New study maps how brain activity patterns trigger and sustain human and animal emotions

Scientists have mapped the brain-wide activity patterns that trigger emotions, revealing how feelings outlast the initial triggers that spark them. Led by Karl Deisseroth at Stanford University, the research team examined how emotions emerge in response to unpleasant sensory stimuli. The study was recently published in the journal Science.

The Eye Puff Test

Researchers used a machine called a tonometer to blow light puffs of air into the left eye of participants in specific sequences. Each puff lasted about 60 milliseconds, with gaps of 3 to 8 seconds between puffs, over a total 5-minute session. The scientists varied the gaps so participants would not tense up in anticipation. A high-speed camera recorded how participants closed their eyes, along with their behavioral and subjective responses. Participants reported that the experience was unpleasant and annoying.

Brain Signals and Ketamine

In a separate group of Stanford University hospital inpatients who had electrodes planted in their brains for epileptic seizures, researchers observed similar behavioral responses. Each air puff caused a fast broadcast signal throughout the brain, followed by a slower, more persistent signal in specific circuits linked to emotional responses. When scientists administered ketamine to some participants, the initial reflexive behavior and first burst of neural activity remained unchanged. However, the subsequent slower brain response dissipated much faster, causing volunteers to describe the experience merely as a tickling of the eyeball rather than an annoying event. Ketamine is known to block a sensor in the brain that integrates signals from different areas.

Experiments With Mice

To study evolutionarily conserved systems, the team repeated the experiments with lab mice using surgically planted electrodes and ketamine injections. The mice showed the same patterns as human participants, with ketamine affecting only the neurons coordinating the slower second phase of neural activity. Fast reflexive responses corresponded to sharp activity spikes in midbrain regions like the thalamus and the periaqueductal grey. The second phase involved the brain's limbic regions and frontal cortex, with activity fading first in the midbrain and last in the frontal cortex.

Context

Emotions are intangible, spontaneous responses that helped early humans dodge dangers, yet the exact mechanisms of how they arise in the brain have remained difficult for scientists to piece together. This study provides new details on how different timescales of neural activity across brain regions shape emotional experiences in both humans and animals.