Questions: Fear Conditioning and Circuit Plasticity

Question 1

A patient with PTSD completes a 12-week exposure therapy program and shows no fear response to trauma-related stimuli at the end of treatment. Three months later, after a stressful period, the fear response fully returns. What does this clinical pattern demonstrate about fear extinction?

• A. The therapy erased the original fear memory, but stress triggered the formation of a new one
• B. Extinction created new inhibitory learning that suppressed the original fear trace; the fear memory remained intact and was reactivated by stress
• C. The patient's amygdala regenerated the damaged circuits that originally stored the fear memory
• D. Stress hormones directly activated fear circuits without requiring a stored fear memory

Answer: (B)

Spontaneous return of extinguished fear after stress is one of the key pieces of evidence that extinction does not erase the original memory — it creates competing inhibitory learning. The prefrontal cortex forms a 'CS predicts safety' association that suppresses amygdala output, but the original LTP-strengthened CS–US association in the lateral amygdala persists. Stress disrupts prefrontal inhibitory control, unmasking the latent fear trace. This is why exposure therapy is effective but relapses occur, and why strengthening extinction consolidation is a major research priority.

Question 2

During fear conditioning, LTP is induced at tone-responsive synapses in the lateral amygdala because:

• A. The unconditioned stimulus activates the central nucleus, which sends feedback to strengthen tone inputs in the lateral nucleus
• B. The conditioned stimulus activates lateral amygdala neurons presynaptically at the same time the unconditioned stimulus strongly depolarizes them postsynaptically, fulfilling the Hebbian conditions for NMDA receptor-dependent LTP
• C. Cortisol released during the shock globally strengthens all synapses in the amygdala through genomic mechanisms
• D. Repeated tone presentations without shock gradually strengthen the tone synapse through homeostatic plasticity

Answer: (B)

This directly applies Hebbian LTP to fear learning. Tone arrives via thalamo-amygdalar and cortical pathways to the lateral amygdala (LA). Shock arrives via somatosensory pathways and strongly depolarizes the same LA neurons. When presynaptic tone activity coincides with strong postsynaptic depolarization from the shock, NMDA receptors at tone synapses are activated (their Mg²⁺ block removed by depolarization), triggering LTP. After learning, the tone alone drives LA neurons strongly enough to trigger fear responses. Option A confuses nucleus locations. Option C is partly true (stress hormones modulate consolidation) but not the primary LTP mechanism.

Question 3

Extinction of a conditioned fear response represents new inhibitory learning rather than erasure of the original fear memory.

• T. True
• F. False

Answer: True

The evidence is compelling: extinguished fear can recover spontaneously after time passes (spontaneous recovery), return after stress, or reappear when tested in a different context from where extinction occurred (renewal). If extinction had erased the memory, none of these phenomena could occur. Instead, extinction produces new cortical learning — prefrontal cortex projects to inhibitory interneurons in the amygdala that suppress central nucleus output. The original fear trace in the lateral amygdala remains; what changes is whether it gets expressed.

Question 4

The amygdala is a specialized fear center whose mainly function is to detect threatening stimuli and produce fear responses.

• T. True
• F. False

Answer: False

The amygdala processes emotional significance broadly, not fear exclusively. It is activated by positive stimuli (food rewards, attractive faces, pleasurable events), by novelty and uncertainty, and by social signals of many kinds. The amygdala's role in reward learning and social cognition is well-established. The 'fear center' label arose because fear conditioning is a methodologically convenient paradigm, but conflating the amygdala's experimental role in fear research with its broader function in evaluating motivational relevance misrepresents the neuroscience. Amygdala damage disrupts reward learning and social judgment as well as fear responses.

Question 5

Why does extinction represent new inhibitory learning rather than erasure of the fear memory, and what evidence from animal studies supports this interpretation?

Model answer: Extinction creates a new association (CS predicts safety) in prefrontal cortex, which projects to inhibitory interneurons in the amygdala to suppress fear expression. The original CS–US fear trace in the lateral amygdala persists. Three phenomena demonstrate this: (1) spontaneous recovery — fear returns after a rest interval with no further shocks; (2) renewal — fear returns when testing occurs in a different context from extinction; (3) reinstatement — a single unsignaled shock after extinction restores fear responses. All three require the original memory to be latent, not erased.

These three phenomena are the canonical evidence against the erasure interpretation. They show that extinction is context-dependent and time-limited in a way that complete erasure would not be. Clinically, this means exposure therapy creates a competing inhibitory memory, not a cure — relapse is predictable when inhibitory control is disrupted by stress, context change, or time. This understanding motivates research into pharmacological agents that enhance extinction consolidation, making the new inhibitory memory stronger and more resistant to disruption.