Importance, limits and caveats of the use of "disorders of consciousness" to theorize consciousness

Abstract

The clinical and fundamental exploration of patients suffering from disorders of consciousness (DoC) is commonly used by researchers both to test some of their key theoretical predictions and to serve as a unique source of empirical knowledge about possible dissociations between consciousness and cognitive and/or neural processes. For instance, the existence of states of vigilance free of any self-reportable subjective experience [e.g. "vegetative state (VS)" and "complex partial epileptic seizure"] originated from DoC and acted as a cornerstone for all theories by dissociating two concepts that were commonly equated and confused: vigilance and conscious state. In the present article, we first expose briefly the major achievements in the exploration and understanding of DoC. We then propose a synthetic taxonomy of DoC, and we finally highlight some current limits, caveats and questions that have to be addressed when using DoC to theorize consciousness. In particular, we show (i) that a purely behavioral approach of DoC is insufficient to characterize the conscious state of patients; (ii) that the comparison between patients in a minimally conscious state (MCS) and patients in a VS [also coined as unresponsive wakefulness syndrome (UWS)] does not correspond to a pure and minimal contrast between unconscious and conscious states and (iii) we emphasize, in the light of original resting-state positron emission tomography data, that behavioral MCS captures an important but misnamed clinical condition that rather corresponds to a cortically mediated state and that MCS does not necessarily imply the preservation of a conscious state.

Keywords: disorders of consciousness; electroencephalography; minimally conscious state; positron emission tomography; vegetative state.

Figure 1.

FDG-PET regional discrimination performance

Figure 2.

Metabolic correlates of CRS-R MCS items

Figure 3.

Schematic timeline of the syndromic taxonomy of disorders of consciousness

Figure 4.

Comparison of the polymorph terminology of disorder of consciousness