Timeline of integrated information theory

This is a timeline of integrated information theory.

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Year (month and date)	Event type	Details
1720	Prelude	Gottfried Wilhelm Leibniz's Monadology appears in print, containing his famous analogy of the mill. Leibniz asks the reader to imagine walking through an enlarged brain as one would walk through a mill, observing all its mechanical operations; nowhere, he contends, would such an observer see a conscious thought, so consciousness cannot arise from mere matter. In his earlier Discourse on Metaphysics (1686), Leibniz had already allowed for infinitely many degrees of consciousness, including unconscious "petites perceptions", and he was the first to distinguish explicitly between perception and apperception (roughly, awareness and self-awareness). Both ideas would resurface in integrated information theory nearly three centuries later. IIT treats consciousness as graded, and in 2015 Giulio Tononi and Christof Koch would cite the mill, alongside John Searle's Chinese Room argument, as anticipating their claim that a computer simulation of a brain would not be conscious.
1948 (July–October)	Prelude	Claude Shannon, a mathematician at Bell Telephone Laboratories, publishes "A Mathematical Theory of Communication" in two installments of the Bell System Technical Journal. Building on earlier work by Harry Nyquist and Ralph Hartley, Shannon defines the fundamental problem of communication as reproducing at one point a message selected at another, and sets aside the meaning of messages as irrelevant to the engineering problem. He adopts a logarithmic measure of information, with units called bits (a word suggested by John Tukey), and introduces the entropy H as a measure of information, choice, and uncertainty. He shows that the entropy of a joint event is at most the sum of the individual entropies, with equality only when the events are independent, and proves fundamental limits on encoding and on channel capacity in the presence of noise. Shannon's statistical framework would supply the mathematical vocabulary for later measures of neural integration, while his exclusion of meaning would become a foil for integrated information theory, which defines information from the perspective of the system itself rather than an external observer.
1987	Prelude	Neuroscientist Gerald Edelman, a 1972 Nobel Prize in Physiology or Medicine laureate, publishes Neural Darwinism: The Theory of Neuronal Group Selection through Basic Books. Its central idea is that each individual's nervous system works as a selective system, resembling natural selection in evolution but operating through different mechanisms. By giving the categorization of things in the world a neural basis, the theory aims to unify perception, action, and learning. It also recasts memory as a dynamic process of recategorization rather than a store that replicates attributes, with implications for psychological states ranging from attention to dreaming. The book draws on developmental and evolutionary biology, anatomy, physiology, ethology, and psychology. Its publisher expects it to renew debate on philosophical questions such as the mind–body problem, the origins of knowledge, and the perceptual basis of language. The theory of neural Darwinism would become the framework within which Edelman and his later collaborator Giulio Tononi study neural integration and consciousness.
1994 (May)	Prelude	Neuroscientists Giulio Tononi, Olaf Sporns, and Gerald Edelman (the latter a Nobel Prize in Physiology or Medicine laureate) publish "A measure for brain complexity: relating functional segregation and integration in the nervous system" in the Proceedings of the National Academy of Sciences. The paper starts from a tension in the brains of higher vertebrates: local areas are anatomically and physiologically specialized, yet they work as an integrated whole during perception and behavior. To capture this interplay, the authors introduce a measure called neural complexity (CN), built from how far subsets of a neural system deviate from statistical independence. CN is high when segregation and integration coexist, and low when components are either fully independent or fully dependent. Computer simulations of cortical areas show that typical cortical wiring (dense connections, strong local connectivity forming neuronal groups, patchy links between groups, and prevalent reciprocal connections) yields high CN. The authors suggest the approach may also apply to gene regulation and embryogenesis. The idea that the brain balances differentiation with integration would become a direct precursor of Tononi's later integrated information theory.
1995	Prelude	Philosopher David Chalmers publishes "Facing Up to the Problem of Consciousness" in the Journal of Consciousness Studies. He argues that progress requires confronting consciousness directly, and isolates its truly hard problem (explaining subjective experience itself) from more tractable problems about cognitive functions and mechanisms. Chalmers critiques recent work that addresses consciousness with reductive methods, arguing that such methods inevitably fail to come to grips with the hardest part of the problem. In the second half of the paper, he proposes that a naturalistic account of consciousness is still possible through a new kind of nonreductive explanation. His candidate is a theory built on principles of structural coherence and organizational invariance, together with a double-aspect theory of information, in which information has both a physical and a phenomenal side. The paper would be reprinted in the 1996 Tucson volume Toward a Science of Consciousness and in Chalmers' 2010 collection The Character of Consciousness. The hard problem framing would become the backdrop against which integrated information theory presents itself, starting from the properties of experience rather than from brain mechanisms.
