“…Some studies use a second-order observational SFL procedure [30], which is analogous to second-order classical FC [31]. Second-order SFL begins with standard observational fear learning procedure in which a neutral CS is paired with the demonstrator’s expression of fear, and then in the subsequent training session, this CS (now called the first-order CS or CS1) is paired with a novel cue (second-order CS or CS2) [30]. The most commonly used measures of learning acquired through observational SFL are changes in autonomic activity, such as skin conductance responses, SCR, which are tested mostly in human studies [23, 25, 32]; behavioral threat responses, such as immobility or freezing, which are used in animal research [26, 33]; and avoidance behavior tested both in humans and animals [27, 34–37].…”