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.
