Pigeons use low rather than high spatial frequency information to make visual category discriminations

Pigeons were trained to discriminate photographs of cat faces from dog faces. They were then presented with test stimuli involving high- and low-pass spatial frequency filtering. Discrimination was maintained with both types of filtered stimuli, though it was increasingly impaired the more information was filtered out, and high-pass filtering impaired discrimination more than low-pass filtering. The pigeons were then exposed to hybrid stimuli in which high-pass filtered dog faces were combined with low-pass filtered cat faces, and vice versa. Response to hybrid stimuli was determined more by the low spatial frequency content than by the high-frequency content, whereas humans viewing the same stimuli at corresponding viewing distance respond more strongly to the high-frequency content. These results are unexpected given that, compared with humans, pigeons' behavior tends to be controlled by the local details of visual stimuli rather than their global appearance.