Sound seems to change the wiring pattern of the brain that processes sound faster than expected

Scientists have not yet answered the old question of whether or how sound shapes the fetal mind in the womb. Pregnant mothers often wonder about the benefits of activities such as playing music during pregnancy. Now, in experiments with newborn mice, Johns Hopkins scientists change the “wiring” pattern of the areas of the brain where sound processes sound, even before the ear canal opens, faster than scientists expected. It reports that it looks like.

The current experiment uses newborn mice that open the ear canal 11 days after birth. In the human foetation, the ear canal opens before birth, about 20 weeks gestation.

Survey results published online on February 12 Science AdvancesUltimately, it can help scientists identify ways to detect and intervene in abnormal wiring in the brain that can cause hearing and other sensory problems.

As scientists, we are looking for answers to basic questions about how we become us. Specifically, we are looking at how our sensory environment shapes us and how early in fetal development this begins to occur. “

Dr. Patrick Canold, Professor of Biomedical Engineering, Johns Hopkins University School of Medicine

Canold began his career in electrical engineering and used microprocessors. It is a natural conduit for the transition to science, studying the circuits of the brain.

The focus of his research is on the outermost part of the brain, the cortex, which is responsible for many functions, including sensory perception. Below the cortex is a white brain substance that, in adults, contains connections between neurons.

During development, the white matter also contains so-called subplate neurons. This is part of the first development in the brain. It is an embryo about 12 weeks gestation in humans and 2 weeks in mice. Johns Hopkins anatomist Mark Moliver is believed to have described some of the first connections between neurons formed in white matter, coining the term subplate neurons in 1973.

These primitive subplate neurons eventually disappear during the development of mammals, including mice. In humans, this happens from just before birth to the first few months of life. But before they die, they establish a connection between the brain’s major gateways for all sensory information, the thalamus, and the middle layers of the cortex.

“The thalamus is the mediator of information from the eyes, ears, and skin to the cortex,” says Canold. “Problems with the thalamus or its connection to the cortex cause neurodevelopmental disorders.” In adults, neurons in the thalamus stretch and project long arm-like structures called axons onto the middle layer of the cortex. However, during fetal development, subplate neurons are located between the thalamus and cortex and act as bridges. At the end of axons, there are communication connections between neurons called synapses. Kanold, who worked with ferrets and mice, previously mapped the circuits of subplate neurons. Kanold also previously discovered that subplate neurons can receive sound-related electrical signals before other cortical neurons.

The current study, initiated by Canold in his previous job at the University of Maryland, addresses two questions: when the sound signal reaches the subplate neurons, something happens and the changes in the sound signal are these. Are you young enough to change your brain circuits?

First, scientists used genetically engineered mice that lack protein in the hair cells of the inner ear. This protein is essential for converting sound into electrical pulses that are sent to the brain. From there, it translates into our perception of sound. Without protein, the brain does not receive signals.

Hearing impaired 1-week-old mice have normal hearing and approximately 25% of connections between subplate neurons and other cortical neurons compared to 1-week-old mice reared in a normal environment. It turned out that it was ~ 30% more. This suggests that sound can alter brain circuits at a very young age, says Canold.

In addition, researchers say these changes in neural connections occurred about a week earlier than usual. Scientists previously believed that sensory experience could alter cortical circuits after thalamic neurons reach and activate the middle layers of the cortex. This is about 11 days when the ear canal opens in the mouse.

“When a neuron is deprived of input such as sound, it reaches out to find another neuron, perhaps to make up for the lack of sound,” says Canold. “This is happening a week earlier than we expected, indicating that lack of sound is likely to reorganize immature cortical connections.”

Just as lack of sound affects brain connections, scientists thought that extra sound could affect early neuronal connections in normal auditory mice.

To test this, scientists placed a normal hearing two-day-old mouse puppy in a quiet enclosure with a beeping speaker or in a quiet enclosure without a speaker. .. Scientists have found that mouse puppies in a quiet beeping enclosure have a stronger connection between subplates and cortical neurons than in a beeping enclosure. However, the difference between beeping and mice housed in a quiet enclosure was not as great as the difference between deaf mice and mice housed in a normal healthy environment.

These mice also have more diversity among the types of neural circuits developed between subplates and cortical neurons compared to normal auditory mouse pups raised in a silent enclosure. I had it. Normal hearing mice raised in a quiet enclosure, like genetically engineered hearing-impaired mice, had neuronal connectivity in the subplate and cortical areas.

“In these mice, we find that differences in early sound experience leave traces in the brain. Exposure to this sound can be important for neurodevelopment,” says Kanold.

The research team is planning additional studies to determine how early exposure to sound affects the brain later in development. Ultimately, they explain how exposure to sound in the womb is important for human development and how these circuit changes are explained when cochlear implants are worn in hearing-impaired children. I want to understand. They also plan to study the brain characteristics of premature babies and develop biomarkers for problems, including miswiring of subplate neurons.