1996 (April)	Prelude	At the second Tucson conference on consciousness, philosopher David Chalmers gives a talk titled "On the Search for the Neural Correlate of Consciousness", published in 1998 in the MIT Press volume Toward a Science of Consciousness II. Chalmers observes that the field has produced a "neural correlate zoo" of competing proposals for the neural correlate of consciousness (NCC). Among them are Francis Crick and Christof Koch's 40-hertz cortical oscillations, Gerald Edelman's reentrant thalamocortical loops, and Bernard Baars's global workspace. He argues that the search faces a foundational obstacle: consciousness cannot be measured directly, and for principled reasons no consciousness meter seems possible. Researchers therefore rely on pre-experimental bridging principles, chiefly verbal report, to judge whether a system is conscious and what it is conscious of. Since these principles are not themselves tested experimentally, Chalmers contends that everyone searching for the NCC is engaged in philosophical reasoning. The absence of a principled measure of consciousness is the gap that integrated information theory would later try to fill, by proposing a quantity to be computed from a system's physical structure.
1998 (December)	Prelude	Giulio Tononi and Gerald Edelman publish "Consciousness and complexity", a review article in Science. They contrast their approach with conventional research on consciousness, which typically looks for the contribution of specific brain areas or groups of neurons. Instead, they ask what kinds of neural processes could account for key properties of conscious experience. They apply measures of neural integration and complexity (extending the neural complexity measure the two had introduced with Olaf Sporns in 1994) and analyze extensive neurological data. This leads them to a testable proposal about the neural substrate of consciousness, which they call the dynamic core hypothesis. The paper shifts the question from where consciousness sits in the brain to what kind of neural process supports it, a framing Tononi would later develop into integrated information theory.
2000	Prelude	Neuroscientists Gerald Edelman, director of the Neurosciences Institute and chairman of the Department of Neurobiology at the Scripps Research Institute, and Giulio Tononi, a senior fellow in theoretical and experimental neurobiology at the same institute, publish the book A Universe of Consciousness: How Matter Becomes Imagination through Basic Books. Aimed at a general readership, the book asks why physical events inside a small mass of brain tissue give rise to the world of conscious experience. Its publisher presents it as the first full-scale theory of consciousness grounded in direct observation of the working human brain, one that challenges much of the conventional wisdom on the subject and bears on language, thought, emotion, and mental illness. The book brings the pair's research program on neural integration and complexity to a broad audience, a few years before Tononi would publish the first formal statement of integrated information theory.
2003 (December 2)	Theory	Giulio Tononi and Olaf Sporns publish "Measuring information integration" in BMC Neuroscience, setting out the measure Φ in detail. They note that communication theory measures how information is transmitted from a sender to a receiver, but not how a distributed network like the brain integrates it. To fill that gap, they introduce effective information: the causal effect one part of a system has on another when the first part's outputs are replaced by noise. Φ is the effective information across a subset's weakest bipartition. A complex is a subset with Φ above zero that is not contained in a subset of higher Φ. The authors explain that the Greek letter evokes information (the "I") within a single entity (the "O"). Simulations of small networks show that Φ is highest when elements are both specialized and densely interconnected, and low when networks are homogeneous or modular. The authors argue that this explains why the thalamocortical system, unlike the modular cerebellum, supports consciousness, and they suggest that basal ganglia loops may underlie unconscious processes. They acknowledge that an exhaustive search is feasible only for about two dozen elements, and they propose probing real brains with transcranial magnetic stimulation. IIT 4.0 would later count this paper, with Tononi's 2004 paper, as IIT 1.0.
2004 (November 2)	Theory	Neuroscientist Giulio Tononi, of the Department of Psychiatry at the University of Wisconsin, publishes "An information integration theory of consciousness" in BMC Neuroscience, the founding paper of what would become integrated information theory (IIT). Starting from two phenomenological properties of experience, differentiation (a vast repertoire of possible experiences) and integration (the unity of each one), Tononi proposes that consciousness corresponds to a system's capacity to integrate information. Thought experiments contrast a person with a photodiode, which gains a single bit when it tells light from dark, and with a megapixel camera, whose sensors distinguish countless states without interacting. He introduces Φ, the effective information across a subset's informational "weakest link", and defines a complex as a subset with Φ above zero that is not part of a larger subset of higher Φ. The quality of experience is set by the informational relationships within the complex. Tononi uses the theory to explain why the thalamocortical system, unlike the cerebellum, supports consciousness, and why consciousness fades in dreamless sleep. He concludes that consciousness is a fundamental, graded quantity, present in infants and animals, and that conscious artifacts should be possible to build.
2005 (September 30)	Experiment	Marcello Massimini, Giulio Tononi, and four colleagues publish "Breakdown of cortical effective connectivity during sleep" in Science, asking why consciousness fades in sleep even though the brain stays active. Combining transcranial magnetic stimulation (TMS) with high-density electroencephalography (EEG), they stimulate the premotor cortex and track how the activation spreads. During quiet wakefulness, an initial response is followed by waves that travel to connected cortical areas several centimeters away. During non-rapid eye movement sleep, the initial response is stronger but quickly dies out without spreading. The authors conclude that the fading of consciousness in some sleep stages may reflect a breakdown in cortical effective connectivity. Tononi's 2004 paper had announced that such TMS–EEG experiments were underway. The same issue of Science carries a commentary on the findings, and a December letter by George Mashour on cognitive "unbinding" in sleep and anesthesia draws an author reply. IIT's proponents would later cite the study among evidence that complexity measures inspired by the theory can distinguish conscious from unconscious states.
