Abstract
The diamond stimulus, introduced by Lorenceau and Shiffrar (1992), contains four occluders and four moving lines that can be perceived as coherent or separate motion. Here we used it to investigate whether attention alone (excluding the effect of fixation) can bias multistable perception, and whether this effect of attention is due to that attended areas look nearer. Our previous research has found that coherent motion perception increased when the occluders were in front of the moving lines, and decreased when the occluders were behind. Therefore, we predicted that coherent motion should be perceived more under the condition of attending to the occluders rather than the moving lines if attended areas look nearer. Observers' intention was controlled in this study in order to reveal the effect of attention better. In experiment 1, we manipulated attention (attending to occluders or moving lines) as an independent variable. Results showed that the percentage of time perceiving coherent motion during one minute trials was higher in attending to occluders condition than attending to moving lines condition significantly, consistent with our prediction. In experiment 2, one more variable was added: the binocular disparity of the moving lines. It was manipulated at four different levels behind the occluders. We predicted that the effect of attention should decrease with increasing depth, because the effect of attention that affects depth would be minor under large binocular disparity. The results were consistent with our prediction. In experiment 3, we added cast shadow for the occluders as a monocular depth cue to enhance the perceived depth of occluders in front of moving lines, and the effect of attention was found to be eliminated compared with the normal condition. These results all consisted with that attention alone can bias multistable perception by making attended areas look nearer.