Emotions serve as the invisible architects of the human experience. They steer our daily interactions, guide our most critical decisions, and dictate our reactions to the world around us. While these internal states are essential for navigating a normal life, they remain one of the most enigmatic frontiers in neuroscience. When emotional responses become disproportionate, misplaced, or pathologically persistent, they cease to be a guide and instead become a source of profound suffering, manifesting as the neuropsychiatric disorders that affect millions worldwide.
Despite decades of intensive research, the precise neural mechanisms that underpin our emotions—how they are generated, how they persist, and how they can lead to mental illness—have remained largely obscured. However, a landmark study from Stanford Medicine, published May 29 in the journal Science, has provided a significant breakthrough. By mapping brainwide neuronal processing, researchers have identified a specific pattern of activity that governs emotional responses to unpleasant experiences. Remarkably, this pattern is not unique to humans; it is shared with mice, suggesting that the fundamental "code" for emotion has been preserved across seventy million years of mammalian evolution.
The study offers more than just a map of the mind; it provides a potential framework for understanding the driving forces behind a spectrum of neuropsychiatric conditions. By pinpointing how the brain integrates sensory data into an emotional state, the research opens new doors for treating disorders characterized by emotional dysfunction.
"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 brought together a multidisciplinary team from across Stanford Medicine’s clinical and laboratory facilities, including senior co-authors Carolyn Rodriguez, MD, PhD; Vivek Buch, MD; and Paul Nuyujukian, MD, PhD. The lead authors included postdoctoral scholars Isaac Kauvar, PhD, and Ethan Richman, PhD, alongside MD/PhD student Tony Liu.
The research was conducted under the auspices of Stanford Medicine’s Human Neural Circuitry (HNC) program. Founded and led by Deisseroth, the HNC program is designed to bridge the gap between basic laboratory science and clinical application, utilizing state-of-the-art methods to measure and perturb human brain activity in real-time within an inpatient medical setting.
The Complexity of Scale: Pulling it all together
The evolutionary journey of mammals is defined by a massive commitment to brain size. This expansion comes with significant biological costs, but it also provides the foundation for a richer and more complex mental existence. To understand the scale of this complexity, one must look at the numbers: a mouse brain contains approximately 100 million neurons, while a human brain contains nearly 90 billion—a thousandfold increase.
"A bigger brain means a richer, more complex mental life," Deisseroth explained. "But there are real constraints once you scale up. The human brain is so big, it takes some time for those rich and complex signals to fully propagate throughout the brain, converge and be properly integrated."
For a brain to function effectively, it must synthesize a staggering amount of data simultaneously. It must weigh sensory inputs against internal goals, physical position, physiological needs, and past experiences. If this integration fails, the resulting decisions and actions are likely to be flawed. Deisseroth suggests that emotions may be the primary mechanism the brain uses to integrate this information over time, creating lasting patterns of behavior.
This integration requires a specific temporal window—a period of persistent communication between distant structures in the brain. 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," Richman said. "This would be akin to the action of a piano’s sustain pedal, which extends the duration of briefly played notes."
If this "sustain pedal" is held too long or released too quickly, the emotional result can become dysfunctional. This timing-based theory provides a new lens through which to view neuropsychiatric disorders.
Leveraging Evolutionary Conservation
Identifying which signals in the brain are truly significant is an immense challenge due to the organ’s sheer complexity. To solve this, the Stanford team employed a clever evolutionary strategy. They reasoned that if a specific pattern of brain activity is found in both humans and mice—species that diverged from a common ancestor 70 million years ago—it is likely essential for survival.
"This approach allowed us to focus our study on the key principles that were shared between mice and humans," said Isaac Kauvar. By searching for activity patterns that were induced by the same stimuli, measured in the same way, and blocked by the same interventions in both species, the researchers could filter out the "noise" of species-specific biology to find the core drivers of emotion.
Deisseroth, who is globally recognized for developing optogenetics—a method of using light to control neurons—noted that this particular study did not rely on that technology. Instead, the team utilized a more direct observation of natural brain dynamics in human patients and animal models.
Puff, Blink, Squint: Measuring the Unpleasant
To study emotion in a laboratory setting, researchers needed a stimulus that was safe, reproducible, and capable of eliciting a consistent emotional response across species. They found their answer in a common tool used by ophthalmologists: the "eye puff" test.
