Perceived motion in orientational afterimages: direction and speed

Two sets of experiments demonstrate new properties of motion in orientation al after-effects. In a previous report, we showed that when observers adapt ed to a static bar grating whose elements varied in size or intensity from one side to the other, offset of the grating resulted in a motion after-eff ect, with the perceived motion in the direction of the largest or most inte nse bar. In the first new experiment, we show that similar results can be p roduced by varying the duration of the bar elements, with the direction of the motion after-effect toward the bar with the longest duration. In the se cond new experiment we demonstrate that the perceived speed of the motion a ftereffect is influenced by the spatial extent of the after-effect, with la rger extents corresponding to faster speeds. The experimental findings are discussed in the context of a neural network theory of visual perception, i n this theory, a moving oriented contour leaves a trail of activity among c ortical cells tuned to orthogonal orientations. We hypothesize that the gra ting stimuli produce after-effects that mimic the pattern of oriented respo nses produced by a true moving contour, and the visual system interprets th is pattern as a cue for motion. We also show how the model connects the pro perties of these motion after-effects to properties of visual persistence. (C) 2001 Elsevier Science Ltd. All rights reserved.