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LYING in the dark, my senses are straining for inputs and finding none. I am floating in warm, salty water that is so close to my body temperature, I can鈥檛 tell where my body ends and the water begins. After a while, my senses go quiet and my focus turns inwards. Now, all I am aware of is my breathing and the surprisingly loud beating of my heart.

I am inside a pod-like floatation tank to try to boost my powers of interoception. According to a growing body of research, interoceptive sensations 鈥� those that originate from within the body, from its tissues, organs and chemicals circulating in the bloodstream 鈥� hold the key not only to better mental well-being, but to revolutionary new treatments for common, yet hard-to-treat conditions like depression, anxiety and eating disorders. With several now in clinical trials, it is a change of direction that could see the focus on the brain alone in mental health become a thing of the past, offering hope of progress for millions.

In recent years, it has become clear that to really understand mental health, you need to factor in just how much the brain cares about what is going on below the neck. For any animal, survival depends on how well it can detect physical changes that may signal a threat and to take appropriate action to get things back on track. Interoception is a bit like our sixth sense 鈥� the ability to detect these bodily changes, from heartbeat to changing concentrations of certain hormones in the blood, as well as the psychological expression of these as feelings and emotions. Integrated by the brain, these changing bodily sensations feed into our mental state and behaviour, consciously or unconsciously, and have a say in every thought and emotion we have. 鈥淚nteroception is fundamental to every brain process and behaviour that there is,鈥� says Hugo Critchley, a neuroscientist at the University of Sussex, UK, who studies the process.

Under the radar

The reason that we generally aren鈥檛 cognisant of this life-sustaining system is because interoceptive messages are below the radar of conscious awareness most of the time. They can and do break the surface when something needs to be addressed, motivating us to seek out food when our stomach rumbles, or get ready to run when we feel a rush of adrenaline.

Yet even when they are being detected at the unconscious level, these signals can affect the way we think, feel and behave. Some scientists believe that the brain鈥檚 integration and interpretation of them is what provides the sense of self, that there is one 鈥淚鈥� living in this moment and experiencing the world in real time, and that any change from the status quo has an important impact on whether that experience feels good or bad. This makes it particularly relevant to mental health. If feelings and emotions are built on bodily sensations, perhaps errant bodily signals, or the brain鈥檚 interpretation of them, are feeding into feelings of emotional distress.

The challenge to unravelling all this is that the brain-body pathways involved are fiendishly complicated. The body is crammed full of sensors, not to mention kilometres of nervous pathways connecting the organs to each other and the brain. Body and brain communicate in both directions, each responding to the other and tweaking things as necessary. This cacophony is broadcast over a huge range of timescales, from the rhythmic beating of the heart over seconds to changes in hormones and other chemicals in the bloodstream over minutes or hours.

To make sense of all this information, the brain employs a certain amount of guesswork, taking mental shortcuts based on past experiences. This predictive processing is a leading explanation for how the brain works in general: it involves making a best guess based on what happened on prior occasions, comparing that prediction with incoming information from the senses and intervening if there is a mismatch between the two.

Where this becomes relevant to mental health is that people vary considerably, not only in the predictions their brains make, but also in their physiological sensitivity to bodily changes and in the threshold at which unconscious signals cross over into conscious awareness. The way people respond to these signals and the meaning they attach to them plays out in many mental health conditions.

Take, for instance, anorexia, which has the highest death rate of all mental health conditions and is notoriously difficult to treat. One aspect that is particularly hard to address is the intense and unpleasant sensations of having food in the gut after eating. 鈥淚鈥檇 be so aware of the food inside of me that it would be hard to walk鈥� How could I move with all of this in me?鈥� says , who wrote .

Sahib Khalsa, a psychiatrist at the Laureate Institute for Brain Research in Tulsa, Oklahoma, was intrigued by an anorexia patient who had told him that spending time in a floatation tank made this uncomfortable sensation go away. 鈥淪he felt that was the only place that she could digest in comfort,鈥� says Khalsa.

This makes sense because floatation therapy can help to selectively remove certain interoceptive inputs. So Khalsa decided to investigate the idea further as a therapy for people diagnosed with anorexia.

鈥淚nteroception is fundamental to every brain process and behaviour that there is鈥�

In a , Khalsa found that after four weekly sessions in a floatation tank, interoceptive messages from the gut were reduced. 鈥淧eople feel their heartbeat and their breath more intensely when they float, but don鈥檛 feel their stomach more intensely,鈥� he says. Participants also reported reduced anxiety and body dissatisfaction ratings after floating, although the effects didn鈥檛 last long.

Oversensitivity to bodily signals can also be an issue in generalised anxiety disorder, a condition that is linked to uncontrolled worrying and an almost constant underlying sense of dread. To get a better insight into this condition, Khalsa injected volunteers who had generalised anxiety with isoproterenol, a drug that acts like adrenaline, but doesn鈥檛 cross the blood-brain barrier. This means any changes in emotional state invoked by the substance could be attributed to bodily sensations rather than a direct effect on brain chemistry.

Sensitive to threats

He found that the dose of isoproterenol needed to raise heart rate and anxiety was significantly lower in people with generalised anxiety disorder than in a control group, suggesting that their bodies are set up in such a way that they ramp up the fight-or-flight response at a much lower level of perceived threat.

