Discrimination of geons by pigeons: The effects of variations in surface depiction

We explored how changes in the depiction of the surface features of a simpl e volume (a geon) affected the pigeon's recognition performance. Pigeons we re trained to make a different keypeck response to each of four computer-re ndered single-geon objects. In Experiment I, the pigeons were tested with i mages of the original stimuli in which the light source was shifted from it s original position, as well as with silhouettes and line drawings of these objects. All three types of stimulus variations resulted in marked drops i n performance: above chance for silhouettes and light-change stimuli, but a t chance for line drawings. In Experiment 2, the pigeons were tested with i mages of the original stimuli in which the contrast levels were either incr eased or decreased. These transformations resulted in very small drops in p erformance (except for the complete absence of contrast-a silhouette). Thes e results indicated that the pigeons attended to the shape of the outside c ontour of an object and to the relative brightness of an object's surface c ontours.