Bengaluru: There are a lot of things to focus on while crossing roads — traffic rules, being on the lookout for potholes (not your phones), etc. But when a vehicle whizzes in out of nowhere, you can sense it and stop in your path.
How does our brain process this ‘salient event’ — sudden events that reorient our attention during routine tasks — even as it is focussed on something else?
Scientists from the National Brain Research Centre (NBRC), Manesar, think they have a part of the answer, and it has to do with an increase in oscillating electrical voltage in the brain.
The team discovered three key ‘nodes’ or parts of the brains that communicate with each other during an interruption, but not during a routine task.
In the long term, the findings have implications for diagnosing mental health disorders, with these inter-regional interactions acting as a metric for daily learning and performance, as well as proxies for understanding the development of anxiety and depression.
The study was published in the May 2021 issue of the peer-reviewed journal NeuroImage.
How the brain processes signals
The brain sends and receives messages across the body as electrical signals passed through nerve cells called neurons.
The neurons, which interact with each other to form neural networks, ultimately produce a specific outcome. Over the years, scientists have identified five different types of oscillating electrical voltages (called brain waves): alpha, beta, gamma, delta and theta.
The brain produces one or more of these waves of different electrical frequencies depending on your state of mind or activity. When you are in a deep sleep, for instance, the brain produces delta waves. When you are doing a task that requires concentration — say, an assignment — the brain produces gamma waves.
“We found that an increase in the energy of alpha rhythms causes the reorientation of attention to salient events,” Arpan Banerjee, additional professor at NBRC, Manesar, who led the study, told ThePrint.
Researchers have shown previously that the brain suppresses distraction and is able to maintain attention on a task by synchronising and desynchronising the alpha waves. However, when a salient event occurs, attention must be reoriented to the event for a short time and then back to the routine task.
Processing salient stimuli
Neuroscientists have implicated three brain regions (also called nodes) in processing salient events — the temporoparietal junction (TPJ), the insula and the lateral prefrontal cortex (lPFC). These nodes are collectively called the ventral attention network (VAN).
The NBRC team also pinned the source of the alpha waves of higher energy to VAN. However, their findings are different from earlier ones, the researchers said.
“The earlier notion was that the VAN nodes remain suppressed in the absence of salient distractors, thus facilitating the execution of the goal-directed task,” Priyanka Ghosh, the lead author of the study, said.
According to the researchers, the earlier studies captured brain activity by using functional Magnetic Resonance Imaging (fMRI). But fMRI can only capture signals in the brain in the order of seconds, a time scale that is much slower than the rate at which salient events are processed. So, the technique may not be quick enough to see how the brain reorients attention during saliency.
In this study, the authors used EEG (electroencephalography) to record changes in brain waves in the order of milliseconds — the rate at which salient events are processed.
“Our findings suggest that the key requirement to process salient events is not the suppression of the overall activity in VAN nodes, but the absence of communication between them,” Ghosh said.
How the findings were derived
The group recorded the brain-wave patterns of 22 healthy individuals aged between 21 and 29 years. The authors asked the participants to perform two tasks.
In the first, the participants had to look at eight 20-second videos of a group of rotating white-coloured dots — 60 per cent in one direction and 40 per cent in the other. The participants had to identify the direction in which most dots rotated as accurately and as quickly as possible.
At 150 milliseconds, a ‘salient dot’ of a different colour would pop up at a random position on the screen but moving in a distinct direction.
The second task involved pairs of still images with a visual cue — a white-coloured ‘+’ mark. The researchers asked participants to go through a bunch of pairs of similar images. The pictures were of natural outdoor or indoor settings without any faces. The participants could see each picture for 20 seconds. As a task, the participants were to draw an imaginary vertical line at the centre of the screen and click the up arrow key if the ‘+’ mark moved to the other half of the screen between two successive images. The participants would click the down arrow key if the ‘+’ mark did not move to the other side. In some pairs, a salient feature — an object or people — would pop up in the second image of the pairs.
In both the tasks, which involved different time scales of processing attention, the researchers looked at EEG recordings to see how the brain waves changed to reorient attention when the participants saw the salient event.
The team found that ‘saliency processing’ required heightened energy in alpha waves from the VAN nodes. Further, the researchers observed that these nodes interacted with each other in both directions when there was a salient event. But in the absence of a salient event, these parts were not interacting with each other.
Ghosh said it was “funny and motivating” to see VAN in action in her daily routine — when a telephone rang in the middle of a lecture or advertisements popped up on her laptop screen while reading literature.
“It was very intriguing to me that we cannot filter salient distractors using attention — as our attention gets reoriented — despite our ability to usually filter several distractors in our perceptual space while performing tasks,” Ghosh said.
“This article will definitely attract large readership and be considered a major contribution in the field,” Supriya Ray, assistant professor at the Centre of Behavioural and Cognitive Sciences, University of Allahabad, Allahabad, said.
He added, however, that a more careful working definition of saliency was needed in the study as saliency refers to conspicuity i.e., how different it is from its surrounding. “The visual system of the brain generates a ‘saliency map’ for an entire scene based on the basic visual features of objects like size, shape, colour, luminance etc. This map eventually drives covert spatial orientation of attention, and overt orientation of gaze,” he said.
While the authors have considered anything that pops out in the visual field as salient in this study, Ray said that it would have helped to connect human behaviour, the physiology of our brain and the saliency of the distractor more rigorously if the saliency of the objects that the authors projected during the experiments was computationally determined and graded.
Implications for mental health, online learning
The team’s findings may have implications in diagnosing mental health conditions in the long run.
“In the long term, communication and cooperation among VAN nodes can be a key marker for a variety of common mental disorders — anxiety, depression, PTSD (post-traumatic stress disorder) and bipolar disorder,” Banerjee said.
It is important, he added, to address whether the interactions among VAN nodes competed with learning and performance. Changes in these interactions with time can be used as a performance metric to evaluate certain tasks.
In the immediate future, Ghosh is planning to investigate whether the heightened energy of alpha waves processing salient events occurs only at the visual level or applies to other senses like hearing as well.
“While we have looked at salient stimuli that act as a distractor, we also want to see if salient stimuli can cooperate in reshaping the goals of an ongoing task,” Banerjee said. “This can have implications for developing online learning tools, an increasing necessity in a pandemic-stricken world. For example, if higher (energy) alpha (waves) to salient distractors means decreased performance and prone to distraction, one can set up appropriate gaming tools to minimise attention to distractors and thereby enhance performance in reading tasks.”
Joel P. Joseph is a science writer
(Edited by Sunanda Ranjan)