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In classical conditioning, a conditioned stimulus (CS, e.g., a tone) is presented just before an unconditioned stimulus (UCS, e.g., a mild toe pinch) in repeated trials, such that the CS will eventually evoke the unconditioned response (e.g., withdrawal reflex) on its own.

Such memories of fear and discomfort are said to be held in the lateral amygdala, with a representation of the CS and its UCS (Díaz-Mataix et al, 2011).

Certain areas of the brain have particular representations of stimuli, e.g., a tone of a given frequency, as the primary auditory cortices have "tonotopic" zones in which different areas have a differential response to separate frequencies.

Since the CS/UCS pairing is more abstract than a single tone, I'm curious as to what type of representation of the CS/UCS is actually encoded within the local subnetworks amygdala. I can't imagine that, for the above example, a particular area holds a representative frequency of a tone used as a CS in tight collaboration with a "pain" signal. It might be realistic, but anatomically, that whole area would also have to reflect an exhaustive set of all of the other possible "life events" that the animal has experienced. So, is this representation in the lateral amygdala just a placeholder to retrieve other pertinent memories stored elsewhere in cortex -- is the hippocampus a major player in keeping this sorted? If, in fact, all these pairings are just weak bindings of diverse stimuli, why is it so difficult to untangle and extinguish more complex types of fears in human patients?

Díaz-Mataix, L., Debiec, J., LeDoux, J.E. & Doyère, V. (2011). Sensory-specific associations stored in the lateral amygdala allow for selective alteration of fear memories. The Journal of Neuroscience, 31, 9538-9543.. PDF

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