Physiology & Behavior, Vol. 32, pp. 847-850. Copyright©Pergamon Press Ltd., 1984. Printed in the U.S.A. 0031-9384/84 $3.00 + .00 Two Eyes Are Better than One: Superior Binocular Discrimination Learning in Pigeons SHIGERU WATANABE, 1 WILLIAM HODOS z AND BRENDA B. BESSETTE Department of Psychology, University of Maryland, College Park, MD 20742 Received 25 July 1983 WATANABE, S., W. HODOS AND B. B. BESSETI'E. Two eyes are better than one: Superior binocular discrimination learning in pigeons. PHYSIOL BEHAV 32(5) 847-850, 1984.--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 monocu- larly. 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. Pigeons Monocular discrimination Binocular discrimination Interhemispheric transfer Intensity discrimination Color discrimination Pattern discrimination DO two brain hemispheres process information better than one? Are two hemispheres capable of summing their activi- ties in a manner something like Lashley's principle of mass action [5]? If two hemispheres are not better than one, this suggests that the two hemispheres function more or less in- dependently in the processing of information from the en- vironment. Differences in learning with both hemispheres and with one hemisphere have been studied by cortical spreading de- pression, surgical hemidecortication and monocular training of mammals [4, 9, 10, 12, 13, 14, 19]. These studies indicated a hemispheric mass action effect in learning and also suggested that restriction of visual input to one hemisphere is necessary to detect a mass-action effect in a monocular- binocular comparison. Fish, which have a completely crossed optic chiasm, re- quired more trials to acquire discrimination tasks monocu- larly than binocularly [17]. However, the results of studies on birds, which also have a completely-crossed optic chiasm, were ambiguous. For example, Levine [6] trained pigeons to discriminate patterns on a jumping stand. His data do not indicate a clear monocular-binocular difference. Siegel [18], who studied ring doves in a similar apparatus, reported no statistically-significant difference between monocular and binocular performance. Meier [7] investi- gated the discrimination performance of pigeons in an oper- ant chamber with two pecking keys. The birds did not show any retardation in monocular discrimination of color and pat- tern stimuli even when their supra-optic commissure was sectioned. Several comparisons of binocular and monocular dis- crimination performance in chickens have been found. Menk- hans [8] reported that in these animals binocular discrimi- nation learning was more rapid than monocular learning. However, Schulte [16] and Rabmann-Esser [12] reported that the monocular discrimination occurred at a more rapid rate. An important procedural difference between these two groups of studies was that Menkhaus covered the occluded eye only during the testing session whereas Schulte and Rahmann-Esser used permanent eye covers, which were worn continuously for the entire duration of the experiment. This continuous occlusion of vision in one eye could have resulted in some form of unilateral deactivation of the visual system that effectively eliminated the action of conflicting stimuli or memory traces from the contralateral hemisphere. In contrast to these earlier findings, Watanabe [20] has recently reported that binocular learning in pigeons is superior to monocular learning. He reported that when pi- geons were trained to discriminate with one eye and simuRa- neously were trained on a reversal of the same task with both 'Present address: Department of Psychology, Keio University, 15-45, Mita 2-Chome, Minatoku, Tokyo 108, Japan. 2Requests for reprints should be addressed to Dr. William Hodos. 847