BMC Neuroscience
November 2, 2004
Giulio Tononi
1,703 citations
Consciousness corresponds to a system's capacity to integrate information, measurable as Phi—the amount of causally effective information integrated across a complex of elements. This theory explains why consciousness arises from the thalamocortical system but not the cerebellum, why it diminishes during dreamless sleep and generalized seizures, and why neural processes underlying consciousness can influence or be influenced by unconscious ones. It also accounts for the unity and differentiation of conscious experiences. The theory implies that consciousness is a graded, fundamental quantity present in infants and animals, and that conscious artifacts could be built.
Nature reviews. Neuroscience
April 20, 2016
Christof Koch, Marcello Massimini, Melanie Boly et al.
1,654 citations
The neural correlates of consciousness—the minimum neural mechanisms sufficient for any one specific conscious percept—are primarily localized to a posterior cortical hot zone that includes sensory areas, rather than to a fronto-parietal network involved in task monitoring and reporting. Some candidate neurophysiological markers of consciousness have proved illusory, while measures of differentiation and integration of neural activity offer more promising quantitative indices of consciousness.
Science
September 29, 2005
Marcello Massimini, Fabio Ferrarelli, Reto Huber et al.
1,603 citations
During non-rapid eye movement sleep, the brain's response to a direct cortical stimulus is stronger but fails to propagate beyond the stimulation site, unlike during quiet wakefulness when the activation spreads to distant connected areas. This breakdown in cortical effective connectivity may explain why consciousness fades during certain sleep stages despite ongoing brain activity.
Biological Bulletin
December 1, 2008
Giulio Tononi
1,522 citations
Integrated information theory (IIT) proposes that consciousness is identical to integrated information. The quantity of consciousness corresponds to the amount of integrated information (Phi) generated by a complex of elements, while the quality of experience is specified by the set of informational relationships within that complex, represented as a shape in qualia space (Q). This framework explains several observations about the neural substrate of consciousness, including its association with certain neural systems, the influence of conscious processes on unconscious ones, the reduction of consciousness during dreamless sleep and seizures, and the role of cortical architecture in shaping experience. Equating consciousness with integrated information carries implications for how we understand nature.
Science Translational Medicine
August 14, 2013
Adenauer G. Casali, Olivia Gosseries, Mario Rosanova et al.
1,299 citations
A new index called the perturbational complexity index (PCI) can objectively measure a person's level of consciousness without requiring them to respond or interact. PCI works by using transcranial magnetic stimulation to briefly perturb the cortex and then measuring the algorithmic complexity of the resulting brain activity patterns. The index was tested on healthy individuals during wakefulness, dreaming, nonrapid eye movement sleep, and under sedation with midazolam, xenon, and propofol, as well as on patients who had emerged from coma. PCI reliably distinguished conscious from unconscious states in single individuals across all conditions, including vegetative state, minimally conscious state, and locked-in syndrome.
Annals of the New York Academy of Sciences
March 1, 2008
Giulio Tononi, Christof Koch
615 citations
This review synthesizes recent findings on the neural correlates of consciousness, distinguishing consciousness from other brain functions. It examines global changes in consciousness during sleep, anesthesia, and seizures, then explores paradigms for studying neural correlates of specific conscious percepts, highlighting the roles of different brain regions. Dynamic aspects of neural activity—sustained versus phasic, feedforward versus reentrant, and neural synchronization—are discussed. The review also considers how theoretical analysis of consciousness's fundamental properties can complement neurobiological research.
Journal of Neuroscience
October 4, 2017
Melanie Boly, Marcello Massimini, Naotsugu Tsuchiya et al.
591 citations
The role of the frontal cortex in consciousness is debated. This Perspective critically reviews clinical and neuroimaging evidence for the involvement of the front versus the back of the cortex in specifying conscious contents and discusses promising research avenues.
Annals of Neurology
September 22, 2016
Silvia Casarotto, Angela Comanducci, Mario Rosanova et al.
