Interoception: The inner sense driving your thoughts

Sometimes our bodies react to the world around us before we realise, so how do internal signals such as a quickening heart or deep breathing affect our thoughts?

It was day 29 of a gruelling 600-mile (966km) canoe journey across the Canadian subarctic when 17-year-old Alex Messenger was savagely attacked by a grizzly bear.

He'd left his camp alone that day in 2005 to climb a nearby peak. And on the way up, head down, his thoughts had meandered between everyday things –his high school reading book, The Liars' Club; the weight of the camera case he was carrying; the vivid little flowers beneath his feet. But while Messenger was daydreaming, the bear was lumbering unseen up the opposite side of the ridge. 

When their paths finally converged, Messenger's body reacted before his mind. "I just saw this kind of brown blob come up over the rise," he says. "I wasn't sure what it was, but this tension ran through my body. My breath quickened, my eyes widened, my pulse doubled immediately, my airways opened."

Now, nearly 20 years on – and after writing about the experience in his memoir The Twenty-Ninth Day ­– Messenger can still recall how long it took for his conscious brain to catch up with the signals his body was bombarding him with. "There was this visceral or subconscious bodily reaction," he says. "And then, later, there was my intellectual and emotional reaction."

At first, he thought the "brown blob" was a musk ox. And it was only as the shape came into sharper focus that he realised his body had already prepared itself to deal with a far more ominous threat: a belligerent, 600lb (272kg) apex predator. The bear charged straight at him and swatted him to the ground with a blinding blow to the head. It locked its jaws around his thigh – causing Messenger to pass out – before leaving him for dead.

Messenger's experience isn't just a miraculous survival story. It also offers an insight into our often overlooked inner sense: interoception. While we're largely familiar with the five outward-facing or "exteroceptive" senses – sight, smell, hearing, taste and touch ­– interoception is our ability to perceive and interpret signals coming from within our own bodies. And it not only helps to keep our bodies in "homeostasis", or balanced working order (by invisibly regulating blood pressure and glucose levels or more overtly encouraging us to eat or drink, for example) but could also be having a profound impact on our thinking, emotions and mental health. It may even be behind our very sense of self.

Over a century before Messenger's terrifying encounter, American philosopher and psychologist William James was exploring the role bodily signals might play in shaping our emotions. He argued that running into a bear doesn't make our heart race because we are scared. Instead, as Messenger's experience seems to suggest, our body responds to the perceived threat by flooding itself with adrenaline, increasing our heart rate and breathing – and that we then interpret these bodily signals as fear. In other words, emotions are born of the body.

As James wrote in his 1884 essay What Is An Emotion?: "We feel sorry because we cry, angry because we strike, afraid because we tremble, and not that we cry, strike, or tremble, because we are sorry, angry, or fearful."

What became known as the James-Lange (Carl Lange was a physician working independently on similar ideas) theory of emotion has been vigorously debated and since evolved – not least to incorporate the idea, outlined in "appraisal theories", that context also plays a key role in determining how physiological signals shape our emotions. After all, if Messenger's heart had started racing on a rollercoaster or during a first date, rather than in a face-off with a bear in a remote wilderness, he may have "appraised" his bodily signals differently and felt excitement rather than terror.

Nevertheless, James laid the early groundwork for how we think about the closely intertwined relationship between brain and body. Indeed, according to Antonio Damasio, professor of psychology, philosophy and neurology at the University of Southern California: "Our thoughts and our feelings and emotions are not just influenced by our bodies but [are] actually inconceivable without them. All of our mental activity is a consequence of body/brain interactions." 

For our recent book, Are You Thinking Clearly?, we explored many of the factors that influence and manipulate the way we think, from genetics, habits and heuristics to technology, time and the bacteria in our gut. And this mysterious inner sense, the subject of a fast-growing research field, proved to be one of the most intriguing.

"Interoception is the processing of bodily signals that come from the inside," says Jennifer Murphy, who researches interoception and how it impacts cognition and mental health at Royal Holloway University of London. "So this would be things like feeling your heart beating, your breathing and knowing when you need to go to the toilet or when you're sick." Hunger and fullness are other examples.

A watertight definition of interoception remains a topic of debate, but the emphasis is on internal signals. "We can gauge whether we're breathless by the sound of our breath," says Murphy. "But that's an exteroceptive, rather than an interoceptive route to perceiving that." Indeed, Sahib Khalsa, a leading interoception researcher at the Laureate Institute for Brain Research in Tulsa, Oklahoma, describes himself as an explorer of our "inner space".

