We conducted two psychophysical experiments to show that mechanisms mediating Café Wall illusion are situated within lower levels of the visual system.

The stimuli for the two experiments consisted of three sinusoidal gratings and two gray lines that were presented on a CRT screen. The task for both experiments was to judge the direction of illusory tilt of gray lines (2AFC). In the first experiment, each stimulus was divided into two parts, the top and bottom gratings and the middle. Subjects observed them through a mirror stereoscope and fused them as one figure. The stimulus subtended 5.7(H) x 4.4(V) deg, and the spatial frequency for the gratings was 0.35 cpd. In the second experiment, the stimuli were flickered in counterphase. The gratings' spatial frequency appears doubled at higher temporal frequencies because of the frequency doubling illusion (Kelly, 1966). We examined whether the apparent frequency, thus phase difference induced by the frequency-doubling illusion affected the direction of illusory tilt. The stimuli subtended 8(H) x 4.48(V) deg, and the spatial frequency for the grating was 0.25 cpd. The modulation frequency was varied between 5 and 60 Hz.

In the first experiment, the dichoptically presented Café Wall figure did not, or very faintly, if any, induce the illusory tilt. In the second experiment, the physical, not the apparent phase difference determined the direction of the illusory tilt.

These results suggested that Café Wall illusion occurs at a level before binocular fusion, and this could be earlier than the cortex.