504 citations
A brain-based measure called the Perturbational Complexity Index (PCI) can reliably distinguish conscious from unconscious individuals, even when they cannot speak or move. The index was first validated in 150 healthy and brain-injured people who could report their conscious state, achieving perfect accuracy in separating conscious from unconscious conditions. Applied to 81 noncommunicative patients, PCI correctly identified 94.7% of those in a minimally conscious state and revealed that 9 of 43 patients diagnosed as vegetative had PCI values overlapping with conscious individuals. These findings suggest that some behaviorally unresponsive patients may retain hidden conscious capacity.
Anesthesiology
February 7, 2012
Robert D. Sanders, Giulio Tononi, Steven Laureys et al.
478 citations
Consciousness—subjective experience—persists during sleep and anesthesia, as evidenced by dreaming. A defining feature of dreaming is disconnection from the environment. Anesthesia aims to prevent the experience of surgery (connected consciousness) by inducing either unconsciousness or disconnection. The isolated forearm technique reveals that consciousness, connectedness, and responsiveness can uncouple during anesthesia; under clinical conditions, a median 37% of patients demonstrate connected consciousness. Potential neurobiological constructs explain this: during light anesthesia, subcortical mechanisms for spontaneous behavioral responsiveness are disabled, but information integration within the corticothalamic network continues producing consciousness, while unperturbed norepinephrinergic signaling maintains connectedness. These concepts emphasize the need for anesthetic regimens and depth-of-anesthesia monitors targeting mechanisms of consciousness, connectedness, and responsiveness.
Journal of Neuroscience
July 1, 1999
Ramesh Srinivasan, D. Patrick Russell, Gerald M. Edelman et al.
438 citations
During binocular rivalry, where each eye sees a different image and perception alternates between them, the brain activity corresponding to the consciously perceived image shows increased synchronization. Red vertical and blue horizontal gratings were presented to each eye, each flickering at a distinct frequency. Magnetoencephalography recorded stronger steady-state magnetic fields at the frequency of the perceived grating over occipital, temporal, and frontal cortices. Coherence analysis revealed increased interhemispheric and intrahemispheric synchronization at the stimulus frequency when that stimulus was perceived. The results demonstrate a direct correlation between conscious perception and synchronous activity of large populations of neocortical neurons.
PLoS Computational Biology
June 12, 2008
David Balduzzi, Giulio Tononi
433 citations
A new time- and state-dependent measure of integrated information, phi, quantifies how much information is generated when a system enters a particular state through causal interactions among its elements, beyond what its parts generate independently. This captures two key properties of consciousness: a large repertoire of experiences that rule out others when one occurs, and the integration of that information into a whole that cannot be decomposed. Applied to discrete networks, phi varies with the state entered, being higher when active and inactive elements are balanced and lower when the network is inactive or hyperactive.
December 1, 2012
Giulio Tononi
378 citations
Integrated information theory (IIT) proposes that consciousness is identical to a maximally integrated structure of cause-effect information, called a quale, which is generated by a complex of elements. The theory starts from phenomenological axioms: every experience is specific (one out of many), unified (cannot be reduced to independent parts), and definite (limited in content and speed). These are formalized into postulates: cause-effect information, integrated information (irreducible to independent components), and exclusion (only maxima matter). A mechanism specifies a concept as a maximally irreducible set of past causes and future effects; a complex is a set of elements whose constellation of concepts specifies a quale. The theory relates integration to metastable neural dynamics and accounts for empirical findings on the neural substrate of consciousness.
PLoS Computational Biology
August 13, 2009
David Balduzzi, Giulio Tononi
272 citations
Integrated information theory proposes that consciousness corresponds to the amount of integrated information generated by a system of elements, and the quality of an experience is determined by the informational relationships within that system. This paper introduces qualia space (Q), where each possible state of a system is an axis, and submechanisms specify repertoires of states. Arrows between these repertoires define informational relationships that together form a shape—a quale—that uniquely characterizes a conscious experience. The quantity of consciousness is the height of this shape (phi). Entanglement measures how irreducible these relationships are. The framework implies that the same activity pattern can yield different qualia in different systems, and vice versa. Experience cannot be reduced to local mechanisms but requires the entire quale.