Some of these bodily signals, which are transmitted from our organs and other body parts to our brains via an intricate "internet" of connections, including the spinal and cranial nerves and chemicals in the bloodstream, are too subtle for our conscious minds to perceive. Others, such as a speeding heart, nervous "butterflies" or hunger, aren't. We are all sensitive to these internal signals to varying degrees, and we can all interpret and respond to them differently, depending on who we are and what we're doing. In fact, disturbances in our sensitivity to and perception of bodily signals may be behind an array of conditions from anxiety and depression to anorexia nervosa.

This is pioneering science and many of the mechanisms behind interoception remain mysterious and hard to test. But researchers such as Murphy, Khalsa, Sarah Garfinkel, professor of cognitive neuroscience at University College London, Anil Seth, professor of cognitive and computational neuroscience at the University of Sussex, and Hugo Critchley, also at Sussex, are gradually drawing back the veil. And the results could have profound consequences for how we understand our minds.

"We know next to nothing about what's happening on the floor of the ocean," says Khalsa. "Yet we know that it's fundamentally important to determining our climate. It's the same thing with interoception. We know so little about what's happening inside of our bodies in relation to how we feel, yet we know that it's important. It cannot be ignored."

So how might interoception apply to our everyday lives?

Take your heartbeat – likely one of the bodily signals you're most often aware of. It's widely recognised that anxiety can cause the heart to quicken. But what if, as James and many others since have suggested, the process also happened in reverse and an increased heart rate could trigger anxiety and cause us to feel fear as well? If this were the case, the degree to which we are "interoceptive" and can perceive our heartbeat – and how we interpret and respond to those signals – might have major implications for our wellbeing and mental health.

Proving such a thing scientifically, however, is extremely difficult. "I've long worried that interoception research is hard because of the difficulty of precise measurement and manipulation of physiological variables and/or interoceptive signals," says Seth.

Nevertheless, headway is being made. Garfinkel has already revealed that our response to fear stimuli can change across a single heartbeat. The study participants were shown fearful and neutral faces when their hearts were in systole (the muscles were contracting) and diastole (the muscles were relaxed). And the results showed that the participants more easily identified the fearful faces – and found them more intense ­– when their hearts were in systole. Their amygdalas – a primitive brain area related to fear response – were also more active. The brain, then, responds to the heart.

But a study published in 2023 by a team of researchers at Stanford University in California went further. They tested whether increased heart rate could induce anxiety and fear responses. The researchers used a non-invasive, optogenetic (a technique which uses light to manipulate cells) pacemaker to precisely increase the heartbeats in mice. They then monitored the rodents to see how willing they were to explore a maze and search for water.

The results were compelling. When their heart rates were elevated, the mice did become more anxious – they were less likely to explore the exposed sections of the maze, instead choosing to remain in the protected areas. Crucially, however, this effect only occurred in "risky contexts" (for example, when there was a threat of a mild shock). Full screens of the rodents' brains also enabled the researchers to accurately pinpoint several brain regions related to this behaviour, including the insular cortex, prefrontal cortex and brainstem.

"Of course, this is a study of mice not people, so the observation of anxiety-like behaviour doesn't necessarily mean that the mice were actually experiencing a mousey version of anxiety," warns Seth. "But the specific finding, that optogenetically enhanced heart rate produces anxiety-like behaviour only in risky environments, is exactly what would be predicted by classic appraisal theory, which extended James/Lange with the idea that perception of physiology in context matters for emotion."

In other words, the study suggests that our "appraisal" or interpretation of bodily signals may play a key role in how they impact our emotions.

Which brings us back to the significance of bodily signals – and how we perceive and respond to them – in our (human) wellbeing, mental health and decision-making. Accurately establishing how interoceptive people are is one of the field's biggest obstacles. "One of the first things you encounter with interoception is quite how complicated it is to test exactly what is happening," Murphy says.

These signals can come from all over our body, from our guts to our lungs. But interoceptive ability is typically measured by asking participants to count their heartbeat and then comparing their reported result with an objective measure. But there are several problems with this method, not least that around 40% of people are unable consciously to perceive their heartbeat.

Nevertheless, an intriguing study by Garfinkel and others found that City of London hedge fund traders who could more accurately perceive their own heartbeats were more likely to make profitable decisions and enjoy longer careers (although it's important to note that the study didn't show causation).

