Visuocortical changes during a freezing-like state in humans.

Area of Science:

• Neuroscience
• Cognitive Psychology
• Sensory Processing

Background:

• Threat detection is crucial for survival and involves optimized sensory cue processing.
• Freezing is an evolutionarily conserved defensive state characterized by immobility, fear bradycardia, and amygdala-periaqueductal grey (PAG) circuit regulation.
• Behavioral evidence suggests freezing enhances visual sensitivity, particularly for coarse visual information, but neural mechanisms are not well understood.

Purpose of the Study:

• To investigate the neural mechanisms underlying enhanced visual sensitivity during a freezing-like state.
• To examine threat-related changes in visuocortical activity and its relationship with fear bradycardia and amygdala-PAG connectivity.

Main Methods:

• A freezing-like state was induced in healthy volunteers using threat of electrical shock.
• Functional MRI (fMRI) measured stimulus-independent (baseline) and stimulus-evoked visuocortical activity to varying spatial frequencies.
• Fear bradycardia and amygdala-PAG connectivity were used to infer the freezing-like state due to immobility measurement limitations in MRI.

Main Results:

• Threat induced a nonspecific increase in baseline visuocortical activity, correlated with amygdala connectivity.
• Increased amygdala-PAG connectivity and a correlation between visuocortical activity and fear bradycardia confirmed the freezing-like state.
• While stimulus-evoked responses were retinotopically specific, individuals with better low-spatial frequency discrimination showed reduced V1 responses under threat.

Conclusions:

• The defensive state of freezing integrates preparatory defensive and perceptual changes.
• A common regulatory mechanism involving the amygdala likely underlies both defensive responses and sensory processing optimization during threat.