The FSN requires participants to find a “magic ball” in one of four quadrants printed onto an acrylic plate. The circular form appears to recede from the surrounding foreground pattern. This effect is achieved by the circular area being printed onto the back of the acrylic plate, relative to the foreground in the target quadrant and the three distracter quadrants which are printed on the front face of the plate. The task provides three levels of difficulty, achieved by plates of various thickness (6 mm, 3 mm and 1.5 mm which is the more “difficult” trial). Using the thickest plate participants were shown how to locate the “magic ball”, by moving the plate laterally to induce motion parallax to highlight the target area in participants that did not spontaneously locate it. For test trials the plates were held at varying distances away from the participant until the participant can no longer reliably detect the “magic ball” on each plate. The plates were randomly rotated for each trial to ensure that the target quadrant was not constant. Stereoacuity (arcsec) was calculated for each participant using their interpupillary distance (mm) and the greatest distance for which the task could be reliability completed (mm) recorded by the experimenter.