Emotions serve as the invisible navigators of the human experience. They steer our daily decisions, color our memories, and guide our social interactions, providing a necessary framework for navigating a complex world. Yet, for all their importance, emotions remain one of the most enigmatic frontiers of science. When they function correctly, they are seamless and adaptive; when they become inappropriate, excessive, or pathologically persistent, they become the foundation of neuropsychiatric suffering.
Despite decades of intensive research, the precise neural mechanisms that transform a sensory experience into a lasting emotional state have remained largely obscured. Psychiatrists and neuroscientists have long struggled to understand the specific brain activity that makes us "tick" during a moment of joy or "sick" during a period of clinical depression.
Now, a landmark study from Stanford Medicine, published May 29 in the journal Science, offers a significant breakthrough. A multidisciplinary team of investigators has successfully mapped the brainwide neuronal processing that underlies the emotional response triggered by unpleasant sensory experiences. Perhaps most strikingly, the study reveals that the fundamental features of this brain activity are shared between humans and mice—an evolutionary conservation that suggests these emotional blueprints have remained virtually unchanged for over 70 million years.
The findings do more than just satisfy scientific curiosity; they provide a potential roadmap for understanding the driving forces behind a wide array of neuropsychiatric disorders, many of which are defined by dysfunctional emotional manifestations.
"Emotional states are fundamental to psychiatry," said Karl Deisseroth, MD, PhD, a professor of bioengineering and of psychiatry and behavioral sciences at Stanford, who led the collaborative effort. The study drew upon the expertise of a diverse team, including senior co-authors Carolyn Rodriguez, MD, PhD; Vivek Buch, MD; and Paul Nuyujukian, MD, PhD, as well as lead co-authors Isaac Kauvar, PhD, Ethan Richman, PhD, and MD/PhD student Tony Liu.
The research was a flagship project of Stanford Medicine’s Human Neural Circuitry (HNC) research program. Founded and led by Deisseroth, the HNC program is designed to bridge the gap between laboratory discovery and clinical application, utilizing an inpatient medical setting to measure and perturb human brain activity with ultra-precise synchronization.
The Computational Cost of a Large Brain
To understand why emotions are processed the way they are, the researchers looked toward the history of mammalian evolution. The move toward larger, more complex brains was a massive evolutionary gamble, bringing with it both immense cognitive advantages and significant biological costs.
"The mammalian lineage has made a huge evolutionary commitment to large brain size, with all its attendant costs and benefits," explained Deisseroth, who is also the D. H. Chen Professor and a Howard Hughes Medical Institute investigator. While a mouse brain contains roughly 100 million neurons, the human brain contains nearly 90 billion. This thousand-fold increase in scale allows for a richer mental life, but it introduces a problem of physics: signal propagation.
In a brain as large as a human’s, complex signals take time to travel, converge, and integrate across distant regions. "Yet, to make accurate decisions, your brain has to pull together your multiple streams of sensory data, your goals, your position in space, your physiological needs and more—all at the same time," Deisseroth said. "If that doesn’t happen, wrong decisions will be made and wrong actions taken."
The team hypothesized that emotions act as a sort of integrative state, holding information in place long enough for the brain to coordinate a response. Deisseroth suggests that emotions may require a specific "window of time" where persistent communication occurs among widely separated brain structures.
Ethan Richman, one of the study’s lead authors, compared this mechanism to a musical instrument. "Tuning the time scale of this communication could be an important aspect of typical brain function," he said. "This would be akin to the action of a piano’s sustain pedal, which extends the duration of briefly played notes."
If the brain’s "sustain pedal" is held too long, or released too quickly, the result could be the emotional dysfunction seen in various psychiatric conditions.
An Evolutionary Shortcut
