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

Consciousness is increasingly understood as a multidimensional phenomenon rather than a simple hierarchy of levels, which complicates the use of brain-based measures to assess patients with disorders of consciousness (DoC). This article reviews studies applying neural measures of conscious level in non-DoC states where conscious experience is better understood. The comparative evidence shows that these measures are more sensitive to some dimensions of consciousness than others and do not straightforwardly rank conscious states. For example, high brain complexity correlates with sensory richness and low attentional constraint but not with goal-directed behavior. The findings indicate current limitations in using such measures to evaluate consciousness as multidimensional and call for closer examination of the link between neural signatures and phenomenology.

Study at a glance

Abstract

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.