Binocular Rivalry Stimuli are Common but Rivalry is not

Recently Arnold (2011) asked “Why is binocular rivalry uncommon?”. He answered in an entertainingly written, provocative article, for which I thank and congratulate him. However, I will argue that Arnold’s answer falls short in two respects and his assumption that rivalry is uncommon is correct for two reasons other than the one he discusses. Binocular rivalry is a phenomenon of human visual perception that is easy to demonstrate in the laboratory by using a stereoscope to present one image to one eye and a different image to the other: one perceives one image rather than both, and the image one perceives alternates between the two at random (Wheatstone, 1838). In answering the question, Arnold (2011) identified two situations outside the laboratory in which the view of one eye differs from that of the other for which he claimed there is no rivalry. The first is when an object, such as the trunk of a small tree, is near both eyes as we fixate on distant objects. The trunk projects a blurry, lowcontrast, low-spatial-frequency (for which Arnold adopted Levelt’s, 1968, umbrella term “stimulus strength”) image onto the temporal region of the right-eye retina and onto the nasal region of the left-eye retina. In the corresponding regions of the other eye’s retina, the distant objects project different, sharp images. The second is when a similar object, the tree trunk again, is closer to one eye, blocking its view. The trunk projects a blurry image onto the fovea of that eye, and the distant objects project different, sharp images onto the fovea of the other eye. In both these situations, Arnold said the sharp images dominate perception forever, preventing rivalry. There are two problems for Arnold with these sorts of examples: