The flash-lag effect. When a light flash (asterisk) is presented in alignment with moving stimulus (A), a moving object (the red bar in this example) is perceived to be leading the flash (B). The apparent lag increases as the speed of the moving object increases (C). The psychophysical function, i.e., the amount of lag as a function of object speed, can be determined by asking subjects to align the flash with their perception of the moving stimulus (see Figure 4).