Neuronal Circuits of Associative Learning

   Herry et al set out to examine the role of neuronal circuits in associative learning such as fear conditioning and extinction in mice. This paper does an excellent job displaying how specific distinct neuronal circuits can trigger a change in behavior. Also, in this paper two distinct classes of basolateral amygdala neurons, which are named the fear and extinction neurons.

   During fear conditioning, an increase in the firing of fear neurons, while during extinction, the opposite happens, an increase in the firing of extinction neurons, and fear neurons would no longer fire. This shows a connection between the activities of these neurons and how they can be responsible to the observed behavior. This shows a whole different perspective than the previous papers we read.  While the other papers used different behavioral tests in order to detect the level of fear and anxiety the animal was experiencing, this paper explored the neuronal circuits involved when it comes to the animal being fearful and anxious. I believe this approach would help us identify the specific circuits involved in certain behaviors. And since we cannot ask a mouse whether they are feeling anxious or fearful, using such methods would help map out the neuronal circuits involved in fear or anxiety. I also believe the experimental protocol used in this paper makes for a better animal model.

   When it came to exploring the connectivity of fear neurons and extinction neurons, researchers of this paper found that fear neurons received input from the hippocampus and projected to the medial prefrontal cortex (mPFC). While extinction neurons would receive input from the medial prefrontal cortex and project to the medial prefrontal cortex, which are connected bi-directionally. This suggests that the mPFC is involved in the consolidation of extinction memory. And the hippocampal input to the basolateral amygdala may be the reason behind allowing fear expression after extinction has taken place. Even though extinction can help decrease the level of fear in an animal, returning from a defensive state to a more exploratory default state, fear cannot be unlearned. And repeated activity of fear neurons over a long time in the basolateral amygdala might consolidate fear memories. 

   Even though long-term memories of conditioned fear and extinction are not ultimately stored in the amygdala, it does play a crucial part when it comes to fear and connected with the hippocampus and mPFC. And hopefully in the future we would be able to identify these neurons involved in fear and extinction.