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Press Release

Hybrid Illusions
by Aude Oliva, Antonio Torralba, and Amanda O'Keefe

September 29, 2007 – September 2008

CAMBRIDGE, MA—Eight Einsteins, a display of unique hybrid images by MIT Assistant Professor of Cognitive Science, Aude Oliva and her colleagues is on display in the new Mark Epstein Innovation Gallery until the beginning of September, 2008.

A hybrid image is a picture that combines the low spatial frequencies of one picture with the high spatial frequencies of another, producing an image with an interpretation that changes with viewing distance or time. The hybrid images are generated by superimposing two images at two different spatial scales: the low spatial scale is obtained by filtering one image with a lowpass filter; the high spatial scale is obtained by filtering a second image with a highpass filter. The final image is composed by adding these two filtered images. Hybrid images are based on the multiscale processing of images by the human visual system.

Emerging Technologies Coordinator for the MIT Museum, Seth Riskin said that "the images Oliva created for the new gallery are engaging because they allow us to almost "feel" our brains working--perceive our own perceiving. We don't immediately understand how our brains "see" the differences in the images and that intrigues people. Oliva's visual perception research has practical applications in several fields especially in the business and medical arenas. The research is exciting for people who study diseases like autism and schizophrenia because it is providing clues about how the brain processes change."

Aude Oliva developed the original images in the 1990s, together with Philippe Schyns at the University of Glasgow. Their technique can be applied to create face pictures that change expression with viewing distance, to display two configurations of a scene in a single picture, and to present textures that disappear when viewed at a distance. By manipulating what you see, they are able to study how the human brain deciphers multiple objects in its visual path.

For example, by flashing hybrid images before people for various lengths of time and recording what the viewers said they saw, they discovered that there is a timing difference when it comes to processing the different resolutions. They found that the processing for coarse features, such as groups of large objects, happens fastest, within 1/20th of a second, giving the viewer an immediate general sense of the scene; then come the finer details like edges. When glimpsing a city street scene, for example, one tends to notice the buildings before the pedestrians.

Later, with the assistance of Antonio Torralba at MIT, they refined hybrid images to produce more enduring and reversible illusions that depend on the viewer's distance from the image. What's more, the distance at which the transition between the images occurs can be controlled quite precisely. Hybrid images are especially powerful when used to create images of faces that change expressions, identity, or pose as the viewing distance is varied. In  Eight Einsteins all the images carry Albert Einstein in the high spatial frequency. At a distance, eight different famous figures will appear. These all exist in the low spatial frequency band.

Two things happen as you step away from the image. First, we lose the perception of the the sharp details of the overlaid picture, giving way to the blurry shape of the other image. Second, the human brain somehow reinterprets this blur as a different person or object - a process known as perceptual grouping. "We have put together two known processes into a way to probe the mechanism of how the brain analyzes a visual image," says Oliva.

This technique is useful because it can trigger the brain to arrive at different interpretations of the same scene. Together with Timothy Brady, a graduate student in the department of Brain and Cognitive Sciences at MIT, Oliva hopes this will allow them to test ideas about how perceptual grouping takes place in the brain, and ultimately which parts of the brain give rise to visual interpretations of a scene.

The display of the series Eight Einsteins triggered a high rating in the blogosphere. Postings of the images have received comments from throughout the world. "We know that people are curious about how their brain works," adds Oliva. "Looking at hybrid images confuses people, and they want to know why."

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