Integration of foveal orientation signals: Distinct local and long-range spatial domains

Human observers can discriminate the orientation of a stimulus configuratio n composed of a pair of collinear visual patterns much better than that of a single component pattern alone. Previous investigations of this type of o rientation signal integration and of other similar visual integrative funct ions have shown that, for closely spaced elements, there is integration onl y for stimuli with the same contrast polarity (i.e., both lighter or both d arker than the background) but, at greater separations, integration is inde pendent of contrast polarity. Is this effect specific to differences in con trast polarity, which is known to be an important parameter in the organiza tion of the visual system, or might there be a cluster of other stimulus di mensions that show similar effects, indicating a more widespread distinctio n between the processes limiting integration at local and long-range spatia l scales? Here, we report a similar distance dependence for orientation sig nal integration across stimulus differences in binocular disparity, directi on of motion, and direction of figure-ground assignment. We also demonstrat e that the selectivity found at short separations cannot be explained only by "end-cuts," the small borders created at the junction of abutting contra sting patterns. These findings imply the existence of two distinct spatial domains for the integration of foveal orientation information: a local zone in which integration is highly selective for a number of salient stimulus parameters and a long-range domain in which integration is relatively unsel ective and only requires that patterns be roughly collinear.