Monocular rivalry exhibits three hallmarks of binocular rivalry

Binocular rivalry occurs when different images are presented one to each eye: the images are visible only alternately. Monocular rivalry occurs when different images are presented both to the same eye: the clarity of the images fluctuates alternately. Could both sorts of rivalry reflect the operation of a general visual mechanism for dealing with perceptual ambiguity? We report four experiments showing similarities between the two phenomena. First, we show that monocular rivalry can occur with complex images, as with binocular rivalry, and that the two phenomena are affected similarly by the size and colour of the images. Second, we show that the distribution of dominance periods during monocular rivalry has a gamma shape and is stochastic. Third, we show that during periods of monocular-rivalry suppression, the threshold to detect a probe (a contrast pulse to the suppressed stimulus) is raised compared with during periods of dominance. The threshold elevation is much weaker than during binocular rivalry, consistent with monocular rivalry’s weak appearance. We discuss other similarities between monocular and binocular rivalry, and also some differences, concluding that part of the processing underlying both phenomena is a general visual mechanism for dealing with perceptual ambiguity. This article may not exactly replicate the final version published. It is not the copy of record. 3 Monocular rivalry exhibits three hallmarks of binocular rivalry: Evidence for common processes We experience the visual world in astounding richness and detail, yet our knowledge of how these conscious percepts arise is still quite poor (cf. Chalmers, 1995). One way to learn more about these processes is to study phenomena in which visual consciousness changes without any change in the stimuli being viewed (Crick & Koch, 1995). Such phenomena are known as perceptually multistable and include binocular rivalry (Porta, 1593, cited in Wade, 1996), reversals of the Necker cube (Necker, 1832), of the Rubin face-vase figure (Rubin, 1915), and of the kinetic depth effect (Wallach & O'Connell, 1953), and motion-induced blindness (Bonneh, Cooperman, & Sagi, 2001). Binocular rivalry is a particularly fascinating example, in which visual consciousness fluctuates randomly between two different images presented one to each eye. It has been studied extensively (for reviews see Alais & Blake, 2005; Blake & O’Shea, In press) and has gone some way to shedding light on how visual awareness arises: Conscious visual experience in binocular rivalry is thought to arise from activation, and suppression, of neurons at a succession of stages in the visual system (e.g., Blake & Logothetis, 2002). Our interest in this paper is in the relationship between binocular rivalry and another phenomenon of perceptual multistability, monocular rivalry. Monocular rivalry was discovered by Breese (1899) in the course of his foundational observations and experiments on binocular rivalry. He found that binocular-rivalry-like behaviour also occurred when a red and a green grating were optically superimposed by a prism and presented to a single eye. Breese called it monocular rivalry to distinguish it from binocular rivalry. He reported that monocular rivalry alternations tended to occur at a slower rate than binocular rivalry alternations and that the perceptual alternations were less vivid: “Neither disappeared completely: but at times the red would appear very distinctly while the green would fade; then the red would fade and the green appear distinctly” (p. 43). One of the unresolved questions in the literature on perceptual multistability is whether common neural mechanisms underlie binocular and monocular rivalry. There are at least three general similarities between the two forms of rivalry that suggest This article may not exactly replicate the final version published. It is not the copy of record. 4 commonality. First, the basic phenomenology is similar in that both involve periods of alternating dominance. Second, both forms of rivalry become more vigorous as stimuli are made more different in colour (e.g., Wade, 1975), or in orientation and spatial frequency (e.g.,Atkinson, Fiorentini, Campbell, & Maffei, 1973; Campbell, Gilinsky, Howell, Riggs, & Atkinson, 1973; O’Shea, 1998). Third, the two forms of rivalry can influence each other, tending to synchronize their alternations in adjacent regions of the visual field (Andrews & Purves, 1997; Pearson & Clifford, 2005). Although monocular and binocular rivalry are similar in these three respects, this is by no means an exhaustive list of possible comparisons between the two rivalries. Here we test whether monocular rivalry shares three other hallmarks of binocular rivalry. First, binocular rivalry can occur between any two images, providing they are sufficiently different. For example, Porta (1593, cited in Wade, 1996) observed rivalry between two different pages of text. Wheatstone (1838) observed rivalry between two alphabetic letters. Galton (1907) observed rivalry between pictures of different faces. Yet, with the possible exception of a study by Boutet and Chaudhuri (2001), monocular rivalry has always been shown between simple repetitive stimuli such as gratings, leading some to suppose that such stimuli are necessary for monocular rivalry (e.g., Furchner & Ginsburg, 1978; Georgeson, 1984; Georgeson & Phillips, 1980; Maier, Logothetis, & Leopold, 2005). In Experiments 1 and 2, we show that monocular rivalry occurs between complex pictures of faces and houses. Second, binocular rivalry has a characteristic distribution of dominance times, a gamma distribution, and the duration of one episode of dominance cannot be predicted by any of the preceding ones (e.g., Fox & Herrmann, 1967; Levelt, 1967). Yet the distribution and predictability of episodes of monocular rivalry dominance are unknown. In Experiment 3, we show that the temporal periods of monocular rivalry are similar to those of binocular rivalry: gamma distributed and independent. Third binocular rivalry suppression is accompanied by a characteristic loss of visual sensitivity. When a stimulus is suppressed during binocular rivalry and becomes invisible, stimuli presented to the same retinal region are also invisible, provided the new stimuli are not so abrupt or so bright as to break suppression (indicative of suppression of the whole eye) (e.g., Fox & McIntyre, 1967; Nguyen, Freeman, & Alais, 2003; Norman, This article may not exactly replicate the final version published. It is not the copy of record. 5 Norman, & Bilotta, 1999; Wales & Fox, 1970). This is usually demonstrated by showing a loss of sensitivity during periods of suppression relative to periods of dominance, however it is unknown whether monocular rivalry also shows such suppression effects. In Experiment 4, we show that monocular rivalry does indeed produce threshold elevations during suppression, although the effect is weaker than in binocular rivalry. The experiments in this paper have been published in abstract form (O’Shea, Alais, & Parker, 2005; O’Shea, Alais, & Parker, 2006; O’Shea & La Rooy, 2004). Experiment 1 Maier et al. (2005) reviewed studies of monocular rivalry, and concluded that monocular rivalry occurs only between simple, faint, repetitive images, such as low-contrast gratings. They observed, however, that alternations in clarity could occur between complex images, such as the surface of a pond and a reflection on it of a tree, although they did not measure rivalry with such stimuli. Boutet and Chaudhuri (2001) optically superimposed two faces that differed in orientation by 90 deg. They reported that the two faces alternated in clarity in a rivalry-like way, but they did not measure rivalry conventionally. They forced observer’s choices about whether one or two faces was seen after brief stimulus presentations of 1 to 3 s. Monocular rivalry, however, usually takes several seconds, or even tens of seconds, before oscillations become evident (e.g., Breese, 1899). We decided to measure monocular rivalry with complex images in a conventional way, by showing observers optically superimposed images for one-minute trials, and asking them to track their perceptual alternations using key presses. We used images of faces and houses. Moreover, we explicitly compared monocular rivalry with binocular rivalry for identical stimuli over a range of stimulus sizes. Method