Purpose: The "frequency-doubling" effect or illusion, in which the apparent spatial frequency of a grating appears to be doubled, is produced when a low spatial frequency sinusoidal grating is counterphase flickered at a high temporal frequency. This study examines changes in apparent spatial frequency of sinusoids of various spatiotemporal content after equating them for detectability.
Methods: Detection contrast thresholds were determined for various spatiotemporal gratings. The perceived spatial frequency was then evaluated using stimuli that were four and six times detection threshold. Match ratios were determined for each spatiotemporal combination, whereby the periodicity of a stationary sinusoid was matched with that of a counterphase flickered grating. The same matching task was repeated under different matching task instructions for five randomly chosen spatiotemporal combinations.
Results: Match ratios were fractional for many spatiotemporal combinations. Apparent spatial frequency ranged from less than veridical to greater than double depending on observer and spatiotemporal stimulus content. The effect of task instruction on match ratios was very small, yet significant.
Conclusions: The appearance of fractional spatial frequency percepts draws into question the notion that frequency-doubling is solely generated by spatially non-linear Y-type magnocellular ganglion cells.