Called the human connectome, this structural system of neural pathways develops as people age. A new study shows transmission speed among brain regions increases into early adulthood. Learning more about neuron transmission may improve the understanding of psychological disorders.

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It often was thought that the speed of information transmitted among regions of the brain stabilized during early adolescence. A new study in Nature Neuroscience by Mayo Clinic researchers and colleagues from the Netherlands found transmission speeds continue to increase into early adulthood.

Because problems such as anxiety, depression and bipolar disorders can emerge in late adolescence and early adulthood, a better understanding of brain development may help clinicians offer therapies to treat these disorders.

"A fundamental understanding of the developmental trajectory of brain circuitry may help identify sensitive periods of development when doctors could offer therapies to their patients," says Dora Hermes, Ph.D., a Mayo Clinic biomedical engineer and senior author of the study.

Called the human connectome, the structural system of neural pathways in the brain or nervous system develops as people age. But how structural changes affect the speed of neuronal signaling has not been well described.

"Just as transit time for a truck would depend on the structure of the road, so does the transmission speed of signals among brain areas depend on the structure of neural pathways," Dr. Hermes explains. "The human connectome matures during development and aging, and can be affected by disease. All these processes may affect the speed of information flow in the brain."

In the study, Dr. Hermes and colleagues stimulated pairs of electrodes with a brief electrical pulse to measure the time it took signals to travel among brain regions in 74 research participants between the ages of 4 and 51. The intracranial measurements were done in a small population of patients who had electrodes implanted for epilepsy monitoring at University Medical Center Utrecht, Netherlands.

The response delays in connected brain regions showed that transmission speeds in the human brain increase throughout childhood and even into early adulthood. They plateau around 30 to 40 years of age.

The team's data indicate that adult transmission speeds were about two times faster compared to those typically found in children. Transmission speeds also were typically faster in 30- or 40-year-old subjects compared to teenagers.

Brain transmission speed is measured in milliseconds, a unit of time equal to one-thousandth of a second. For example, the researchers measured the neuronal speed of a 4-year-old patient at 45 milliseconds for a signal to travel from the frontal to parietal regions of the brain. In a 38-year-old patient, the same pathway was measured at 20 milliseconds. For comparison, the blink of an eye takes about 100 to 400 milliseconds.

The researchers are working to characterize electrical stimulation-driven connectivity in the human brain. One of the next steps is to better understand how transmission speeds change with neurological diseases. They are collaborating with pediatric neurosurgeons and neurologists to understand how diseases change transmission speeds compared to what would be considered within the normal range for a certain age group.

The research is supported by the National Institute of Mental Health of the National Institutes of Health (R01MH122258). The authors reported no relevant disclosures. A complete list of authors and affiliations is in the research article.

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