Week 5

            The 2008 Herry et al paper was our first look at extinction in relation to the fear circuitry and optogenetic manipulations that we had seen up to this point. The goal of the paper was to show that two distinct populations of neurons, extinction and fear, are at work regulating the response to conditioned stimuli during fear paradigms.

            I thought that the experimental design was lacking at times as they made broad claims and backed up with little evidence. The first fault I had was with their explanation that these fear and extinction neurons were two totally separate neuronal populations. The questions they posed was a good one, whether individual extinction neurons can function as fear neurons later with a different CS perhaps. The problem was when “demonstrating” that this was not the case and they are in fact two separate classes of neurons, all they showed was a classic fear paradigm. In the example, it stated that extinction neurons responded to the extinguished CS but not the unextinguished CS (obviously) and that the fear neurons responded to the unextinguished CS and not the extinguished CS (also obviously). This does not prove their point that these two neurons cannot later become a extinction neuron or fear neuron after having played the other role prior. This is later remedied a little with their physiological data showing that the two different types of neurons do have some distinct differences. This helped the validity of their assertions much more than their designed experiment made it seem. After these facts, and a few other procedural mishaps such as using multiple contexts to condition and extinguish fears, the paper made a valid case for microarchitecture of these cells providing a switch of behavior when confronting fear stimuli.

            The second paper from Courtin et al was much more thorough in its language and figures providing data for their identification of subpopulations of neurons from type 2 INs the PVINs. These figures made this paper easier to follow and the story came across more convincingly. The synchronicity of the neurons firing being the defining feature of information encoding makes it a prime target to explore when attempting to find see behavioral expression come from neuronal circuits. Their findings that synchronization of neurons could elicit a sustained fear response and that resetting to the optimal firing relationship could be used as a sort of therapy was exciting.