The Interoception/Exteroception Boundary
Almost every page in this wiki uses the word interoceptive as though its extension were settled. It is not, and the sources disagree in a way that is easy to miss because none of them argues with the others.
The conventional answer, and why it fails
The default criterion is anatomical: the skin is the border, and interoception is the processing of signals generated below it. It has the great virtue of being checkable and the fatal defect of getting several important cases wrong in both directions.
- The vestibular system sits in the same sensory organ as audition, above any plausible internal border, and yet what it reports is the organism’s own balance. Anatomy files it outside; function puts it inside.
- Proprioception arises in muscles, tendons and connective tissue — anatomically internal — but reports body configuration in a way that has traditionally been grouped with somatosensation rather than with visceral sensing. See proprioception.
- Taste receptors sit at a surface that is topologically outside the body, sensing substances that have not yet entered it, yet gustatory afferents project to the NTS alongside unambiguously visceral ones.
- Chemoreception detached from the mouth entirely: functional bitter-taste receptors (TAS2Rs) on lung and vascular smooth muscle and on cortical neurons, possibly responding to bacterial ligands (Berntson & Khalsa 2021). Receptor type does not draw the line either.
- affective-touch: CT afferents lie in the skin, the paradigm exteroceptive organ, but share interoceptive fibre morphology and project to dorsal posterior insula rather than to S1/S2.
Craig’s 2002 widening was the first major redrawing — pulling temperature, pain, itch, sensual touch, hunger, thirst and air hunger inside on the grounds that they share the lamina I afferent route to the insula. That answer is pathway-based: interoceptive means carried by the homeostatic afferent system. It is the criterion most of this wiki has been implicitly using, and Berntson & Khalsa weakened it by showing that lamina I is one route among vagal, cranial, sacral, humoral and direct central chemosensing — some of which reach behaviour with no afferent fibre at all.
Three functional criteria, proposed within one year
All three appear in the Trends in Neurosciences 44(1) special issue, in different articles, with no cross-reference between them.
| source | criterion | what it makes the boundary about |
|---|---|---|
| Chen et al. 2021 | whether signals and responses represent, rather than originate from, the internal or external world | the content of the representation |
| Petzschner et al. 2021 | the inferred state the signal informs, not the channel it arrives on | the inferential role of the signal |
| Berntson & Khalsa 2021 | an act — ingestion is a decision to move a substance into the body, so the swallow may mark the transition | a behavioural transition point |
They agree on the important negative claim — the line is not drawn by anatomy or by receptor type — and they are not obviously inconsistent with each other. But they are answers to subtly different questions, and they come apart in practice. Chen et al.’s criterion makes the vestibular system interoceptive because balance is a fact about the organism. Petzschner et al.’s makes a signal interoceptive if it updates a model of bodily state, which could include a visual signal about one’s own body. Berntson & Khalsa’s locates the boundary at a moment in time rather than in a signal at all.
Why it matters rather than being a definitional quibble
Three concrete consequences, each already live on other pages.
It determines what the measures measure. If interoception is a natural kind unified by pathway, a cardiac accuracy score is a sample of a general capacity. If it is unified by what the signal represents, cardiac accuracy is a sample of one channel with no guarantee of generality — which is what Ferentzi et al. (2018) found empirically, reported on interoceptive-taxonomy and berntson-2021-neural-circuits: interoceptive sensitivity does not generalize across channels.
It determines the scope of the clinical claims. Chen et al. fold descending body regulation into interoception outright, which brings blood pressure, baroreflex and organ function inside as outcome measures. Whether a therapy “works through interoception” depends heavily on where this line is drawn — see weng-2021-interventions-of-interoception, where the organizing claim is exactly that and is nowhere measured.
It is quietly load-bearing for the constructionist arguments. If interoception is a party to multisensory-integration rather than a channel that terminates and then influences things (Quigley et al.), then asking whether a given percept is interoceptive or exteroceptive may be a badly-formed question about an assembled product. That is a stronger claim than any of the criteria above, and it dissolves the boundary rather than locating it.
Status
Open, and not currently contested — which is the odd part. Three criteria were proposed in one journal issue by authors who cite each other’s articles without engaging each other’s definitions. This page exists to keep them side by side rather than letting whichever was read most recently silently become the wiki’s working definition.