During fear conditioning, a cue (CS) signals an inevitable distal threat (US) and evokes a conditioned response that can be described as attentive immobility (freezing). The organism remains motionless and monitors the source of danger while startle responses are potentiated, indicating a state of defensive hypervigilance. Although in animals vagally mediated fear bradycardia is also reliably observed under such circumstances, results are mixed in human fear conditioning. Using a single-cue fear conditioning and extinction protocol, we tested cardiac reactivity and startle potentiation indexing low-level defensive strategies in a fear-conditioned (n = 40; paired presentations of CS and US) compared with a non-conditioned control group (n = 40; unpaired presentations of CS and US). Additionally, we assessed shock expectancy ratings on a trial-by-trial basis indexing declarative knowledge of the previous contingencies. Half of each group underwent extinction under sham or active transcutaneous vagus nerve stimulation (tVNS), serving as additional proof of concept. We found stronger cardiac deceleration during CS presentation in the fear learning relative to the control group. This learned fear bradycardia was positively correlated with conditioned startle potentiation but not with declarative knowledge of CS-US contingencies. TVNS abolished differences in heart rate changes between both groups and removed the significant correlation between late cardiac deceleration and startle potentiation in the fear learning group. Results suggest, fear-conditioned cues evoke attentive immobility in humans, characterized by cardiac deceleration and startle potentiation. Such defensive response pattern is elicited by cues predicting inevitable distal threat and resembles conditioned fear responses observed in rodents.

中文翻译：

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面对不可避免的远端威胁时注意不动——惊吓增强和恐惧心动过缓作为情绪和注意力的指标

在恐惧条件反射期间，提示 (CS) 发出不可避免的远端威胁 (US) 信号，并引发条件反应，可描述为注意力不活动（冻结）。有机体保持静止并监视危险源，同时加强惊吓反应，表明处于防御性过度警戒状态。虽然在这种情况下也可靠地观察到动物迷走神经介导的恐惧心动过缓，但结果在人类恐惧条件反射中是混合的。使用单提示恐惧条件反射和消退方案，我们在恐惧条件（n  = 40；CS 和 US 的配对呈现）与非条件对照组（n = 40; CS 和 US 的未配对演示文稿）。此外，我们在逐次试验的基础上评估了冲击预期评级，并索引了先前意外事件的声明性知识。每组中有一半在假或主动经皮迷走神经刺激 (tVNS) 下经历了消退，作为额外的概念证明。我们发现，相对于对照组，在恐惧学习中的 CS 演示期间心脏减速更强。这种习得性恐惧心动过缓与条件性惊吓增强呈正相关，但与 CS-US 突发事件的陈述性知识无关。TVNS 消除了两组之间心率变化的差异，并消除了恐惧学习组中晚期心脏减速和惊吓增强之间的显着相关性。结果表明，以恐惧为条件的线索会引起人类注意力不集中，其特征是心脏减速和惊吓增强。这种防御反应模式是由预测不可避免的远端威胁的线索引发的，类似于在啮齿动物中观察到的条件性恐惧反应。