Reverse lateralization of visual discriminative abilities in the European starling

Previous experiments on visual feature discrimination abilities have consistently shown a right-eye system lateralization in pigeons, Columba livia, and young domestic chickens, Gallus gallus domesticus, both nonpasserine species. Recently, however, it has been shown that photoreceptor distribution in the left and right retinas are asymmetrical in the European starling, Sturnus vulgaris, a passerine species. Single cone receptors are significantly more abundant in the left retina, which suggests that starlings should perform visual discrimination tasks more proficiently with the left eye, in contrast to previous findings with nonpasserines. We tested this hypothesis using the technique of monocular occlusion. In the first experiment, starlings were tested on a simultaneous visual discrimination task in three conditions: binocular (both eyes), left monocular (left eye only) and right monocular (right eye only). Subjects in the binocular and left-monocular conditions achieved significantly higher performance scores on the discrimination task than birds in the right-monocular condition. A second experiment found similar results, with birds in the left-monocular condition learning the discrimination task more than twice as quickly as those in the right-monocular condition. Subsequent tests with the alternative eye for both groups indicated no interocular transfer. These findings suggest that visual discriminative abilities in starlings are asymmetrical, and that they are lateralized in the opposite eye system than has been reported for all other species tested to date.