“Stream of consciousness” has become a catch phrase, one used in literature to describe the work of writers like James Joyce and in psychology to describe the way our conscious minds interpret the world.
But what if this long-held notion of consciousness functioning as a continuously flowing “stream” is incomplete or perhaps even wrong? A new research line at the Beckman Institute is challenging that concept, so far at least when it comes to how the visual systems in our brains function in certain states.
A study published by Beckman researchers in 2009 revealed the discovery of a pulsed inhibition mechanism in the brain that explained how the visual system often fails to perceive normally detectable visual stimuli from the environment because the brain samples the visual environment in rhythmic “frames” rather than continuously.
Now a second paper in this research line is reporting that these rhythmic frames can be entrained to external signals and induce an increase in visual sensitivity at the precise moment in time in which a visual event is expected to happen.
The results of experimental findings from this most recent study, the researchers write, “suggest a plausible mechanism of temporal attention.” That is, “the brain can synchronize its waves of consciousness to predictable rhythmic events,” said Diane Beck, a co-author of the paper.
“There is this idea that we look out into the world and see this ongoing flow of consciousness that has been compared to a stream,” said the paper’s lead author, Kyle Mathewson. “This evidence and other evidence are starting to show it might not be like that, it might be more discrete.”
The paper, titled Rescuing stimuli from invisibility: Inducing a momentary release from visual masking with pre-target entrainment, appears in the journal Cognition. Authors of the paper are Mathewson, Beck, Monica Fabiani, Gabriele Gratton, and Alejandro Lleras of Illinois’s Psychology Department and the Beckman Institute.
This latest study showed that the effect they observed can be controlled through entraining the brain. The authors write that “this entrainment may represent a mechanism by which temporal attention is tuned to produce temporally-precise peaks in visual sensitivity, to both anticipate and optimize visual processing of brief visual events. Rhythmic visual entrainment has a dramatic effect on visual sensitivity, providing us with a powerful technique to experimentally control with fine temporal precision whether or not near-threshold stimuli reach conscious awareness.”
Mathewson said that this is the first time that research has demonstrated that these brain rhythms can be controlled and harnessed in this way. “These results are the first evidence that you can actually entrain the brain rhythms of the visual system that we measured in the past experiment so that you can have control over visibility of stimuli.
“In our previous paper we found that, depending on what state of this ongoing phase the brain is in, you will be more or less likely to see a target or it will escape your awareness. Now we are able to actually control the really precise timing of those events. It’s almost like we can set up a filter through which people can look at the world that has a timing aspect to it.”
The experiment employed a faint, rhythmic light stimulus and the technique of backward masking, which 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 and the first is at near-threshold levels of being seen, causing its visual information to be lost.
In their abstract, the authors write that the results show how “consciousness of otherwise masked stimuli can be experimentally induced by sensory entrainment” and that the data reveals how “awareness of near-threshold stimuli can be manipulated by entrainment to rhythmic events, supporting the functional role of induced oscillations in underlying cortical excitability, and suggest a plausible mechanism of temporal attention.”
Lleras said that the researchers were trying to see if, by presenting the brain with rhythmic stimuli, it was possible for it to somehow be sensitive to the rhythmicity of the stimuli, and respond with the same rhythm as the stimulus presented to test subjects.
“The entrainment allows the brain to be more sensitive at a particular moment in time – the time where it is expecting the world to show something,” Lleras said.
The discovery means the technique could have future applications, for example, in a power plant where maintaining the attention level of plant operators is imperative.
“If you present the stimulation at that particular point in time then we are able to increase the sensitivity of the brain to outside stimulation,” Lleras said. “Now you will be able to see things that otherwise would be much less likely to register in your brain.”
The first study in this research line used electrodes to measure brain activity. Lleras joined the research line for this paper, adding his expertise in behavioral studies in order to test whether the timing of the pulsed inhibition wave can be manipulated. He said this ongoing research line is answering questions about our visual systems that previously were mystifying to researchers.
“The issue is that up until they started their research on the alpha it was fairly unknown why it is that if you do the same trial twice, present exactly the same little very dim light to a person, one time they see it and one time they don’t,” Lleras said. “Why is it that I cannot predict with total certainty whether you will or will not see this faint light.
“For a long time this has been portrayed as random fluctuations in the brain, neural noise and things like that. Things we can never get a handle on, stochastic processes in the brain. But now we seem to be starting to get a handle on that issue and discovering ways in which we can get better at saying whether you will in fact see that little light and increasing your chances of seeing that light. The magnitude of our effect is pretty large.”
Mathewson, whose work first revealed this mechanism, said there is much more to come in this research area because this mechanism might be more common than previously thought.
“We’re just flashing something on the screen but if you think about it many of the things around us are rhythmic: our speech has certain rhythmicity to it, cars moving down the street, movement has rhythmicity to it,” Mathewson said. “So this might be a more common mechanism throughout the brain by which the brain starts to process the environment because it picks up on the rhythmicities that are everywhere.
And that means a research line that has many possibilities.
“The result is it opens a whole new direction that research can take in creating these rhythms that are ongoing in the brain and harnessing them as a way of looking at how these rhythms influence brain processing,” Mathewson said.
