Using a "virtual illusion" to put parallax in its place

Visual illusions and ambiguous figures have provided researchers with important insights into how the human visual system interprets its visual environment. The advent of computers capable of producing compelling virtual scenes has enabled researchers to use complex environments whilst retaining the controlability and repeatability of traditional experimental designs. Having control of the environment permits certain rules which apply in the real world to be broken, creating new types of visual illusions. In this experiment, we moved an object in a room in antiphase with the simulated motion of the observer, ie we manipulated motion-parallax information for this object relative to all others. Under monocular viewing, observers perceived the object not as moving, but as closer and smaller than before. Thus, the perceived depth, size, and location of an object can be controlled via the motion-parallax cue. We also tested stereo viewing and found that, if the object remained within the useful stereo range, the object's real position was re-established and it was correctly perceived as moving. The results indicate that motion parallax is an important cue for deciding the distance of an object from the observer, overriding other cues such as prior experience (a chair is usually on the floor and not toy-sized). However, it is not sufficient to counteract these cues in conjunction with correct stereo information. We discuss these results and go on to consider how virtual reality opens the door to a new generation of virtual visual illusions, tailored to investigate specific perceptual tasks