HUMANS COMBINE THE OPTIC FLOW WITH STATIC DEPTH CUES FOR ROBUST PERCEPTION OF HEADING

The retinal flow during normal locomotion contains components due to r otation and translation of the observer. The translatory part of the f low-pattern is informative of heading, because it radiates outward fro m the direction of heading. However, it is not directly accessible fro m the retinal flow. Nevertheless, humans can perceive their direction of heading from the compound retinal flow without need for extra-retin al signals that indicate the rotation. Two classes of models have been proposed to explain the visual decomposition of the retinal flow into its constituent parts. One type relies on local operations to remove the rotational part of the flow field. The other type explicitly deter mines the direction and magnitude of the rotation from the global reti nal flow, for subsequent removal. According to the former model, nearb y points are most reliable for estimating one's heading. In the latter type of model the quality of the heading estimate depends on the accu racy with which the ego-rotation is determined and is therefore most r eliable when based on the most distant points. We report that subjects underestimate the eccentricity of heading, relative to the fixated po int in the ground plane, when the visible range of the ground plane is reduced. Moreover we find that in perception of heading, humans can t olerate more noise than the optimal observer (in the least squares sen se) would do if only using optic flow. The latter finding argues again st both schemes because ultimately both classes of model are limited i n their noise tolerance to that of the optimal observer, which uses al l information available in the optic flow. Apparently humans use more information than is present in the optic flow. Both aspects of human p erformance are consistent with the use of static depth information in addition to the optic flow to select the most distant points. Processi ng of the bow of these selected points provides the most reliable esti mate of the ego-rotation. Subsequent estimates of the heading directio n, obtained from the translatory component of the flow, are robust wit h respect to noise. In such a scheme heading estimates are subject to systematic errors, similar to those reported, if the most distant poin ts are not much further away than the fixation point, because the ego- rotation is underestimated.