<title>Spatial summation of face information</title>

Do all parts of the face contribute equally to face detection or are some parts more detectable than others? The task was to detect the presence of normalized frontal-face images within in aperture windows of varying extent. We performed such a face summation study using two-alternative forced-choice psychophysics. The face stimuli were scaled to equal eye-to- chin distance, foveated on the bridge of the nose. The images were windowed by a fourth-power Gaussian envelope ranging from the center of the nose to the full face width. Eight faces (4 male and 4 female) were presented in randomized order, intermixed with 8 control stimuli consisting of phase- scrambled versions of the images with equal Fourier energy. The integration functions for detection of random images did not deviate significantly from a log-log slope of -0.5, suggesting the operation of a set of ideal integrators with probability summation over all aperture sizes. The data for face detection showed that observers were not ideal integrators for the information in the face images, but integrated linearly up to some small size and failed to gain any improvement for information beyond some larger size. This performance suggested the operation of a specialized face template filter at detection threshold, differing in extent among the observers.