Figure 1. Experimental methods.

(A). Examples of the three stimulus types used (from left to right: unfamiliar female monkeys, illusory faces, non-face objects). The non-face objects were selected from the public domain on the basis that they matched the examples of illusory faces for object content. (B). The results of the human experiment. Here, the rows represent individual subject data (N = 10) and columns represent the 45 images comprising the stimulus set. Importantly, none of the non-face objects was rated as being “face-like” (>100) on a 200-point scale (M_{non-face objects} =5.24; SEM =.45). Two pairwise contrasts confirmed that the non-face objects had a significantly smaller average score than either the monkey faces (P <.01, η² =.99) or illusory faces (P <.01, η² =.99). (C). The trial procedure for the three conditions of interest in the monkey experiment. Each trial consisted of three time periods: fixation, free viewing, and reward after successful trial completion or time out after trial aborts.