Mechanisms of Consciousness

Defining Consciousness

• There is no singular scientific definition of consciousness, which makes it challenging to study. Some key features often used in describing consciousness are awareness, subjectivity, and temporal continuity.
• The “hard problem” is a term coined by David Chalmers that refers to the difficulty of explaining how neural mechanisms and neuronal connectivity give rise to the subjective experience of consciousness.¹
  Essentially, there is a knowledge gap between neuroanatomical structures, pathways, and networks and the routine experiences of daily human life.

Consciousness: Wakefulness and Awareness

• The two key components of consciousness are wakefulness and awareness. They are essential components of full consciousness; if either is lacking, consciousness declines.

• Both awareness and wakefulness arise from distinct yet interconnected neural networks.
• Awareness (sometimes described as the content of consciousness) is highly dependent on the brain’s sensory regions, particularly the posterior cerebral cortex.
• Awareness is derived from horizontal connectivity and relies on frontoparietal communication.
• The degree of wakefulness is controlled by vertical connectivity from the brainstem through the hypothalamus to cortical structures.
• Initially, the reticular activating system in the brainstem was identified as a key brain region that supports wakefulness (or arousal).
• There is now significant evidence for extrareticular areas in the brainstem, hypothalamus, and thalamus that promote arousal and are collectively termed the AAN.
• Numerous subcortical structures believed to modulate arousal and signal to the cortex, promoting wakefulness, are currently under study. When damaged, these structures can result in a comatose patient.
• Additionally, specific neurotransmitters that modulate arousal have been identified.³

NCC

• NCC are the minimum set of neural mechanisms necessary for consciousness.
• NCC can be further broken down into content-specific NCC and full NCC. Content-specific NCC are the neural mechanisms underlying particular phenomenal contents, such as colors, individuals, locations, or thoughts. Full NCC is defined as neural mechanisms required for overall cognition, encompassing all content-specific NCC.⁴
• To identify the NCC, investigators have compared brain activity in fully conscious subjects with that in subjects with reduced consciousness, such as during sleep, anesthesia, or in individuals with disorders of consciousness.
• To address the complexity of consciousness, numerous theoretical frameworks have been proposed to explain and study it.
• Some of the more common theoretical frameworks include global workspace theory (GWT)-widespread neural interactions work in concert to create consciousness. A single area is not solely responsible for consciousness.⁵
• Integrated information theory (IIT) posits that neural systems integrate information, and the greater this integration, the higher the level of consciousness.
• Both the GWT and IIT rely on the existence of large neural networks throughout the brain.

Key Brain Regions and Networks for Consciousness

• Although numerous theories address the neural components of consciousness, investigators have identified regions central to consciousness. They include the following:
• Thalamocortical systems: The thalamus and its projections to the cortex play a broad role in both awareness and wakefulness. Thalamic projections can be classified into core and matrix neurons, which relay information to the cortex. Core cortical loops are believed to maintain consciousness and assist with the consistency of perception, whereas matrix cortical loops support wakefulness and the threshold for perception.⁶
• Posterior cortical areas (occipital, parietal, and temporal lobes)- also defined as “posterior hot zones.” When activity is suppressed, consciousness fades even if arousal systems remain active. The posterior cortex is believed to be responsible for maintaining consciousness.⁷
• Prefrontal cortex: This area is thought to be responsible for higher-order awareness, meta-cognition, and executive functioning. Authors debate whether this area is absolutely necessary for consciousness or enhances it. Damage to the prefrontal cortex causes higher-order dysfunction and social disturbances.⁸
• AAN: Structures within the brainstem, basal forebrain, and diencephalic areas, which, when activated, increase cortical activity and alertness. The suprachiasmatic nucleus controls circadian rhythm. Wake-promoting circuits also regulate this cycle and are located in the lateral hypothalamus, pedunculopontine and laterodorsal tegmental nuclei, dorsal/median raphe nuclei, and ventral tegmental nuclei.⁹

Consciousness and Anesthetic Agents

• Anesthesia is, by definition, a pharmacological induction of loss of consciousness. It is reversible and distinct from natural sleep or a coma. The electroencephalogram pattern of patients under general anesthesia (progressive increase in low-frequency, high-amplitude activity) suggests widespread suppression of cortical activity. It is similar to that of a comatose patient.¹⁰
• Different anesthetic agents induce loss of consciousness through distinct perturbations of neurobiological connectivity that underlie consciousness. Each agent targets specific neural circuits, and either enhance inhibitory pathways or suppresses arousal systems, leading to unconsciousness.¹⁰
• Historically, anesthetic agents were thought to act by globally depressing cortical function. More recently, specific nuclei have been identified as targets of anesthetic agents that induce unconsciousness.
• The mesopontine tegmental anesthesia area (MPTA) is a small brainstem nucleus that has been identified as a “gatekeeper” mechanism, such that when exposed to very small doses of GABAergic agents, the subject will be in a state of surgical anesthesia, including akinesia, analgesia, amnesia, and loss of consciousness.¹¹
• The current model suggests that within the MPTA, inhibitory interneurons tonically inhibit effector neurons at the ascending reticular activating system, thereby maintaining wakefulness (a so-called “brake”). Once the MPTA is exposed to GABAergic anesthetics, the brakes are released, and the effector neurons are no longer inhibited. Their activation results in widespread functional cortical suppression, and loss of consciousness occurs.¹²