Fig. 1: A simplified summary of the fear learning paradigms most often used in humans and rodents.

A FPS fear-potentiated startle. US unconditioned stimulus, often an electric shock to the hand (annoying but not painful), an airblast directed towards the larynx or an aversive sound, especially used in children. CS conditioned stimulus. The CS+ is the stimulus paired with the US whereas the CS− is not paired with the US. The startle probe is a brief, sudden, loud noise that elicits an automatic response to close the eyes. The startle response is assessed by placing an electrode under the eye to measure action potentials with electromyography of the orbicularis oculi muscle. The pairing of the CS+ and the startle probe elicits an increase in the CS+ response. B SCR skin conductance response. US unconditioned stimulus, often an electric shock to the hand (annoying but not painful). CS conditioned stimulus. The CS+ is the stimulus paired with the US whereas the CS− is not paired with the US. In the non-dominant hand, this SCR measures emotional arousal to rewarding and aversive stimuli. FPS and SCR can be measured within the same experiment but often only FPS or SCR are measured. C US unconditioned stimulus, an electric footshock. A few mild electric footshocks are delivered in an experimental box. Animals are tested for fear extinction and extinction recall in the same experimental box keeping the context as similar as possible. D The cue is often an acoustic tone, an odor, or a light, which are paired with mild electric footshocks. US unconditioned stimulus, an electric footshock. The fear extinction and fear extinction recall are tested changing the conditions of the experimental box used for fear acquisition so that the rodent presents no contextual fear but cued-fear conditioning. For example, between the different fear phases, the color and lighting of the box should be changed. E The conditioned response is often the FPS or SCR in humans and freezing in rodents. CS blocks groups of CSs binned together.