The motion aftereffect without motion: how adaptation to non-directional flicker creates a directional aftereffect in the motion system

We describe an unusual motion aftereffect that probes early stages of motion coding psychophysically. Adapting contrast flickers in sawtooth fashion over time, but neither the adapter nor the test pattern actually moves. This compelling aftereffect, unlike the classic one, reverses direction when test contrast is inverted. Its strength, measured by nulling, is nearly proportional to adapting contrast and is a bandpass function of adapting flicker rate, peaking at 4 Hz. Standard motion models combine non-directional filters that encode spatial and temporal gradients of the input image. In our modified model the non-directional input filters also adapt, and evoke negative afterimages that represent illusory temporal gradients. These combine with spatial gradients of the test image to form illusory motion signals. Complex 2-D and 3-D illusory motions can be evoked, but the source of adaptation is probably retinal or LGN cells that sense the way local image brightness changes over tim