Abstract
Previous research has shown that flickering elements can be grouped on the basis of temporal phase differences. We investigated the spatial and temporal parameters that permit temporal phase segmentation. The stimulus consisted of a regular square lattice of Gaussian spots modulated in luminance against a background of 54 cd m−2. Subjects identified one quadrant which was modulated 180° out of phase to the other quadrants. The modulation threshold was found for temporal frequencies between 4 and 30 Hz. Four Gaussian spot sizes with standard deviations from 0.05 to 0.43 deg were used: lattice spacing was proportional to dot size.
We found that detection of phase differences between large spots failed at about 10 Hz. However, for the smallest dots the task was possible at frequencies of 30 Hz. Furthermore, the elements at the border of the quadrant were necessary and sufficient for maximal sensitivity. Using solid quadrants modulated at 15 Hz, we found that the maximum separation between the flickering quadrants lies between 0.5 to 1.0 deg. The results suggest that there are two systems that use temporal phase for segmentation. The first system operates below 10 Hz, where phase can be consciously followed and integrated over a large area. The second system operates at higher frequencies but only within confined regions of the visual field. Image segmentation for stimuli modulated at high temporal frequencies is not determined by long-range phase synchronisation. We suggest that detection may be mediated by a second-order nonlinearity.
Get full access to this article
View all access options for this article.