Frequency-doubling illusion under scotopic illumination and in peripheral vision

The authors sought to determine whether frequency-doubling illusion (FDI) could be perceived under scotopic illumination at central and peripheral retinal locations. For comparison, perception of the FDI at the central and peripheral retina under photopic illumination was also evaluated.

Five subjects matched the apparent spatial frequency of counterphase flickering sinusoidal gratings with stationary sinusoidal gratings presented foveally and out to 20 degrees eccentricity under photopic and scotopic illumination conditions. Two spatial frequencies (0.25 and 0.50 cpd) were used at four temporal frequencies (2, 8, 15, and 25 Hz). Subsequent experiments explored the range of spatial and temporal frequency stimulus conditions under which the scotopic FDI might be observed.

Under scotopic illumination conditions, the apparent spatial frequency of eccentrically presented 0.25- and 0.50-cpd flickering gratings gradually increased as a function of flicker frequency and approaches "doubling" at 15 Hz. Under photopic conditions, the apparent spatial frequency of 0.25-cpd flickering at 25 Hz was approximately doubled in all four primary meridians at central and peripheral eccentricities. The final experiment showed that the spatiotemporal range under which the scotopic FDI could be seen was similar to the photopic illumination condition reported earlier.

Scotopic FDI is similar to photopic FDI at the central and the peripheral retina. This suggests that similar mechanisms are responsible for generating the illusion under both photopic and scotopic illumination conditions.