Probing visual consciousness: Rivalry between eyes and images Department of Psychology, University of Otago, Dunedin, New Zealand Rishi Bhardwaj Department of Psychology, University of Otago, Dunedin, New Zealand Robert P. O’Shea Department of Psychology, The University of Sydney, Australia David Alais Department of Psychology, The University of Sydney, Australia Amanda Parker During binocular rivalry, one stimulus is visible (dominant), while the other stimulus is invisible (suppressed); after a few seconds, perception reverses. To determine whether these alternations involve competition between the eyes or between the images, we measured suppression depth to monocular probes. We did so in conventional rival stimuli and in rival stimuli swapping between the eyes at 1.5 Hz (both sorts of rivalry were shown either with or without 18-Hz flicker). The conventional conditions cause rivalry that could involve either competition between the eyes or between the images or both. The eye-swapping conditions cause rivalry that could involve competition between the images. Probes were either a small spot or a contrast increment to one of the rival stimuli. Using both yes–no and forced-choice procedures, we found that conventional conditions yielded large suppression depth and that eye-swapping conditions yielded small suppression depth. Weak suppression during image rivalry is consistent with conventional rivalry’s involving competition between eyes and between images and flicker-and-swap rivalry’s involving little, if any competition between eyes and mainly competition between images. Keywords: binocular rivalry, visual awareness, visual consciousness, flicker and swap rivalry, stimulus rivalry, visual detection Citation: Bhardwaj, R., O'Shea, R. P., Alais, D., & Parker, A. (2008). Probing visual consciousness: Rivalry between eyes and images. Journal of Vision, 8(11):2, 1–13, http://journalofvision.org/8/11/2/, doi:10.1167/8.11.2. Introduction During binocular rivalry, visual consciousness fluctuates randomly between two incompatible stimuli imaged on corresponding retinal regions of the two eyes. One stimulus is visible for few seconds (dominant) while the other stimulus is invisible (suppressed); after a few seconds, visibility reverses. This process continues for as long as one looks. Two broad approaches to the neural processing mediating rivalry can be distinguished: rivalry could involve competition between the representations of the two images tagged with low-level, eye-of-origin informa- tion (so-called eye rivalry) or between representations of the two images at some higher level of the visual system (so-called image rivalry). To demonstrate eye rivalry, Blake, Westendorf, and Overton (1980) presented a vertical grating to one eye and a horizontal grating to the other and asked the observer to press a key when the vertical grating, say, was exclusively dominant. This key press swapped the gratings between the eyes: observers reported seeing the horizontal grating. Blake et al. concluded that it was the region of an eye that dominates during rivalry, not a particular stimulus. Similarly, Shimojo and Nakayama (1990) presented a stereogram with an interocularly unpaired region consist- ing of non-identical patterns to induce rivalry. When that region had a disparity consistent with partial occlusion, they found no rivalry; when that region had the opposite disparity, they found rivalry. Shimojo and Nakayama concluded that rivalry critically depends on which eye receives an unpaired stimulus. Furthermore, Ooi and He (1999), investigating the role of attention in binocular rivalry, found that presenting a pop-out cue to one eye prior to the onset of rivalry made that eye’s stimulus dominant in subsequent rivalry. They concluded that dominance during rivalry critically depends on which eye receives the pop-out cue. All these results are consistent with eye rivalry theory (Blake, 1989; Lehky, 1988; Nguyen, Freeman, & Wenderoth, 2001; Wolfe, 1986). To demonstrate image rivalry, Diaz-Caneja (1928, translated by Alais, O’Shea, Mesana-Alais, & Wilson, 2000) presented a left field of horizontal lines and a right field of concentric semicircles to the left eye and the opposite pattern to the right eye. He found that observers occasionally reported coherent alternations between circles and horizontal gratings, combining these images from the components in the two eyes. Kova ´cs, Papathomas, Yang, Journal of Vision (2008) 8(11):2, 1–13 http://journalofvision.org/8/11/2/ 1 doi: 10.1167/8.11.2 Received January 14, 2008; published August 5, 2008 ISSN 1534-7362 * ARVO