Does binocular disparity or familiar size information override effects of relative size on judgements of time to contact?

Abstract

Previous studies indicate that non-tau sources of depth information, such as pictorial depth cues, can influence judgements of time to contact (TTC). The effect of relative size on such judgements, the size-arrival effect, is particularly robust. However, earlier studies of the size-arrival effect did not include binocular disparity or familiar size information. The effects of these cues on relative TTC judgements were measured. Results suggested that disparity can eliminate the size-arrival effect but that the amount of disparity needed to do so is greater than typical stereoacuity thresholds. In contrast, familiar size eliminated the size-arrival effect even when disparity information was not available. Furthermore, disparity contributed more to performance when familiar size was present than when it was absent. Consistent with previous studies, TTC judgements were influenced by multiple sources of information. The present results suggested further that familiar size is one such source of information and that familiar size moderates the influence of binocular disparity information.

Acknowledgments

This work was supported in part by the Texas Advanced Research Program under grants Nos. 003644-0017-1997 and 003644-0081-2001.

I thank Erin L. Braddock, Jason M. Bush, Eric W. Holder, and Elizabeth M. Williamson for assistance with various aspects of scene development, data collection, or data entry. I also thank Robert E. Todd of ESI Technology for technical assistance.

Notes

¹The longer duration was used to maximize the effectiveness of binocular disparity when it was added in Experiment 3. The time necessary to perceive depth in a stereoscopic display is greater for complex displays (such as those used here) than for simple displays (Julesz, 1971).

²Unexpectedly, the size-arrival effect also did not occur for scenes with nontextured spheres, yet it occurred for scenes with textured spheres. One possible explanation for these findings is that, in the absence of explicit familiar-size information, some observers nevertheless may have assumed that the nontextured spheres represented objects familiar to them. If these assumed objects approximated the ratios represented by the marble and beach ball in the familiar-size scenes, the occurrence of the size-arrival effect would not be expected. In contrast, when both objects contained the same checkerboard texture, observers may have assumed that the objects were equal in virtual size, resulting in the size-arrival effect. According to this explanation, one might argue that the scenes with line-drawn squares should not have resulted in the size-arrival effect because observers assumed that the squares represented familiar objects (as they putatively did for the nontextured spheres). However, this seems less likely. Whereas spheres comprise a large number of moving objects encountered in the real world (balls), squares do not. In any case, twice as many observers reported the size-arrival effect for the nontextured spheres as for the familiar-size scenes (8 observers vs. 4 observers).

³Relative binocular disparities were computed from the parameters of the virtual scenes with formulas published previously (Cormack & Fox, 1985; Kaufman, 1974). Participants viewed the displays from the canonical distance (DeLucia, 1991a; Hochberg, 1986) so that the optical properties subtended at the participant's eyes approximated those subtended at the virtual eyes.

⁴According to constructivist theories, familiar size is the only depth cue that, by definition, requires past experience (Hochberg, 1971), and an effect of familiar size implicates cognitive mechanisms associated with traditional empirical theories of depth perception (Hochberg, 1978). Results of earlier studies also implicated cognitive processes in TTC judgements (DeLucia, 2004a; DeLucia & Liddell, 1998; DeLucia & Novak, 1997). However, known size has been characterized as a powerful cue to distance perception without reference to such traditional theories (Wann, Mon-Williams, McIntosh, Smyth, & Milner, 2001).