ELIZABETH SVOBODA, New York Times, 13 Feb 07
“I saw a face looking up at me; it was the Virgin Mary staring back,” she told reporters in 2004. “I was in total shock.”
After holding onto the stale relic for 10 years, Ms. Duyser put it up for sale on eBay. The auction generated so much excitement that the sandwich eventually sold for $28,000, proving that she was not alone in seeing a face where none should reasonably exist. (Efforts to locate her to comment for this article were unsuccessful.)
Such faces made headlines again near the end of 2006, when Mars Express, an orbiter from the European Space Agency, captured the highest-quality three-dimensional images to date of what looks like a face in the Cydonia region of Mars. The photos reignited conspiracy theories that governments on Earth are trying to hide the existence of intelligent life on Mars.
Why do we see faces everywhere we look: in the Moon, in Rorschach inkblots, in the interference patterns on the surface of oil spills? Why are some Lay’s chips the spitting image of Fidel Castro, and why was a cinnamon bun with a striking likeness to Mother Teresa kept for years under glass in a coffee shop in Nashville, where it was nicknamed the Nun Bun?
Compelling answers are beginning to emerge from biologists and computer scientists who are gaining new insights into how the brain recognizes and processes facial data.
Long before she had heard of Diana Duyser’s grilled-cheese sandwich, Doris Tsao, a neuroscientist at the University of Bremen in Germany, had an inkling that people might process faces differently from other objects. Her suspicion was that a particular area of the brain gives faces priority, like an airline offering first-class passengers expedited boarding.
“Some patients have strokes and are then able to recognize everything perfectly well except for faces,” Dr. Tsao said. “So we started questioning whether there really might be an area in the brain that is dedicated to face recognition.”
Dr. Tsao used functional magnetic resonance imaging to record which areas of the brain were activated when macaque monkeys were presented with stimuli including fruits, gadgets, scrambled patterns — and faces. She discovered almost immediately that groups of cells in three regions of the brain’s temporal lobe seemed to be strongly attuned to faces.
“The first day we put the electrode in, it was shocking,” Dr. Tsao said. “Cell after cell responded to faces but not at all to other objects.” Her results were published in October in the journal Science.
Dr. Tsao’s investigation yielded a surprising related finding: areas of the brain she had identified as face-specific occasionally lighted up in response to objects that bore only a passing resemblance to faces.
“Nonface objects may have certain features that are weakly triggering these face cells,” she said. “If you go above a certain threshold, the monkeys might think that they’re seeing a face.” In the same way, she said, objects like cinnamon buns, rocky outcroppings and cloud formations may set off face radar if they bear enough resemblance to actual faces.
Pawan Sinha, a cognitive scientist at the Massachusetts Institute of Technology, has devoted years of research to figuring out just what attributes touch off these face-specific pings. Security software that is being developed for identifying potential terrorists or detecting intruders must be able to reliably recognize faces. In teaching the software to do this, Dr. Sinha and his colleagues have arrived at unexpected insights into the question of why we sometimes see a cinnamon bun as a cinnamon bun, and other times as the earthly incarnation of a sainted nun.
To develop detector software optimized to pick out any human face, even in less-than-ideal surroundings, Dr. Sinha began by putting into his computer hundreds of faces as varied as those in a Benetton advertisement famous for its diversity.
As the computer amassed the information, it was able to discover relationships that were of great significance to almost all faces, but very few nonfaces. “These turn out to be very simple relationships, things like the eyes are always darker than the forehead, and the mouth is darker than the cheeks,” Dr. Sinha said. “If you put together about 12 of these relationships, you get a template that you can use to locate a face.”
Most people think of the cartoon smiley face, with its discrete eyes, nose and mouth, as the quintessential face template, but Dr. Sinha’s computer can identify faces even when the pictures are of low resolution.
When he presented human subjects with blurry face images, containing only 12 by 14 pixels’ worth of visual information, they performed similarly well, recognizing 75 percent of the face images accurately. This suggests that like the computer, the human brain processes faces holistically, like coherent landscapes, rather than one feature at a time.
These images are just “ dark blobs on a big blob,” Dr. Sinha said. “So clearly there’s not enough diagnostic information in the individual features. Yet something about the overall organization of the image, the gestalt, is still allowing us to recognize the face.”
Once in a while, the computer emits a false alarm. “This is a good analogy for what the human brain might be doing,” Dr. Sinha said. “Like the computer, it’s trying to determine what the regularities are in all of these faces to create a prototype.
“But this prototype is not perfect,” he said. “Sometimes genuine faces do not match these regularities, and sometimes nonfaces satisfy them.”
In other words, if the pattern of light and dark patches on a brindle cow happens to correspond to our conceptions of what a face should look like, we may interpret the coincidence as a visitation from Jesus Christ or Marilyn Monroe.
While the human tendency to see faces in other objects is rooted in neural architecture, the large number of actual faces we see every day may also be partly responsible for the Nun Bun phenomenon, said Takeo Watanabe, a neuroscientist at Boston University. His studies of learning processes show that after the brain is bombarded with a stimulus, it continues to perceive that stimulus even when it is not present.
To demonstrate this effect, Dr. Watanabe had subjects sit in front of a computer screen with faint dots cascading across it. At first, the participants could not figure out which direction the dots were moving. Then they went through another round of tests in which they were to identify letters superimposed on the dots as they moved across the screen.
When the subjects were then presented with a blank screen and asked to describe what they saw, a strange thing happened: not only did they insist they were seeing dots, but they tended to say the dots were moving in the direction they had been moving during the previous session.
Dr. Watanabe says the results suggest that subliminally learning something “too well” interferes with perceptions of reality. “As a result of repeated presentation, the subjects developed enhanced sensitivity to the dots,” he said. “Their sensitivity got so high that they saw them even when there was nothing there.”
Because faces make up such a significant part of the visual backdrop of life, he added, they may fall into the same category as the dots: people have gotten so used to seeing faces everywhere that sensitivity to them is high enough to produce constant false positives. This tendency to become hyperattuned to common stimuli may represent a survival advantage. “If you lived in primeval times, for instance,” Dr. Watanabe said, “it would be good to be very sensitized to tigers.”
Dr. Sinha of M.I.T. says that whether the hair-trigger response to faces is innate or learned, it represents a critical evolutionary adaptation, one that dwarfs side effects like seeing Beelzebub in a crumpled tissue.
“The information faces convey is so rich — not just regarding another person’s identity, but also their mental state, health and other factors,” he said. “It’s extremely beneficial for the brain to become good at the task of face recognition and not to be very strict in its inclusion criteria. The cost of missing a face is higher than the cost of declaring a nonface to be a face.”