With rivalry, the size of this modulatory field can be directly controlled by changing the size of a stimulus in one eye relative to the other. With standard models of binocular normalization, introducing a stimulus in a competing eye should contribute to the find protocol pooled inhibitory component of normalization (Ding and Sperling, 2006; Moradi and Heeger, 2009), which predicts shifts in contrast gain (strongest effects at

mid-contrasts), but not in response gain (strongest effects at high contrasts), regardless of size (Supplemental Information). However, if rivalry also includes a process that behaves like attention, the shape of contrast response functions for attenuated signals should differ depending on the size of the dominant stimulus in the other eye—a manipulation that would alter the size of the modulatory field. Specifically, when the dominant stimulus is substantially larger than the

stimulus in the other eye, thereby evoking a large modulatory field, the normalization framework of attention predicts a reduction in contrast gain for the probe stimulus (Figure 1A). However, when LY2835219 mouse the dominant stimulus evokes a small modulatory field, the contrast response functions should transition toward a reduction in the response gain (Figure 1B). To explore whether normalization modulates visual competition, we examined how psychometric functions

change for an attenuated stimulus under rivalry, and whether those changes depend on the size of the putative modulatory field. We measured observers’ ability to discriminate fine changes in the orientation of a probe stimulus (4° clockwise or counterclockwise) that was either presented monocularly, or was suppressed under binocular rivalry (Figure 2). To control the size of the modulatory field in the rivalry conditions, we manipulated the size of the dominant competing stimulus such that in some trials, it was either the same size as the probe (small: 1.5°), somewhat larger than the probe (medium: 2.5°), or substantially larger (large: 8°). The rms contrast of the probe stimuli ranged from 0.8%–23%, almost allowing us to measure the entire psychometric function, a behavioral measure that scales proportionally to the signal-to-noise ratio of the underlying contrast response function (Herrmann et al., 2010; Pestilli et al., 2009). Specifically, changes in the neural contrast response function under this framework directly impacts an observer’s ability to discriminate orientation changes in the probe, which would, in turn, be reflected in corresponding changes to the behavioral psychometric functions. Rivalry had a substantial impact on psychometric functions (Figure 3A).