Unpaired unconditional stimuli during fear extinction at full and reduced intensity reduce re-acquisition.

Presenting unpaired unconditional stimuli (US) during extinction has been shown to reduce the contextual renewal of conditional fear and to slow re-acquisition. The present study investigated whether this reduced return of fear is also observed if the intensity of the US presented during extinction is lower than that presented during acquisition. Three groups of participants (N = 121) were trained in a differential fear conditioning procedure that employed habituation, acquisition, extinction, renewal test, and re-acquisition phases. To induce renewal, the context was changed during extinction training in an ABA design. Group Standard received no US presentations during extinction training whereas group Unpaired received five unpaired USs during extinction at the physical intensity used during acquisition. The intensity of the unpaired USs was halved in group Reduced. Electrodermal responses in the three groups did not differ during habituation, acquisition, extinction or the renewal test where no renewal was observed in any group. However, significant differential electrodermal responses were observed on the first block of re-acquisition training after standard extinction, but not after unpaired extinction regardless of US intensity. This suggests that unpaired US presentations can strengthen extinction learning even if presented at a reduced intensity. This finding opens the possibility of translating the unpaired US extinction approach into applied settings.