Figure 1. Schematic drawing of the inter- and intraconnectivity of the three major regions involved in fear conditioning and extinction.

Fear behaviour is a complex phenomenon which primarily involves connections between the amygdala, hippocampus and prefrontal cortex. The known connections between the amygdala, hippocampus and medial prefrontal cortex are shown schematically. Triangles representing principal neurons and interneurons are shown as black or white circles, indicating the wide variety of these neurons. The simplified medial prefrontal cortex (mPFC) shows L2/3 and L5/6 of the prelimbic prefrontal cortex (PLPFC) and the infralimbic prefrontal cortex (ILPFC). The hippocampus (HPC) is lumped together as the HPC, except for the ventral hippocampal region (vHPC). The amygdala is divided into the input regions, the basolateral amygdala (BLA) where inputs from conditioned (CS) and unconditioned stimulus (US) converge. The output zone, the central amygdala is divided into three regions: the central capsular (CLC); central lateral (CeL); and central medial. Two types of recently identified neurons in the CeL are shown, which respond differently to CS stimulation. The numbers refer to the following citations. (1) Wang et al. 2006; (2) Hoover & Vertes, 2007; (3) Vertes, 2004; (4) Van Aerde et al. 2008; (5) Parent et al. 2009; (6) Pikkarainen et al. 1999; (7) Kishi et al. 2006; (8) Mcdonald et al. 1996; (9) Mcdonald, 1998; (10) Condéet al. 1995; (11) Pinto & Sesack, 2008; (12) Pinard et al. 2012; (13) Royer et al. 1999; (14) Delaney & Sah, 2001; (15) Smith & Paré, 1994.