Herry et al. and Reznikov et al.

Herry et al. (2008) studied how two different neuronal populations function during fear and extinction in the basal amygdala.  They found that fear and extinction neurons are differently innervated with the hippocampus and mPFC.  Extinction neurons ended up being bi-directional with the mPFC, whereas fear neurons can only receive input from the hippocampus.  Their experiment showed that fear and extinction neurons are functionally specialized.  In addition, the basal amygdala neurons appeared to be necessary for extinction, as fear was not extinguished with their silencing.  They are also necessary for the renewal of previously extinguished fear responses, as muscimol did not increase freezing levels in extinguished mice after the basal amygdala was removed.  I think that this paper is good for showing the mechanism by which fear is shut on and off, but it does not account for the fact that organisms have different levels of fear and extinction.

            Reznikov et al. (2015) researched individual variations in response to stress, and I liked how the rats were separated into those with weak extinction (WE) and those with strong extinction (SE).  They claimed that the weak extinction cohort was a good preclinical model for PTSD.  The researchers measured anxiety in these two groups using the open field test, novelty suppressed feeding (NSF), and elevated plus maze (EPM).  They found that, after fear conditioning, all these tests confirmed more anxiety in the WE group, which correlated to a lower amount of corticosterone in the mice before the fear conditioning.  I like how this model is so applicable to human PTSD models, especially since humans have cortisol, which is similar to corticosterone.