Summary: New evidence suggests the anterior insular cortex functions as a gate that helps determine which sensory signals reach conscious awareness.
Source: University of Michigan
Every moment we are awake, the brain receives a steady stream of sensory inputs of varying intensity. Neuroscientists have long asked why some of these inputs become part of our conscious experience while others remain unnoticed. What neural mechanism decides which signals pass into awareness?
A new study from the Department of Anesthesiology and the Center for Consciousness Science at Michigan Medicine identifies a cortical region that appears to act as this gate. The research implicates the anterior insular cortex (AIC) in regulating when sensory information gains access to conscious processing.
“Information processing in the brain happens on two levels: automatic sensory processing that occurs without awareness and higher-order processing that emerges when a stimulus reaches conscious significance,” says Zirui Huang, Ph.D., a research investigator in the Department of Anesthesiology. The team, led by Anthony Hudetz, Ph.D., set out to test whether the AIC functions as the switch between these two modes.
Researchers recruited healthy volunteers and used fMRI to monitor brain activity while participants were given carefully titrated doses of the anesthetic propofol to control their level of consciousness. Throughout induction and recovery, participants performed mental imagery tasks—imagining playing tennis, walking along a path, or squeezing their hand—and occasionally performed a real motor task, squeezing a rubber ball. Mental imagery was chosen because it reliably activates the same motor and sensory networks that actual movement engages, while avoiding confounds related to overt behavior.
As participants lost behavioral responsiveness under propofol, the typical pattern of brain activations associated with mental imagery changed. During wakefulness, imagining a movement produced clear activation in movement-related cortical areas while other networks deactivated as attention focused on the task. With deeper sedation, these task-related activations occurred less often and eventually disappeared when consciousness was lost. As participants recovered, the activation patterns returned in parallel with the restoration of awareness.
Across these transitions, the researchers found that activity in the anterior insular cortex correlated with whether brain regions switched between task-related activation and deactivation. When the AIC remained active, the brain supported the network dynamics that underlie conscious access. When the AIC was suppressed, those transitions were disrupted and conscious perception failed to occur.
Hudetz explains that a sensory input typically triggers activity in the anterior insula. When consciousness is suppressed, the AIC becomes deactivated and the broader network dynamics that sustain awareness break down. In this view, the AIC functions as a filter that permits only the most salient or timely information to penetrate into conscious experience.
To further probe the AIC’s role, the team used a classical perceptual test in awake volunteers. A face was flashed on a screen for a very brief interval—just three hundredths of a second—and immediately followed by a high-contrast masking image intended to interrupt conscious perception of the face. Participants reported whether they had seen a face. Whether the brief face reached consciousness correlated with the level of pre-stimulus activity in the anterior insula: higher AIC activity just before the flash made conscious detection more likely.
“Anterior insula activity fluctuates constantly,” Huang notes. “Whether a stimulus is perceived depends in part on the AIC’s momentary state when the information arrives. If that activity is elevated at the time of stimulus presentation, the image is more likely to enter awareness.” Taken together, the propofol and masking experiments support the conclusion that the anterior insular cortex acts as a gate for conscious access.
About this consciousness research news
Source: University of Michigan
Contact: Kelly Malcom – University of Michigan
Image: The image is in the public domain
Original Research: Open access. “Anterior insula regulates brain network transitions that gate conscious access” by Zirui Huang et al., published in Cell Reports.
Abstract
Anterior insula regulates brain network transitions that gate conscious access
Highlights
- • Dysfunction of the anterior insula during anesthesia disrupts brain network transitions that support consciousness
- •Pre-stimulus activity in the anterior insula predicts whether a visual stimulus will reach conscious awareness
- •Evidence supports the anterior insula acting as a gate for conscious access to sensory information
- • This cortical gate sits at an intermediate level within the brain’s hierarchical processing architecture
Summary
Conscious access to sensory input appears to be controlled at an intermediate cortical site between primary sensory areas and transmodal association cortices. Using functional neuroimaging and a volitional mental imagery paradigm that avoids motor confounds, the authors show that titrated propofol-induced loss of responsiveness coincides with dysfunction in the anterior insular cortex (AIC) and impaired transitions between the default-mode and dorsal attention networks. Candidate subcortical arousal or gating regions, including the thalamus and basal forebrain, did not show the same association. Complementary experiments in awake participants demonstrate that pre-stimulus AIC activity near perceptual threshold predicts conscious access. Altogether, these findings support the hypothesis that the anterior insula, positioned midway in the cortical hierarchy, regulates network transitions that gate conscious access to sensory information.