Two eyes are better than one: superior binocular discrimination learning in pigeons.

Four groups of pigeons each were trained to perform four visual-discrimination tasks of varying difficulty. Two groups received the training binocularly and the other two monocularly. Two types of stimulus displays were used. In one display the two pecking keys were mounted in a horizontal arrangement, whereas, in the other display, the two keys were mounted in a vertical arrangement. In both displays binocular learning was more rapid than monocular learning. This difference increased with problem difficulty. Moreover, training with the horizontal key arrangement resulted in more rapid learning with either viewing condition. These results suggest that the inferiority of monocular learning was not caused by restriction of the visual field along the horizontal plane, and the failure to detect monocular-binocular differences in learning in previous reports may have resulted from inadequate task difficulty. Finally, the results suggest that monocularly-viewing birds should not be regarded as "natural split-brain" preparations.