Have you ever watched carefully a rotating barberspole ? 

Have you ever wondered why the stripes appear to move continuously upwards although it is evident that the red&white stripe pattern painted on the rotating cylinder is actually moving horizontally ?

 
This effects is related to a puzzle that is known in visual sciences as the 'aperture problem'. 

Note that you cannot see in which direction the two lines are moving if your view is restriced to the inside of the red circles (an 'aperture') - they both seem to move in oblique direction. Only if you see the line ends, you realise that the left line is moving horizontally, and the right line is moving up and down !

 
The only way for the visual system to find out in which of many possible directions a line or contour is actually moving, thus is to focus at the line endings, or the intersection of contours with boundaries. 

 
In the Barbers Pole, more stripes intersect with the vertical boundaries than with the horizontal boundaries, leading to a stronger vertical than horizontal motion signal. Therefore the visual system (the brain) interprets a diagonal stripe pattern moving behind a vertical slit as moving downwards: this is an illusion ! 

 
We can use this motion illusion a s a tool to study how the brain combines ('integrates') motion signals from different parts of the display. 

 
For instance, in this cross-shaped configuration of a horizontal and a vertical narrow slit the same diagonal stripe pattern appears to move rightwards and upwards, respectively. 

Which direction do you perceive at the intersection of the two slits? 

It turns out that motion in regions rather distant from the intersection can drive the perceived direction at that intersection point (for details, click here). 

 
Using this kind of artificial motion stimuli, leading to sometimes surprising illusions, we can study how motion information is processed in the human visual system and combined with other information to guide adequate behaviour, which is crucial to survive in complex environments, like everyday traffic.