Apparatus
The experimental apparatus consisted of a wooden background panel with a diameter of 35 cm
(see Fig. 1). The panel was round on top and covered with black felt to provide a homogeneous background
without landmarks. A shaft went through the center of the panel. At the front end of the shaft,
a bracket with magnets allowed fast and effortless mounting of the stimulus glasses, which then could
be turned in the frontoparallel plane. At the back end of the shaft, a pointer indicated the angular position
of the shaft on a scale. The experimenter, who was seated to the side of the apparatus, could see
the scale, but the panel hid the scale from the participants’ view.