To quantify these

effects, we fit the data for each observer with Naka-Rushton functions (Herrmann et al., 2010; Pestilli et al., 2009; Naka and Rushton, 1966; Ling et al., 2010), for which two key parameters are predicted to change under the normalization framework: C50 and d′max. These parameters have been used in previous psychophysics studies as metrics for changes in contrast Selleck Olaparib gain and response gain. The C50 parameter corresponds to the semi-saturation constant, and changes in this parameter with rivalry suppression indicate a contrast gain shift. The d′max parameter corresponds to the asymptotic response at high contrasts, and changes in this parameter indicate a response gain reduction. Parameter estimates revealed a pattern consistent with predictions of the normalization model of attention: C50 shifted toward higher contrasts for dominant stimuli regardless of their size, whereas d′max was attenuated the most when the dominant stimulus was the same size as the probe stimulus. Consistent CP-690550 with these results, response gain-like modulation has previously been found with rivalry when similar-sized stimuli are pit against each other, both in single-unit (Sengpiel and Blakemore, 1994) and behavioral studies

(Ling et al., 2010; Watanabe et al., 2004). Fitting the data separately for each individual yielded a similar pattern of results (Figures 3B and 3C; Figure S1 available online). When the dominant stimulus was large, there was solely a change in C50 for all observers (Figure 3B), with no change in d′max (Figure 3C). However, as the size

of the competitor approached that of the probe, changes in both C50 and d′max emerged. While standard normalization models would only predict a contrast gain shift (Moradi and Heeger, 2009), our results indicate that an additional Adenosine mechanism is needed to account for our results; indeed, the conjoint reduction in both contrast gain (C50) and response gain (d′max) when the dominant stimulus is small is a prediction borne from the normalization model of attention for scenarios where the probe is small and the modulatory field is roughly the same size (Reynolds and Heeger, 2009). One alternative explanation for the large competitor’s inability to suppress high contrast probes is center-surround interactions that plausibly could weaken the strength of the center region of the competing stimulus. Although center-surround inhibition has been shown to be least effective in the fovea (Petrov et al., 2005), the retinal region targeted by our stimuli, we sought to rule out this alternative explanation explicitly by performing an additional control experiment, where we measured the degree to which the surround region of the large stimulus attenuated its center portion (Figure S2).