2008 (June 13)	Theory	Giulio Tononi and David Balduzzi publish "Integrated Information in Discrete Dynamical Systems: Motivation and Theoretical Framework" in PLOS Computational Biology. Earlier versions of the theory measured integrated information only for stationary systems modeled as Gaussian processes, leaving out time and changing dynamics. The paper introduces a time- and state-dependent measure, φ, of how much information a system generates as a whole when it enters a particular state through causal interactions among its elements, above and beyond what its parts generate independently. In small simulated networks, φ peaks in balanced states and collapses when a network is inactive or hyperactive, which the authors liken to coma and generalized seizures. A system that merely replays an activity sequence without interacting generates no integrated information, while modular, homogeneous, feedforward, and lattice architectures prove poor or inefficient integrators. The authors describe φ as an intrinsic, observer-independent property, contrasting it with the traditional notion of information sent from a source to a receiver. They also acknowledge that the combinatorial explosion of possible partitions makes computing φ for the human brain unfeasible, a limitation later critics would press.
2008 (December)	Theory	Giulio Tononi publishes "Consciousness as Integrated Information: A Provisional Manifesto" in The Biological Bulletin, setting out the framework explicitly as integrated information theory (IIT). As in his 2004 paper, the theory starts from phenomenology and thought experiments and claims that consciousness is integrated information. The quantity of consciousness corresponds to the integrated information (Φ) generated by a complex of elements, defined as the information the complex generates above and beyond its parts. The quality of experience is specified by the set of informational relationships within the complex. To capture this, Tononi introduces qualia space (Q), in which each axis is a possible state of the complex and the informational relationships generated by its causal mechanisms form a shape that completely specifies a particular experience. The manifesto argues that the framework explains why consciousness depends on some neural systems and not others, why it fades in dreamless sleep and generalized seizures, and how different cortical architectures shape the quality of experience. It closes by noting that equating consciousness with integrated information has broader implications for our view of nature.
2009 (August 14)	Theory	Giulio Tononi and David Balduzzi publish "Qualia: the geometry of integrated information" in PLOS Computational Biology, developing the qualia space (Q) from Tononi's 2008 manifesto into a formal framework for the quality of experience. Q has an axis for each possible activity state of a complex. Each submechanism of the complex specifies a point in Q (a repertoire of system states), and arrows between these points define informational relationships. Together the arrows form a quale, a shape that completely characterizes the quality of a conscious experience, while Φ corresponds to the height of that shape. A new quantity, entanglement, measures how irreducible informational relationships are to their components and is used to specify concepts and modes. Among the corollaries: two systems with identical activity patterns can generate different qualia; inactive elements contribute to experience but inactivated ones do not; and modalities and submodalities correspond to subshapes in Q. Specific qualities such as the "redness" of red are generated by a local mechanism but cannot be reduced to it. The authors suggest the framework may offer a principled way to translate qualitative properties of experience into mathematics.
2011 (October 13)	Measure	Neuroscientist Anil Seth and colleagues Adam Barrett and Lionel Barnett publish "Causal density and integrated information as measures of conscious level" in Philosophical Transactions of the Royal Society A. They describe an outstanding challenge for neuroscience: developing measures of conscious level that are both theoretically grounded and practically applicable, high in alert wakefulness and low in dreamless sleep, coma, and general anesthesia. The paper reviews measures of dynamical complexity, focusing on causal density and integrated information, which capture in different ways how far a system's dynamics are simultaneously differentiated and integrated. Because conscious scenes share these features, the authors argue, such measures are good candidates for tracking conscious level. New simulations show where the measures agree and where they diverge, and the authors discuss remaining obstacles to applying them to empirical data. The paper treats integrated information as one candidate measure among several rather than as a definition of consciousness. Both the 2013 paper introducing the perturbational complexity index and the 2016 Nature Reviews Neuroscience overview of IIT would cite it.
2011 (October 20)	Experiment	Jeffrey Edlund, Nicolas Chaumont, Arend Hintze, Christof Koch, Giulio Tononi, and evolutionary biologist Christoph Adami publish "Integrated information increases with fitness in the evolution of animats" in PLOS Computational Biology. Noting that the ability to integrate disparate sources of information is a hallmark of biological organisms, yet hard to quantify, they evolve simulated agents, or animats, over thousands of generations to solve a navigation task in a simple maze. Along the evolutionary line of descent, measures of information integration rise together with fitness as the animats adapt. The authors compare these measures with more conventional information-processing measures and conclude that high fitness requires both processing and integration, with integration the better predictor when the task demands memory. They suggest that such measures can quantify an organism's functional complexity even without fitness data. A 2014 follow-up by Larissa Albantakis and colleagues would extend the approach to environments of increasing complexity. In 2015, Tononi and Koch would cite these simulations as IIT's account of why consciousness is adaptive and therefore evolved.
