**6.1.4 Procedure**

Participants were shown the apparatus with the surface in the horizontal position and the procedure was explained to them prior to signing an informed consent form. Participants were shown where to stand (directly in front of the apparatus) and then asked put on the blindfold. Before each trial, the surfaces were set to the required orientation manually by the experimenter according to a computer instruction. The participant then explored the surface as in Experiment 1. When the participant gave the forced choice response ("closer to vertical" or "closer to horizontal"), the experimenter pressed either the up-arrow key or the down arrow key on a keyboard, causing the computer to record the trial and update the staircase. The computer then gave instruction to the experimenter concerning the orientation of the next stimulus.
