Abstract
Upon increasing levels of threat, animals activate qualitatively different defensive modes, including freezing and active fight-or-flight reactions. Whereas freezing is a form of behavioural inhibition accompanied by parasympathetically dominated heart rate deceleration, fight-or-flight reactions are associated with sympathetically driven heart rate acceleration. Despite the potential relevance of freezing for human stress-coping, its phenomenology and neurobiological underpinnings remain largely unexplored in humans. Studies in rodents have shown that freezing depends on amygdala projections to the brainstem (periaqueductal grey). Recent neuroimaging studies in humans have indicated that similar brain regions may be involved in human freezing. In addition, flexibly shifting between freezing and active defensive modes is critical for adequate stress-coping and relies on fronto-amygdala connections. This review paper presents a model detailing these neural mechanisms involved in freezing and the shift to fight-or-flight action. Freezing is not a passive state but rather a parasympathetic brake on the motor system, relevant to perception and action preparation. Study of these defensive responses in humans may advance insights into human stress-related psychopathologies characterized by rigidity in behavioural stress reactions. The paper therefore concludes with a research agenda to stimulate translational animal–human research in this emerging field of human defensive stress responses.This article is part of the themed issue ‘Movement suppression: brain mechanisms for stopping and stillness’.
Highlights
Imagine you are standing in your office and all of a sudden a man walks in and attacks you with a knife
What neural mechanisms support flexible shifting between passive ‘freezing’ and active ‘fight-or-flight’ modes? Do we see individual differences in these automatic action tendencies and might it be possible to influence or ‘train’ them? Before addressing these questions, I first describe the phenomenology of freezing and fight-or-flight reactions as well as the psychophysiological and neural mechanisms associated with these threat-related defensive states
In a visual discrimination task, Lojowska et al [11] observed that fear bradycardia induced by threat of shock was associated with improved detection of low-spatial frequency (LSF) cues, at the expense of high spatial frequency detection
Summary
Imagine you are standing in your office and all of a sudden a man walks in and attacks you with a knife. Most people tend to fall back on primary ‘freeze–fight –flight’ tendencies and have great difficulty controlling their actions or shifting flexibly between passive freezing and active fight-or-flight. Insight into how these defensive reactions are controlled in the brain is relevant for individuals in high-risk professions who have to perform optimally under stress. This article reviews recent insights into the phenomenology of threat-induced freezing in humans and animals It focuses on how we can control automatic defensive threat reactions. I first describe the phenomenology of freezing and fight-or-flight reactions as well as the psychophysiological and neural mechanisms associated with these threat-related defensive states. Threat continuation of behaviour false alarm orienting fight/flight freezing no success tonic immobility no success
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Schedule a callMore From: Philosophical Transactions of the Royal Society B: Biological Sciences
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