Consciousness: forms, functions and properties

In modern neuroscience, it's more convenient to describe consciousness not as a "thing," but as a set of processes that create subjective experience and make information accessible for flexible behavior: self-awareness, understanding of events, purposeful choice, and communication. This definition immediately distinguishes consciousness from simple automatisms that can occur without explicit experience or control. [1]

In clinical practice, consciousness is most often assessed by two relatively independent components: the level of wakefulness (the extent to which the ability to maintain "alertness," reactivity, and sleep-wake cycles is preserved) and awareness (the presence of meaningful experiences and signs of understanding oneself or the environment). This framework explains why "open eyes" do not necessarily prove awareness and why "drowsiness" does not always equate to "unconsciousness." [2]

It is important to distinguish "no external responses" from "no consciousness." Locked-in syndrome and some forms of cognitive-motor dissociation demonstrate that awareness can be maintained with extremely limited motor skills and communication, and diagnostic errors are particularly dangerous here. [3]

Consciousness is also conveniently viewed as a multidimensional phenomenon: in addition to level and content, we distinguish the degree of self-reference, attentional control, coherence of experience, the ability to report, and the vividness of images. This perspective helps explain why dreams, delirium, and the effects of certain psychoactive substances provide an "experience" but with impaired logic, memory, and coherence. [4]

Table 1. 2 clinical axes of consciousness and typical examples

Component	What does it mean?	An example where the component is saved	An example where the component is reduced
Level of wakefulness	General alertness and ability to maintain wakefulness	Normal wakefulness, slight drowsiness	Coma, deep sedation, heavy anesthesia
Mindfulness	The presence of meaningful experience and signs of understanding	Locked-in syndrome, lucid wakefulness	Unresponsive wakefulness syndrome, deep coma

[5]

Neurobiology of consciousness: what maintains level and what forms "content"

Awakening is largely maintained by structures in the brainstem and basal forebrain, which regulate cortical activation and switch the brain between states of sleep, wakefulness, and sedation. Damage to these systems can cause consciousness to "switch off" even with relatively intact cortex. [6]

Awareness and the content of experience are more strongly linked to the interactions between the cerebral cortex and thalamus, as well as large networks that integrate sensory information, memory, attention, and self-reference. Research on consciousness has focused on the interactions between the fronto-parietal attentional systems and the so-called default mode network, which is associated with inner speech, autobiographical memory, and self-image. [7]

Transitions between states of consciousness are often accompanied by changes in functional connectivity, that is, how well distributed areas of the brain function together. General anesthesia, heavy sedation, and certain brain injuries all exhibit characteristic changes in activity and connectivity, which can be assessed using electroencephalography and neuroimaging. [8]

A separate important idea: consciousness does not have to be "all or nothing." In some conditions, fragmented processing of stimuli, isolated islands of perception, or "hidden awareness" may persist, which is more difficult to detect behaviorally but can sometimes be detected instrumentally. This is why current guidelines emphasize the need for standardized assessment and repeated examinations. [9]

Table 2. Key neural systems associated with consciousness

System	The main role	What happens if there is a violation?
Stem cell activation systems	Supports wakefulness and reactivity	Drowsiness, stupor, coma
Thalamocortical circuits	"Transmission and coordination" of information between the cortex and subcortex	Decreased awareness, dissociation of processes
Fronto-parietal attention networks	Access to information for monitoring and reporting	Disorganization of attention, control errors
Passive mode network	Self-reference, inner speech, autobiographical memory	Disruption of self-image, “loss of internal coherence”

[10]

Normal states of consciousness: waking, sleep and dreaming

In waking consciousness, consciousness is usually most stable: the external environment is processed quickly, attention can shift, and speech and goal-directed behavior are available. Even in waking consciousness, some actions can be performed automatically, especially with familiar routes and skills, but if necessary, the ability to regain control and consciously modify an action is usually retained. [11]

Sleep is not a complete "shutdown of consciousness." Sleep includes stages of slow-wave sleep and rapid eye movement (REM) sleep, and the transitions between them are strictly regulated by neural circuits. Clinical sleep staging rules are standardized because sleep structure is associated with recovery, memory, and the risk of sleep-disordered breathing. [12]

Dreaming is not limited to REM sleep: reports of dreamlike experiences also occur in non-REM sleep, although the phenomenology often differs. Modern reviews emphasize that the "REM sleep equals dreaming" connection is oversimplified, and that the role of REM sleep in mental well-being is more complex and cannot be reduced to a single function. [13]

Lucid dreaming is interesting because it introduces an element of meta-consciousness within the dream: the awareness that it is a dream and partial control. Recent reviews describe neurophysiological markers and possible clinical potential for nightmares, but emphasize that induction methods and therapeutic effects depend on the protocol and individual characteristics. [14]

Table 3. Sleep states and what the clinic usually sees

State	What is typical	What does polysomnography see most often?
N1	Transition from wakefulness to sleep	Slowing down of activity, easy awakenings
N2	The bulk of sleep in adults	Sleep spindles and K complexes
N3	Deep slow wave sleep	High-amplitude slow waves
REM	REM sleep	Rapid activity, muscle atonia, rapid eye movements

[15]

Altered and impaired states: from delirium to coma and anesthesia

Delirium is a common cause of "altered consciousness" in hospital: the condition develops acutely, its severity fluctuates, attention and orientation are impaired, and perception and thinking are often impaired. For diagnosis, it is important to remember that delirium can be either excited or "quiet," and it is the latter that is often overlooked. [16]

Intoxication and drug effects can reduce the level of wakefulness or alter the content of experience, ranging from lethargy to hallucinations. Clinically, the key difference from "psychological trances" is that there is usually a physiological cause, risks to respiration and hemodynamics, and management requires an urgent assessment of vital functions. [17]

