Integrated information theory: from consciousness to its physical substrate

Journal name:
Nature Reviews Neuroscience
Year published:
Published online


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.

At a glance


  1. An experience is a conceptual structure.
    Figure 1: An experience is a conceptual structure.

    According to integrated information theory (IIT), a particular experience (illustrated here from the point of view of the subject) is identical to a conceptual structure specified by a physical substrate. The true physical substrate of the depicted experience (seeing one's hands on the piano) and the associated conceptual structure are highly complex. To allow a complete analysis of conceptual structures, the physical substrate illustrated here was chosen to be extremely simple1, 2: four logic gates (labelled A, B, C and D, where A is a Majority (MAJ) gate, B is an OR gate, and C and D are AND gates; the straight arrows indicate connections among the logic gates, the curved arrows indicate self-connections) are shown in a particular state (ON or OFF). The analysis of this system, performed according to the postulates of IIT, identifies a conceptual structure supported by a complex constituted of the elements A, B and C in their current ON states. The borders of the complex, which include elements A, B, and C but exclude element D, are indicated by the green circle. According to IIT, such a complex would be a physical substrate of consciousness (Supplementary information S1 (figure)). The conceptual structure is represented as a set of stars and, equivalently, as a set of histograms. The green circle represents the fact that experience is definite (it has borders). Each histogram illustrates the cause–effect repertoire of a concept: how a particular mechanism constrains the probability of past and future states of its maximally irreducible purview within the complex ABC. The bins on the horizontal axis at the bottom of the histograms represent the 16-dimensional cause–effect space of the complex — all its eight possible past states (p; in blue) and eight possible future states (f; in red; ON is 1 and OFF is 0). The vertical axis represents the probability of each state (for consistency, the probability values shown are over the states of the entire complex and not just over the subset of elements constituting the purview). In this example, five of seven possible concepts exist, specified by the mechanisms A, B, C, AB, AC (all with Φmax>0) in their current state (which are labelled as Ac, Bc, etc.). The subsets BC and ABC do not specify any concept because their cause–effect repertoire is reducible by partitions (Φmax=0). In the middle, the 16-dimensional cause–effect space of the complex is represented as a circle, where each of the 16 axes corresponds to one of the eight possible past (p; blue arrows) and eight possible future states (f; red arrows) of the complex, and the position along the axis represents the probability of that state. Each concept is depicted as a star, the position of which in cause–effect space represents how the concept specifies the probability of past and future states of the complex, and the size of which measures how irreducible the concept is (Φmax). Relations between two concepts (overlaps in their purviews) are represented as lines between the stars. The fundamental identity postulated by IIT claims that the set of concepts and their relations that compose the conceptual structure are identical to the quality of the experience. This is how the experience feels — what it is like to be the complex ABC in its current state 111. The intrinsic irreducibility of the entire conceptual structure (Φmax, a non-negative number) reflects how much consciousness there is (the quantity of the experience). The irreducibility of each concept (Φmax) reflects how much each phenomenal distinction exists within the experience. Different experiences correspond to different conceptual structures.

  2. Identifying the elements, timescale and states of the physical substrate of consciousness (PSC) from first principles.
    Figure 2: Identifying the elements, timescale and states of the physical substrate of consciousness (PSC) from first principles.

