Interoceptive perception is imprecise

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Chapter 4 endnote 21 & 22, from How Emotions are Made: The Secret Life of the Brain by Lisa Feldman Barrett.
Some context is:

[note 21] From your brain’s point of view, locked inside the skull, your body is just another part of the world that it must explain. Your pumping heart, your expanding lungs, and your changing temperature and metabolism send sensory input to your brain that is noisy and ambiguous.

[note 22] Of course, there are times when you directly feel a headache, a full stomach, or your heart pounding in your chest. But your nervous system isn’t built for you to experience these sensations with precision...

Some interoceptive changes in the body are represented with great precision, such as temperature and certain types of nociception.[1] For example, your brain must have a fairly precise representation of your heart's function or you would likely die.[2] That being said, much of sensory input from your body is not represented in precise detail (e.g., you don't feel your kidneys cleaning your blood). This is a good thing. Think about the last time you banged your toe on something, and for a moment you couldn’t focus on anything else in the world except wanting to smash whatever object brought you this pain. If you could actually feel the universe of events inside your body all the time, you’d never pay attention to anything outside your skin.

The reasons why interoceptive perceptions are imprecise are complex and not well understood. In part, it has to do with how predictions are structured in the cerebral cortex. Interoceptive predictions cross only one synapse from body-budgeting (visceromotor) neurons (in the cingulate cortices and anterior insula to primary interoceptive cortex (in mid and posterior insula). Compare this to visual, auditory and somatosensory predictions flow across many synaptic connections before arrive to the primary exteroceptive cortices; this provides many opportunities for the prediction to become elaborated with details. Also, primary visual, auditory, and somatosensory cortices have a very well developed layer IV in the primate brain, which means that we are better able to compute prediction error and learn from sensory inputs in a more detailed way; by contrast, layer IV in primary interoceptive cortex is not as well developed. This may be why you can see, hear, and feel with your skin in more with more precision (with much more detail) than you can feel what is going on inside your body (which is lower in dimensionality and usually experienced as affect).

Imprecise interoceptive perceptions may also have to do with how sensory input is delivered to the cortex for prediction error to be computed. Visceral sensations ascend to the cortex via the unmyelinated portion of the vagus nerve so that signals from different neurons can influence one another as their signals move along the axons themselves.[3][4] The eminent neuroscientist Bud Craig argues that there are distinct “labeled lines” for sharp pain, cold, warmth, itch, sensual touch, muscle ache, cardiac pain, colonic pressure, bladder urgency, etc., although Craig also admits that:

  1. It is not clear whether these sensations are represented distinctly in the cortex, and
  2. Many of the neurons that carry this information to the brain carry multiple sources of information.

Notes on the Notes

  1. Craig, A. D. 2015. How Do You Feel? An Interoceptive Moment with Your Neurobiological Self. Princeton, NJ: Princeton University Press.
  2. E.g., Furio Colivicchi, Andrea Bassi, Massimo Santini and Carlo Caltagirone. 2005. "Prognostic Implications of Right-Sided Insular Damage, Cardiac Autonomic Derangement, and Arrhythmias After Acute Ischemic Stroke." Stroke 36 (8): 1710-1715.
  3. Damasio, Antonio, and Gil B. Carvalho. 2013. “The Nature of Feelings: Evolutionary and Neurobiological Origins.” Nature Reviews Neuroscience 14 (2): 143–152.
  4. Niewnhuys et al [full reference to be provided]