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Adjusting a rod to the gravitational vertical in the presence of a large square or rectangular tilted frame leads to a significant deviation towards the orientation of the frame. According to the «vectional hypothesis», the tilted frame produces a self-tilt illusion in the observer, in a direction opposite to that of the frame. This self-tilt illusion is similar, by analogy, to the self-motion induced by rotating peripheral displays. A compensatory process, engaged in response to the illusory body tilt induced by the tilted frame, would explain the deviation of the subjective vertical towards the direction of the frame. However, although widely held, there is little direct evidence supporting this hypothesis. Moreover, other empirical observations support an alternative possibility that the frame effect, like the Dietzel-Roelofs effect, could arise because of a tilt of the apparent median plane of the observer towards the frame. This hypothesis may be referred as «the apparent-median-plane-tilt hypothesis». The aim of this paper is threefold. First, we describe a number of different hypotheses about the frame effect. We focus on two : the vectional hypothesis and the apparent-median-plane-tilt hypothesis. Second, we attempt to explore the generality of the vectional hypothesis and third, we try to distinguish between the vectional hypothesis and the apparent-median-plane-tilt hypothesis.
