Brightness borders can trigger motion sensitive receptors. The motion receptors are just a few neural links up into the nervous system from the brightness receptors in the eye. They are mostly on our visual periphery. Eye movements cause them to be

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Brightness borders can trigger motion sensitive
receptors. The motion receptors are just a few
neural links up into the nervous system from the
brightness receptors in the eye. They are mostly
on our visual periphery. Eye movements cause them
to be triggered by contours in the display.The
display has black and white patches flanked by
coloured patches. Between each patch is a border
or contour, which is a big step in brightness if
it is between black and light green, or between
white and dark blue. The brightness step is a
smaller one if it is between black and dark blue,
or white and light green,If our eyes move a
little, the brightness borders sweep across the
motion receptors. The big brightness steps 
trigger more motion  impressions. The big-step
borders  are curved, in opposite directions. One
part of the curve will sweep directly across the
motion receptors. The other part will just slide
along, not sweeping directly across. Only the
part that sweeps directly across gives a strong
motion signal. Hence there is a bias in the
signal from different parts of the contour. The
bias gives the overall motion a direction. The 
motion sensitive receptors are mostly linked to
the visual periphery. Hence, if you fixate the
centre, the motion there is less. And if you hold
your eyes steady, the motion can come to a stop,
but if you move your eyes the motion increases.
Each eye motion makes the brightness borders
sweep across motion receptors.