Stare at a spinning ceiling fan for a while, and it will eventually -- just for a moment -- appear to spin in the opposite direction. Although this seems like the kind of visual hiccup that might be observed and just as quickly forgotten, the laboratory of Dr. David Eagleman recognizes glitches of visual perception as crucial for understanding the normal function of the brain.

"Perceptual illusions are often our richest inroads into understanding what is running under the hood," said Eagleman, an assistant professor of neuroscience at Baylor College of Medicine.

Illusions in perceived motion

Eagleman and two of his students, Giovanni Piantoni and Keith Kline, asked participants to watch a series of dots moving in one direction. Occasionally, participants suddenly reported the illusory perception that the dots moved in the other direction. Using electroencephalography (EEG), a method used to record the activity of large groups of neurons, the researchers were able to measure the difference in brain activity during correct and illusory perception.

"It's a powerful approach: nothing about the display is changing, and yet your perception of what you're seeing occasionally reverses entirely," Eagleman said. "This allows us to ignore the low-level mechanics of the visual system and instead measure the neural activity that underpins the content of visual awareness."

Neural coalitions compete

The researchers found that they could detect the activity of one coalition of neurons break down -- and another coalition build up -- whenever the perception of motion switched. These changes in the neural activity correlated with what participants reported seeing.

This provided direct evidence that the neural populations underlying the two possible perceptions compete with one another for dominance, and whichever one wins determines the content of consciousness, Eagleman said.

"Think of the two populations of neurons as political parties, each fighting for control of the senate," said Eagleman. "When one party gets its strategies aligned, it can topple the other and take control for a short time."

What causes both coalitions to be activated even though the motion is only in one direction? Earlier research from Eagleman's laboratory suggests that continuous motion has a small effect of "tickling" motion detectors that represent the opposite direction. This allows the opposite direction to enter the fray.

"Even though an observer is presented with leftward motion and most of her visual system gets that correct, a small population of neurons will insist that the motion is in the other direction. That is when the rivalry starts, each coalition fighting the other for power," Eagleman said. "The larger coalition wins most of the time, but occasionally there is a brief, surprise overturn."

Binocular rivalry

To confirm their results, the researchers also examined binocular rivalry, a phenomenon that occurs when each eye is presented with a different image.

Rather than perceiving an overlap of the two images, the brain only perceives one, then switches to see the other, and so on back and forth. Just as with the illusory motion reversal, the EEG measurements revealed that two neural populations were constantly battling for control. In this case, the two populations were of equal size, and as a result the amount of time they were in power was evenly shared.

Funding for the study came from the National Institutes of Health.

The findings appear in the current edition of the Journal of Vision.

The science of competing populations in the brain is the topic of Eagleman's upcoming book, Incognito: The Brains Behind the Mind, which will be published in ten languages in 2011.