2012	Publication	Neuroscientist Christof Koch publishes Consciousness: Confessions of a Romantic Reductionist with MIT Press, a book that is part scientific overview, part memoir, and part futurist speculation. Koch recounts his collaboration with Francis Crick in the 1990s and 2000s and the emergence of consciousness, once considered a fringe subject, as a legitimate topic of scientific investigation. He places a handful of colleagues at the center of that shift, including Ned Block, David Chalmers, Stanislas Dehaene, Giulio Tononi, and Wolf Singer. Alongside chapters on attention and awareness, neurons that respond to celebrities, free will, and the loss of his belief in a personal God, Koch devotes one chapter to arguing that consciousness is a fundamental property of complex things. That echoes Tononi's 2004 claim that consciousness is a fundamental quantity. Another chapter outlines an electromagnetic gadget to measure consciousness. The book signals Koch's embrace of the framework he would later co-develop with Tononi, whom he joins as co-author of the 2016 Nature Reviews Neuroscience overview of IIT.
2012	Publication	Giulio Tononi publishes Phi: A Voyage from the Brain to the Soul through Pantheon Books, a 364-page book addressed to a general readership. A 2017 review in the Spanish-language philosophy journal Mutatis Mutandis presents it as one of the relevant contributions to a problem that has become one of neuroscience's greatest challenges: consciousness, a concept grasped intuitively but whose definition involves debates spanning neuroscience, psychology, anthropology, and philosophy.
2012 (June–September)	Theory	Giulio Tononi publishes "Integrated information theory of consciousness: an updated account" in the Archives Italiennes de Biologie. Building on thought experiments, the article recasts IIT as a set of phenomenological axioms (information, integration, exclusion) paired with "ontological postulates". The information postulate holds that only "differences that make a difference" from the intrinsic perspective of a system matter: a mechanism generates information if its present state has specific past causes and specific future effects within the system. The integration postulate admits only information that cannot be partitioned among independent components. The exclusion postulate keeps only maxima, so each mechanism specifies a single maximally irreducible set of causes and effects, called a concept. A complex is a set of elements specifying a maximally irreducible constellation of concepts, evaluated at its optimal spatio-temporal scale, and this structure, or quale, is identical with an experience. The article also links the temporal grain of integration to metastable dynamics in the corticothalamic system. A corrected version is republished in the journal's December 2012 issue. In 2014, the formal presentation of IIT 3.0 would point to this account as its precursor.
2012 (August)	Publication	Giulio Tononi, professor of neuroscience at the University of Wisconsin, publishes Phi: A Voyage from the Brain to the Soul through Pantheon Books, a 384-page book that presents integrated information theory to a general readership. Rather than an exposition, the book is a literary journey in three parts: an aging Galileo Galilei is guided by three famous scientists. Francis Crick, co-discoverer of the structure of DNA, shows him how portions of the brain function separately and together. Alan Turing, a founder of artificial intelligence, helps him link these facts into "a scientific theory of consciousness". Charles Darwin explores how consciousness evolves. Notes at the end of each chapter, written in a voice separate from the narrator's, expand on the themes; the last one remarks that the book's closing symbol "must hold some great significance to the author but could not be deciphered." Publishers Weekly calls the book "both playful and philosophical" and "extravagant", praising its lavish artwork and literary references as showing how fully complementary art and science can be. The book appears the same year as Christof Koch's Consciousness: Confessions of a Romantic Reductionist.
2013 (January–March)	Critique	Philosopher John Searle reviews Christof Koch's Consciousness: Confessions of a Romantic Reductionist in The New York Review of Books (January 10 issue), arguing that information theory cannot explain consciousness: consciousness is observer-independent, but information exists only relative to an observer. In a letter published on March 7, Koch and Giulio Tononi reply that this is exactly IIT's own starting point. IIT, they write, introduces a "non-Shannonian" notion of integrated information, "differences that make a difference" to a system from its intrinsic perspective. Against Searle's objection that consciousness "cannot spread over the universe like a thin veneer of jam", they add that only local maxima of integrated information exist, so there is no superordinate consciousness of the United States. Searle answers by quoting their own words. He cites Tononi's claim that a binary photodiode "enjoys exactly 1 bit of consciousness", and Koch's book, which calls IIT "an elaborate version of panpsychism" and finds a modicum of Φ even in protons. Searle maintains that the integrated information in such systems is observer-relative too, and concludes that the theory "does not seem to be a serious scientific proposal."
