Table 3 Indicators of consciousness and respective ethical implications in disorders of consciousness

Goal-directed behavior (GDB) and model-based learning

GDB is a behaviour aimed at achieving specific goals on the basis of two essential conditions: the ability to represent the consequences of subjective actions; the knowledge that those particular actions are instrumental for achieving desired goals.

Model-based learning is defined as the capacity for an internal model of the subjective spatiotemporal condition, including particular connections between automatic and voluntary behaviours and their outcome

GDB and model-based learning are indicative of the ability to have conscious interests, to recognize the relevance of external inputs to fulfill those interests, and to act on the basis of those inputs for fulfilling interests. Checking for their presence is ethically required, possibly through a multimodal assessment

Brain anatomy and physiology

In mammals, consciousness depends on the structural and functional integrity of specific anatomic structures in the brain with a characteristic physiology. Similar brain structures indicate the presence of consciousness

In the case of DoCs, structural and functional damages of thalamocortical systems impair consciousness. The main ethical issue is whether consciousness is switched off or is still present to a limited degree. Caution in inferring absence of consciousness from brain damage is recommended

Psychometrics and meta-cognitive judgment

Psychometric curves for stimulus detection and discrimination, as well as the ability of some meta-cognitive judgments on perceived stimuli, show similarities between humans and some animals (e.g., monkeys, rodents, and birds). The same is arguably true for patients with DoCs: if behavioural or instrumental tests show psychometric curves similar to healthy subjects, consciousness might be inferred

Stimulus detection and relative meta-cognitive judgment are evidence of retained awareness in DoCs. Exploring strategies for detecting them, both through behavioural tests and instrumental decoding of cortical activity, is ethically required

Episodic memory

This type of memory is defined as autobiographical or narrative memory, i.e. memory of facts (‘‘what’’) that are spatiotemporally specified, i.e. experienced at a particular place (‘‘where’’) and time (‘‘when’’)

Exploring strategies for assessing relevant neuronal structures, looking for correlates of episodic recall (e.g., relevant hippocampal regions), is ethically required

Acting out one’s subjective, situational survey: illusion and multistable perception

Studies with non-human primates and cats showed their susceptibility to illusions and perceptual ambiguity, and there are evidences for rodents and birds as well

Exploring possible strategies for assessing relevant neuronal structures is ethically required. If the patient is behaviourally unresponsive, autonomous indicators (e.g., electrodermal activity, galvanic skin response, pupils) or stimuli reactions in relevant brain regions might be considered

Acting out one’s subjective, situational survey: visuospatial behaviour

Conscious subjects perceive objects as stably situated, even when they explore their environment with their gaze

Its relevance to patients with DoCs seems limited (if at all relevant) unless we look for replacement measures: no overt behaviour but decoding of internal brain activity indicating the use of one’s body map and the planning of a spatial response