Orientation-tuned suppression in binocular rivalry reveals general and specific components of rivalry suppression Experimental Psychology, Helmholtz Institute, Utrecht University, The Netherlands Sjoerd M. Stuit School of Psychology, University of Sydney, Australia John Cass Experimental Psychology, Helmholtz Institute, Utrecht University, The Netherlands Chris L. E. Paffen School of Psychology, University of Sydney, Australia David Alais During binocular rivalry (BR), conflicting monocular images are alternately suppressed from awareness. During suppression of an image, contrast sensitivity for probes is reduced by È0.3–0.5 log units relative to when the image is in perceptual dominance. Previous studies on rivalry suppression have led to controversies concerning the nature and extent of suppression during BR. We tested for feature-specific suppression using orthogonal rivaling gratings and measuring contrast sensitivity to small grating probes at a range of orientations in a 2AFC orientation discrimination task. Results indicate that suppression is not uniform across orientations: suppression was much greater for orientations close to that of the suppressed grating. The higher suppression was specific to a narrow range around the suppressed rival grating, with a tuning similar to V1 orientation bandwidths. A similar experiment tested for spatial frequency tuning and found that suppression was stronger for frequencies close to that of the suppressed grating. Interestingly, no tuned suppression was observed when a flicker-and-swap paradigm was used, suggesting that tuned suppression occurs only for lower-level, interocular rivalry. Together, the results suggest there are two components to rivalry suppression: a general feature-invariant component and an additional component specifically tuned to the rivaling features. Keywords: vision, binocular rivalry, suppression, orientation, spatial frequency Citation: Stuit, S. M., Cass, J., Paffen, C. L. E., & Alais, D. (2009). Orientation-tuned suppression in binocular rivalry reveals general and specific components of rivalry suppression. Journal of Vision, 9(11):17, 1–15, http://journalofvision.org/9/11/17/, doi:10.1167/9.11.17. Introduction Presenting conflicting visual information to each eye results in perceptual alternations between the two monocular images such that each undergoes suppression in a stochastic alternation. This phenomenon of fluctuating perception with constant visual input is referred to as binocular rivalry (BR) and has been a popular topic with visual scientists for nearly two hundred years (Wheatstone, 1838). Many different kinds of images will induce BR provided their features differ sufficiently. BR has been demonstrated in many primary stimulus domains, for instance, motion (Moutoussis, Keliris, Kourtzi, & Logothetis, 2005), orientation (Polonsky, Blake, Braun, & Heeger, 2000), spatial frequency (Julesz & Miller, 1975), and color (Breese, 1909), as well as between global motion and complex visual objects such as houses and faces (Alais & Melcher, 2007; Alais & Parker, 2006; Tong, Nakayama, Vaughan, & Kanwisher, 1998). Even though the kinds of stimuli used can differ greatly between experiments, the dynamics of BR and the basic phenomenology of perceptual suppression are highly similar. The lack of awareness during suppression and the diversity of stimuli that engage in rivalry have led to two major questions within BR research: Where in the brain is rivalry initiated, and what is the fate of the suppressed stimulus? In an attempt to answer the first question and identify the level at which rivalry occurs, two conflicting theoret- ical viewpoints on the nature of rivalry suppression have been developed. One theory proposes that BR is a low- level process where there is competition at an early stage between monocular visual channels to determine which eye will dictate perception (Blake, 1989). This view is often referred to as “eye-rivalry” and has been supported by functional imaging studies (Polonsky et al., 2000; Tong & Engel, 2001) as well as psychophysical experiments (Blake & Fox, 1974; Blake, Westendorf, & Overton, 1980). The second theory holds that rivalry is a higher- level process that occurs between representations of visual Journal of Vision (2009) 9(11):17, 1–15 http://journalofvision.org/9/11/17/ 1 doi: 10.1167/9.11.17 Received April 27, 2009; published October 16, 2009 ISSN 1534-7362 * ARVO