Meanwhile, individuals who struggle to perceive their bodily feelings find it more difficult to articulate and regulate their emotions. "We have really good theories and we also have really good reason to expect that there might be disruption in interoception across a variety of both physical and mental health conditions," says Murphy – who conducted a recent review of the evidence. These conditions include depression, anxiety, obesity, anorexia nervosa and autism.

There are, of course, large variations in how interoceptive individuals are. In one wide-ranging review, for example, Murphy found there are differences in how cisgender men and women perceive bodily signals, finding women were much less accurate at perceiving their heartbeat in tasks than men. While the reasons for this are unclear – and could include genetic, hormonal and environmental factors – the findings could provide clues as to why women are more likely than men to suffer from depression and other common mental health issues. Indeed, Murphy will now be researching how interoception and mental health vary across the menstrual cycle.

Interoception research is also informing new potential treatments for a range of conditions. Khalsa, for example, has developed a vibrating capsule that can be swallowed, allowing researchers to determine people's sensitivity to gut sensations and how they interpret them. This could help scientists understand and treat those with anorexia nervosa, for example, who appear to experience disruptions in their perception of bodily signals and often report feeling bloated or prematurely full after eating just small amounts.

Garfinkel, Critchley and colleagues, meanwhile, have applied interoceptive techniques to the treatment of anxiety in autistic adults. Autistic people suffer more anxiety than the general population and may be less accurate at perceiving and interpreting bodily signals and less able to understand emotions. In a randomised controlled trial, autistic adults received either a control treatment or a novel "Aligning Dimensions of Interoceptive Experience (Adie)" therapy involving interoceptive heartbeat detection tasks, feedback on their performance and exercises to moderately increase their heartbeat. After the trial, those on the Adie therapy reported significantly lowered anxiety compared with the control group. It seems the therapy enabled the participants to more accurately predict and interpret their bodily signals, such as a quickening of heart rate, which in turn helped them to regulate their anxiety.

Remarkably, this intriguing internal sense may also be behind our very sense of self. After all, as Seth explains, our most basic sense of selfhood is "being a body". But what drives this? We often picture our "self" as the captain of a ship or, in Seth's words, "a mini-me within the skull that 'does' the perceiving, and then decides what to do next".

Seth, however, believes that this is the wrong way of looking at it. Instead, he argues that what we experience as the "self" is more of a process – an amalgam of the brain's predictions about and perceptions of signals coming from the exterior world and, perhaps even more importantly, our own bodies.

Ultimately, the brain's number one goal is to keep the body, and therefore itself, alive. But while it must do its best to map and manage its external surroundings (to avoid being eaten by a bear, for example) and maintain favourable conditions in the body (to avoid glucose levels becoming too high or blood pressure too low, for example) it can't directly access these two environments. It is effectively locked away, even though we don't experience it like that, and must rely on indirect signals it can't guarantee the cause of.

Instead, the brain creates a model of the body based on a whole range of parameters necessary for survival. It then constantly makes predictions, which it tests for errors and corrects against the sensory inputs it is receiving, allowing it to regulate the system. Seth believes that emotions – which are "variations on the theme of things feeling good or feeling bad" – emerge out of this process of "interoceptive inference" as a means to keep the body where it needs to be to stay alive.

So how does this pioneering theory play out in a scenario such as the one Messenger found himself in? As Seth explains in his book, Being You: "The experience of fear I feel as a bear approaches is a control-oriented perception of my body – more specifically 'my body in the presence of an approaching bear' – that sets in train the actions that are best predicted to keep my essential variables where they need to be. Importantly, these actions can be both external movements of the body – like running – and internal 'intero-actions' such as raising the heart rate and dilating blood vessels."

But whether we're on a quest to discover the elusive source of consciousness, better understand our emotions, ease our anxiety or tame our butterflies, we could all benefit from being more in tune with what our bodies are telling us. And for Seth, a good place to start is meditation. "One thing you're doing when you learn to meditate is to pay attention to your body, to what's happening in your body, and not just think, 'I feel this way, and then project your emotions back out into some narrative about what's happening in your life'."

Just don't focus on it too much. "You can potentially become overly sensitive to every little thing that's happening in your body and I imagine that could drive some kind of anxiety as well," Seth warns. 

When it comes to understanding interoception, a great many mysteries remain. But as science delves ever deeper, watch this inner space.

* Miriam Frankel and Matt Warren are science journalists and authors of Are You Thinking Clearly?

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