We’ve heard of proprioception, the awareness of our limbs in space and time (see “Living in a 3-D World”), but there’s another sense that is lesser known: interoception. Interoception is a companion sense with separate signals and pathways that helps us maintain organ homeostasis. Both senses cooperate to give us awareness of our internal and external states. One might say proprioception gives us awareness of the external self, and interoception gives us awareness of the internal self. Every organism needs both, and both are semi-automatic, resulting in responses tending to bring stability in a fluctuating environment. Research on interoception is trending, promising whole body health.
Watch for It
The word interoception is not yet found at Dictionary.com, but it is appearing more frequently in scientific literature, so it is good to become familiar with the concept. One might think of it as “internal perception” or how the brain perceives the state of the body’s internal organs. As we shall see, there are design implications. Meanwhile, Darwinians are going to encounter worse headaches explaining what amounts to a “system of systems” arranged hierarchically in a functionally coherent way.
The Cell Press journal Trends in Neurosciences gave a worthy introduction to interoception in an open-access review paper, “The Emerging Science of Interoception: Sensing, Integrating, Interpreting, and Regulating Signals within the Self.” Lead author Wen G. Chen with 12 co-authors led a special issue in the journal on “The Neuroscience of Interoception,” exemplifying the rising tide of interest in the subject.
Interoception refers to the representation of the internal world, and includes the processes by which an organism senses, interprets, integrates, and regulates signals from within itself.
The brain communicates with internal organs via the peripheral nervous system and non-neuronal systems.
Key components of a unified research framework of interoception include interoceptive signals, interoceptors, ascending and descending pathways, central interpreters, central integrators, central regulators, and interoceptive effectors. [Emphasis added.]
As we hear these words, our design sense begins tingling. Like systems biology, the field of interoception views an organism as a system rather than a collection of parts. For researchers and practitioners in medicine, this new perspective can shift their approach from treating this or that organ or symptom in isolation. It steps back and takes more of a wide-angle view of the whole organism and how it communicates within itself with a flurry of signals.
Psychologists were talking about interoception in a special issue of Biological Psychiatry back in 2018, mapping ways that this “new” scientific approach could aid mental health. Chen et al. pointed out that interest in interoception accelerated in earnest at the NIH in 2019:
Neuroscience has progressed tremendously in the past decades in clarifying how we sense and interact with the external world. On the sensory side, this line of research, sometimes referred to as ‘exteroception’, encompasses (according to most definitions) the primary sensory systems of vision, audition, olfaction, taste, and somatosensation. Less is known about the interoceptive system — the ability of the nervous system to represent our own internal world. On April 16/17 2019, the NIH Blueprint for Neuroscience Research convened a 2 day workshop entitled ‘The Science of Interoception and Its Roles in Nervous System Disorders’. At the workshop a distinguished group of investigators highlighted recent findings and discussed a wide range of topics crucial for the future of interoception research.
Comparing and Contrasting Proprioception and Interoception
The sensors for proprioception reside in the muscles and joints. The sensors for interoception reside in our organs, informing our brains of states such as hunger, the need to go to the bathroom, or emotional states such as anger, fear, or stress. Whereas proprioception signals limb positions through large fibers in the central nervous system (CNS) to the somatosensory cortex in the brain, interoception signals internal status through mechanoreceptors and other sensors scattered throughout the body to the insular cortex in the brain. Yet both cooperate and interact.
Scripps Research posted a short video introduction to interoception that covers the basic concepts of what this extra sense does for us; watch the 2-minute clip on YouTube.
It mentions mechanosensory proteins Piezo1 and Piezo2 that I discussed last year here. Scripps found that these touch-sensitive receptors are found in organs throughout our body, prompting signals to the brain.
For a specific example, the Champalimaud Foundation in Lisbon, Portugal, issued a press release about our “second brain” in the gut that “decides between attack and repair,” calling it “your gut’s railway switch.” Experiments on mice revealed an unexpected orchestration directed by VIP (vasoactive intestinal peptide, a neurochemical messenger):
When the team experimentally activated VIP-releasing neurons in mice, the gut’s epithelial cells began producing cytokines that stimulate a type 1 immune response — the body’s “killer mode”, used to destroy bacteria and infected cells. But when they blocked the receptor VIPR1 in epithelial cells, the balance flipped: the killer response weakened, while a type 2 immune response — the one that drives repair and fights parasites — grew stronger.
The shift was striking. Mice lacking VIPR1 in their epithelial cells became more vulnerable to bacterial infections like Salmonella, yet more resistant to parasitic worms. “This was the big surprise”, says Roksana Pirzgalska, first author of the study. “We realised the neurons of the gut are not just fine-tuning immunity locally — they are orchestrating completely different immune programmes”.
