Anstis' Contour Adaptation
from Michael’s Visual Phenomena & Optical Illusions
This bicycle will loose its wheels… Press the button Flicker, and keep your gaze steadily on the green cross in the center. When the flicker stops, the wheels are gone – only to reappear after a few seconds.
The contrast slider on the right should be set such that the wheels of the bicycle are just –but clearly– visible. The duration of the flicker is pre-set to 8 s, longer times increase the suppression effect, it then works on higher contrasts. You can also check out various flicker frequencies, pre-set to 5 Hz.
If you shift your gaze slightly right after the flicker stops, the wheels will reappear immediately.
Inspired by Stuart Anstis’ presentation at the ECVP 2012, adapted with his kind permission (a little later, in early 2013, his pertinent paper in JOV came out, see below).
This is an application of contrast adaptation: the high contrast during the flicker phase shifts the contrast transfer function (locally!) to the right. Thus the small luminance step of the contour that defines the wheels becomes sub-threshold. It’s interesting (but fits well with our understanding of early vision) that loss of step-perception leads to loss of the entire area (the wheels in this example). This type of contrast gain control with a time constant of some seconds probably operates not in the eye, but in the primary visual cortex (Heinrich & Bach 2001).
Related phenoma on this site are here: Contrast Gain Control and here: Contrast Constancy.
Anstis S (2013) Contour adaptation J Vis:Vol 13, No. 2, 25
Heinrich TS, Bach M (2001) Contrast adaptation in human retina and cortex. Invest Ophthalmol Vis Sci 42:2721–2727 [PDF]