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The Global Workspace (GW) Theory of Consciousness and Epilepsy

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The Global Workspace (GW) Theory of Consciousness and Epilepsy Behavioural Neurology 24 (2011) 67–74 67 DOI 10.3233/BEN-2011-0313 IOS Press The global workspace (GW) theory of consciousness and epilepsy Fabrice Bartolomeia,b,c,∗ and Lionel Naccached,e,f aINSERM, U751, Laboratoire de Neurophysiologie et Neuropsychologie, Marseille, F-13005, France bUniversite´ de la Mediterran´ ee,´ FacultedeM´ edecine,´ Marseille, F-13005 France cCHU Timone, Service de Neurophysiologie Clinique, Assistance Publique des Hopitauxˆ de Marseille, Marseille, F-13005, France dAP-HP, Groupe hospitalier Pitie-Salp´ etriˆ ere,` Departments of Neurophysiology & Neurology, Paris, France eINSERM, ICM Research Center, UMRS 975, Paris, France f Universite´ Paris 6, FacultedeM´ edecine´ Pitie-Salp´ etriˆ ere,` Paris, France Abstract. The global workspace (GW) theory proposes that conscious processing results from coherent neuronal activity between widely distributed brain regions, with fronto-parietal associative cortices as key elements. In this model, transition between conscious and non conscious states are predicted to be caused by abrupt non-linear massive changes of the level of coherence within this distributed neural space. Epileptic seizures offer a unique model to explore the validity of this central hypothesis. Seizures are often characterized by the occurrence of brutal alterations of consciousness (AOC) which are largely negatively impacting patients’ lives. Recently, we have shown that these sudden AOC are contemporary to non-linear increases of neural synchrony within distant cortico-cortical and cortico-thalamic networks. We interpreted these results in the light of GW theory, and suggested that excessive synchrony could prevent this distributed network to reach the minimal level of differentiation and complexity necessary to the coding of conscious representations. These observations both confirm some predictions of the GW model, and further specify the physiological window of neural coherence (minimum and maximum) associated with conscious processing. Keywords: Consciousness, temporal lobe epilepsy, global workspace, synchrony 1. Introduction ized seizures and frequently occur in partial seizures. In this review we will focus on the mechanisms leading Epilepsy is one of the most frequent brain disease to AOC in partial seizures, and particularly in the most affecting near 1% of the general population and nega- frequent type of chronic drug resistant partial epilep- tively impacting quality of life [28]. sies, the temporal lobe epilepsies (TLE). Thereis a classical and operativedistinction between AOC is the most dramatic clinical manifestation of generalized epilepsies in which the epileptic process TLE, causing important handicap and potential source affect bilaterally and widely the cerebral cortex and of injury. In line with this, the international classi- partial (or focal) seizures in which the seizures start fication of epileptic seizures has made impaired con- from a limited part of the cerebral cortex and secon- sciousness the cornerstone by which the main cate- darily spread to other cortical and subcortical regions. gories of partial seizures, simple and complex, are dis- Alteration of consciousness (AOC) is usual in general- tinguished [12]. It emphasizes an essential characteris- tic that in many cases has significant repercussions on the quality of life of the epileptic patient. ∗ Corresponding author: Pr Fabrice Bartolomei, MD, PhD, Ser- Whereas the structural and functional changes ob- vice de Neurophysiologie Clinique, CHU Timone-264 Rue st Pierre, 13005-Marseille, France. Tel.: +33 491 385 833; Fax: +33 491 385 served in TLE have been largely studied, the mecha- 826; E-mail: [email protected]. nisms leading to AOC are poorly known [8]. Several ISSN 0953-4180/11/$27.50 2011 – IOS Press and the authors. All rights reserved 68 F. Bartolomei and L. Naccache / Global workspace theory and epilepsy theories/studies have proposed scenario about AOC in creases in spectral power in the gamma band, 3) in- TLE (see below). creases in long-distance phase synchrony in the beta Recently we have proposed that AOC in temporal range, and 4) increases in long-range Granger causali- lobe seizures could be explained by an alteration of the ty [22]. “global workspace”functioning[3,13,17],a theorythat On the other hand ‘intransitive consciousness’ refers we will developed in the next paragraphs. to the mere existence of a functional conscious GW, – irrespective of its particular content –, in contrast with non conscious states such as deep sleep, comatose, 2. The GW model of consciousness general anesthesia, vegetative state, and generalized or complex partial seizures with AOC. Crucially, GW The GW