Attentional allocation and the pan-field color illusion.

Humans subjectively experience a scene as rendered in color across the entire visual field, a visual phenomenon called "pan-field color" (Balas & Sinha, 2007). This experience is inconsistent with the limited color sensitivity in the peripheral visual field. We investigated the effects of visual attention allocated to the peripheral visual field on the pan-field color illusion. Using "chimera" stimuli in which color was restricted to a circular central area, we assessed observers' tendency to perceive color throughout images with achromatized peripheral regions. We separately analyzed sensitivity and response bias in judging the color content of the scene image as full-color, chimera, or gray. Using a dual-task paradigm, we manipulated observers' attentional allocation by controlling the stimulus presentation time of the central task, making the foveal attentional load change. The slope of the foveal load-sensitivity function suggests that attention was modulated by foveal load even in the peripheral visual field. Bias was affected by the size of the central colored area, such that the tendency to answer "full-color" to the chimera image increased with eccentricity. Based on these effects of attention on sensitivity and bias, we suggest that the pan-field color illusion cannot be fully explained by the decrease of sensitivity that is modulated by attentional allocation in the periphery. Our results rather indicate that the pan-field color illusion at least partly reflects a liberal bias in peripheral vision.

Discounting mechanism underlies extinction illusion.

Humans can perceive a coherent visual scene despite a low spatial resolution in peripheral vision. How does the visual system determine whether an object exists in the periphery? We addressed this question by focusing on the extinction illusion in which a disk becomes subjectively invisible when presented at the intersection of grids. We hypothesized that the disk would go unnoticed when the stimuli with and without the disk produced the same strength of visual signals. The visual system would miss the disk by confounding the target signals with the intersection signals that should be discounted. Computational analysis revealed that the energy ratio between the stimuli with and without the disk decreased with stimulus eccentricity and such energy ratio could successfully explain the observer's d' to detect the disk. These results indicate that the discounting mechanism relying on stimulus energy determines the awareness toward a peripheral object.