Observers never reported any afterimage of the surrounding circles, indicating that only afterimages of the central inducing stimulus were generated by the illusory display during the adaptation phase. The afterimage of the inducer persisted for approximately 10 s.
Figure 2 shows mean perceived diameter of the static target and the afterimage for the two display conditions. A two-way analysis of variance (ANOVA) was carried out on manual estimates with Phase (adaptation versus test) and Display (small flickering annulus versus big flickering annulus) as main factors. The main effect of Phase was significant (
F (1,5)= 14.194,
p = 0.013) as was the main effect of Display (
F (1,5)= 24.268,
p = 0.004), but their interaction was not (
F (1,5) = 0.635,
p = 0.462). The magnitude of the illusion as measured by the manual estimation task was 7.7% for the static target and 6.3% for its afterimage. These values are compatible with the range of 5–10% found on average (McCready,
1985; Rose & Bressan,
2002). Overall, an afterimage was perceived 9.7% smaller than its original stimulus possibly reflecting differences in luminance contrast between the adaptation and test phases (Weintraub, Tong, & Smith,
1973). Results from this experiment demonstrate that the apparent size of an afterimage is modulated by size-contrast illusions even when surrounding elements are no longer present on the retina. Moreover, the fact that the illusion operated with a flickering surround strongly suggests that lateral inhibition operating at the level of the retina was not the primary mechanism underlying the illusion.