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Fig. 7.1. Demonstration of the tilt aftereffect. Fixate your gaze on the circle inside the central diagram for at least 30 seconds, moving your eye slightly inside the circle to avoid developing strong afterimages. Now fixate on the diagram at the left. The vertical lines should appear slightly tilted clockwise; this phenomenon is called the direct tilt aftereffect. If you instead fixate upon the horizontal lines at the right, they should appear barely tilted counterclockwise, due to the indirect tilt aftereffect. Adapted from Campbell and Maffei 1971; reprinted from Bednar and Miikkulainen 2000b.