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Abstract

Purpose: Judging the orientation of bilateral objects, e.g. faces, is a task that humans perform everyday. In order to achieve some understanding of the mechanisms underlying this ability, we investigated the influence of directional light on perceived orientation. Changing the illuminant's position induces a substantial apparent orientation shift. We documented this phenomenon and measured its size as a function of the mean orientation of the face and the angle between the two light directions. To assure that the phenomenon is based on perceived orientation of the face rather than on gaze direction, we repeated the experiment using surface models with constant albedo.
Methods: Using a 2AFC paradigm, the participants decided whether a sequence of two images appeared to rotate to the left or to the right. The images showed the same face in different orientations and illuminated from different directions. Between the two images, a distractor was shown for 2 sec. The physical rotation nullifying the illumination induced apparent orientation shift was determined.
Results: Depending on the set of parameters used, an apparent orientation shift of up to 10 degs can be induced. The effect is minimal when the face is shown in a frontal view and maximal if the face is oriented 30 to 45 degs from frontal. The increase of the apparent orientation shift with the angle between the two light directions saturates at an angle of 60 degs between the light directions. The phenomenon does not require photorealisticly textured stimuli. Using surface models with constant albedo yields similar results.
Conclusions: The results are discussed in the context of possible strategies for orientation judgement. The finding that the magnitude of the effect is dependent on the mean orientation of the face implies that the effect is unlikely to be based only on local surface attitude judgements. Rather, we favor a model which assumes that first the symmetry plane of the face is detected, and then a comparison is made between the visible parts on both sides of this plane to estimate orientation.