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.”
Ro said the results suggest that conscious awareness of the same visual event can be highly variable.
“For example, if something happens during a certain phase of ongoing brain activity, we may not see that event that we might be readily able to detect in other phases of brain activity just fractions of a second later,” Ro said. “This has large implications in that it provides evidence that what is seen or not seen by someone can be as much of a function of their brain alpha-phase as what is physically present.”
The discovery came about when a separate project Mathewson was working on showed anomalous electroencephalographic (EEG) results involving alpha brain waves. Their EEG data showed, as they write, that “when a visual target presentation coincides with the trough of an alpha wave, cortical activation is suppressed as early as 100 ms after stimulus onset, and observers are less likely to detect the target. Thus, during one alpha cycle lasting 100 ms, the human brain goes through a rapid oscillation in excitability, which directly influences the probability that an environmental stimulus will reach conscious awareness.”
This rapid oscillation effect of pulsed inhibition on conscious awareness led to the ABS analogy that Gratton came up with and that the researchers used in the paper.
“In the past it was thought that when you put the brakes on the environment, you were constantly braking,” Mathewson said. “But what we found support for is that it’s not a constant brake; it’s more like the ABS of a car system where you are braking but you are keeping in touch with the environment. You are sampling every hundred milliseconds to make sure that there is nothing important that you need to be attending to in the environment.”
The researchers say the pulsed inhibition mechanism kicks in due to natural limits on our cognitive abilities, such as when tiredness affects the visual awareness of a driver.
“I like to think of it as two routes to (target) detection,” Beck said. “You can put a lot of effort into detecting something but the brain is not going to be able to do this continuously. So there is this other possibility, this pulsation, so you don’t lose contact completely with the environment. You might not be as fully focused as you were before but you still have this pulsating contact with the environment.”
Fabiani said the original project was based on the concept that the sensory world is not received passively but is instead constantly being interpreted by a system that uses the brain’s higher cortical regions for inhibiting or enhancing the sensory regions that receive signals from the environment.
“This shows that if attention is heightened then alpha is suppressed altogether,” she said. “But if you are relatively passively viewing things or there is this relatively passive task at hand then there is pulsating control.
“The idea is that there are top-down inferences, interpretations that come down, and the question is where these top-down inferences come from. The idea behind this study and behind a series of studies that we are now running is that we need to understand where these inferences are coming from, whether it’s frontal areas or parietal areas, and there are several candidates.”
Fabiani said the pulsed inhibition mechanism is probably just one of several affecting visual awareness.
“One of these mechanisms is the alpha rhythm which people have known for a long time to be there,” she said. “But people haven’t understood necessarily that it works this way. This is the first time that it shows this specific mechanism, and I’m sure there are others.”
The researchers revealed the pulsed inhibition mechanism in their experiments through the phenomenon of masking. Masking involves a second visual stimulus that masks the stimulus that preceded it, so that the second stimulus is the one that reaches the conscious mind.
“The masking itself makes it so that the first stimulus is quite hard to see or it’s right on the threshold of being seen or not seen,” Mathewson said. “Somehow it works back in time to make the first one harder to see.”
“The basic idea is that there is some buffer in which we keep information and if we override this buffer at a certain point this buffer is read by our perceptual system, or cognitive processing system,” Gratton said. “Then we may lose some information.”
Beck said masking was important for detecting the mechanism during alpha states.
“The critical thing about using the masking is you need to get perception right on the edge of, are you going to detect it or are you not, and it’s not until you are at that point that you can see the effects of these alpha rhythms,” she said.
The researchers say that the implications of the paper are many. For one, it informs us that seeing or not seeing things in our environment is not always a conscious choice.
“It should be interesting to people to know why it is that sometimes they miss things,” Beck said. “What this says is it’s not really their fault. There is a cycle and if (the event) comes at the wrong point in the cycle you are going to miss it and if it comes at the right point you are going to get it.”
Ro said that is why knowing that the pre-stimulus alpha phase is predictive of visual awareness is important.
“For example, by being able to predict whether or not someone will see something, we should be able to implement better ways of delivering information to people to ensure that they will detect it,” Ro said. “This might then enhance safety, reduce errors, and prevent mishaps that frequently occur because people fail to see something that is right in front of them. This knowledge might also provide some clues to help treat individuals with certain deficits in visual attention and awareness.”