Binocular rivalry is a phenomenon that occurs when two incompatible images are presented to the two eyes. Even though the visual stimuli remain constant, visual perception alternates between the two monocular stimuli. Binocular rivalry is therefore extremely useful for understanding mechanisms underlying the selection of visual inputs for perception. We are studying the mechanisms of perceptual selection in binocular rivalry as well as the effects of predictive context.
Roles of magnocellular and parvocellular systems in perceptual selection
The magnocellular and parvocellular streams are two early visual pathways that are specialized for the processing of motion and form, respectively. We have documented differential contributions of these two processing streams to perceptual alternations during binocular rivalry (Denison and Silver, 2012). We studied a phenomenon called interocular switch rivalry in which rival images that are periodically exchanged between the two eyes can generate different types of perceptual alternation. By varying the spatial, temporal, and luminance properties of the rivalrous stimuli, we could bias processing towards either the magnocellular or parvocellular stream. These stimulus manipulations produced reliable changes in the type of perceptual alternation experienced by subjects during interocular switch rivalry, establishing a framework for investigating the differential contributions of the magnocellular and parvocellular systems to visual perception.
Predictive context in binocular rivalry
In this work, we present stimuli that engender expectations of forthcoming stimuli by the subject and then measure how these expectations influence which of the rivaling images is selected by the brain for conscious perception. In one study, presentation of a moving stimulus to both eyes creates the expectation that the stimuli will continue moving in the same direction. After exposure to the moving stimulus, the subject is presented with a pair of rival images, one consistent with the expectation and one that is inconsistent. We have found that the expected image is more likely to be perceptually selected (Denison et al., 2011).
More recent studies involve the use of statistical learning to create unconscious “expectations” about upcoming visual stimuli. In these experiments, subjects are exposed to sequences of stimuli that have patterns embedded in them. The participants in this research are unaware of the patterns, but they nevertheless implicitly learn these patterns.
In the next phase of the experiment, we present a portion of the pattern followed by a rivaling display containing both the expected stimulus, given the learned pattern, and an unexpected stimulus. Does the brain perceptually select the stimulus that it “expects” to see, or does it perceptually prioritize the unexpected, surprising stimulus? We have obtained evidence for both of these mechanisms, and one possibility is that the effects of predictive context on perceptual selection in binocular rivalry depend on the level in the visual processing hierarchy at which the conflict between the rivaling stimuli is resolved (Denison et al. 2016; Piazza et al. 2018; Lawler and Silver, 2023).
Hemispheric specialization in perceptual selection in binocular rivalry
The double filtering by frequency (DFF) theory developed by Lynn Robertson and Rich Ivry proposes that the two cerebral hemispheres process different spatial frequency components of the visual environment. Specifically, the left hemisphere is specialized for higher spatial frequencies, with the right hemisphere preferentially processing lower spatial frequencies. We have tested whether this theory also applies to perceptual selection. We biased processing of rivalrous gratings of different spatial frequencies to one of the hemispheres by presenting them in either the left or right visual hemifield. We found that the likelihood of perceptual selection of a particular spatial frequency depends on whether the rivalrous stimuli are primarily processed by the left or right hemisphere, in accord with the DFF theory (Piazza and Silver, 2014). In addition, these hemispheric differences in perceptual selection of spatial frequencies are based on a relative comparison of the spatial frequencies that are being processed by the visual system at any given time, not on the absolute spatial frequencies of the stimuli (Piazza and Silver, 2017).