Once the fight-or-flight response is under way, the very fact of having a raised heart rate then primes the brain to be more sensitive to threat, setting up a vicious cycle of overactive bodily signalling and negative interpretation that boosts interoceptive sensitivity. In , Critchley and Sarah Garfinkel, then also at the University of Sussex, found that fearful images shown as the heart contracted were rated as more intense than if the same images were shown as the heart relaxed. This suggests that a racing heart could make a bad situation worse. 鈥淛ust being in that constantly aroused state means that fear processing and fear memories are augmented,鈥� says Garfinkel, now at University College London.

One potential way to break this cycle is to short-circuit the link between the cardiovascular system and fear processing in the brain, an idea that is being tested in people with post-traumatic stress disorder (PTSD), where an overactive fight-or-flight response leads to a racing heart, fast breathing and other debilitating panic-related symptoms in situations that are, objectively, safe. A clinical trial of a blood pressure-reducing drug, losartan, as a treatment for PTSD is under way in hospitals across the US. The idea is that by reducing the cardiovascular overreaction, people with PTSD stand a better chance of being able to calm down before the vicious cycle gets out of control.

Another potential approach is to help people to learn how to reinterpret their body sensations, giving them a better chance of regulating their emotions.

Research has shown that problems with interoception often arise not only when a person has inaccurate body signals, but when this is combined with a negative interpretation of what those signals mean. In anxiety in particular, the combination of being worse at accurately tuning into body signals, combined with a propensity to expect bad things to happen, , again setting off a vicious cycle.

Critchley wanted to find out whether addressing this would help reduce anxiety in autistic people. They experience anxiety at up to five times the rate of the general population and cognitive or group therapies often aren鈥檛 suitable for them.

In a , participants were trained to better tune in to their heartbeat, either by counting the beat or by trying to work out if an audible beep was in sync with their heart. The training also involved light exercise, to raise the heartbeat and give them practice of detecting it in non-threatening situations.

鈥淭hey could get into the zone where they could feel their heartbeat and then gradually relax and still feel their heartbeat while tuned into it,鈥� says Critchley.

Increasing awareness of heartbeat seemed to be enough to short-circuit rising anxiety. Participants reported a significant drop in anxiety scores over three months, and around a third no longer qualified as having a clinical anxiety disorder. Interoceptive heartbeat signals were rated as less intrusive, and people reported being better able to put the sensation into context. 鈥淲hen I notice the impacts of anxiety on my body, I am more aware of them and am able to reassure myself that it is just a physical reaction. I am better at taking deep breaths and trying to slow my breathing and heart rate down,鈥� said one participant.

With these early promising results, the hope is that the training can be extended to other conditions. It could be used to , for example, which has also been linked to problems with interoception.

Another group who could benefit are people with hypermobile joints 鈥� ones that extend beyond the usual range of motion. This affects as many as 1 in 5 people, and anxiety and panic disorders are significantly more common in this group. There are also links with chronic pain, fatigue and eating disorders.

The extreme bendiness of the joints is caused by a more flexible form of collagen, but collagen is also found in blood vessels. Laxer collagen here causes blood to pool in the lower legs, which forces the heart to beat faster to maintain blood pressure. This racing heart can be interpreted as anxiety, even when there is no obvious cause for concern. , also at the University of Sussex, is now investigating if training people to interpret these signals differently may bring some relief.

More direct ways to calm the body are also being developed. The descending branch of the vagus nerve is part of the parasympathetic nervous system, which returns the body鈥檚 arousal back to a calm baseline after stress. Electrically stimulating the vagus nerve via an implant in the neck has been shown to . Non-invasive ways to stimulate the nerve are now in the works, although there is little hard evidence so far that this non-implanted method works for mental health conditions.

A deeper understanding of the complexities of interoception will also help to inform future treatments. Khalsa is working on a way to explore gut sensitivity in eating disorders and how it manifests in the brain. He has designed a vibrating capsule that can be swallowed and used as a probe to measure gut sensitivity. Combined with electroencephalography, which measures brain activity using sensors on the scalp, this will allow mapping of possible differences in the brain鈥檚 response to gut sensation.

, which correlated with increased activity in parts of the brain that are thought to be a gastric detection network. Khalsa is working on a trial in people who are hospitalised with anorexia to see if differences in their gut sensitivity predict their chances of recovery over the long term. The ultimate end point would be a way to intervene in this process to improve chances of recovery.

Khalsa hopes that we will eventually understand interoception so well that we can 鈥済o through the process stepwise, identifying where in the nervous system is the issue鈥�, he says. When it is easier to identify where problems arise, new targets for treatments will follow. And with more knowledge of how to intervene for ourselves comes the possibility of using forms of self-care that improve our mental health 鈥� whether that is a session in a floatation tank or doing star jumps to get used to how your heart feels when it is pumping hard and how to use deep breaths to calm it down afterwards.

While it is early days for evidence-based interoceptive treatments, Garfinkel believes that the time for ignoring the body鈥檚 role in mental health is well and truly over. 鈥淢ental health conditions, especially those with some degree of emotional disturbance, have their origin in aberrant interoceptive mechanisms and any treatment that targets the body has the potential to help treat these conditions. I honestly believe it鈥檚 a potential revolution in mental health treatments,鈥� she says.