How can we go from a state of inattentiveness to a state of utmost attention? The locus coeruleus, literally the “bruise”, is a tiny collection of cells at the base of the brain. As the main source of the neurotransmitter norepinephrine, it helps us control our focus of attention. Scientists at the Max Planck Institute for Human Development and the University of Southern California have now combined evidence from animal and human studies and developed a novel framework that describes how the bruise regulates our brain’s sensitivity to relevant information in situations, attention require. Their findings were published in an opinion piece in the journal Trends in Cognitive Sciences.
Our attention fluctuates. Sometimes we are distracted and things escape our awareness, while sometimes we can easily focus on the essentials. Imagine going home after a day at work; perhaps you are mentally preparing the grocery list for dinner – you are in a state of inattentiveness. However, if a car suddenly honks that you haven’t noticed, it’s easy to get your attention and react to this new situation. But how does the brain switch from a state of inattention to a state of focused attention?
In states of inattention, our brain is dominated by slow, rhythmic fluctuations in neural activity. In particular, neural rhythms with a frequency of around 10 Hertz, so-called alpha oscillations, are intended to suppress the active processing of sensory impressions in the event of inattentiveness. Alpha oscillations can thus be understood as a filter that regulates our brain’s sensitivity to external information.
“While the relationship between the increasing and decreasing alpha oscillations and attention has been known for some time, less is known about what makes these rhythmic fire patterns arise and go,” says Markus Werkle-Bergner, Senior Scientist at the Center for Lifespan Psychology at the Max Planck Institute for Human Development and co-author of the opinion article.
To investigate this question, the researchers focused on the bruise (Locus coeruleus), a tiny cell structure that is located deep under the cerebral cortex in the brain stem. This cluster of cells is only about 15 millimeters in size, but is connected to most of the brain by an extensive network of long-range nerve fibers. The bruise is made up of neurons, which are the main source of the neurotransmitter norepinephrine. By regulating neural communication, norepinephrine helps control stress, memory, and attention.
Due to its small size and its location deep in the brain stem, it was previously hardly possible to examine the noradrenergic nucleus non-invasively in living people. Fortunately, animal research in recent years has shown that fluctuations in pupil size are related to the activity of the bruise. Thus, our eyes can be seen as a window to a region of the brain that has seemed inaccessible for a long time. “
Mara Mather, Professor of Gerontology, University of Southern California and co-author of the opinion piece
To investigate whether bruise norepinephrine might be a factor that regulates alpha oscillations, the researchers combined records of pupil size and neural vibrations while the participants solved a demanding attention task. As expected, in moments of larger pupil size, indicating higher noradrenergic activity, the alpha oscillations disappeared. In addition, participants who showed stronger pupil and alpha responses were better able to solve the attention task. These results, published in a 2020 article in the Journal of Neuroscience, suggest that by modulating alpha oscillations, the bruise can help us focus our attention.
What went unanswered in this study is how norepinephrine affects alpha oscillations. To approach this question, the authors also drew on earlier animal experiments that recorded neuronal activity directly from neurons in the thalamus, a region in the middle of the brain that acts as a pacemaker for the alpha rhythm. Importantly, the rhythmic firing of these neurons at rest leads to the cortical alpha oscillations observed during states of inattentiveness. However, the addition of norepinephrine to these neurons cancels their rhythmicity.
“By combining the results from multiple studies, we were able to describe how norepinephrine and the thalamus might interact to control rhythmic alpha activity. We suspect that the bruise norepinephrine regulates our brain’s sensitivity to processing relevant information by suppressing alpha generators in the thalamus, ”says Martin Dahl, postdoc at the Center for Lifespan Psychology, Max Planck Institute for Human Development and the University of Southern California and lead author of the opinion article.
In situations that require a sudden change in attentiveness, a rush of norepinephrine helps us refocus and quickly dodge the approaching car.
Further long-term studies examining both the locus coeruleus and the thalamus in the same participants may shed new light on neural mechanisms of attention and its decline with age and disease.
Max Planck Institute for Human Development