Apparent distance reduction with moving stimuli (Tandem Effect): evidence for an attention-shifting model.

When two vertical rods move through a horizontal window in close succession, the Tandem Effect can be observed. It consists of a spatial illusion (distance between the rods looking smaller than it actually is) and a temporal illusion (under certain conditions both rods are seen simultaneously in the window, though the first rod has left the window before the second rod enters it). We report six experiments that explored the distance-reduction illusion and tested an attentional model of the effect. It assumes that attention is initially focused on the first rod and then shifted to the second, when it enters the window. The percept of the pair of rods is integrated from the first rod's position at the beginning, and the second rod's position at the end, of the focus shift. Consequently their subjective distance will be smaller than their physical distance by the distance that they travel during the focus shift. Experiments 1 and 2 established the Tandem Effect as an empirical phenomenon and showed that its size depends on stimulus parameters such as window size and movement speed. Experiments 3-5 tested specific predictions from the attentional model. Experiment 6 examined a further illusion, the Fröhlich Effect, and showed that it can be subsumed under the model. The experiments produced some unexpected effects and some predictions from the model were only partly confirmed. It is shown that the main findings can be combined into two quantitative functions that describe the course of focusing. One implication is that visual attention does not "move" from one object to another; rather all attention shifts originate in the fovea. We discuss several alternative interpretations of our data and show that they are less satisfactory than the attentional model.