Johns Hopkins Medicine scientists have shown, in a study carried out in mice and published in the journal “Science Advances”, that sounds influence the brain of babies while they are in the womb. “As scientists, we seek answers to basic questions about how we become who we are. Specifically, we are looking at how our sensory environment shapes us and how early in fetal development it begins to occur, ”the experts said.
In development, the white matter also contains so-called subplate neurons, some of the first to develop in the brain, at around 12 weeks gestation in humans and the second embryonic week in mice. These neurons in the primordial subplate eventually die during development in mammals, including mice. In humans, this occurs shortly before birth during the first few months of life, although before they die they make connections between a key gateway in the brain for all sensory information, the thalamus, and the middle layers of the cortex. .
In adults, neurons in the thalamus stretch out and project long arm-like structures called axons into the middle layers of the cortex. although in fetal development the neurons of the subplate are located between the thalamus and the cortex, acting as a bridge. At the end of the axons there is a communication link between neurons called a synapse. Working in ferrets and mice, the experts previously mapped the circuits of the neurons on the subplate.
First, the scientists used genetically modified mice that lack a protein in the hair cells of the inner ear. Protein is integral to transforming sound into an electrical pulse that reaches the brain; hence it translates into our perception of sound. Without the protein, the brain does not receive the signal. In the 1-week-old deaf mice, the researchers saw between 25 percent and 30 percent more connections between neurons in the subplate and other neurons in the cortex, compared to one-week-old mice with normal hearing and raised in a normal environment. This suggests that sounds can change brain circuits at a very young age.
Furthermore, the researchers say, these changes in neural connections occurred about a week earlier than is normally seen. Scientists had previously assumed that sensory experience can only alter cortical circuits after neurons in the thalamus extend and activate the middle layers of the cortex, which in mice is around the time when the ear canals open ( about 11 days).
‘When neurons are deprived of information, such as sound, neurons seek out other neurons, possibly to make up for the lack of sound. This is happening a week earlier than we think, and it tells us that the lack of sound probably reorganizes the connections in the immature cortex “, have detailed the experts. In the same way that the lack of sound influences brain connections, the scientists thought it was possible that the extra sounds could also influence early neural connections in mice with normal hearing.
To test this, the scientists placed 2-day-old mouse pups with normal hearing in a quiet enclosure with a speaker that beeps or in a quiet room without a speaker. The scientists found that mouse pups in the silent enclosure without the beep had stronger connections between the subplate and cortical neurons than in the enclosure with the beep.
However, the difference between mice housed in beeping and silent enclosures was not as great as between deaf mice and those raised in a normal loud environment. These mice also had more diversity between the types of neural circuits that developed between the subplate and cortical neurons, compared to pups of mice with normal hearing raised in a silent enclosure without sound.
Mice with normal hearing reared in the silent enclosure also had neuronal connectivity in the subplate and cortex regions similar to that of genetically modified deaf mice. «In these mice, we see that the difference in the experience of early sound leaves a trace in the brain, and that this exposure to sound may be important for neurodevelopment “, they have settled.
Sign up for newsletter de Familia and receive our best news free of charge every week