Nuanced Definitions
Chen et al. point out the blurry line between interoception and exteroception. The senses work together in ways that overlap. One example they give is the vestibular system that overlaps with the auditory system. Another example of overlap is the fact that “neural activities in subcutaneous tissues, including muscles and connective tissues, that contribute to proprioception, are a form of interoception.” Although the concept of interoception can be traced back 150 years or so, “interoception research mostly regained momentum in recent years, partly because of the availability of high-resolution, multimodal tools for interrogating interoceptive processes.”
A modern nuanced definition of interoception requires understanding that signals between organs and brain are bidirectional, with ascending pathways sharing information to the brain and descending pathways regulating responses through the integrated representation of the internal world from all the senses. “One key difference between this revised definition and some more traditional definitions of interoception is the inclusion of the descending body regulation component,” they say. Moreover, this bidirectional nature of interoception expands beyond the CNS into the vascular, endocrine and immune systems. The liver, for instance, “talks to the brain” via “nutrients, hormones, and cytokines, potentially via the liver-innervating vagal sensory neurons,” according to a paper in the PLOS Biology series discussed below.
Prospects for New Research
Chen et al. end with a list of outstanding research questions. Clearly interoception is a lively “new” field ripe for investigation from many angles. PLOS Biology published a special issue on interoception last month. In the lead editorial, “The body sends a signal: Perspectives on interoception,” Lucas K. Smith introduced four articles in the issue that review current research on hepatic, gut, and cardiovascular interoception as well as the trending concept of “Whole Body Health.” In another entry in the PLOS Biology special edition, the same Wen G. Chen, lead author of the 2021 Trends in Neurosciences paper, along with Helene Langevin (both working at NIH) discuss “Interoception as a central mechanism in Whole Person Health.” They treat interoception as “not only an emerging and important topic in neuroscience but also a fundamental biological interface central to the health of the whole person.” Their perspective article offers a 4M framework (Measure, Map, Monitor, Modulate) for bringing interoceptive concepts to the attention of medical researchers to improve patients’ overall health and quality of life.
Cautions and Encouragements
Because of its holistic approach, interoception as a unifying concept with therapeutic implications is being co-opted by various self-help groups, including philosophies, psychologies, mystical religions, and even new-age cults. These promise that their techniques, including yoga, meditation, and the trendy term “mindfulness,” can bring “balance” or “wholeness” when our interoceptic signals are out of whack (example, example). While I would not deny someone’s personal experience who has benefited from following such advice, I see two problems. First, these counselors are often weak on empirical science. They tend to borrow terms and apply them to their pre-existing approaches. Second, few of them account for the origin of the complex systems that comprise interoception. They tend to take this complex, integrated system for granted.
Intelligent design overcomes those weaknesses. Many in the ID movement have scientific prowess at the PhD level in biology, anatomy, physiology, neuroscience, zoology, botany, microbiology, biochemistry, and engineering. And design advocates have a necessary and sufficient cause in their explanatory toolbox for the complexities of interoception: intelligence. This is especially helpful for hierarchically integrated designs like interoception.
It’s instructive that scientific papers on interoception rarely mention Darwinian evolution. The only thing evolving is the science itself: e.g., from Chen et al.,
The definition of interoception has evolved over the years…. In the mid-20th century, the idea evolved to reflect the more dynamic concept of homeostasis. More recently, interoception has been commonly referred to as the process by which the nervous system senses and integrates information about the inner state of the body.
Kluger et al. in the PLOS Biology series echo this usage:
The rapidly evolving field of research on interoception, meaning the conscious and unconscious detection of visceral signals, ticks all the boxes of a ‘hot topic’ in modern-day neuroscience.
Applications
Interoception is not only a hot topic; it is ID-friendly. For young researchers looking to grow their careers and contribute to biological sciences, here is a good field to consider. Design advocates might want to steer young researchers into projects about interoception that investigate its nuances in a scientifically rigorous way. And medical researchers or practitioners should keep abreast of the findings to expand their treatment options. Rather than approaching the body as a collection of parts, and focusing on the one part that is ailing, interoception-based modalities will consider the whole person and look for signals — even remote from the site of trouble — that could be causing the problem. For everyone seeking to understand the amazing human body, interoception is a field to watch.
Plus, think of this: every organism, from cell to sauropod, needed interoception from the start. That’s another difficulty for evolution that extends all the way back to the origin of the first cell. What “switched on” a protocell’s awareness of its internal and external states such that it could respond and adapt? Viewing an organism in light of Douglas Axe’s term “functional coherence” (Undeniable, Chapter 10), wherein each living thing takes in external signals and communicates with itself, liberates biology from mechanistic reductionism and elevates it to the wonder that attracts scientific inquiry. As Neil Thomas writes in False Messiah: Darwinism as the God that Failed, “The sheer exceptionalism of our terrestrial biosphere stands in sharp contrast to the life-denying deadness of the outer cosmos.”