Spatial structure of the frequency doubling illusion

We sought to determine (1) the perceived spatial frequency of flickering sinusoidal gratings as a function of temporal frequency at contrasts down to detection threshold levels, and (2) the influence of two different matching criteria on reported size matches to identify changes in the apparent spatial structure introduced by counterphase flicker. To determine if spatial structure was observable at detection threshold contrasts, we simultaneously measured contrast detection and orientation–identification thresholds of flickering gratings. We then measured the apparent size of flickering grating bars at sub-threshold to supra-threshold contrast levels using two matching criteria: (1) by adjusting the spatial frequency of a stationary comparison grating and (2) by adjusting the distance between two parallel thin lines. For all subjects, contrast detection and orientation–identification 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 periodicity of low (0.25 and 0.50 cpd) spatial frequency test gratings increased. At detection threshold contrast levels and below, all observers reported difficulty in performing the match and results were variable. Size matches using stationary gratings were consistently smaller than those made using the line targets, which is suggestive of distortion in perceived spatial structure at the nodal region of flickering gratings.