If a tired driver on a slick road brakes too hard and their vehicle starts to go into a skid, the anti-lock braking system (ABS) assumes control. When it comes to the human visual system, researchers from the Beckman Institute have found that we have our own ABS mechanism – a discovery that explains why we sometimes miss what is right in front of our eyes.

In a paper published by the Journal of Neuroscience the researchers and a collaborator report, for the first time, on a “pulsed inhibition” mechanism that explains how the visual system often fails to perceive stimuli from the environment that at other times would be readily detectable. When this pulsed inhibition mechanism is in operation, such as when we are tired, we perceive the visual world not as a continuous stream but more like the frames of a film. These “frames” form the apparently seamless movie of our conscious awareness, even though we are in fact often missing important visual targets in the environment.

Co-authors of the paper are Biological Intelligence research theme faculty members Monica Fabiani, Gabriele Gratton, Diane Beck, graduate student Kyle Mathewson, and Tony Ro from the City University of New York. The paper, titled To See or Not to See: Pre-stimulus Alpha Phase Predicts Visual Awareness, was published in the March 4 issue of the Journal of Neuroscience and posted online.

Gratton said others have proposed similar theories of visual awareness, but added that this new paper provides experimental data to prove the contention, and it shines a light on a previously unknown mechanism that enables the process to work.

“Though we see the world as a continuous event, it really is not a continuous event,” Gratton said. “There are small episodes that are connected to each other and then formed together, just like in a movie you have frames of what you see that are connected together to form actions and movements.

“There are people who had already thought about this as a possibility, but here we have some type of mechanism that shows that this is actually happening. It’s not just a theory or an idea; it is an actual physiological event that occurs with this particular time course.”

In the paper, the researchers write that their theory of cortical inhibition asserts that “increased alpha power represents a ‘pulsed inhibition’ of cortical activity that affects visual awareness” and that this “cortical excitability level may mediate target detection” in the visual system.

The researchers report that because the visual stimulus is unchanged, this failure to see readily detectable objects is likely due to brain function. The implications of this discovery are important, Fabiani said.

“This is not an irrelevant mechanism in the real world,” she said. “Once you explain the mechanism and you know how it works then it becomes a way to monitor attention in an operator whose errors could be very costly, like a bus operator or someone who monitors at a power plant.”

In their paper, the researchers write that their results show the “influence of oscillatory microstates of cortical activity, manifested by alpha phase, on subsequent neural activity and visual awareness.” Gratton says this oscillation mechanism comes into play when the “top down” regions of the brain that are in control in attentive states give way during relaxed states in which alpha waves are present.     

“What I believe is that alpha is sort of a fallback mechanism, a way of functioning for the cortex,” Gratton said. “Normally when you start paying attention and are awake this disappears, there is this top down control, and this phenomenon doesn’t occur. This is a fallback system for when you don’t have active control by this top down mechanism.”