Proceedings of the National Academy of Sciences
March 17, 1998
Giulio Tononi, Ramesh Srinivasan, D. Patrick Russell et al.
259 citations
During binocular rivalry, where each eye sees a different image but only one is consciously perceived at a time, neuromagnetic responses in the brain show activity at frequencies corresponding to both stimuli, even for the unseen one. However, when a stimulus is not consciously perceived, the power of its neural response is reduced by 50-85% across many brain regions, including areas outside the visual cortex. This modulation is widespread but not global, indicating that conscious perception correlates with the strength of synchronized neural activity in large neuronal populations.
The International Journal of Neuropsychopharmacology
June 7, 2012
Wallace C. Duncan, Simone Sarasso, Fabio Ferrarelli et al.
253 citations
A single infusion of the NMDA receptor antagonist ketamine rapidly reduces depressive symptoms in patients with treatment-resistant major depressive disorder. In 30 patients, ketamine increased electroencephalogram slow wave activity during early non-REM sleep and raised plasma levels of brain-derived neurotrophic factor. The occurrence of high amplitude slow waves and their slope also increased, indicating enhanced synaptic strength. Changes in BDNF levels correlated with changes in EEG parameters, but only in patients who responded to ketamine. This suggests that enhanced synaptic plasticity, reflected by increased slow wave activity and BDNF, is part of the mechanism behind ketamine's rapid antidepressant effects.
Cognitive Neuroscience
July 1, 2010
Marcello Massimini, Fabio Ferrarelli, Michael J. Murphy et al.
205 citations
During the first REM sleep episode of the night, transcranial magnetic stimulation (TMS) triggered widespread and differentiated patterns of cortical activation on electroencephalography (EEG), similar to those observed during wakefulness. In contrast, during NREM sleep, cortical activations became more local and stereotypical, indicating impaired intracortical dialogue. These findings suggest that TMS combined with high-density EEG can probe the internal dialogue of the thalamocortical system, potentially offering a method to assess brain function in patients who are unable to move or communicate.
Frontiers in Psychology
January 1, 2013
Francesca Siclari, Joshua J. LaRocque, Bradley R. Postle et al.
184 citations
Consciousness during sleep changes dramatically across the night. Seven healthy participants were awakened 15–30 minutes apart across 44 nights and asked about their most recent conscious experience before the alarm. Recall with content occurred in 34% of non-REM awakenings and 77% of REM awakenings. Each sleep stage showed a distinct profile of features including thinking, perceiving, self-relatedness, and reflective consciousness. The serial awakening method yielded a large, representative sample of sleep consciousness with minimal disruption from sleep fragmentation, and may be especially useful when combined with brain imaging techniques.
Clinical EEG and Neuroscience
January 1, 2014
Simone Sarasso, Mario Rosanova, Adenauer G. Casali et al.
164 citations
Consciousness requires both functional integration and differentiation in the brain, a property termed brain complexity. Transcranial Magnetic Stimulation combined with electroencephalography (TMS/EEG) can quantify this complexity. Studies consistently show that the complexity of the cortical response to TMS collapses during loss of consciousness in deep sleep, anesthesia, and vegetative state after severe brain injury. Complexity recovers when consciousness returns during wakefulness, dreaming, the minimally conscious state, or locked-in syndrome. This approach may help understand the pathophysiology of disorders of consciousness and requires careful methodological attention.
Entropy
November 27, 2019
Andrew Haun, Giulio Tononi
158 citations
Experience feels the way it does for a reason, and spatial experience offers a promising starting point for investigation because it is more accessible to introspection than qualities like color or pain. Much of experience is spatial, from bodily sensations to the visual world, which appears as an extended canvas. To feel extended, this canvas must consist of countless spots related through connection, fusion, and inclusion, with each spot having a location, size, boundary, and distance from others. The authors propose an account based on integrated information theory (IIT), showing that a simulated grid-like network of units yields a cause-effect structure that explains main properties of spatial experience. This suggests spatial experience is supported by brain areas with grid-like connectivity, and changes in connectivity should warp experienced space.
