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.

The major brain networks of human visual consciousness

“…The later events along this timeline, especially for TPN, merge into report-dependent, post-perceptual processing needed for subsequent report. Overall our present results agree with recent work suggesting that visual perception begins with early signal detection 30 followed by a pulse of arousal, attentional and limbic signal amplification 36 leading to network switching and a wave of processing to form conscious experiences 4,9 .…”

“…Our identification of specific reportindependent subcortical and cortical systems overlapping in space and time is an important step forward towards understanding the neural mechanisms of consciousness. These systems can be placed along a proposed timeline consistent with the three major networks found in our data 4 .…”

“…Our results identify subcortical and cortical networks designed for signal detection, attentional amplification, and perceptual processing that together can explain visual consciousness. These findings provide evidence that understanding consciousness can be reframed as requiring multiple overlapping brain networks to produce consciousness of visual events 4 .…”

“…Specifically, conscious awareness of discrete events lies at the temporal nexus of attention and memory, both major fields in modern neuroscience that contribute to understanding consciousness. We hypothesize that systems crucial for consciousness include: (1) attention mechanism mediating signal detection, dynamic modulation of arousal, and bottom-up plus top-down attentional control, overlapping in space and time with; (2) systems that limit competing signals (e.g., through reduced default mode network activity); and, finally, (3) hierarchically organized systems that fully process signals for memory encoding and subsequent report 4 .…”

Disruption in structural–functional network repertoire and time-resolved subcortical fronto-temporoparietal connectivity in disorders of consciousness

“…Our results are in agreement with several hypothesis and theories for the emergence of consciousness, of which most share the importance of thalamo-cortical connectivity for consciousness ( Blumenfeld, 2021 ; Dehaene et al, 2011 ; Schiff, 2010 ). The mesocircuit hypothesis states that deafferentation between the frontal cortex and subcortical regions is crucial in explaining the failure of recovery of consciousness ( Schiff, 2010 ).…”