an image is presented to one eye and a blank field to the other, introducing a different image onto the blank field usually results in that image being seen immediately. This is referred to as flash suppression. History Binocular rivalry was discovered by Porta. Porta put one book in front of one eye, and another in front of the other. He reported that he could read from one book at a time and that changing from one to the other required withdrawing the "visual virtue" from one eye and moving it to the other. According to , binocular colour rivalry was first reported by . also recorded it when looking at different colours from spectra in the bevel of a mirror. The clearest early description of both colour and contour rivalry was made by . To experience colour rivalry Dutour either crossed his eyes or overdiverged his eyes (a form of free fusion commonly used also at the end of the 20th century to view Magic Eye stereograms) to look at differently coloured pieces of cloth or differently coloured pieces of glass . To experience contour rivalry Dutour again used free fusion of different objects or used a prism or a mirror in front of one eye to project different images into it. The first clear description of rivalry in English was by . Wheatstone invented the stereoscope, an optical device (in Wheatstone's case using mirrors) to present different images to the two eyes. Early theories Various theories were proposed to account for binocular rivalry. Porta and Dutour took it as evidence for an ancient theory of visual perception that has come to be known as suppression theory. Its essential idea is that, despite having two eyes, we see only one of everything (known as singleness of vision) because we see with one eye at a time. According to this theory, we do not normally notice the alternations between the two eyes because their images are too similar. By making the images very different, Porta and Dutour argued, this natural alternation can be seen. Wheatstone, on the other hand, supported the alternative theory of singleness of vision, fusion theory, proposed by Aristotle. Its essential idea is that we see only one of everything because the information from the two eyes is combined or fused. Wheatstone also discovered binocular stereopsis, the perception of depth arising from the lateral placement of the