We shall first show qualitative demonstrations of contour adaptation, followed by some quantitative measurements. All stimulus movies were programmed in Director 11 (Adobe, San Jose, CA) and displayed on the monitor of a 2700 iMac computer (Apple, Cupertino, CA). They were viewed from a distance of 57 cm in a dimly lit room. All screen luminances were calibrated with a Minolta Chromameter II photometer (Konica Minolta, Ramsey, NJ). (In this article, all the movies have been exported into Quicktime [Apple, Cupertino, CA]. All movies should be run in a loop mode).
Movie 1 demonstrates contour adaptation. Six contour-rich eight-pointed stars are grouped around a central fixation point. The three light-gray stars on the right are all the same, with a Michelson contrast of about +7% (depending on your monitor). The three dark-gray stars on the left are all the same, with a Michelson contrast of about −7%. (We use positive and negative numbers for the Michelson contrast of spatial increments and decrements.) All stars are about equally salient. But now run the movie. First you see a dynamic adapting field, in which the two top stars flicker between black and white at 5 Hz, while below it the edges that outline the two bottom stars also flicker between black and white. Since the time-averaged black and white have the same mean luminance as the mid-gray surround, these flickering stimuli generate no visible afterimages. However, they do leave an invisible area of reduced sensitivity to contrast.
After a few seconds of adaptation the original static test stars reappear in
Movie 1 (after which the adapting cycle restarts.) The two unadapted lateral stars, positioned at nine and three o'clock, are kept visible for comparison purposes, but adaptation makes the other four test stars disappear from view, even though they are still present on the screen. Note that both types of adapting fields—flickering solid stars and flickering outlines—reduce the sensitivity of the visual system enough to make both the light and dark test stars subjectively disappear.
It is not necessary to use thin spiky stars in which all parts of the star are near to an edge. In
Movie 2, outline circles that flicker at different rates are fixated for 3 s, followed by low-contrast gray test disks of the same size and of Michelson contrast ∼5%. Result: The test disks completely disappear from view. This works for disks of radii between 1° and 10°. Test disks of higher contrast (not shown) look like very blurry blobs following adaptation.
More complex shapes can adapt.
Movie 3 shows two identical test patterns side by side, namely low-contrast lith portraits of Dr. Michael Webster. Fixate the central red spot while a flickering
outline of the right-hand portrait adapts your visual system. When the two test portraits reappear, the right-hand one will have apparently vanished, though it is still present on the screen.