Interoception is a whole-brain process

Chapter 4 endnote 24, from How Emotions are Made: The Secret Life of the Brain by Lisa Feldman Barrett.
Some context is:

Interoception is actually a whole-brain process anchored in [the interoceptive] network.

Perhaps you’ve noticed that I am being a hypocrite. In How Emotions are Made, I spill a lot of ink criticizing the notion that each part of the brain has a functional essence—the mistaken belief that “cognition happens over here, while emotion happens over there, with perception way over there.” And yet, I’ve just told you that interoception occurs in a distinct brain network. This seemingly hypocritical state of affairs developed because I’ve been hiding part of the story of interoception to simplify things. Interoception is a whole-brain process, but when we discuss interoception, the regions in Figure 4-4 get our focus.

In fact, any psychological process can be considered a function of the whole brain, with some networks playing primary roles and other playing more supporting roles. For instance, the visual network plays a supporting role in interoception, and likewise the interoceptive network plays a supporting role in vision. Neurons have the capacity to be multimodal. This is because neurons have a common language of firing across all modalities –they encode information with temporal patterns of firing, like Morse code. This allows they to carry information from more than one modality. For example, if you damaged your visual cortex for some reason, you might be able to train your primary somatosensory cortex to “see.” A camera on the forehead captures the images and a chip on the tongue applies pressure sensations to the tongue. The images (patterns of pixels) are then translated into patterns of pressures points onto the tongue. These patterns are then represented in somatosensory cortex. The brain soon learns how to predict and perceive with these patterns.

In fact, as interoception and visual processing happen simultaneously, both are distributed throughout the whole brain, with certain regions playing primary roles.

When you watch a movie, your primary visual cortex (V1), which is the main sensory region for vision, is also helping you to hear the dialogue. Your primary auditory cortex (A1) is also helping you to see the images. Scientists once considered V1 and A1 to be the very definition of localized function, with V1 exclusively dedicated to vision and A1 to hearing. But now we know that neurons can serve more than one function, and these regions convey information about many different senses, playing supporting roles for one another.

While in my lab, my colleague Ajay Satpute showed evidence that “sensory” parts of the brain show consistent increases in activity during interoception. While people looked at neutral visual images such as hillside scenes, brain activity increased in primary visual cortex; but when they looked at an unpleasant image that would involve an increase in sympathetic nervous system activity, such as a gun, the increase was even larger. The more evocative images produced activity not only in the expected interoceptive network regions, but also in the primary sensory regions for the stimulus, as if visual neurons are serving as support regions for the interoceptive network, carrying information relevant to interoception. Other factors like the contrast or complexity of stimuli did not account for this finding. We observed similar findings for the auditory cortex (when hearing evocative sounds, like screams and cries), somatosensory cortex (like pin pricks and pinches), and gustatory cortex (like sour tastes) as well.

The “supporting role” discovery is a major reason why neuroscientists now think about the brain as one big network of neurons, rather than a collection of blobs.

Most neurons have more than one function. This is not equipotentiality, the obsolete belief that every neuron can perform every function. I’m just saying that any given neuron has multiple functions (i.e., more than one receptive field); they are multi-purpose.