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P.T. Quaid, T. Simpson, J.G. Flanagan; Monocular and Dichoptic Masking Effects on the Frequency Doubling Illusion . Invest. Ophthalmol. Vis. Sci. 2003;44(13):54.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To assess the effects of temporal and spatial frequency masking on the frequency doubling (FD) illusion. Methods: The right eye of 10 normal, healthy subjects (5 female, mean age 25, SD 1.549) was examined at 10 degrees nasal and at fixation. The FD stimulus was generated using a custom Unix based system and a Sony 20" monitor. The stimulus was masked using a second monitor with a full-field, sinusoidal flicker (10, 16.67, 25, and 50Hz) of the same mean spatial and temporal luminance. Monocular experiments were performed with a 50:50 beam splitter in front of the right eye. Dichoptic experiments were performed with a front surface mirror placed in front of the left eye (viewing the masking stimulus). The right eye was used to determine the FD thresholds using a modified "rapid estimation by binary search" thresholding paradigm. Similar experiments were performed with a spatial mask using a slowly drifting grating (0.3 degrees/sec) with spatial frequencies of 0.1, 0.2, 0.3, 0.4 and 0.5 cycles per degree (cpd). Results: Temporal tuning effects were observed peaking at 25Hz dichoptically (p<0.03) and at 10 degrees monocularly (p<0.002). The monocular experiment at fixation exhibited an increase in threshold at 25 and 50Hz (p<0.003). Monocular spatial frequency tuning effects were observed peaking around 0.25cpd (p<0.03). Dichoptic spatial frequency masking exhibited peak tuning at the doubled frequency of 0.5cpd (p<0.02). Conclusions: These results suggest that thresholds determined for the FD illusion are dominated by a flicker component and the dichoptic masking results point to a cortical origin of the FD illusion.
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