2013 (August 14)	Measure	Adenauer Casali, Marcello Massimini, and colleagues, including Steven Laureys and Giulio Tononi, publish "A Theoretically Based Index of Consciousness Independent of Sensory Processing and Behavior" in Science Translational Medicine. Motivated by the lack of an objective bedside measure of consciousness for unresponsive brain-injured patients, they introduce the perturbational complexity index (PCI). Extending the team's earlier TMS–EEG work, PCI perturbs the cortex with magnetic pulses to engage distributed interactions (integration) and then compresses the resulting electrical responses to measure their algorithmic complexity (information). Applied to existing recordings, PCI ranges from 0.44 to 0.67 in 32 awake healthy subjects but drops to 0.18–0.28 in non-REM sleep, with similarly low values under midazolam, xenon, and propofol anesthesia. Among patients who emerged from coma, those in a vegetative state score 0.19–0.31, those with locked-in syndrome 0.51–0.62, and those in a minimally conscious state in between. The journal's editorial summary cautions that clinical validity still requires prospective trials. The 2016 Nature Reviews Neuroscience overview of IIT would cite PCI among the theory's contributions to assessing consciousness in non-communicative patients.
2014 (May 8)	Theory	Masafumi Oizumi, Larissa Albantakis, and Giulio Tononi publish "From the Phenomenology to the Mechanisms of Consciousness: Integrated Information Theory 3.0" in PLOS Computational Biology. The paper calls the earlier stationary-system version IIT 1.0 and the state-dependent version IIT 2.0. It starts from axioms about experience (existence, composition, information, integration, and exclusion) and turns them into postulates about how physical mechanisms such as neurons or logic gates must be configured. Information becomes "differences that make a difference" within a system, measured with the earth mover's distance. The central identity holds that an experience is a maximally irreducible conceptual structure, a constellation of concepts in qualia space, whose irreducibility ΦMax sets its quantity. Worked examples yield several corollaries: inactive elements can contribute to experience; a two-element "photodiode" with feedback is minimally conscious; and a feedforward network can be a "zombie", unconscious yet matching a conscious system's input–output behavior. The authors build that zombie by unfolding a recurrent system's elements over time. Five years later, critics would turn this very construction against the theory in the unfolding argument.
2015 (May 19)	Publication	Giulio Tononi and Christof Koch publish "Consciousness: here, there and everywhere?" in Philosophical Transactions of the Royal Society B, setting out what IIT 3.0 implies about which systems are conscious. They argue that behavioral and neural correlates cannot settle hard cases such as brain-damaged patients, preterm infants, distant animal species, and machines. The authors acknowledge that IIT shares central intuitions of panpsychism: consciousness is a fundamental property, graded, likely widespread among animals, and present in small amounts even in very simple circuits. Unlike panpsychism, however, IIT holds that not everything is conscious. Its exclusion postulate answers the combination problem: two people talking, a nation, or a sand dune form no further conscious entity. Purely feedforward networks, such as those used in deep learning, are unconscious however capable, so no Turing test can suffice. Even a digital computer faithfully simulating a human brain would likely experience next to nothing, just as a simulated star does not bend space–time, while neuromorphic hardware might not face this limit. The authors propose validating the theory on human consciousness first and extrapolating outward from there.
2014 (May 21)	Critique	Theoretical computer scientist Scott Aaronson publishes "Why I Am Not An Integrated Information Theorist (or, The Unconscious Expander)" on his blog Shtetl-Optimized, answering a challenge from physicist Max Tegmark, an enthusiastic adopter of IIT. Aaronson coins the "Pretty-Hard Problem of Consciousness" (deciding which physical systems are conscious) and calls IIT a serious attempt at it that nonetheless fails. He shows that a system that merely multiplies its input by a Vandermonde matrix, or that applies error-correcting codes built on expander graphs, can have an arbitrarily large Φ, so IIT would deem it "unboundedly more conscious than humans are". In the comments, a reader notes that IIT 3.0, published two weeks earlier, had dropped the normalization his construction exploits. Aaronson nonetheless places IIT in the top 2% of mathematical theories of consciousness, precisely because it can be shown wrong. Tononi replies in a guest post on the blog, accepting that a large enough grid of simple logic gates could exceed a brain's Φ but arguing that such a grid would indeed be conscious, and that common-sense intuitions cannot test a fundamental theory.
2015 (September 17)	Critique	Michael Cerullo publishes "The Problem with Phi: A Critique of Integrated Information Theory", a Perspective in PLOS Computational Biology, the journal that had carried several of IIT's own formulations. He argues that IIT rests on a principle of "information exclusion" (that consciousness grows with the number of alternatives a system rules out), for which Tononi offers no justification beyond intuition. To test IIT's claimed empirical support, Cerullo invents a deliberately trivial rival, "Circular Coordinated Message Theory", which ties consciousness to information circulating in feedback loops and reproduces IIT's predictions about split-brain patients and the cerebellum. Because IIT rejects computational functionalism, he adds, it is vulnerable to David Chalmers's "fading" and "dancing" qualia thought experiments. Drawing on the 2014 exchange between Scott Aaronson and Tononi, in which Tononi accepted that a large grid of logic gates could exceed a brain's Φ and maintained that it would be conscious, Cerullo concludes that IIT describes at most a noncognitive "protoconsciousness", a form of partial panexperientialism. He recalls that after Tononi's talk at the 2014 Science of Consciousness meeting, Ned Block remarked: "You have a theory of something, I am just not sure what it is."