model, initiated by Bernard Baars’ the- theory predicts that transitions between such non con- oretical work [2,3] proposes that at any given time scious states and conscious mode of processing follow many modular cerebral networks are active in paral- non-linear threshold functions. More precisely, GW lel and process information in an unconscious manner, predicts that in response to progressive linear neuro- while consciousness would correspond to the broad- modulatory increases (or decreases) of global cortical casting ofinformationto a globalworkspace. Dehaene, activation by the Ascending Reticular Activating Sys- Changeux, Naccache and colleagues developed these tem, long-distance cortico-cortical coherence would principles, and elaborated a plausible functional neural show a non-linear ‘all-or-none’ mode of activation (or architecture to the GW [15–17]. In particular they pro- deactivation). posed that conscious access would be causally related These ideas were recently implemented in a compu- to a mechanism of top-down attentional amplification tational model of GW network [14] in which ascend- into a self-sustained brain-scale state of coherent ac- ing brain stem nuclei send globally depolarizing neu- tivity that involves many neurons distributed through- romodulatory signals to a thalamic and cortical hierar- out the brain. The long-distance connectivity of these chy. Simulations confirmed that progressive increase “workspace neurons” can, when they are active for a in spontaneous firing of cortico-thalamic structures as minimal duration, make the information available to a function of neuromodulatorrelease evolves into what a variety of processes including perceptual categoriza- is known in dynamical systems theory as a Hopf bi- tion, long-term memorization, evaluation, and inten- furcation: spontaneous firing increases continuously in tional action. According to this model, global avail- intensity, but high-frequency oscillations appear sud- ability of information through the workspace is what denly in the gamma band (20–80 Hz). we subjectively experience as a conscious state. Neu- Taken together, these results strongly suggest that rophysiological, anatomical, and brain-imaging data long-distance cortico-cortical synchronization in the strongly argue for a major role of prefrontal cortex, an- beta and gamma band play a fundamental and causal terior cingulate, and the areas that connect to them, in role in conscious processing and top-down [10,11,21, creating the postulated brain-scale workspace. 26]. Importantly, two aspects of consciousness have to be The exploration of AOC contemporary to seizures distinguished. On the one hand ‘access consciousness’ can provide precious specifications of the range of neu- refers to the neural mechanisms enabling an initially ral coherence associated with conscious processing. In non-conscious perceptual representation, encoded in a particular, excess of coherences are predicted to be a specialized local cortical processor, to be broadcasted cause of conscious impairment, by reducing GW com- to the GW network as a conscious percept. In other plexity and differentiation to dramatically poor val- terms, the issue of ‘access consciousness’ explores the ues [37]. transitions between non-conscious processing and con- scious processing in a conscious subject. Many exper- iments in healthy controls and in various neurological 3. Alteration of Consciousness in Epilepsy: A disorders such as blindsight, neglect, visual hallucina- summary of putative mechanisms tions and agnosias can be accounted for by GW theo- ry [34]. We could recently demonstrate that conscious Temporal lobe seizures are the most frequent cause access correlates with four neural signatures: 1) sus- of partial seizures with AOC. Seizures in TLE are char- tained voltage changes over distributed regions, partic- acterized by epileptic discharges originating from one ularly in prefrontal cortex, 2) large and sustained in- or several regions of the temporal lobe (often from the F. Bartolomei and L. Naccache / Global workspace theory and epilepsy 69 Table 1 The Consciousness Seizure Scale (CSS) Criteria Assessment of the criteria 1. Unresponsiveness (0 or 1). The patient does not execute simple verbal commands (ex: “clap your hands”, “open the mouth”, “close your eyes”). 2. No visual attention (0 or 1). The patient presents no adequate visual response to external stimuli (ex: the patient does not look at the examiner during examination). 3. No interaction with the examiner (0 The patient does not present any signs (other than visual attention) of response to the examiner. or 1). 4. No consciousness of the seizure (0 The patient does not report to be in seizure state at any time of the seizure course (ex: he/she does or 1). not call the examiner at the beginning of the seizure). 5. Inappropriate behavior
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