Beak Shift: an Explanation for Interocular Mirror-image Reversal in Pigeons

Abstract

MELLO¹ has reported that pigeons, trained with one eye open to peck at a key displaying a stimulus pattern, tend to peck most often to the lateral mirror-image of that stimulus when tested with the other eye open. We propose here an explanation for this phenomenon based on the premise that a bird viewing monocularly responds on the basis of cues on one side of the key only—the side of the seeing eye. On the right half of a key, for example, a 45° line would occupy the upper portion of the key, while its mirror-image, a 135° line, would occupy the lower portion. A bird trained with its right eye open therefore might, if it attended only to the right half of the key, learn to discriminate the lines on the basis of up–down cues rather than of orientation per se. Furthermore, the up–down cues are reversed on the left half of the key, so that the 135° line now occupies the upper portion and the 45° line the lower portion. If the bird is now tested with its left eye open and attends to the left half of the key, it should therefore favour the mirror-image of the stimulus it preferred with its right eye open.