Faces in the cloud: Fourier power spectrum biases ultrarapid face detection.

Recent results show that humans can respond with a saccadic eye movement toward faces much faster and with less error than toward other objects. What feature information does your visual cortex need to distinguish between different objects so rapidly? In a first step, we replicated the "fast saccadic bias" toward faces. We simultaneously presented one vehicle and one face image with different contrasts and asked our subjects to saccade as fast as possible to the image with higher contrast. This was considerably easier when the target was the face. In a second step, we scrambled both images to the same extent. For one subject group, we scrambled the orientations of wavelet components (local orientations) while preserving their location. This manipulation completely abolished the face bias for the fastest saccades. For a second group, we scrambled the phases (i.e., the location) of Fourier components while preserving their orientation (i.e., the 2-D amplitude spectrum). Even when no face was visible (100% scrambling), the fastest saccades were still strongly biased toward the scrambled face image! These results suggest that the ability to rapidly saccade to faces in natural scenes depends, at least in part, on low-level information contained in the Fourier 2-D amplitude spectrum.