Reversed phi revisited.

Two briefly flashed lines shown in rapid succession evoke a sensation of motion in human observers. This is examined quantitatively such that line separation, temporal offset, and contrast polarity are varied. Line pairs are presented on a gray background and are either both bright or both dark (equal contrast polarity) or one line is bright and the other is dark (opposite contrast polarity). Observers are instructed to indicate the perceived direction of motion. With foveal viewing, perceived direction is veridical for line pairs with equal contrast polarity but is reversed for line pairs with opposite contrast polarity, with spatial separations between 0 and 12 arcmin, and with temporal offsets between 8 and 33 ms. When separations, temporal offsets, or both are further increased, perception reverts to veridical. When lines of opposite contrast polarity are presented in the parafovea, reversal of perceived direction is also observed. The psychophysical results correlate well with those of recent intracellular recordings from directionally selective simple cells in the primary visual cortex of anesthetized cats (N. J. Priebe & D. Ferster, 2005).