Mona Lisa Smile
Mona Lisa’s captivating smile is perhaps the most renowned art mystery of all time. Margaret Livingstone, a neurobiologist at Harvard Medical School, showed that Mona Lisa’s smile appears and disappears due to different visual processes used by the brain to perceive information in the center versus the periphery of our vision. Look directly at Mona Lisa’s lips and notice that her smile is very subtle, virtually absent. Now look at her eyes, or at the part in her hair, while paying attention to her mouth. Her smile is now much wider. The movement of our eyes as we gaze around Mona Lisa’s face make her smile come alive, flickering on and off from perception. The center and periphery of the visual field have this differential effect on perception because the neurons at the center of our vision see a very small portion of the world, giving us high resolution vision. Conversely, the neurons in the periphery see much larger pieces of the visual scene and thus have lower resolution.

The Da Vinci Code of Perception
Mona Lisa’s smile can be explained by the fact that images are blurred in the periphery of our vision, and her smile is only seen when blurred. Livingstone solved this mystery by simulating how the visual system sees Mona Lisa’ smile in the far periphery, the near periphery, and the center of our gaze (panels left to right). The simulation was done in Adobe Photoshop by simply blurring and deblurring the painting to simulate the change in resolution from the center of vision to the far periphery. The smile appears on the left and middle panels (far and middle visual periphery), but is gone on the right panel (center of gaze). The effect is similar to those in slides 3 through 5, and it is also explained by the fact that different retinal neurons are tuned to different spatial frequencies. In a sense, Leonardo Da Vinci painted the Mona Lisa as a hybrid, with a happy Mona Lisa superimposed on a sad one, each having different spatial frequency content.

Coffee Face
Our face-detection neural machinery can be overloaded. There’s a man’s face hidden in this image. But before we spill the beans about its location, look around and see if you can find it yourself. It’s difficult! Don’t give up too quickly: finding the face may take you a few minutes the first time you look. But once you have seen it, you will always find it immediately in every subsequent search.

Scientific American
Optical illusions are all the fad now.

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