Facilitation of contrast detection by flankers without perceived orientation

To extract meaningful structure from noisy input signals, the human visual-processing system uses elementary structures, such as contours, to extract more complex informative structures. The first step in contour processing involves identifying local orientation. The phenomenon of collinear facilitation is important for understanding how orientation detection is implemented; at the fovea, a stripe near the contrast threshold (target) is easier to detect when it is collinearly flanked by stripes with the same orientation (flankers). This facilitation requires collinear alignment and presumably reflects cortical mechanisms in the early visual cortex. Strong collinear orientation signals are said to help in detecting a feeble signal and in establishing a smooth conscious linkage of orientations. However, contrary to this notion, we show here that relatively small but significant facilitation occurs even when the flankers have no perceived orientation. One such case involves concentric flankers that have unbiased luminance energies in all orientations. When collinearly surrounding an oriented target, these flankers facilitated detection of the target. In another case, oriented flankers that were made invisible through interocular suppression and that were monocularly surrounding an oriented target yielded collinear facilitation even though the flankers themselves were completely masked by random patterns presented to the other eye. These findings indicate that automatic, preconscious processing of orientation information at some early stage can improve the visibility of local linear elements. They also indicate the usefulness of latent visual information in detecting orientation and constructing our visual world.