Insular cortex
A lobe of the brain folded away beneath the side of the cerebrum, wired to the structures that handle body-state, emotion, and attention. It is the one cortical region that takes in a complete picture of signals coming from inside the body, and the region whose damage can wipe out a specific felt urge such as nicotine craving. For this reason it is the part of the brain most closely tied to how internal bodily states become conscious feelings.
The cortical lobe buried beneath the lateral sulcus of the human cerebrum, organised into posterior (granular), middle (dysgranular), and anterior (agranular) subfields, and connected reciprocally with the thalamus, the amygdala, the anterior cingulate cortex, the ventromedial prefrontal cortex, and the visceromotor periphery. In Craig's interoception model the dorsal posterior insula receives the spinothalamic-tract afferent stream from lamina I and re-represents homeostatic body-state, with the right anterior insula re-re-representing that signal as the substrate of conscious feeling.
Etymology§
Insula, Latin for island. Named for its appearance — a buried cortical island visible only when the operculum (the surrounding frontal, temporal, and parietal cortex) is parted. Island of Reil refers to Johann Christian Reil, the German anatomist who described the structure in 1809; the eponym is now mostly historical, retained as an alternative in clinical-neuroanatomy texts. The structure was a comparative-anatomy curiosity for most of the nineteenth and twentieth centuries; the functional architecture worked out across the late twentieth century is the substrate of Augustine 1996 and the modern functional literature.
The insula is the cortical lobe that does the most work for interoception literature, and not by accident: it is the only cortical region that receives a complete representation of the interoceptive signal stream — spinothalamic afferents from lamina I, vagal afferents via the nucleus tractus solitarius — and the only cortical region whose lesion specifically disrupts a felt-state outcome (the loss of nicotine craving after insular damage in Naqvi and Bechara 2007).
The functional partitioning matters. The dorsal posterior insula in Craig's model is the primary interoceptive cortex — the first cortical re-representation of body- state — while the right anterior insula is the re-representation layer, where the felt-state substrate is generated. The cytoarchitectonic basis for the distinction (granular posterior, agranular anterior) is documented in Augustine 1996 and is the same anatomy that Barrett and Simmons's EPIC model uses to argue the opposite: that the agranular anterior insula is sending top-down predictions down the granular gradient, not receiving feed-forward signal up it.
The von Economo neurons in the anterior insula — large bipolar projection cells first emphasised by John Allman at Caltech and given pride of place in Craig 2009 — are a comparative-neuroanatomy detail with disproportionate citation life. They are concentrated in humans, great apes, elephants, and cetaceans, and have been treated by some authors as the substrate of consciousness; the strong version of this claim is contested. The insula is the right place to look for the substrate of feeling; the von Economo neurons are not necessarily the substrate.
Discussed in§
- Circuitry and functional aspects of the insular lobe in primates including humans
- How do you feel? Interoception — the sense of the physiological condition of the body
- How do you feel — now? The anterior insula and human awareness
- Neural systems supporting interoceptive awareness
- Damage to the insula disrupts addiction to cigarette smoking