with strong fear responses and infants with milder reactions.

These fear responses — how someone reacts to a scary situation — in early life can be indicators of future mental health.

And there is growing evidence tying neurological well-being to the microbiome in the gut. (…)

“Fear reactions are a normal part of child development. Children should be aware of threats in their environment and be ready to respond to them” said Knickmeyer, who also works in MSU’s Institute for Quantitative Health Science and Engineering, or IQ.

“But if they can’t dampen that response when they’re safe, they may be at heightened risk to develop anxiety and depression later on in life.”

On the other end of the response spectrum, children with exceptionally muted fear responses may go on to develop callous, unemotional traits associated with antisocial behavior, Knickmeyer said. (…)

The researchers also discovered that the content of the microbial community at 1 year of age related to fear responses.

Compared with less fearful children, infants with heightened responses had more of some types of bacteria and less of others.

The team, however, did not observe a connection between the children’s gut microbiome and how the children reacted to strangers who weren’t wearing masks.

Knickmeyer said this is likely due to the different parts of the brain involved with processing potentially frightening situations.

“With strangers, there is a social element. So children may have a social wariness, but they don’t see strangers as immediate threats,” Knickmeyer said.

“When children see a mask, they don’t see it as social. It goes into that quick-and-dirty assessment part of the brain.”

As part of the study, the team also imaged the children’s brains using MRI technology.

They found that the content of the microbial community at 1 year was associated with the size of the amygdala, which is part of the brain involved in making quick decisions about potential threats.