2015 (November)	Event	New York University's Center for Mind, Brain and Consciousness, co-directed by David Chalmers and Ned Block, co-hosts a two-day workshop on integrated information theory, at which Giulio Tononi, Christof Koch, Chalmers, and ten other speakers present to about 30 scientists and philosophers. After Tononi's 90-minute tutorial and Koch's talk, Scott Aaronson, styling himself the "official IIT skeptic", reprises his argument that simple error-correcting systems can have enormous Φ. He charges IIT's proponents with invoking common sense when it suits them (the unconscious cerebellum) but rejecting it when it does not (the conscious grid). Tononi replies that Aaronson critiqued an outdated version of Φ and that IIT is a work in progress, while physicist Max Tegmark presents half a dozen alternative definitions of Φ, which Aaronson says his analysis also covers. Participants are particularly puzzled by the exclusion postulate. Reporting for Scientific American's blog network, science journalist John Horgan writes that the workshop strengthened his mysterianism. He endorses John Searle's 2013 objection that information presupposes a conscious observer, and predicts that IIT's panpsychist implications will limit its popularity.
2016 (May 26)	Publication	Giulio Tononi, his Wisconsin colleague Melanie Boly, Marcello Massimini of the University of Milan, and Christof Koch, now at the Allen Institute for Brain Science, publish "Integrated information theory: from consciousness to its physical substrate", an Opinion article in Nature Reviews Neuroscience. The article presents the theory in its axiomatic form to a broad neuroscience readership. It starts from five essential properties of experience, called axioms (intrinsic existence, composition, information, integration, and exclusion), and derives postulates that any physical substrate of consciousness must satisfy, concluding that this substrate must be a maximum of intrinsic cause–effect power. On this basis, the authors argue that the theory can determine, in principle, both the quality and the quantity of experience. They note that it leads to some counterintuitive predictions and argue that it can be used to develop new tools for assessing consciousness in patients unable to communicate. The work is supported in part by the Templeton World Charity Foundation, which would later fund an adversarial collaboration testing IIT against a rival theory.
2017 (September 7)	Measure	Mathematician Shun-ichi Amari and neuroscientists Naotsugu Tsuchiya and Masafumi Oizumi post "Geometry of Information Integration" on arXiv. By this point multiple measures of integrated information have been proposed, each quantifying in its own way how much information is lost when a system is split into parts and the information transmission between them is removed. Using information geometry, the authors analyze four of these measures and clarify how they relate: two define the split model on dually flat manifolds and two on curved manifolds. They show that the measures form a hierarchy and give explicit expressions for each. The paper offers a unifying mathematical view of a growing family of approximations, needed because, as Balduzzi and Tononi had acknowledged in 2008, exact integrated information cannot feasibly be computed for systems as large as the brain.
2018 (June 29)	Critique	Philosopher Tim Bayne publishes "On the axiomatic foundations of the integrated information theory of consciousness" in Neuroscience of Consciousness. Calling IIT one of the most influential scientific theories of consciousness and a guiding framework for attempts to build a consciousness meter, Bayne asks whether its foundations are secure. He notes that the IIT literature describes the postulates as "derived" from the axioms, yet never weighs rival explanations, as an abductive inference would require. Taking the five axioms in turn, he argues that none can play its intended role. Some are self-evident but impose no substantive constraint, such as the claim that each experience is "the particular way it is". Others constrain theories but are not self-evident, such as the claim that consciousness is purely intrinsic, which externalist philosophers reject. He cites associative agnosia against the integration axiom, and vague contents, like the indeterminate number of a tiger's visible stripes, against exclusion. Bayne proposes instead a "natural kind" approach modeled on the study of diseases such as hepatitis, within which, he suggests, IIT could still be developed. IIT 4.0 would later describe the step from axioms to postulates as an "inference to a good explanation".
2018 (June 25)	Measure	Pedro Mediano, Anil Seth, and Adam Barrett post "Measuring Integrated Information: Comparison of Candidate Measures in Theory and Simulation" on arXiv; the paper would later appear in the journal Entropy. IIT is built around measures of how much more information a system generates than the sum of its parts, but the authors note that little is known about how the several existing candidate measures of integrated information (Φ) compare on non-trivial networks. They give intuitive descriptions of six distinct measures and test them in simulations of eight-node networks with Gaussian linear autoregressive dynamics. The results show a striking diversity: no two measures agree consistently across all analyses, and only some appear to genuinely reflect dynamical complexity, in the sense of simultaneous segregation and integration among system components. The study extends Seth and Barrett's 2011 comparison of causal density and integrated information, and complements the previous year's mathematical comparison of four measures by Amari, Tsuchiya, and Oizumi. Its results underscore that operationalizing IIT depends on which measure is chosen.
