Your brain is a taskmaster that often makes individual neurons perform multiple operations at the same time. Like any other overworked laborers forced to juggle too many responsibilities, overwrought nerve cells are prone to make mistakes.
Focus on the star in the center of Figure A. Slowly move your head toward the page, then away from it. The rotation you perceive is called the Pinna-Brelstaff illusion. Vision researchers Baingio Pinna and Gavin Brelstaff theorize that illusory rotation arises from the brain's strategy of making certain neurons responsible for detecting both the orientation and
the direction of movement of visual lines and curves.
Neurons in the visual cortex of the brain are organized into subgroups, each of which responds best to lines oriented at a specific angle. Neurons that "prefer" the particular angle of an object viewed at any given moment are more active than those preferring other orientations. A subgroup of visual neurons gets most excited when a line with a preferred orientation is in motion and the direction of that motion is at a right angle to the line's orientation.
Just as the brain determines the orientation of objects by "looking" at which groups of orientation-selective neurons are active, it also assesses the direction of motion of objects by the activity of those same nerve cells. This doubling up of orientation and motion detection works great if a line is moving at right angles to its orientation, but if the line is moving in any other direction, the brain gets confused.
When you move your head toward Figure A, both circles appear to expand. Each bar slides outward across your retina, stimulating cells tuned to that bar's particular tilt. Because those cells are doing double duty, the stimulation convinces them that the bars are moving in a perpendicular direction as well. The two motions, added together, create an illusion of circular motion. Figure B reveals how this works for Figure A's inner circle, which appears to move clockwise. The reverse tilt of the bars in the outer circle of Figure A makes it appear to move counterclockwise.
When you move your head away from the page, both motions reverse direction, reversing the direction of the illusion.
Once again, stare at the star in Figure A, but this time hold the page up between both hands and rotate it as if you were steering an imaginary car. Keep your viewing distance constant as you rotate the page. The inner circle should appear to expand, again because your brain thinks each diagonal bar is moving at right angles to itself. If you repeat Experiment 1, you'll notice some of this expansion effect here as well, but this magnification is masked by the more powerful illusory rotation effect. Actual rotation of the figure, on the other hand, masks whatever illusory rotation might be present, teasing out the radial component of the illusion.
Repeat Experiment 1, but this time focus on the star in Figure B. The rotation and magnification effects will be harder to see, because there is no apparent counter-rotating ring outside the circle of lines to highlight the illusion. This illustrates why the brain gets by with its frugal ways. Under normal circumstances, less-than-optimal performance by individual neurons in the visual cortex is good enough. And thank goodness: If the brain demanded perfection, it would need so many neurons that our heads would be the size (and weight) of medicine balls.
Check out an animation of the Fechner color illusion: dogfeathers.com/java/fechner.html