Abstract
With mounting evidence that an extra-retinal eye movement signal is used to disambiguate depth order in motion parallax (MP) (Nawrot, 2003a; 2003b; Naji & Freeman, 2004; Nawrot et al. 2004), the perception of vivid and unambiguous MP in motion pictures presents an interesting and important challenge to this eye movement theory. It is unclear how a stationary observer viewing a motion picture on stationary screen might be generating the prescribed extra-retinal eye movement signal. Filmmakers have long recognized the depth-enhancing value of parallax created by camera translation. An under-appreciated aspect of this camera translation is the broad background motion field created behind the object of interest at center-of-frame. We hypothesized that this background motion drives a reflexive OKR signal and a countermanding pursuit signal is generated to maintain fixation on the object of interest. This pursuit signal is then available to disambiguate the perception of depth from MP. This hypothesis was tested using a random-dot MP stimulus (Rogers & Graham, 1979) presented upon a translating OKR grating background. Observers remained stationary and were required to maintain fixation on a stationary point on the display (monitored by video based eye tracking). Results show that background translation was sufficient to disambiguate the perceived depth of this MP stimulus. Perceived depth order was identical to that found in all other MP conditions; dots moving on the retina in the same direction as the pursuit signal are perceived as nearer than fixation. A static grating did not disambiguate the perceived depth of the MP stimulus. We conclude that the eye movement theory provides a parsimonious explanation for unambiguous depth from MP in many different conditions. Indeed, the neural mechanism might be found in MSTd where many neurons respond to broad field motion in one direction and to pursuit eye movements in the opposite direction (Komatsu & Wurtz, 1988).
Work supported by NIH EY12541