Snakes and ladders: the role of temporal modulation in visual contour integration

We investigated temporal aspects of the cortical mechanisms supporting visual contour integration by measuring observers' efficiency at detecting fragmented contours, composed of Gabor micropatterns, embedded in a field of distractor elements. Gabors consisted of a static Gaussian enveloping a sinusoidal carrier which was temporally modulated by motion or counter-phase flicker. The elements forming the path could be oriented either parallel ('snakes') or perpendicular to the contour orientation ('ladders'). Sensitivity to contour structure (estimated by measuring the maximum tolerable element orientation jitter supporting contour detection) was increased when the elements were drifting or flickering. Snakes were more detectable than ladders under all conditions. The increase in sensitivity conferred by drifting carriers was present even when the elements in the same stimulus were drifting at a range of speeds spanning almost three octaves. These results lend further support to the notion that the contour integration system receives separate transient and sustained input.