Size constancy in face discrimination: Synthetic faces & principal components

There is some evidence that faces are harder to recognize across a range of sizes using face photographs (Kolers et al., 1985). This may be because the face photographs would have provided more detailed information at larger sizes. We investigate whether thresholds for face discrimination exhibit size constancy using synthetic faces which all have the same information content for face identification. Thresholds for face discrimination were measured using 4-D synthetic face cubes (Wilson et al., ARVO, 2001). The experiment consisted of a 110 ms presentation of one face followed by a noise mask for 110 ms. Then two comparison faces appeared on the screen and a subject selected the face matched to the flashed face in a 2 AFC paradigm. We varied the linear size ratio (1:1, 2:1, or 4:1) between the target face and the comparison faces. When constructing regular face cubes, thresholds for face discrimination tended to remain constant across the size changes between target and comparison faces for both front and 20 deg side views of faces. When constructing face cubes based on principal components of our entire face population, thresholds for face discrimination tended to be unaffected by the size change between the flashed face and the comparison faces over a considerable range of the principal components. Only when the small face was flashed, the thresholds in higher and lower order principal components were affected by the size change. A control experiment in which small faces were flashed for 250 ms restored size constancy. These findings indicate that thresholds for face discrimination exhibit size constancy over a 4:1 size range for most principal components and that size constancy breaks down only when a small face defined by high spatial frequencies is briefly flashed. A longer processing time for small high frequency faces restores size constancy.