    It is possible to determine maxima of cause–effect power within the central nervous system by perturbing and observing neural elements at various micro- and macro-levels18. High cause–effect power is reflected in deterministic responses and low cause–effect power is reflected in responses that vary randomly across trials. a | To identify the spatial grain of the elements of the PSC supporting consciousness, a schematic example shows how optogenetic perturbation and unit recording could be applied to a subset of neurons (here, 3 out of 36 neurons) to establish maxima of cause–effect power. For each of three trials, the left panel shows the effects of the perturbation on the entire system at the micro-level. Grey neurons are unaffected, blue neurons decrease their firing rates, red neurons increase their firing rates and purple neurons respond with burst firing. The right-hand panel shows the effects of the perturbation at the macro-level after coarse-graining of the 36 neurons into nine groups of four cells each. Macro-states are defined according to the rule that if ≥50% of the neurons in the group are in a given micro-state (such as low firing, high firing or bursting), then the group is considered to be in that state at the macro-level. In this example, the macro-level (groups of neurons) has higher cause–effect power than the micro-level (single neurons), because the response is deterministic at the macro-level (as evidenced by the consistent colour scheme), whereas there are variations between trials at the micro-level (inconsistent colours). b | To identify the temporal grain of neuronal activity supporting consciousness, a possible experimental setup would be one in which one neuron (the top trace) is optogenetically excited while recording from other neurons (labelled N1–N4) across three trials, shown in the upper panel at the 10 ms timescale (micro-scale). Grey shading indicates no effects on neuron firing in the 10 ms following the stimulation compared with the 10 ms before the stimulation, blue shading indicates decreased firing and red shading indicates increased firing. The lower panel shows the same data after temporal coarse-graining over 100 ms intervals. Macro-states are defined according to the rule that if a neuron increases (or decreases) its firing rate by >50% within 100 ms post-stimulus compared with the baseline, the macro state is considered to be high (or low) firing. In this example, the macro-level (100 ms intervals) has higher cause–effect power (more deterministic responses) than the micro-level (10 ms intervals). c | To identify the neural states that support consciousness, optogenetic perturbations could be used to drive one neuron to fire either at low frequency, high tonic frequency or bursting (top trace) resulting in spectral peaks at 2 Hz (green), 50 Hz (red) and 150 Hz (yellow) for neurons N1–N4 (data are shown as a firing rate histogram). For each trial, the upper panel shows the responses of the other four neurons to each stimulation frequency at the micro-scale level in the spectral domain (micro-bins, only a few of which are represented). The coloured bars indicate coincidence, within a micro-bin, between the frequency of stimulation and the spectral peak of the responses. The lower panel of each trial shows the effect of the perturbation at the corresponding macro-level after spectral coarse-graining. Macro-states map into micro-states as indicated below the frequency bins. Here, spectral coarse-graining (binning firing rates into three levels, low, high and burst firing) results in higher cause–effect power (responses that are more deterministic) than at the micro-level.

  3. Identifying the physical substrate of consciousness (PSC) from first principles.
    Figure 3: Identifying the physical substrate of consciousness (PSC) from first principles.

    The complex of neural elements that constitutes the PSC can be identified by searching for maxima of intrinsic cause–effect power. a | For example, assume that the elements, timescale and states at which intrinsic cause–effect power reaches a maximum have been identified using optogenetic and unit recording tools (Fig. 2). Here, the elements are groups of neurons, the timescale is over 100 ms and there are three states (low, high and burst firing). b | In a healthy, awake participant, the set of neural elements specifying the conceptual structure with the highest Φmax is assumed, based on current evidence, to be a complex of neuronal groups distributed over the posterior cortex and portions of the anterior cortex5. Empirical studies can, in principle, establish whether the full neural correlates of consciousness5 correspond to the maximum of intrinsic cause–effect power, thereby corroborating or falsifying a key prediction of integrated information theory. c | The boundaries of the PSC (green line) may change after cortical lesions, such as those causing absolute achromatopsia, resulting in a smaller PSC. d | The PSC boundaries may also move as a result of changes in excitability and effective connectivity, as might occur during pure thought that is devoid of sensory content. e | The PSC could also split into two large local maxima of cause–effect power (represented here by green and blue boundaries) as a result of anatomical disconnections, such as in split-brain patients, in which instance each hemisphere would have its own consciousness. f | The PSC may also split as a result of functional disconnections, which may occur in some psychiatric disorders and perhaps under certain dual-task conditions — for example while driving and talking at the same time. g | The coexistence of a large major complex with one or more minor complexes that may support sophisticated, seemingly unconscious performance could be a common occurrence in everyday life.

