Brain ‘Rewired’ by Sound Early in the Womb
Playing music to foetuses in the womb to enhance their brain development is a popular practice even if thus far not grounded in science, but new research has shown that there may be some effect even at very early stages.
New research from John Hopkins University indicates that ‘wiring changes’ made in response to sounds occur even earlier than thought before. The ear canals of newborn mice only open after 11 days, while in humans, the ear canals open at 20 weeks’ gestation. The researchers used the mice as a model for human foetuses, and examined their neural connections at one week old.
“As scientists, we are looking for answers to basic questions about how we become who we are,” said Patrick Kanold, PhD, professor of biomedical engineering at The Johns Hopkins University and School of Medicine. “Specifically, I am looking at how our sensory environment shapes us and how early in foetal development this starts happening.”
Prof Kanold started out in electrical engineering before switching to neuroscience. His field of research is on the cortex, the outer layer of neurons underneath which lies the white matter which consists of connective neurons.
In developing foetuses, in the white matter, subplate neurons can be found at 12 weeks in human gestation and the second embryonic week in mice. This subplate neurons are the precursors to neurons and die off over a period lasting from before birth to several months old. Before they disappear, they make a connection between the thalamus, which is an important sensory gateway, and the middle layers of the cortex.
“The thalamus is the intermediary of information from the eyes, ears and skin into the cortex,” explained Prof Kanold. “When things go wrong in the thalamus or its connections with the cortex, neurodevelopmental problems occur.”
The subplate neurons respond to sound before the cortical neurons, prompting two questions for Prof Kanold: When sound signals reach the subplate neurons, does anything happen, and can a change in sound signals reflect changes in the brain circuits at these young ages?
To answer these questions, the researchers used mice genetically engineered to be deaf, unable to convert sound into nerve signals. In deaf, week-old mice, there were 25-30% more connections between subplate and cortical neurons.
“When neurons are deprived of input, such as sound, the neurons reach out to find other neurons, possibly to compensate for the lack of sound,” said Prof Kanold. “This is happening a week earlier than we thought it would, and tells us that the lack of sound likely reorganises connections in the immature cortex.”
To compare the difference extra auditory stimuli made, the researchers put 2-day old pups in a quiet enclosure or an enclosure with a constant beeping sound. There were differences between the subplate neuron connections for beeping and quiet enclosure mice, but not as great as between the deaf and hearing mice. The quiet enclosure mice had stronger connections between the subplate and cortical neurons, similar to the deaf mice. The mice in the beeping enclosure also had a greater diversity in neural circuitry.
“In these mice we see that the difference in early sound experience leaves a trace in the brain, and this exposure to sound may be important for neurodevelopment,” explained Prof Kanold.
The researchers are planning to examine how sound in early development impacts the brain in later life, as well as how sounds in the womb influences neural wiring. This has application for cochlear implants for children born deaf. They also plan to study premature infants neural wiring problems and develop biomarkers for abnormal subplate neuron development.
Source: Medical Xpress
Journal information: Early peripheral activity alters nascent subplate circuits in the auditory cortex, Science Advances (2021). DOI: 10.1126/sciadv.abc9155
