A perturbational approach for evaluating the brain's capacity for consciousness
Section snippets
Evaluating a subject's level of consciousness
The bedside evaluation of patients affected by disorders of consciousness (DOC) relies on repeated behavioural observation by trained personnel. During the examination, spontaneous and elicited behaviour in response to multisensory stimulation is recorded in accordance with specific scales (Giacino et al., 2004; Gill-Thwaites and Munday, 2004; Kalmar and Giacino, 2005; Shiel et al., 2000). Regardless of the scale employed, the examiner typically looks for (1) evidence of awareness of the self
Evaluating a brain's capacity for consciousness
In this chapter, we propose an additional level at which consciousness can be studied even when no communication whatsoever (behavioural or neural) can be established with the subject. This paradigm does not aim at probing the subject in order to elicit wilfull behaviours or neural activations; rather, it involves probing directly the subject's brain to gauge core properties that are theoretically relevant for consciousness. This option requires (1) starting from a theory that suggests which
Theoretical guidelines: the integrated information theory of consciousness
The IITC takes its start from phenomenology and, by making a critical use of thought experiments, argues that subjective experience is integrated information. Therefore, according to the IITC, any physical system will have subjective experience to the extent that it is capable of integrating information. In this view, experience, i.e. information integration, is a fundamental quantity that is, in principle, measurable, just as mass or energy is. Information and integration are, on the other
Employing TMS/hd-EEG to evaluate thalamocortical integration and information capacity
Different methods have been proposed in order to infer on a subject's level of consciousness solely based on brain activity. Some of these methods, such as spectral analysis (Berthomier et al., 2007) and the proprietary “bispectral index” (Myles et al., 2004), seem to correlate empirically with consciousness but have no clear theoretical foundation. Other measures, such as neural complexity (Tononi et al., 1994) and causal density (Seth, 2005), are theoretically motivated (Seth et al., 2008)
TMS/hd-EEG detects changes in the brain's capacity for integrated information during sleep
Sleep is the only time when healthy humans regularly lose consciousness. Subjects awakened during slow-wave sleep early in the night may report short, thought-like fragments of experience, or often nothing at all (Hobson et al., 2000). Sleep also exposes several interesting paradoxes about the relationships between consciousness and the brain. For instance, it was thought that the fading of consciousness during sleep was due to the brain shutting down. However, while metabolic rates decrease in
TMS/EEG in DOC patients: some predictions
Given the variety of brain lesions and conditions that are associated to DOC (Laureys et al. (2004), Laureys et al. (2009)), it is very difficult to predict what kind of results TMS/hd-EEG might give in individual DOC patients. However, an informed guess can be adopted at least in some specific cases. For instance, it is conceivable that TMS-evoked activations similar to the ones described during slow-wave sleep may also be found in patients that are in a coma caused by a lesion in the
Future perspectives
We attempted at identifying an objective marker of consciousness that is theoretically grounded and practically measurable. The core message of this chapter is that using by TMS/hd-EEG it is possible to detect clear-cut changes in the capacity of human thalamocortical circuits to integrate information, a theoretical requirement to generate conscious experience, when the level of consciousness fluctuates across the sleep–wake cycle. The implication of this finding is that TMS/hd-EEG may be
Acknowledgements
This work was supported by European grant STREP LSHM-CT-2205-51818 to Marcello Massimini. Melanie Boly is Research Fellow at the Belgian National Fund for Scientific Research (FNRS). We thank Andrea Soddu, Silvia Casarotto and Fabio Ferrarelli for their help and comments.
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The strength of weak integrated information theory
2022, Trends in Cognitive SciencesCitation Excerpt :Overall, these studies found that when subjects are unconscious, the brain’s response to TMS is stereotypical across electrodes and/or remains local to the site of stimulation (at standard stimulus intensities), whereas the response of conscious subjects is more diverse across electrodes and spreads across larger regions of cortex. In this way, the probed neural dynamics when the participant is conscious appear both to be more diverse and to play out across a wider network, which implies greater differentiation and greater integration, suggesting a natural connection between PCI and IIT and representing an ‘implicitly weak’ approach to IIT, as highlighted in recent work [5,38]. While PCI has been presented as a proxy for ΦMax, and its success has been described as support for IIT [1,25], PCI’s general behaviour does not comply with the strict requirements of strong IIT for a measure of integrated information.
EEG as a marker of brain plasticity in clinical applications
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