Figure 1.

Classes of visual motion and their corresponding space-time plots of luminance. (A) A simple moving object, such as a flying bee, produces for viewers approximate first-order motion, luminance in one position that correlates with luminance at an offset position and a later time. (B) If the luminance flickers, such as with fluttering butterfly wings, the luminance correlation can break down, and luminance-related signals can end up uncorrelated or negatively correlated after a spatial and temporal offset. This illustrates a case of second-order motion. (C) If the internal motion of an object is opposite to its motion as a whole, as with the wing of this maple fruit when it is twirling in the opposite direction of its trajectory, it produces another type of second-order motion called theta motion, in which the first-order luminance correlations and second-order motion cues occur in opposite directions. Whereas stimuli in both (B,C) contain some first-order motion, it is weak by comparison to the higher-order cues.