When you look at the twisted cord illusion it should seem to you that the lines are bent one way or another, and tend to be bowed near the center of the window. The lines are straight and parallel.
The twisted cord effect is likely due to orientation-sensitive simple cells in the striate cortex, which interact to combine closely-spaced tilted lines into a single tilted line. You can test the effects of changing the width of the lines with the Increase Line Width and Decrease Line Width menu commands. Use the Increase Cord Spacing and Decrease Cord Spacing commands to change the distance between the lines. Use the Increase Cord Length and Decrease Cord Length commands to change the length of the individual black and white strands. Use the Invert command to reverse the direction of tilt.
References
Coren, S. & Girgus, J.S. (1978). Seeing is Deceiving: The Psychology of Visual Illusions. Hillsdale, New Jersey: Lawrence Erlbaum Associates.
Luckiesh, M. (1965). Visual Illusions, Their Causes, Characteristics and Applications. New York: Dover.
