Revealing boundary-contour based surface representation through the time course of binocular rivalry

Abstract

We varied the surface boundary-contour properties of binocular rivalry (BR) stimuli to measure the rivalry percept as a function of stimulus duration. Experiment 1 compared perception from BR stimuli with monocular boundary contour (MBC) and binocular boundary contour (BBC). We found global dominance is achieved with stimulus duration as short as 30ms for the MBC rivalry stimuli, whereas it takes more than 150 ms for the BBC rivalry stimuli. This shows that global dominance can occur rapidly in the absence of a corresponding boundary contour in one half-image. Experiment 2 measured the detection of a monocular Gabor probe located centrally on a 1.5° versus 3.0° MBC rivalry stimulus. We found reliable binocular suppression is observed earlier with the 1.5° MBC stimulus, presumably because of the probe being spatially located nearer to the boundary contour. These findings, in conjunction with those in Su et al. (2011), support the notion that the representation of the dominant surface begins at the MBC and spreads toward the center of the image.

Figure 1

Stimuli used in Experiment 1 and the predicted percepts. (a) A binocular rivalry (BR) stimulus typically used in the laboratory. Note the disc in each half-image is clearly delineated by a boundary contour. We thus coin this stimulus a BR stimulus with binocular boundary contour (BBC). (b) A variant of a BR stimulus with BBC. Here, the boundary contour of the right half-image is produced by a 180 deg phase shift between the central and surrounding gratings. (c) The predicted percepts of the BBC rivalry stimulus (disc) from its onset to the development of global dominance. (d) A BR stimulus with monocular boundary contour (MBC), where only the left half-image has a boundary contour delineating the vertical grating disc. The right half-image has horizontal grating but no boundary contour at the corresponding retinal area. (e) The predicted percepts of the MBC rivalry stimulus (disc) from its onset to the development of global dominance.

Figure 2

Two examples of MBC stimuli used in Experiment 2. (a) A 1.5° MBC rivalry stimulus with vertical grating disc. (b) A 3.0° MBC rivalry stimulus with horizontal grating disc. Not shown at the counterparts of these stimuli, namely, a 1.5° MBC rivalry stimulus with horizontal grating disc and a 3.0° MBC rivalry stimulus with vertical grating disc. All four stimulus conditions were tested.

Figure 3

Results of Experiment 1. Percentages of seeing the four percepts as a function of stimulus duration for the BBC rivalry stimuli (rows a and b) and MBC rivalry stimulus (row c). The four columns of graphs plot the data from the four disc sizes (diameters) tested. The percentage of perceiving global dominance is evident at 30 ms, increases with stimulus duration, and is significantly earlier and higher with the MBC rivalry stimulus. In contrast, with the BBC rivalry stimuli, the percentage of perceiving global dominance is not significantly different from zero even at 150 ms.

Figure 4

Results of Experiment 2. The top panel above the graphs illustrates examples of stimuli tested in the dominance and suppression conditions, for the 1.5° (left) and 3.0° (right) MBC rivalry stimuli. The monocular Gabor probe is presented on the MBC disc half-image in the dominance condition, while it is presented on the homogeneous grating half-image in the suppression condition. Graphs (a) and (b) plot the sensitivity index ($\widehat{d}\prime$) for detecting the Gabor probe in the dominance and suppression conditions as a function of SOA, respectively, for the 1.5° and 3.0° MBC rivalry stimuli. For each SOA, $\widehat{d}\prime$ for the dominance condition is higher for the 1.5° MBC rivalry stimulus. Furthermore, significant binocular suppression is observed as early as 30 ms with the 1.5° MBC rivalry stimulus but not until 80 ms with the 3.0° MBC rivalry stimulus. Graphs (c) and (d) plot the reaction time to detect the Gabor probe. For the 1.5° MBC rivalry stimulus, RT is longer for detecting the probe in the suppressed than dominance condition and is evident as early as 80 ms. Such RT difference between the suppression and dominance conditions reveals the effect of binocular suppression. In contrast, for the 3.0° MBC rivalry stimulus, the RT difference becomes significant only at 120 ms SOA.