Table 1.

Proposed taxonomy of behaviorally unresponsive disorders of consciousness (DoC): salient conceptual, clinical, and research considerations.

Taxonomy	Clinical concept of consciousness	Exemplative neurotechnological indicator(s)	Clinical implications	Research implications
Covert cortical processing (CCP)	Intact association cortex responses to stimuli in the absence of behavioral evidence of stimulus processing (e.g. language comprehension) or command-following.	Stimulus-based fMRI response Stimulus-based EEG response	Patients diagnosed with coma, vegetative state/unresponsive wakefulness syndrome (VS/UWS), or low-level minimally conscious state (MCS-) by behavioral criteria could exhibit signs of association cortex responsiveness on stimulus-based testing that exceed what is implied by a behavioral diagnosis. Outcomes (prognosis) and capacity for experience (phenomenology) may differ from those who lack such responses. Prevailing behavior-based approaches to clinical diagnosis of DoC are inherently insensitive to CCP and should be interpreted accordingly. To enhance the reliability of clinical DoC assessment, CCP should be queried when possible, and surrogate decision-makers should be counseled about what is known and unknown about CCP.	Underexplored opportunities exist for further empirical studies of phenomenology and prognosis in CCP. Studies of unresponsive patients with DoC that do not include measures of CCP risk erroneous miscategorization or conflation of heterogeneous subpopulations.
Cognitive-motor dissociation (CMD)/covert consciousness	Intact volitional modulation of brain activity, despite absence of language function on routine bedside behavioral evaluation (Schiff 2015)	Task-based fMRI response Task-based EEG response	Patients who fail to respond to routine behavioral testing at the bedside may still harbor capacities to volitionally modulate brain activity or consciously respond only through advanced testing. Prevailing behavior-based approaches to clinical diagnosis and prognosis of DoC are inherently insensitive to CMD and should be interpreted accordingly. Patients diagnosed with low-level MCS (MCS-), VS or coma by prevailing behavioral criteria may exhibit signs of awareness that exceed their behavioral diagnosis. Generally more favorable prognosis for functional recovery as compared to behaviorally unresponsive patients without CMD (Egbebike et al. 2022, Claassen et al. 2019)	Further empirical studies of phenomenology, mechanisms, and correlates of CMD are needed. Studies of unresponsive patients with DoC that do not include measures of CMD risk erroneous miscategorization or conflation of heterogeneous subpopulations, including mislabeling patients who are aware and covertly responsive as unconscious.
Covert brain complexity (CBC)	Intact ability of a patient’s brain to sustain complex dynamics resembling those seen in conscious states despite a behavioral diagnosis of coma or VS/UWS, and absence of discernible responses to sensory-based stimuli.	Transcranial Magnetic Stimulation—Electroencephalography Perturbational Complexity Index (TMS-EEG PCI)	Some patients may demonstrate patterns of preserved brain complexity (i.e. high PCI value in response to TMS perturbation) but due to sensory, motor and/or cognitive impairments exhibit no other covert or overt signs of consciousness. In patients with known sensory, motor or cognitive impairments, tests that bypass afferent and efferent pathways and causally probe network connectivity implicated in human consciousness may prove more reliable (Edlow et al. 2023)	Further studies comparing diagnostic and prognostic value of complexity measures with a composite reference standard including behavioral, task-based, stimulus-based and resting-state measures are needed, and may help to illuminate optimal approaches to DoC classification and prediction. Whether the brain’s capacity to sustain complex dynamics as indexed by PCI is a necessary condition for CCP has yet to be empirically studied, but is a theoretically plausible hypothesis.