**6.1.3 Design**

Each participant gave responses to individual stimuli selected from an up-down staircase procedure. There were 10 blocks of 6 trials each in which two trials from each of three staircases were randomly interleaved. The starting values for the three staircases were either approximately 12°, 42°, and 72° (N=6) or 18°, 48° and 78° (N=6). The step size of each staircase was 18°. That is, if the presented orientation was deemed closer to vertical, the next orientation presented by that staircase was 18° lower, and if the presented orientation was judged closer to horizontal, the next presented orientation was 18° higher. The three staircases together sampled orientation space with a resolution of 6° and approximated a method of constant stimuli that was centered on the apparent bisection point.
