What does ‘cross on a screen’ mean in this sentence:
He and Arjen Alink at the Max Planck Institute for Brain Research in
Frankfurt, Germany, asked 12 volunteers to focus on a cross on a screen,
above and below which bars flashed on and off to create the illusion of
movement.
Does it mean center of a screen or it means the actual image of the geometrical figure ‘cross’ on a screen?
And one more, what does ‘bars’ mean, from the same sentence? Is that some kind of geometrical figure?
I found it here.