Brain Mechanisms of Conscious Awareness: Detect, Pulse, Switch, and Wave

Abstract

Consciousness is a fascinating field of neuroscience research where questions often outnumber the answers. We advocate an open and optimistic approach where converging mechanisms in neuroscience may eventually provide a satisfactory understanding of consciousness. We first review several characteristics of conscious neural activity, including the involvement of dedicated systems for content and levels of consciousness, the distinction and overlap of mechanisms contributing to conscious states and conscious awareness of transient events, nonlinear transitions and involvement of large-scale networks, and finally the temporal nexus where conscious awareness of discrete events occurs when mechanisms of attention and memory meet. These considerations and recent new experimental findings lead us to propose an inclusive hypothesis involving four phases initiated shortly after an external sensory stimulus: (1) Detect-primary and higher cortical and subcortical circuits detect the stimulus and select it for conscious perception. (2) Pulse-a transient and massive neuromodulatory surge in subcortical-cortical arousal and salience networks amplifies signals enabling conscious perception to proceed. (3) Switch-networks that may interfere with conscious processing are switched off. (4) Wave-sequential processing through hierarchical lower to higher cortical regions produces a fully formed percept, encoded in frontoparietal working memory and medial temporal episodic memory systems for subsequent report of experience. The framework hypothesized here is intended to be nonexclusive and encourages the addition of other mechanisms with further progress. Ultimately, just as many mechanisms in biology together distinguish living from nonliving things, many mechanisms in neuroscience synergistically may separate conscious from nonconscious neural activity.

Keywords: attention; consciousness; mechanisms; memory; subcortical; thalamus.

Figure 1.

Brain systems for content and level of consciousness. The content or substrate of consciousness is provided by hierarchically organized sensory, motor systems, and cognitive systems, and by brain systems for memory, emotions, and drives. The level of consciousness acts on all of these different modules and is regulated by the consciousness system (see Fig. 2). Reproduced with permission from Blumenfeld (2022).

Figure 2.

The consciousness system. Anatomical structures involved in regulating the level of alertness, attention, and awareness. (A) Medial view showing cortical (blue) and subcortical (red) components of the consciousness system. (B) Lateral cortical components of the consciousness system. Note that other circuits not pictured here, such as the basal ganglia, nucleus accumbens, claustrum, and cerebellum, may also play a role in attention and other aspects of consciousness. Reproduced with permission from Blumenfeld (2022).

Figure 3.

The timeline of consciousness. An event such as a brief external stimulus is indicated by the vertical arrow at time zero. Neural precursors of consciousness are shown in blue, and consequences of consciousness in purple, while the neural mechanisms of the conscious event itself are shown in green. Reproduced with permission from Blumenfeld (2022).

Figure 4.

Detect, Pulse, Switch, and Wave hypothesis for consciousness. Sequence of neural mechanisms proposed to produce conscious awareness of events, using an external visual stimulus activating primary visual cortex as an example. Reproduced with permission from Blumenfeld (2022).