Utilisation of spatial frequency information in face search.

In previous studies the utilisation of spatial frequency information in face perception has been investigated by using static recognition tasks. In this study we used a visual search task, which requires eye movements and fast identification of previously learned facial photographs. Using Fourier phase randomisation, spatial information was selectively removed without changing the amplitude spectrum of the image. Fourier phase was randomised within one-octave wide bands of nine different centre spatial frequencies (2-32 c/face width, 0.63-10.1 c/deg). In a control condition no randomisation was used. All stimuli had similar contrast. Search times and eye movements during the search were measured. The removal of spatial information by phase randomisation at medium spatial frequencies resulted in a considerable increase of search times. In the main experiment the maximum of the search times occurred between 8 and 11 c/face width. The number of eye fixations behaved similarly. In an additional experiment with a threefold viewing distance the search times increased and the maximum of the search times shifted slightly to lower object spatial frequencies (5.6-8 c/face width). This suggests that the band of spatial frequencies used in face search is not completely scale invariant. The results show that information most important to face search is located at a limited band of mid spatial frequencies. This is consistent with earlier studies, in which non-dynamical face recognition tasks and low-contrast stimuli have been used.