Do low spatial frequencies explain the extremely fast saccades towards human faces?

The visual perception of human faces by man is fast and efficient compared to that of other categories of objects. Using a saccadic choice task, recent studies showed that participants were able to initiate fast reliable saccades in just 100-110ms toward an image of a human face, when this was presented alongside another image without a face. This extremely fast saccadic reaction time is barely predicted using classical models of visual perception. Thus, the present research investigates whether this result might be explained by the low spatial frequency content of images. Using the same paradigm, with two images simultaneously presented to the left or right visual fields, participants were asked to make a saccade towards a target image. The target was defined as an image belonging to one category: human face, animal or vehicle. The other image corresponded to the distractor and belongs to the other categories. We compared performance to saccade toward one category of target. The two images were displayed either in color, gray-level, low-pass filtered or high-pass filtered. As previous studies, we found that the shortest SRT was observed for saccades towards faces rather than towards animals or vehicles. Analysis of saccadic reaction time distributions showed that, in 130-140ms, participants were able to make more correct than incorrect saccades towards faces for unfiltered (color and gray-level) and low-pass filtered images whereas they needed more time for high-pass filtered images. In contrast, the minimum time participants needed to correctly saccade towards animals and vehicles was longer for low-pass and high-pass filtered than for unfiltered images. The analysis of the image statistics in the Fourier domain revealed that the amplitude spectrum of faces was mainly contained in the low spatial frequencies. Consistent with a coarse-to-fine processing of visual information, our results suggest that extremely fast saccades towards faces could be initiated by low spatial frequencies.