Study: A Few Seconds Make Big Impact in Brain’s Learning Circuitry

Training a dog to sit works best with a treat in hand. Similarly, learning in the mammalian brain at the cellular level also relies on a timely reinforcement, and new findings from University of Texas at Dallas researchers suggest just how fast that reinforcement must occur.

A new study shows how a brief window for chemical reinforcement signals plays a critical role in shaping learning and memory. The findings demonstrate that a time-dependent mechanism previously observed at the level of single synapses also operates in the broader neural networks of the brain.

Crystal Engineer BS'03, MS'05, PhD'08, assistant professor of neuroscience in the School of Behavioral and Brain Sciences, and Dr. Seth Hays, professor of bioengineering in the Erik Jonsson School of Engineering and Computer Science and director of preclinical research in the Texas Biomedical Device Center (TxBDC), published their research in the January-February issue of Brain Stimulation. The study sheds light on how precisely timed chemical release guides the brain's ability to learn and remember.

"When neurons are activated, a short window opens during which reinforcement can strengthen neural connections and circuits," said Hays, who is a senior author of the study. "We found that if a chemical signal arrives within two or three seconds, the connection is strengthened proportionate to how much time is left. While we understood that timing made a difference, now we know more about the duration of that window and how it functions."

"We found that if a chemical signal arrives within two or three seconds, the connection is strengthened proportionate to how much time is left. While we understood that timing made a difference, now we know more about the duration of that window and how it functions."

Dr. Seth Hays, professor of bioengineering in the Erik Jonsson School of Engineering and Computer Science

Understanding how the brain recognizes which recent activity should be reinforced - a concept called credit assignment - has been a challenge.

Eligibility-Trace Principle

Credit assignment is rooted in synaptic plasticity, the mechanism by which communication between neurons is strengthened or weakened by neuromodulators, which are messenger chemicals that carry signals between nerve cells. Disruption in these chemicals, including dopamine and norepinephrine, contributes to neurodevelopmental disorders, such as autism and Rett syndrome.

To clarify the role of timing in neuromodulator release, lead author Brendan Williams PhD'25, an incoming postdoctoral research associate at the University of North Carolina at Chapel Hill, turned to the concept of an eligibility trace - the brain's mechanism for linking an event to a reward or punishment.

"The concept of the eligibility trace was initially borrowed from computer science," Williams said. "Subsequently, we discovered that this is similar to how the brain works."

The eligibility-trace principle has been demonstrated in previous studies to operate at the level of a single synapse - the space between neurons where chemical signaling takes place.

"The electrical activity traveling through the synapse can trigger a molecular signaling cascade," Williams said. "The duration of the signaling cascade is the eligibility trace. If neuromodulator release occurs during this window, the brain can connect cause with consequence."

The new study, which was conducted in rodents, showed that in addition to occurring at single synapses, the eligibility-trace mechanism also occurs across hundreds and thousands of cells.

"We now know that cortical plasticity, the ability to modify broad populations of neurons, is regulated by the same temporal constraints as in single synapses," Engineer said. "It's a brief opportunity for neuromodulators to strengthen or weaken synapses, but it quickly tapers off."

To measure the temporal boundaries of an effective eligibility trace, the researchers used vagus nerve stimulation (VNS) to control precisely the timing of neuromodulator release.

The vagus nerve travels up the neck from the chest and abdomen, and regulates digestion, heart rate and respiration by carrying information from the body to the brain and back. Previous UT Dallas studies have shown that stimulating the nerve with electrical pulses during physical rehabilitation can rewire areas of the brain damaged by stroke and lead to improved recovery. In the new study, VNS was used to reinforce neural activity at specified time intervals.

"Brendan's work clearly showed what we always thought might be true about VNS: that it can provide the reinforcing signal to modify synapses, but only when delivered during a narrow eligibility window," Hays said. "Dissociating VNS from the stimulus by just three seconds was enough to block induced plasticity."

The study provides new insights on the basic neuroscience and also has important clinical implications. Clarifying how VNS works will help refine clinical implementation, said Engineer, who is principal investigator of the grant from the National Institute on Deafness and Other Communication Disorders (R01DC017480), a component of the National Institutes of Health, which funded the study.

"Our findings show that VNS is not a 'magic bullet,'" Hays said. "There is a precise timing mechanism at the synapses that mediates neuroplasticity. This should remove uncertainty about how VNS works."

Other study authors include Michael Borland MS'10, PhD'17; TxBDC lab manager Tanya Danaphongse; neuroscience junior Lasya Pasapula; and Jayant Rajagopal BS'25, a Collegium V Honors graduate who is now a student at the Joe R. & Teresa Lozano Long School of Medicine at UT Health San Antonio.

Media Contact: Stephen Fontenot, UT Dallas, 972-883-4405, [email protected], or the Office of Media Relations, UT Dallas, (972) 883-2155, [email protected].