2018 (July 26)	Measure	William Mayner, Giulio Tononi, and colleagues publish "PyPhi: A toolbox for integrated information theory" in PLOS Computational Biology. The paper describes an open-source Python package that implements IIT's framework and unfolds the full cause–effect structure of discrete dynamical systems of binary elements. Released on GitHub under the GNU GPLv3 license, with online documentation and a web-based graphical interface, the software serves as an up-to-date reference implementation of IIT's formalisms. Its authors note that it has also been applied in research on complexity, emergence, and certain biological questions. The paper is candid about limits: because the numbers of states, subsystems, mechanisms, purviews, and partitions all grow exponentially, computation time scales as O(n53n), restricting practical analysis to about 10 to 12 elements. A chain of seven stochastic majority gates takes roughly 2.75 hours on 32 processor cores. The authors also warn that the analysis assumes Markovian systems with conditionally independent elements, assumptions that may not hold for data such as EEG recordings. PyPhi would later be used to compute the worked examples in the 2023 IIT 4.0 paper.
2019 (May 9)	Critique	Adrien Doerig, Aaron Schurger, Kathryn Hess, and Michael Herzog publish "The unfolding argument: Why IIT and other causal structure theories cannot explain consciousness" in Consciousness and Cognition. They distinguish theories that explain consciousness through brain functions, which many patterns of connectivity could realize, from causal structure theories such as IIT and Recurrent Processing Theory, which identify consciousness with causal structure itself, so that feedforward systems are never conscious and feedback systems always are. Drawing on theorems from the theory of computation, they argue that any recurrent network can be "unfolded" into a feedforward network with identical input–output behavior, so no experiment relying on reports could tell the two apart. Their conclusion is that causal structure theories are either false or outside the realm of science. Replies follow, including a 2020 call by Naotsugu Tsuchiya, Thomas Andrillon, and Andrew Haun to move "beyond functionalism/behaviorism". In 2022, Herzog, Schurger, and Doerig respond that first-person experience cannot rescue such theories: on their reading, IIT makes consciousness fully dissociated from anything measurable and therefore epiphenomenal, a position they call "dissociative epiphenomenalism".
2021	Publication	Christof Koch publishes "Reflections of a natural scientist on panpsychism" in the Journal of Consciousness Studies. Admitting a long-standing "secret crush" on panpsychism, Koch writes that IIT shares several of its intuitions: consciousness is an intrinsic, fundamental property of reality, is graded, and is present in small amounts in simple physical systems. Unlike panpsychism, he argues, IIT specifies which systems are and are not conscious and addresses the combination problem through its exclusion postulate, which determines whether aggregates such as ant colonies, beehives, or murmurating flocks form a single conscious entity. He adds that IIT explains why consciousness is adaptive, and faults panpsychism and other philosophical "isms" for failing to offer a sustained research program that yields new experiments. Against philosopher Philip Goff's claim that quantitative science cannot capture qualitative experience, Koch points to IIT's mathematical account of how the experience of empty two-dimensional space corresponds to the causal powers of a simple grid-like network, work published by Andrew Haun and Giulio Tononi in 2019. The piece restates the position Koch and Tononi had set out six years earlier.
2022 (January 12)	Critique	Michael Herzog of the École Polytechnique Fédérale de Lausanne, Aaron Schurger of Chapman University, and Adrien Doerig of the Donders Institute publish "First-person experience cannot rescue causal structure theories from the unfolding argument" in Consciousness and Cognition, answering critics of their 2019 unfolding argument. They identify the crux of the debate as whether first-person experience, independent of third-person data, can ground a theory of consciousness, and argue that it cannot: science relies on measurement, and experience is measurable only through reports, brain activity, and the relation between them. No experiment based on third-person data, they contend, can therefore test IIT as a theory of consciousness: "IIT may be a good theory of something, but not of consciousness." Because IIT dissociates consciousness from anything measurable, they argue, it makes both the level and the content of consciousness epiphenomenal, a "dissociative epiphenomenalism" in which even reports of experience cannot be trusted. Since such reports are the foundation of IIT's axioms, they conclude that accepting IIT leads to rejecting its own axioms.
2022 (March 11)	Experiment	A team including Yong Wang and Xiaoli Li publishes "Application of Fast Perturbational Complexity Index to the Diagnosis and Prognosis for Disorders of Consciousness" in IEEE Transactions on Neural Systems and Rehabilitation Engineering. Noting that TMS–EEG is an effective tool for measuring the level of consciousness, but that an accurate method for quantifying TMS-evoked activity is still lacking, the study applies a fast variant of the perturbational complexity index (PCIst) to recordings from 30 healthy participants and 181 patients with disorders of consciousness. PCIst distinguishes healthy participants from patients in a minimally conscious state and patients with unresponsive wakefulness syndrome, at low computational cost and even with data down-sampled to 250 Hz. Using one-year follow-up scores on the Coma Recovery Scale-Revised for 114 of the patients, the authors find that PCIst in the 9–12 Hz band performs best for both diagnosis and prognosis. Carried out by researchers outside the groups that introduced PCI, the study tests the approach on a patient sample far larger than that of the original 2013 paper.
