HENS USE OCCLUSION TO JUDGE DEPTH IN A 2-DIMENSIONAL PICTURE

Hens (Gallus gallus domesticus) were taught to peck at a touch screen. On the screen was a grid on which a square and a circle were depicted . The square and the circle were given different positions at random f or each trial, but were never overlapping. The hens were rewarded for pecking at the symbol that was higher up on the grid/screen, ie at the one that to a human observer was seen as being further away. Every te nth trial was a probe trial in which the animals were presented with e ither the circle overlapping the square or vice versa. The hens were n ever rewarded during the probe trials. As mentioned, the hens had lear ned to peck at the symbol that appeared to be further away during the nonprobe trials. During the probe trials the hens pecked at the symbol that was occluded, ie in the absence of any other cues they used occl usion to determine which of the two symbols was further away. The resu lts suggest that not only can nonhuman animals use image height as a c ue but that they can generalise this to situations in which occlusion is the only depth cue present.