NBIO 220:    Central Mechanisms in Visual Perception
VISUAL ILLUSIONS

Illusory motion
Hermann grid
Checker shadow



Illusory motion

Concepts being illustrated:  Visual perception is generated by the brain, is not just a reflection of the elements out in the world.  Not every percept has a direct physical correlate.  Not every percept accurately reflects what is really going on, because the visual system has to make a number of assumptions, and once in a while, the assumptions fail.


 



Hermann grid

This is a variation of the Hermann grid illusion, named after L. Hermann. While he was reading a book on sound by John Tyndall, Hermann (1870) saw spots in the intersections of spaces among the figures that Tyndall had arranged in a matrix.
Concepts being illustrated:  Context dependent visual perception, lateral inhibition.


 



Checker shadow (by Edward H. Adelson)

Which check do you think is brighter, the check in the shadow or the check outside the shadow?  These two checks are in fact exactly the same gray value.
Concepts being illustrated:  Context dependent visual perception.