Under typical conditions, norepinephrine will stimulate the activity of already highly active neurons, while "silencing" less active ones.
At a neural level, this helps us to stay focused and ignore distractions under conditions of stress.
But as we age, it seems that we become more easily distracted by irrelevant stimuli, and even more so while in a situation that registers as stressful.
Mara Mather and other researchers at the University of Southern California in Los Angeles decided to try to test whether older people are, in fact, less focused than younger people, and to find out why and how that happens.
The new study — which has been published in the journal Nature Human Behavior — confirms that the older we get, the more difficult it becomes for us to stay focused in less than ideal conditions.
"Trying hard to complete a task increases emotional arousal," explains Mather, "so when younger adults try hard, this should increase their ability to ignore distracting information."
"But for older adults, trying hard may make both what they are trying to focus on and other information stand out more."
She and her colleagues explain that this suggests that the locus coeruleus begins to function less effectively over time.
A key locus in cognitive decline
Mather's previous research also tied the locus coeruleus to something else: Alzheimer's disease. More specifically, this is where tangles of tau protein — which is a main characteristic of this neurodegenerative disease — first start to form in the brain.
"Initial signs of this pathology," she says, "are evident in the locus coeruleus in most people by age 30."
"Thus," Mather continues, "it is critical to better understand how locus coeruleus function changes as we age."
So, for the purpose of the new study, the researchers worked with 28 younger adults, aged 18–34, and 24 older adults, aged 55–75, whose locus coeruleus activity and emotional arousal they monitored during a set of attention experiments.
They did so using both brain scans and by assessing pupil dilation, which has been deemed a good indicator of locus coeruleus activity.
Smooth communication in younger brains
The tests consisted of showing the participants pairs of images: one featuring a building, and the other depicting a type of object. In some instances, the building was highlighted and the object appeared faded, and at other points it was the other way around.
In each case, participants were asked to note which image was accentuated. But, in order to add an element of stress to the experiment, every now and again an announcement was made to the participants that they might receive an electric shock at the end of the exercise. At other times, though, an announcement would be made that no electric shock was forthcoming.
It was revealed that in the case of the younger participants, when the picture of a building was highlighted, the stress of expecting a shock actually increased brain activity in an area called the "parahippocampal place area."
This brain region gets its name from the fact that it is activated when we look at places and map out spaces.
The same sort of brain activity decreased when they were shown unhighlighted images, indicating that the communication between the locus coeruleus, the parahippocampal place area, and the frontoparietal network — another brain structure tied to attention — ran smoothly, without any "hiccups."
Older adults struggle to focus under stress
The same did not hold true in the case of older study participants. Under conditions of stress — when expecting a mild electric shock — the older adults exhibited less intense activity of the frontoparietal network, even when looking at highlighted images.
This, the researchers explain, suggests that the link between this brain network and the locus coeruleus may stop functioning so well later in life.
At the same time, the older participants displayed increased activity in the parahippocampal place area in stressful conditions, regardless of whether they were looking at highlighted or faded images of buildings.
This indicates that the younger adults' attention become more selective when they experienced emotional arousal, allowing them to focus only on the pictures that stood out.
By contrast, in older adults, the parahippocampal place area "lit up" indiscriminately when they felt stress, suggesting that they were unable to focus on one prominent stimulus.
These findings, the study authors note, provide another building block in our understanding of how cognitive functions change over time, and they may, in the future, help us to prevent neurodegeneration.
"Deciphering exactly how these changes in the brain occur as we age could one day help us uncover how to protect the brain from cognitive decline and loss of function," concludes Mather.