Figure 1.

Illustration of the experimental design. Four emotionally-neutral faces (2 male, 2 female) used with permission in the study were selected from the series of faces developed by Matsumoto and Ekman⁶¹. [Available as the Japanese and Caucasian facial expressions of emotion and neutral faces (JACNeuF). D Matsumoto, P Ekman - Human Interaction Laboratory, University of California, 1988. (https://www.humintell.com/for-use-in-research/; ekmansf@itsa.ucsf.edu). For each participant, two faces (e.g., first and second in figure) were conditioned with forearm brush strokes during their presentation and therefore represent CS+ faces; the other two faces (e.g., the third and fifth in figure) were never paired with brushing and represent CS− faces. Faces assigned CS+ vs CS− status were randomized across participants, but each CS type consisted of a male and female face. The US-CS pairing accords to a 50% partial reinforcement: only half of the presentations of the CS+ ’s were accompanied by the US (e.g., first and last faces in figure [CS+ paired]) and half were not (e.g., second and fourth faces in figure [CS+ unpaired]. Affective ratings of CS+ faces were collected in unpaired trails and thus were uncontaminated by direct effects of the US.