- Amygdala
- An almond-shaped subcortical structure in the medial temporal lobe, comprising several nuclei. The basolateral complex (BLA) receives sensory input and learns threat associations; the central nucleus (CeA) coordinates defensive responses via projections to the hypothalamus, brainstem, and autonomic nervous system. The amygdala is necessary for the acquisition and expression of conditioned fear in animal models.
- Two-route model (LeDoux)
- Joseph LeDoux proposed two pathways for threat processing. The "low road" (thalamus → amygdala) is fast and unconscious — a coarse sensory representation triggers a rapid defensive response before full cortical processing. The "high road" (thalamus → cortex → amygdala) is slower but allows more nuanced evaluation. The low road explains why we flinch at a stick before realising it's not a snake.
- Fear conditioning
- A form of Pavlovian conditioning in which a neutral conditioned stimulus (CS, e.g. a tone) is paired with an aversive unconditioned stimulus (US, e.g. a shock), causing the CS alone to elicit a conditioned fear response (CR, e.g. freezing, increased heart rate). Fear conditioning in rodents requires the BLA; extinction reduces but does not erase the original association.
- Extinction
- The reduction of a conditioned fear response through repeated presentation of the CS without the US. Extinction is not erasure — it is new inhibitory learning that competes with the original fear association. This is why extinguished fears can return through spontaneous recovery, reinstatement, or renewal when the context changes — a finding with direct implications for exposure-based treatments.
- Preparedness theory
- Martin Seligman's 1971 proposal that humans are evolutionarily prepared to acquire fears of ancestrally relevant threats (snakes, spiders, social rejection, heights) but not of modern dangers. Prepared fears are acquired faster, are more resistant to extinction, and may develop without explicit conscious pairing — providing a biological account of common phobia patterns.
- Freeze-flight-fight response
- The sequence of defensive reactions to threat: initial freezing (reducing detection by the predator), flight (escape if possible), and fight (if escape is blocked). Controlled by the periaqueductal grey (PAG) in the brainstem, with amygdala input. Freezing also appears in anxiety — the behavioural inhibition system (Gray) suppresses ongoing activity in the face of signals of punishment or non-reward.
- Fear generalisation
- The spread of conditioned fear from the original CS to stimuli that resemble it — a gradient effect in which more similar stimuli elicit stronger responses. Excessive generalisation — where the person responds fearfully to stimuli bearing only superficial resemblance to the original threat — is a core feature of PTSD and phobia.
- Subjective fear vs defensive behaviour
- LeDoux's later work distinguishes between the amygdala's role in coordinating defensive behaviour (freezing, avoidance, physiological arousal) and the subjective feeling of being afraid. He argues the amygdala mediates the former, while subjective fear is a cortical construct. This challenges simple equation of amygdala activation with "feeling afraid."