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The perception produced by this illusion occurs because of the way that neurons in the brain interact. Some neurons make excitatory connections, meaning that they excite other neurons. These "neural extroverts" are a loud, raucous bunch, and while they get most of our attention, they are not the only neurons in the brain. Inhibitory neurons are the dark, serious cells of neural society, and are responsible for a great deal of the information processing that the brain performs. Excitatory and inhibitory neurons balance each other, allowing the visual system to very quickly detect features such as bright and dark contrasts.
The café wall illusion exposes the limitations of this balancing act. In very bright areas inhibitory cells act to diminish the perceived brightness causing very bright areas to appear dimmed. The reverse is also true, with excitatory neurons causing dark areas to appear brighter.
In Figure 1, notice that the part of the mortar line indicated by an blue circle is surrounded by a large white region. As the figure indicates, these areas will be dimmed in the retina. The part of the line indicated by a orange circle is surrounded by dark regions, and will be slightly brightened in the retina. The result is that each line of mortar acquires a faint diagonal striping, like a candy cane.
The illusion depends on these "candy cane" lines, which develop into converging slopes, with one end of the bricks appearing wide, and the other narrow. This part of the illusion is due to processing at the back of the brain, in the primary visual cortex. The primary visual cortex contains excitatory and inhibitory neurons that detect the orientation of lines.
In Figure 2, we've isolated the striped mortar lines for clarity. This is the sort of image that the retina might send to the primary visual cortex. It is to this image that the neurons in the visual cortex will react. When they see the top mortar line, all the stripes going down and right will trigger excitatory neurons. Inhibitory neurons will act to quiet the excitatory cells, causing the down-left lines to dominate. The consequence is that the visual cortex perceives the entire top mortar line as sloping down and to the left.
Similarly, the bottom mortar line in figure 2 is strongly dominated by stripes sloping down and left. This causes inhibition of the down-left oriented neurons in primary visual cortex, so that the bottom mortar line is perceived to slope down and right.
Each mortar line in the café wall illusion is perceived as sloping down and right or down and left in an alternating sequence. The final result is that each layer of bricks appears narrow at one end and wide at the other, also in an alternating sequence.
Ashish Ranpura earned his bachelor's degree in neuroscience at Yale University, where he studied the cellular basis of learning and memory. He began his career in science journalism at National Public Radio's "Science Friday," and continues to be deeply interested in promoting public understanding of science. He is currently conducting research on cognitive development underlying number perception and arithmetical skills.
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