By Susana Martinez-Conde and Stephen L. Macknik / Scientific American: Mind Matters
The Illusion of Sex
The Illusion of Sex, by Harvard psychologist Richard Russell, won Third Prize at the 2009 Best Visual Illusion of the Year Contest. The two side-by-side faces are perceived as male (right) and female (left). However, both of them are versions of the same androgynous face. The two images are exactly identical, except that the contrast between the eyes and mouth and the rest of the face is higher for the face on the left than for the face on the right. This illusion shows that contrast is an important cue for determining the sex of a face, with low-contrast faces appearing male and high-contrast faces appearing female. And it may also explain why females in many cultures darken their eyes and mouths with make-up. A made-up face looks more feminine than a fresh face.
Mona Lisa SmileMona 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 PerceptionMona 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.
Mooney FacesOur nervous systems are hardwired to detect and process faces rapidly and efficiently, oftentimes with very scarce details available. The pictures in the accompanying slide are often referred to as Mooney faces, after cognitive psychologist Craig Mooney, who used similar images in his research on perception. Mooney faces illustrate how little visual information it takes to “see” a face.