While not painful, a quick puff of air to the eye is inherently unpleasant. For the study, the researchers administered series of precisely timed eye puffs to participants. Human subjects described the sensation as "annoying," "unpleasant," and "uncomfortable." When the puffs were delivered in rapid succession, the feeling of annoyance increased and lasted long after the stimuli stopped.
Deisseroth noted that this lingering negative state is adaptive. "Any repeated series of negative events is important to the brain, to be considered in guiding future behavior," he said.
To capture the brain’s response to these puffs at high resolution, the team recruited a cohort of patients at Stanford Hospital. These individuals were already undergoing treatment for medication-resistant epilepsy and had electrodes surgically implanted deep within their brains to locate the origin of their seizures. This clinical necessity provided a rare and serendipitous opportunity to record human brain activity with extreme precision.
As the patients participated in the eye-puff experiments, researchers observed a consistent two-phase response. Immediately following a puff, there was a reflexive blink. This was followed by a secondary behavior—prolonged squinting or additional rapid blinking. This second phase was a quantifiable manifestation of the emotional state triggered by the unpleasant stimulus.
Simultaneous brain recordings revealed a matching two-phase neural pattern. The first 200 milliseconds after a puff saw a sharp, brainwide burst of activity—essentially the "news" of the event being broadcast. This was followed by a longer, second phase lasting roughly 700 milliseconds, localized to specific circuits associated with emotion. This second phase provided the extended window for integration that Deisseroth and his team had hypothesized.
When the same experiment was conducted on mice, the results were nearly identical. Mice also showed the two-phase neural pattern and, after a series of puffs, entered a generalized negative state, evidenced by a decreased interest in seeking rewards—a classic hallmark of emotional persistence.
The Ketamine Effect: Releasing the Sustain Pedal
To prove that the second phase of brain activity was indeed the key to the emotional state, the researchers turned to ketamine. At low doses, ketamine is an FDA-approved antidepressant known to cause dissociation—a state where a person is aware of a sensory experience but feels no emotional connection to it.
"Ketamine recipients are fully aware of sensory experience, but they often don’t have typical emotions about that experience," Deisseroth said. "It’s as if it’s happening to someone or something else."
Under carefully controlled clinical conditions, the researchers administered ketamine to the human subjects. The results were striking. The participants still felt the air puffs and still showed the initial reflexive blink, but their subjective sense of annoyance vanished. One participant even described the sensation as "entertaining," while another said it felt like "little whispers on my eyeballs."
Crucially, the brain recordings showed that while the first phase of neural activity (the 200ms "news" broadcast) remained intact, the second phase was dramatically shortened. Ketamine effectively "sped up" the decay of the emotional integration phase. It was as if the drug had released the sustain pedal on the brain’s emotional response. The same effect was observed in mice, who stopped their self-protective squinting but maintained their reflexive blinks.
Toward a New Science of Emotion and Timing
The implications of these findings extend far beyond the laboratory. By identifying that emotional states are dependent on the timing and persistence of brainwide communication, researchers may have found a new way to categorize and treat mental health disorders.
Deisseroth suggests that many neuropsychiatric conditions may be the result of this timing mechanism being "out of tune." For example, if the decay of integrative brain activity is too fast—as seen with ketamine—information cannot be properly integrated. This could explain symptoms in schizophrenia, where patients may perceive their own self-generated actions as being controlled by outside forces.
Conversely, if the second wave of activity is too strong or lasts too long, it could lead to hyperstabilized brain states. This could manifest as the intrusive thoughts of OCD, the persistent despair of depression, or the lingering trauma of PTSD. Even the social and language processing challenges associated with autism might be linked to a brain state that is too "stable" to keep up with the rapid-fire exchange of social information.
"These are fascinating possibilities, which we are now exploring," Deisseroth said. "It’s amazing what an unbiased brainwide screen can reveal, especially with the right technology and across millions of years of evolution."
The study was supported by the National Institutes of Health, the AE Foundation, and anonymous donors. Stanford University’s Office of Technology Licensing has filed a patent for intellectual property related to the study’s findings, signaling a potential shift in how we might one day diagnose and treat the temporal roots of human emotion.
