Figure 1.

Task stimuli and design. (A) Three emotional expression continua were created by morphing between sad and fearful, surprised and sad, and surprised and fearful expressions, using six 16.7% morph steps. These continua were created for both a male and a female actor, faces were taken from the Pictures of Facial Affect set (Ekman and Friesen, 1976). The actors' neutral expressions were also used as stimuli, but not included in the continuum construction. For the main fMRI analysis, using a categorical model, faces were labeled according to the end of the continuum to which they belonged (“A” or “B,” e.g., “surprise” or “fear”) with a third label being given to “50/50” morphs that comprised equal amounts of each end expression. (B) Within the fMRI task, participants were shown faces with expressions from one continuum at a time, presented together with neutral faces and double length null trials in a pseudo-random fashion using a “type 1 index 1 sequence” (Figure S1). Participants were asked to respond by key press to presentation of the neutral face. (C) In the behavioral task, an equivalent type 1 index 1 presentation sequence was used but participants were asked to respond to all faces except the neutral trials, categorizing the faces as showing “mainly” expression “A” or expression “B” for a given continuum (e.g., “mainly surprise” or “mainly fear” for the surprise-fear continuum, as shown in the example.)