Spatial limitations of temporal segmentation.

We investigated the spatial parameters that permit temporal phase segmentation. Subjects identified a stimulus quadrant which was modulated 180 degrees out of phase with the rest of the stimulus at temporal frequencies between 2 and 30 Hz. We determined the modulation sensitivity for regular square lattices of Gaussian spots and a stimulus made from solid quadrants with varying separation. Sensitivity declined rapidly when spatial separation of the modulating areas was approximately 0.4 degree, but was relatively unchanged by further spatial separations. The results suggest that there are two systems that can detect temporal phase differences. The first is a segregation process that operates below 10 Hz, where phase can be consciously followed and compared across large retinal distances. The second system is a segmentation mechanism that operates at higher temporal frequencies but only over a short range.