The human brain has long been known to perceive things that aren't there—from phantom limbs to patterns in chaos. But a new study from the Massachusetts Institute of Technology (M.I.T.) shows for the first time that it is surprisingly quick to bend reality when normal perception is disrupted. The results were published yesterday in The Journal of Neuroscience.

A case study from 2007 found that a stroke patient was experiencing distorted vision after having lost the optical pathways from the upper left field of his vision. The patient's mind was apparently striving to compensate for the loss, but in doing so things viewed in the lower left field appeared to be stretched vertically toward the blank area. A square would, for example, appear to be a tall rectangle. Functional magnetic resonance imaging (fMRI) showed that the part of the brain that had been deprived of the information was taking on info from an adjacent area.

But researchers wondered how—and how long after the loss—the brain had been trying to compensate for the missing pathways.

Other than to satisfy simple curiosity, the time element could help them pinpoint how the change happened: Were new pathways in the visual cortex being built or existing but quiet ones being utilized?

Daniel Dilks, a postdoctoral researcher at M.I.T.'s Kanwisher Lab, who was an author on both the 2007 case study and this paper, says that because he didn't begin working with the patient until six months after the stroke, he didn't know when the changes in the brain had occurred.

In animal experiments optical pathways are physically cut away to study the process, he says, but even in those studies, time must be allotted for the healing process, time that could be the precious evidence for the mechanisms at work. He wanted to find a way to test more rapidly, and in humans, so he decided to make use of people's naturally occurring blind spots.

In a series of experiments, Dilks and his team placed an adhesive eye patch on the left eyes of volunteers with normal vision and had them look at a series of shapes with their right eyes (the right blind spot is usually filled in by information from the left eye and vice versa). The researchers found that, like the stroke patient, volunteers perceived squares stretching toward their blind spots, turning into rectangles. And, perhaps more important, they found that the change was happening almost instantaneously—not within weeks or hours as Dilks had hypothesized.

"I was flabbergasted," he says, after finding that the volunteers reported seeing the rectangles as fast as the researchers could record it: two seconds.

This speed beats any other recorded bending of perception (known as "referred sensation"). The illusion of a phantom limb has been documented within about 24 hours of amputation, and similar cases of other senses, such as auditory, have yet to be examined, Dilks notes.

The surprising rapidity "is the most consistent with some kind of functional change—and not physical change," he explains. The results point to the idea that there are underlying silent connections that are already present. The brain, he points out, must be flexible to receive the constantly changing information from the world at large. "I think [the discovery] really is about how the visual system handles extreme changes in input."

A similar phenomenon called "filling in" has been known for some time. It happens when the brain "fills in" missing information in a person's blind spot if the other eye is covered. An optical illusion of broken lines can become continuous if the break falls in the blind spot. But the researchers aren't yet sure that the two occurrences happen for precisely the same reasons.

They do know, however, that the findings will help to shed light on how flexible the adult brain can be. "Our results highlight the stunning ability to adapt to moment-to-moment changes in experience even into adulthood," Dilks and his colleagues concluded in the paper.