Abstract
18 informed female Os were studied under 3 viewing conditions: free inspection without fixation cross, free inspection with central fixation cross, and fixation with central fixation cross. Transversal angles of 22.5°, 45.0°, 67-5°, and 90.0° were investigated under each viewing condition and constant error measures of the illusion obtained. Addition of the fixation cross did not significantly change the illusion in the free inspection conditions for any of the angles. Illusion magnitude again varied as a function of angle. Fixation of the cross significantly reduced but did not abolish the illusion for the 22.5°, 45.0°, and 67.5° angles. No significant magnitude of illusion was found under free inspection for the 90.0° angle; however, a small significant magnitude of illusion was observed under fixation for the 90.0° angle in a direction opposite to that observed for the smaller angles.
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