The brain's 'dimmer switch' could help explain why attention drifts
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McGill researchers have discovered that serotonin, a brain chemical best known for its role in mood and mental health, also helps regulate attention. "Our findings suggest serotonin-producing neurons deep in the midbrain act like a dimmer switch on attention," said Arjun Krishnaswamy, associate professor in the Department of Physiology, who co-led the study with associate professor Erik Cook. The findings were published in Nature Communications.
More than a century of research has revealed how attention shapes behavior, but the biology that controls attention remains poorly understood. "Everyone has moments of sharp focus and moments of drift. This work identifies a specific brain chemical that selectively controls the moment-to-moment strength of attention to salient stimuli," he said.
How serotonin shapes focus
In the study, Ph.D. student Jonas Lehnert trained mice to find a faint hidden visual target, much like how a person might pick out a friend's face in a crowd. When researchers reduced serotonin activity, the mice focused better. Increasing serotonin had the opposite effect. "That was surprising because it's often assumed that increasing a brain signal will improve performance," said Cook.
The results suggest the level of serotonin helps control the moment-to-moment strength of attention. Rather than turning attention on or off, serotonin appears to act more like a dimmer switch, dialing its strength up or down.
These insights could help researchers better understand such conditions as ADHD. They also raise new questions about how medications that act on serotonin—including common antidepressants—may affect the brain.
The authors caution their findings don't mean lower serotonin generally leads to better attention in people. In their study, serotonin was manipulated with a level of precision that differs significantly from how medications such as SSRIs affect the brain.
Focus versus flexibility
While people often think of distraction as a problem, occasional lapses in focus may serve an important purpose.
"From an evolutionary perspective, the ability to quickly shift attention to a new threat or opportunity was probably beneficial for survival," said Krishnaswamy.
"Today, that same system may be at work when you're focused on your job but still monitoring your phone for an important message. The challenge is finding the right balance between focusing strongly and remaining responsive to changes around us."
Publication details
Jonas Lehnert et al., Rapid dynamics of dorsal raphe serotonin neurons regulate the strength of visual attention, Nature Communications (2026). DOI: 10.1038/s41467-026-70658-y
Journal information: Nature Communications
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NeurologyPsychology & Mental healthPsychiatry Provided by McGill University Who's behind this story?
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