General anesthesia and deep sedation create a controlled decline in consciousness. Neurophysiological markers on electroencephalography (EEG) are important for the physician: typical patterns of sedation, and, in cases of extreme sedation, the "burst suppression" phenomenon, which occurs both in hypothermia and in severe brain injury. Recent reviews emphasize that EEG interpretation is becoming an important part of safe patient management. [18]

Coma is a state in which wakefulness and awareness are absent: there are no sleep-wake cycles, no purposeful responses to stimuli. Various trajectories are possible: recovery, transition to unresponsive wakefulness syndrome (previously called a vegetative state), or transition to minimal consciousness. Guidelines emphasize that accurate classification requires standardized scales, repeated examinations, and caution in prognosis. [19]

Locked-in syndrome is fundamentally different from coma and disorders of consciousness: consciousness and cognitive functions may be preserved, but communication is severely limited due to paralysis. Therefore, an assessment of consciousness always requires a section on possible motor limitations and consideration of communication technologies, including brain-computer interfaces for certain patients. [20]

Table 4. Main clinical conditions with impaired consciousness

State	Level of wakefulness	Mindfulness	Key clinical sign
Delirium	Often preserved but unstable	Distorted	Fluctuations, impaired attention
Coma	Sharply reduced	Absent	No sleep-wake cycles
Unresponsive wakefulness syndrome	Partially restored	No signs	There are cycles, no targeted responses
Minimal consciousness	Partially saved	Minimal signs	Inconstant but reproducible traits
Locked-in syndrome	Saved	Saved	Paralysis with preserved consciousness

[21]

How consciousness is measured in medicine

The most common initial assessment tool for acute loss of consciousness is the Glasgow Coma Scale. It assesses eye opening, speech, and motor responses and helps standardize the description of dynamics, but it does not replace a full neurological examination and does not always adequately distinguish complex conditions with limited speech or movement. [22]

For prolonged disorders of consciousness, the Coma Recovery Scale-Revised is considered the gold standard for behavioral assessment because it clearly differentiates reflexive and cognitively mediated responses across multiple modalities. In recent years, more rapid screening tools have been developed and validated to reduce the risk of missing minimal signs of consciousness in real-world clinical settings. [23]

Instrumental methods are used when behavior does not provide a reliable response or there is a suspicion of "hidden awareness." Electroencephalography assesses rhythms, reactivity, signs of sedation, and deep depression of activity; positron emission tomography and functional magnetic resonance imaging can demonstrate the preservation of metabolism and the task response network in individual cases. [24]

A key practical lesson from guidelines on disorders of consciousness is that errors are more likely to arise from a single examination and misinterpretation of motor limitations than from "poor scales." Therefore, repeated assessments, standardized protocols, a multidisciplinary approach, and careful discussion of prognosis with the family are recommended. [25]

Table 5. Tools for assessing consciousness and their place

Tool	What does it measure?	Where it is especially useful	Restrictions
Glasgow Coma Scale	Basic reaction level	Emergency care, trauma	Dependent on speech and movements
Coma Recovery Scale, Revised	Fine behavioral diagnostics	Long-term disorders of consciousness	Requires training, takes time
Simplified assessment of disorders of consciousness	Rapid screening	High-traffic areas	Does not replace the full protocol
Electroencephalography	Rhythms, reactivity, depth of sedation	Sedation, coma, monitoring	Interpretation requires experience
Positron emission tomography and functional magnetic resonance imaging	Metabolism and response networks	Complex cases, "hidden awareness"	Availability and indications are limited

[26]

Theories of consciousness: why they are needed and what new tests have shown

Theories of consciousness are useful not only for philosophy: they provide predictions about which structures and types of connectivity should be critical for experience, and thus help design research on sleep, anesthesia, and disorders of consciousness. Among the most discussed models are the global neural workspace and integrated information theory, as well as competing approaches, including recurrent processing theories. [27]

The global neural workspace emphasizes the broad "dissemination" of information across frontoparietal systems, making content available for reporting, memory, and control. Integrated information theory emphasizes the degree of causal integration of the network and often links critical substrates to posterior association areas, leading to different empirical expectations. [28]

In 2025, the results of coordinated tests of the predictions of competing theories were published, in which researchers compared the expected "signatures of consciousness" with neuroimaging and electroencephalography data in several laboratories. These studies did not "close the matter," but they showed that simple versions of the theories require refinement, and that consciousness is likely associated with the dynamic interaction of multiple networks, rather than a single point. [29]

In practice, this means two things. First, the assessment of consciousness must be multi-faceted, because a single indicator is rarely sufficient. Second, states of consciousness are best described in terms of specific functions and risks, rather than in vague categories of "higher" and "lower," because clinical tactics depend on the cause and reversibility of the condition. [30]

Table 6. Comparison of leading theories of consciousness in 1 table

Approach	The main idea	What predicts more often	Where is testing done specifically?
Global neural workspace	Consciousness is linked to global access to information	The importance of frontoparietal interactions	Attention, report, masking, anesthesia
Integrated information theory	Consciousness is associated with the integration of the causal structure	The Importance of Integration and Rear Hot Spots	Sleep, anesthesia, and network integration comparison
Recurrent processing	Consciousness arises from recurrent feedback loops in sensory systems.	The role of local processing cycles	Perception, visual paradigms, masking

[31]

When is an urgent medical evaluation needed for altered consciousness?

A sudden decrease in alertness, new marked confusion, seizures, respiratory distress, high fever with disorientation, suspicion of intoxication, and “most unusual behavior” require not waiting but urgent assessment of the causes, because some conditions are reversible only with rapid intervention. [32]