2022 (June 4)	Publication	Giulio Tononi, Christof Koch, and four Wisconsin colleagues, including Larissa Albantakis and Melanie Boly, post "Only what exists can cause: An intrinsic powers view of free will" on arXiv, an essay that applies integrated information theory to the problem of free will. According to IIT, the presence of consciousness corresponds to a maximum of cause–effect power in the brain, and the way an experience feels to how that cause–effect power is structured. If the theory is right, the authors argue, people have free will in a genuine sense: they have alternatives, reasons, and values; they make decisions; and they, not their neurons or atoms, are the cause of their willed actions and bear responsibility for them. The argument rests on what the authors call an "intrinsic powers ontology": what exists absolutely, in physical terms, are intrinsic entities, and only what exists can cause. The essay extends IIT's claim to describe what truly exists into ethics and agency. It would remain under revision for years, reaching a fifth version in September 2026.
2023 (June 23)	Adversarial collaboration	Initial results of the first adversarial collaboration funded through the Templeton World Charity Foundation's Accelerating Research on Consciousness initiative, a $30 million program, are presented at the annual meeting of the Association for the Scientific Study of Consciousness. Led by Lucia Melloni of the Max Planck Institute for Empirical Aesthetics, the project pits IIT against the Global Neuronal Workspace theory. Six independent laboratories ran experiments to test predictions agreed in advance by both camps, following a method pioneered by psychologist Daniel Kahneman. IIT locates the substrate of consciousness in a posterior cortical "hot zone", while the workspace theory centers on prefrontal networks that broadcast information across the brain. The results match neither theory perfectly. Activity in the posterior cortex is consistent with IIT, but the predicted sustained synchronization between brain regions does not appear; some of the workspace theory's prefrontal predictions hold, but not all. Three judges conclude that IIT fared slightly better. The foundation, which describes the findings as "groundbreaking", stresses that neither theory has been ruled out and that further experiments are under way.
2023 (October 17)	Theory	A team at the University of Wisconsin led by Larissa Albantakis, with Giulio Tononi as senior author, publishes "Integrated information theory (IIT) 4.0: Formulating the properties of phenomenal existence in physical terms" in PLOS Computational Biology. Presented as the successor to IIT 1.0 (2003–2004), 2.0 (2008–2009), and 3.0 (2014), the paper takes the existence of experience as a "zeroth" axiom and identifies five essential properties: intrinsicality, information, integration, exclusion, and composition. Each is restated as a postulate about the cause–effect power a physical substrate must have. New elements include an intrinsic difference measure, said to be uniquely consistent with the postulates, and the explicit assessment of causal relations. The theory proposes an explanatory identity: an experience is the Φ-structure of distinctions and relations unfolded from a maximal substrate. Arguing that functionally equivalent systems need not be phenomenally equivalent, the authors conclude that digital computers running even artificial general intelligence programs might experience nothing, since consciousness is about being, not doing. They concede that exhaustive calculation is infeasible for realistic systems. Tononi discloses a financial interest in Intrinsic Powers, Inc., a company developing a clinical device to assess consciousness.
2025 (April 30)	Adversarial collaboration	The Cogitate Consortium publishes "Adversarial testing of global neuronal workspace and integrated information theories of consciousness" in Nature, with theory leaders including Giulio Tononi, Christof Koch, and Stanislas Dehaene among its co-authors. Theory-neutral teams at twelve institutions on three continents tested three preregistered predictions, with pass/fail criteria agreed in advance, in 256 participants using fMRI, magnetoencephalography, and intracranial EEG. Information about conscious content appeared in visual, ventrotemporal, and inferior frontal cortex, in line with some predictions of each theory. For IIT, the most direct challenge is the absence of sustained synchronization within the posterior cortex, which contradicts its claim that network connectivity specifies consciousness. For the workspace theory, the prefrontal cortex showed no "ignition" when a stimulus disappeared and carried little information about stimulus identity or orientation. The authors stress that they tested the theories' proposed brain implementations rather than their mathematical cores, so proponents could revise the former while keeping the latter. Rather than issue a verdict, they invite readers to weigh the evidence, publishing the adversaries' own interpretations alongside the consortium's.
2026 (June 16)	Critique	Vikas O'Reilly-Shah, Alessandro Selvitella, and Aaron Schurger publish "A caveat regarding the unfolding argument: implications of plasticity" in Neuroscience of Consciousness. The unfolding argument, introduced in 2019 by Doerig, Schurger, and colleagues, holds that because any recurrent neural network can be "unfolded" into a functionally equivalent feedforward network, theories that tie consciousness to causal structure, such as IIT, are either false or unscientific. The new paper offers mathematical proofs that this equivalence breaks down when a recurrent network's connection weights change rapidly through plasticity: its input–output function drifts over time, retains traces of perturbations indefinitely, and outgrows any finite set of static networks. The authors argue that this restores empirical testability to theories that build in fast plasticity, while stressing that it lends no theory credibility. For IIT specifically, they conclude that the original challenge stands, since IIT's causal structure can still be expressed in an unfolded network, and suggest that IIT could escape only by incorporating plasticity for principled rather than ad hoc reasons. Schurger is a co-author of both the original argument and this caveat.

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