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4 1 Introduction

(a) (b)

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X X X X X X X X O X X X O X
X X X X X X X O X X O X X O
X X X X X X X X X O X O O X
X X X X X X X O X X O X X X
X X X X X X X X O X X X O X
X X X X X X X O X X O X X O
X X X X X X X X O X X X O X
X X X X X X X X X X O O X X
X X X X X X X X O X X X O X
(c) (d)

Figure 1.3 Some common optical illusions and what they might tell us about the visual system: (a) The classic
M¨ uller-Lyer illusion, where the length of the two horizontal lines appear different, probably due to the imagined
perspective effects. (b) The “white” square B in the shadow and the “black” square A in the light actually have
the same absolute intensity value. The percept is due to brightness constancy, the visual system’s attempt to
discount illumination when interpreting colors. Image courtesy of Ted Adelson, http://web.mit.edu/persci/people/
adelson/checkershadow illusion.html. (c) A variation of the Hermann grid illusion, courtesy of Hany Farid, http:
//www.cs.dartmouth.edu/ farid/illusions/hermann.html. As you move your eyes over the figure, gray spots appear
∼
at the intersections. (d) Count the red Xs in the left half of the figure. Now count them in the right half. Is it
significantly harder? The explanation has to do with a pop-out effect (Treisman 1985), which tells us about the
operations of parallel perception and integration pathways in the brain.