Figure 1.

Visual stimuli. A, Original face images consisted of three different monkeys displaying three different facial expressions (open-mouth, neutral, and pout-lips). B, Bandpass filtered face images. Bandpass filters with different center image-based SFs were applied to an original face image (top left in A) in seven steps. C, An example of changes in stimulus size. Size manipulation was applied to a filtered face image (center in B) in five steps. Size manipulation was applied to all filtered face images; thus, a stimulus set consisted of 35 images.