  4. Phenomenal content and access content.
    Figure 4: Phenomenal content and access content.

    The content of an experience is much larger than what can be reported by a subject at any point in time. The left-hand panel illustrates the Sperling task41, which involves the brief presentation of a three by four array of letters on a screen, and a particular row being cued by a tone. Out of the 12 letters shown on the display, participants correctly report only three or four letters — the letters cued by the tone — reflecting limited access. The top middle panel illustrates a highly simplified conceptual structure that corresponds to seeing the Sperling display, using the same conventions as outlined in Fig. 1. The myriad of positive and negative, first- and high-order, low- and high invariance concepts (represented by stars) that specify the content of this particular experience (seeing the Sperling display and having to report which letters were seen) make it what it is and different from countless other experiences (rich phenomenal content). The bottom panel schematically illustrates the physical substrate of consciousness (PSC) that might correspond to this particular conceptual structure (its boundary is represented by a green line). The PSC consists of neuronal groups that can be in a low firing state, a high firing state or a bursting state. Alone and in combination, these neuronal groups specify all the concepts that compose the conceptual structure. Stars that are linked to the PSC by grey dashed lines represent a small subset of these concepts. The PSC is synaptically connected to neurons in Broca's area by means of a limited capacity channel (dashed black arrow) that is dynamically gated by top-down connections (shown as solid black arrows) originating in the prefrontal cortex to carry out the instruction (that is, to report the observed letters 'OSA').


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Author information


  1. Department of Psychiatry, University of Wisconsin, 6001 Research Park Boulevard, Madison, Wisconsin 53719, USA.

    • Giulio Tononi
  2. Department of Psychiatry, University of Wisconsin, 6001 Research Park Boulevard, Madison, Wisconsin 53719 USA; and at the Department of Neurology, University of Wisconsin, 1685 Highland Avenue, Madison, Wisconsin 53705, USA.

    • Melanie Boly
  3. Department of Biomedical and Clinical Sciences 'Luigi Sacco', University of Milan, Via G.B. Grassi 74, Milan 20157, Italy; and at the Instituto Di Ricovero e Cura a Carattere Scientifico, Fondazione Don Carlo Gnocchi, Via A. Capecelatro 66, Milan 20148, Italy.

    • Marcello Massimini
  4. Allen Institute for Brain Science, 615 Westlake Ave N, Seattle, Washington 98109, USA.

    • Christof Koch

Competing interests statement

The authors declare no competing interests.

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Author details

  • Giulio Tononi

    Giulio Tononi is a neuroscientist and psychiatrist at the Department of Psychiatry at the University of Wisconsin, Madison, USA, where he works on the nature of consciousness and the functions of sleep.

  • Melanie Boly

    Melanie Boly is a neurologist and neuroscientist at the University of Wisconsin, Madison, USA. She uses neuroimaging and theoretical approaches to understand the neural substrate of experience and its alterations in sleep, anaesthesia and disorders of consciousness.

  • Marcello Massimini

    Marcello Massimini is Associate Professor of Neurophysiology at the University of Milan, Italy. He was trained as a medical doctor and devotes his research to understanding the neuronal mechanisms of loss and recovery of consciousness in sleep, anaesthesia and in patients with brain injuries.

  • Christof Koch

    Christof Koch is the President and Chief Scientific Officer of the Allen Institute for Brain Science in Seattle, Washington, USA. He trained as a physicist. On a quest to understand the physical roots of consciousness before his brain stops functioning, he published his first paper on the neural correlates of consciousness with Francis Crick a quarter of a century ago.

Supplementary information

PDF files

  1. Supplementary information S1 (figure) (284 KB)

    Axioms and postulates of IIT.

  2. Supplementary information S2 (box) (83 KB)

    IIT Pseudocode

  3. Supplementary information S3 (figure) (189 KB)

    Integrated information, neuroanatomy and neurophysiology.

  4. Supplementary information S4 (box) (785 KB)

    Neuronal bistability impairs information integration during slow wave sleep

  5. Supplementary information S5 (box) (246 KB)

    IIT and other theories of consciousness

Additional data