Visually Perceived Eye Level and Horizontal Midline of the Body Trunk Influenced by Optic Flow

Abstract

The eye level and the horizontal midline of the body trunk can serve, respectively, as references for judging the vertical and horizontal egocentric directions. We investigated whether the optic-flow pattern, which is the dynamic motion information generated when one moves in the visual world, can be used by the visual system to determine and calibrate these two references. Using a virtual-reality setup to generate the optic-flow pattern, we showed that judged elevation of the eye level and the azimuth of the horizontal midline of the body trunk are biased toward the positional placement of the focus of expansion (FOE) of the optic-flow pattern. Furthermore, for the vertical reference, prolonged viewing of an optic-flow pattern with lowered FOE not only causes a lowered judged eye level after removal of the optic-flow pattern, but also an overestimation of distance in the dark. This is equivalent to a reduction in the judged angular declination of the object after adaptation, indicating that the optic-flow information also plays a role in calibrating the extraretinal signals used to establish the vertical reference.

The linear perspective information in ground surface representation and distance judgment

“…These data were obtained in a previous study with a different group of observers in our laboratory (B. Wu, 2004). Clearly, these data reveal that distance is underestimated in the dark when the ground surface is not visible Philbeck & Loomis, 1997;J. Wu et al, 2005).…”

“…We believe this main effect of target distance is likely to be due to the observer's fixation position, because in a separate study at our laboratory (J. Wu et al, 2005) we found that the eye fixation position can affect the judged eye level. Recall that during the judged eye-level measurement in the present experiment, observers fixated their eyes on a reference target on the floor (immediately below the test target).…”

“…(For simplicity, in the present article we will assume that the implicit surface has a constant slant.) The implicit surface causes the distance of a dimly lit target on the level ground surface to be underestimated in the dark Philbeck & Loomis, 1997;J. Wu et al, 2005).…”

“…J. Wu et al (2005) revealed that the position of the FOE influences the perceived eye level. The study also showed an adaptation effect of the optic flow on the perceived eye level.…”

“…For example, our laboratory recently showed that optic flow information also influences the perceived eye level (J. Wu et al, 2005). When one walks on a level floor surface with a steady, straight-ahead eye fixation, the focus of expansion (FOE) of the optic flow coincides with the observer's eye level.…”