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Integrated information theory: from consciousness to its physical substrate


In this Opinion article, we discuss how integrated information theory accounts for several aspects of the relationship between consciousness and the brain. Integrated information theory starts from the essential properties of phenomenal experience, from which it derives the requirements for the physical substrate of consciousness. It argues that the physical substrate of consciousness must be a maximum of intrinsic cause–effect power and provides a means to determine, in principle, the quality and quantity of experience. The theory leads to some counterintuitive predictions and can be used to develop new tools for assessing consciousness in non-communicative patients.

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Figure 1: An experience is a conceptual structure.
Figure 2: Identifying the elements, timescale and states of the physical substrate of consciousness (PSC) from first principles.
Figure 3: Identifying the physical substrate of consciousness (PSC) from first principles.
Figure 4: Phenomenal content and access content.


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The authors thank L. Albantakis, C. Cirelli, L. Ghilardi, W. Marshall, W. Mayner, A. Mensen, M. Oizumi, U. Olcese, B. Postle, S. Sasai and other colleagues for their various contributions to the work presented here. This work was supported by the Templeton World Charity Foundation, the McDonnell Foundation and the Distinguished Chair in Consciousness Science (University of Wisconsin) (to G.T.), and by the James S. McDonnell Scholar Award 2013 (to M.M.).

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Correspondence to Giulio Tononi.

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Integrated Information Theory

PowerPoint slides

Supplementary information

Supplementary information S1 (figure)

Axioms and postulates of IIT. (PDF 340 kb)

Supplementary information S2 (box)

IIT Pseudocode (PDF 135 kb)

Supplementary information S3 (figure)

Integrated information, neuroanatomy and neurophysiology. (PDF 219 kb)

Supplementary information S4 (box)

Neuronal bistability impairs information integration during slow wave sleep (PDF 842 kb)

Supplementary information S5 (box)

IIT and other theories of consciousness (PDF 322 kb)



A condition in which a person is unable to perceive colours.


A condition in which a person has a neurological deficit, but is unaware of it.


Properties that are self-evident and essential; in integrated information theory, those that are true of every possible experience — namely, intrinsic existence, composition, information, integration and exclusion.

Background conditions

Factors that enable consciousness, such as neuromodulators and external inputs that maintain adequate excitability.

Cause–effect repertoire

The probability distribution of potential past and future states of a system that is specified by a mechanism in its current state.

Cause–effect space

A space with each axis representing the probability of each possible past and future state of a system.

Cause–effect structure

The set of cause–effect repertoires specified by all the mechanisms of a system in its current state.


A set of elements in a state that specifies a conceptual structure corresponding to a maximum of integrated information (Φmax). A complex is thus a physical substrate of consciousness.


The cause–effect repertoires specified by a mechanism that is maximally irreducible (Φmax).

Conceptual structure

The set of all concepts specified by a system of elements in a state with their respective Φmax values, which can be plotted as a set of points in cause–effect space.

Content-specific NCC

Neural elements, the activity of which determines a particular content of experience.


The minimum constituents of a system that have at least two different states (for example, being on or off), inputs that can affect those states and outputs that depend on them.

Full NCC

The neural elements constituting the physical substrate of consciousness, irrespective of its specific content.

Integrated information

(Denoted Φ). Information that is specified by a system that is irreducible to that specified by its parts. It is calculated as the distance between the conceptual structure specified by the intact system and that specified by its minimum information partition.


Any subset of elements within a system that has cause–effect power on it (that is, that constrains its cause–effect space).

Neural correlates of consciousness

(NCC). The minimum neuronal mechanisms jointly sufficient for any one specific conscious experience.


Properties of experience that are derived from the axioms of integrated information theory and that must be satisfied by the physical substrate of consciousness — namely, to be a maximum of irreducible, specific, compositional, intrinsic cause–effect power (intrinsic cause–effect power for short).


The subsets of elements of a complex, the past and future states of which are constrained by a mechanism specifying a concept.


The qualitative feeling of phenomenal distinctions within an experience (for example, seeing a colour, hearing a sound or feeling a pain).


Maximally irreducible overlaps among the purviews of two or more concepts.

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Tononi, G., Boly, M., Massimini, M. et al. Integrated information theory: from consciousness to its physical substrate. Nat Rev Neurosci 17, 450–461 (2016).

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