Perceived depth from observer-induced motion parallax determined with size-distance invariance

Purpose. The perceived size of an object of fixed angular subtense increases with perceived distance. Previously, Nawrot & Blake (1994, ARVO, 35/4, p. 1985) used size-distance invariance to generate an objective measure of depth perceived in kinetic depth (KD) and dynamic stereopsis (DS) displays. The current study extends the same 2AFC size discrimination paradigm to include the perception of depth from computer generated motion parallax (MP) yoked to horizontal head movements in the fronto-parallel plane. If the depth perceived in a MP display is similar to other cues for depth perception (KD and DS), then perceived depth should influence perceived size and provide a means for comparing MP with KD and DS. Methods. Observers monocularly viewed a CRT depiction of a vertical square wire framework viewed on-edge. Observer's head movements, digitized from a mechanical head movement device, produced changes in the CRT depiction of this frame stimulus thereby generating the perception of depth from MP. Two smaller squares, oriented fronto-parallel to the observer, appeared rigidly attached to opposite sides of the frame, one near the top and the other near the bottom. On any given trial, one square was physically larger than the other, and the observer's task was to indicate which square - top or bottom - this appeared to be. The perceived depth of each square, one nearer than fixation and the other farther than fixation, should influence their perceived size and the relative size judgement. Results and Conclusion. The perceived size of the squares is influenced by the depth perceived in the MP display. This is similar to the effect previously found for both DS and KD. That is, to achieve size matches, the square appearing nearer must be physically larger than the square appearing farther away. This is the effect of size-distance invariance. Furthermore, from the resulting size matches, the magnitude of perceived depth may be calculated. The depth perceived in this MP display is similar to that perceived in the analogous KD display, and is less than in the analogous DS display.