A model of the perceived relative positions of moving objects based upon a slow averaging process.

We extend the local energy model of position detection to cope with temporally varying position signals and the perception of relative position. The extension entails two main components. First, a form of persistence for the position signal based on the temporal impulse response function of the visual system. Secondly, we hypothesise that the perceived relative position of two objects is determined by a slow average of the difference of the objects' position signals. The model explains why briefly flashed static dots are perceived to lag behind continuously visible moving dots, without the need for a motion extrapolation process [Nijhawan, R. (1994). Nature, 370, 256-257]. The dependence of this illusion on parameters such as the velocity, duration, frequency and number of flashes of the motion trajectories is accurately captured by the model. Furthermore, the model makes two predictions. First, briefly flashed dots on a staircase trajectory should lead dots with a long duration. Secondly, it should be possible to abolish the lag-effect between continuously visible and stroboscopically moving objects by halting the continuously visible dots during the interflash interval of the stroboscopic dots. Both predictions are corroborated in experiments.