Abstract
Spatial frequency-doubling occurs when a low spatial frequency sinusoidal grating is modulated at high temporal rates — its apparent spatial frequency increases. What does this mean for estimates of the local feature size of such a stimulus? We sought to determine: 1) the perceived size of local features as a function of temporal frequency and contrast down to detection thresholds, & 2) the influence of two different criteria on reported size matches. To determine if spatial structure was observable at detection threshold contrasts, we simultaneously measured contrast detection and orientation discrimination threshold of flickering gratings (500 ms exposure) using a 2-AFC, method of constant stimuli task. In subsequent experiments, the apparent size of 0.25 or 0.5 cpd (randomly interleaved) flickering grating bars was determined at 3 contrast levels (1x, 2x and 8x-detection threshold) by two methods: 1) by adjusting the spatial frequency of a stationary grating & 2) by adjusting the distance between two parallel thin lines. The initial spatial frequency of the stationary grating and separation between the lines was randomised before each setting was made. Four observers made 50 size settings under each condition at 3 temporal frequencies (1, 10, & 21Hz). For all subjects, contrast detection & orientation discrimination thresholds were similar, indicating that sufficient spatial structure is observable at detection threshold to make judgements of local spatially defined features. With increasing flicker rate, the apparent size of local features shrink. At detection threshold, all observers indicated that size of local features are smaller, however results were variable and all subjects reported difficulty in performing the match. At higher contrasts, the perceived size of the local features increased but was still smaller than that for stationary gratings. Size matches using static gratings were consistently smaller than those made using the line targets.