Consciousness as a multidimensional phenomenon: implications for the assessment of disorders of consciousness

Abstract

Disorders of consciousness (DoCs) pose a significant clinical and ethical challenge because they allow for complex forms of conscious experience in patients where intentional behaviour and communication are highly limited or non-existent. There is a pressing need for brain-based assessments that can precisely and accurately characterize the conscious state of individual DoC patients. There has been an ongoing research effort to develop neural measures of consciousness. However, these measures are challenging to validate not only due to our lack of ground truth about consciousness in many DoC patients but also because there is an open ontological question about consciousness. There is a growing, well-supported view that consciousness is a multidimensional phenomenon that cannot be fully described in terms of the theoretical construct of hierarchical, easily ordered conscious levels. The multidimensional view of consciousness challenges the utility of levels-based neural measures in the context of DoC assessment. To examine how these measures may map onto consciousness as a multidimensional phenomenon, this article will investigate a range of studies where they have been applied in states other than DoC and where more is known about conscious experience. This comparative evidence suggests that measures of conscious level are more sensitive to some dimensions of consciousness than others and cannot be assumed to provide a straightforward hierarchical characterization of conscious states. Elevated levels of brain complexity, for example, are associated with conscious states characterized by a high degree of sensory richness and minimal attentional constraints, but are suboptimal for goal-directed behaviour and external responsiveness. Overall, this comparative analysis indicates that there are currently limitations to the use of these measures as tools to evaluate consciousness as a multidimensional phenomenon and that the relationship between these neural signatures and phenomenology requires closer scrutiny.

Functional geometry of the cortex encodes dimensions of consciousness

“…This interpretation supports a multidimensional definition of consciousness, where global states (as distinct from local, content-involving states) of consciousness can be conceived of as regions within a multidimensional space 5 . Notably, our work represents a conceptual departure from cognitive-behavioral definitions of consciousness, such as a traditional two-dimensional representation (awareness vs. wakefulness) 1 , or multiple cognitive representations such as attention, memory, sensory processing, executive function, and meta-awareness 5 – 8 . We propose that a given state of consciousness can be represented, independently of cognitive-behavioral description, by a family of cortical gradients derived from the principled characterization of the brain’s functional geometry.…”

“…This two-dimensional scheme was recently extended by the inclusion of a behavioral dimension (i.e., ability to produce motor responses) 2 , allowing for a better characterization of cognitive-motor dissociation 3 or covert consciousness 4 , which occurs in certain behaviorally non-responsive patients with neuropathological disorders. Although multidimensional representations of consciousness have been proposed from a theoretical perspective 5 – 8 , an important knowledge gap has been the identification of what these dimensions are in neural terms and how they are associated with the brain’s neurofunctional properties.…”

Are we really unconscious in “unconscious” states? Common assumptions revisited

“…Descriptive approach: Extending the pragmatic approach, we can systematically describe states and conditions functionally, behaviorally, cognitively, and physiologically (e.g., as in Sanders et al, 2012 ), to obtain a more complete and nuanced picture of the states, while remaining agnostic as to whether consciousness is truly present/absent (in the Nagelian sense) (see also Bayne et al, 2016 , 2020 ; Engemann et al, 2018 ; Walter, 2021 ). For example, a moderate level of propofol anesthesia could be characterized as a state of partial responsiveness and partially connected awareness (IFT shows command following), partial/occasional recall (it is possible to obtain some dream reports), impaired explicit memory formation (ability to report dreams quickly declines after awakening), and partial capacity for communication (through IFT).…”

Towards modern post-coma care based on neuroscientific evidence

“…Initially based on bedside clinical signs and behaviors that would denote the presence or absence of consciousness, this taxonomy is starting to include measures derived from neurophysiology and neuroimaging ( Edlow et al, 2017 ; Schiff, 2015 ; Thibaut et al, 2021 ). Despite these technological advances in our understanding of consciousness and its impairments, there is still an active debate on the clinical signs warranting the presence of conscious awareness or the minimal brain activity requirements to maintain consciousness ( Walter, 2021 ). The ever-changing state of the science and the unresolved conceptual debates on neurophysiological substrates pose major challenges to researchers and clinicians alike ( Giacino, Fins, Laureys, & Schiff, 2014 ).…”