HOUSTON — 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.
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, 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.”
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.
This provided 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.
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.