Neuroscience of Consciousness
January 1, 2016
Erik Hoel, Larissa Albantakis, William Marshall et al.
153 citations
Causal interactions in complex systems like the brain can be examined at different spatiotemporal levels. While it is often assumed that the micro level is causally complete, this work shows that causal power can be stronger at macro levels. Using a measure called ΦMax, developed within integrated information theory, the authors systematically evaluated causal power at micro and macro levels in simplified neuronal-like systems. For systems with indeterminism or degeneracy, ΦMax peaked at a macro level when coarse-graining micro elements produced macro mechanisms with high irreducible causal selectivity.
Journal of Neuroscience
September 10, 2018
Francesca Siclari, Giulio Bernardi, Jacinthe Cataldi et al.
148 citations
Dreaming during non-rapid eye movement (NREM) sleep is linked to fewer, smaller, and shallower slow waves and faster spindles, especially in central and posterior brain regions. A minority of very steep, large slow waves in frontal areas, occurring against a background of reduced slow wave activity and accompanied by high-frequency power increases (local microarousals), preceded successful dream recall. The findings suggest that the brain's ability to generate experiences during sleep is reduced when neuronal off-states are present in posterior and central regions, and that dream recall may be aided by intermittent activation of arousal systems during NREM sleep.
Annals of the New York Academy of Sciences
May 1, 2008
Giulio Tononi, Marcello Massimini
135 citations
Consciousness fades during deep NREM sleep early in the night even though cortical neurons remain active and receive sensory inputs. According to integrated information theory, what matters for consciousness is not firing rates or synchronization but the brain's ability to integrate information—having a large repertoire of available states that cannot be decomposed into independent subsystems. Experiments using transcranial magnetic stimulation and high-density electroencephalography (TMS/hd-EEG) in humans tested this prediction. The sleeping brain, though active and reactive, loses its ability to enter states that are both integrated and differentiated; it either breaks into causally independent modules, responding with short local activation, or produces an explosive, aspecific slow wave.
IEEE Spectrum
May 29, 2008
Christof Koch, Giulio Tononi
134 citations
Within a generation, people may have the option to upload their minds—thoughts, memories, and personality—to a computer, offering an alternative to death. Once consciousness is reduced to electronic patterns, it could be copied, edited, sold, pirated, bundled with other minds, or deleted. The text raises the question of whether one would sell their soul on eBay, framing the ethical and practical implications of digital immortality and the commodification of consciousness.
PLoS ONE
August 28, 2013
Fabio Ferrarelli, Richard Smith, Daniela Dentico et al.
125 citations
Long-term Buddhist meditators with about 8,700 mean lifetime hours of practice show increased gamma power (25-40 Hz) in parietal-occipital regions during non-rapid eye movement sleep compared to meditation-naive individuals. This increase is specific to gamma frequencies, unrelated to spontaneous arousal levels during NREM sleep, and positively correlated with the length of lifetime daily meditation practice. The findings indicate that meditation practice produces measurable changes in spontaneous brain activity and suggest that EEG gamma activity during sleep may serve as a sensitive marker of long-lasting plastic effects of meditative training on brain function.
Nature Communications
February 25, 2022
Minji Lee, Leandro Sanz, Alice Barra et al.
120 citations
A deep-learning-based explainable consciousness indicator (ECI) uses EEG responses to transcranial magnetic stimulation and resting-state EEG to separately quantify arousal and awareness. Tested during sleep (n=6), general anesthesia (n=16), and severe brain injury (n=34), ECI distinguishes states such as ketamine-induced anesthesia and rapid eye movement sleep, which combine low arousal with high awareness. Parietal brain regions are most relevant for these measurements. The indicator offers a way to disentangle the two components of consciousness across physiological, pharmacological, and pathological conditions.