Long before a baby's ears are functional, the brain is already building the circuitry needed for hearing, according to new research from Johns Hopkins University.
Published in the journal Science Advances, the study in mice identifies a previously unknown neural "shortcut" that organizes the auditory system before birth, offering new insight into how the auditory system prepares to process sound and eventually learn language.
While it's well-known that sound travels from the ear to the auditory cortex, the brain's hub for hearing, Hopkins researchers discovered a new neural circuit that bypasses the ear entirely. Their findings show that instead, the frontal cortex—the region involved in vocalization—sends signals directly to the auditory cortex, allowing the developing brain to activate hearing-related circuits before external sounds can be heard.
"Our results provide the first direct functional evidence of this biological shortcut that doesn't go through hearing," says senior author Patrick Kanold, professor of biomedical engineering and neuroscience at Johns Hopkins. "It's a novel brain activity source that can shape the earliest development in mammals, like interpreting information and discerning complex sounds."
The findings may also help researchers better understand conditions affecting speech and language, including autism spectrum disorders, schizophrenia, and central hearing impairments, where this neural signaling may not develop normally.
To uncover this, the team studied newborn mice at an early developmental stage. Because their ear canals are sealed at birth, mice are temporarily deaf in their first few weeks of life, providing an ideal model to observe how the auditory system organizes itself before hearing begins.
For the current study, the scientists recorded neuron activity in the young mice when they squeaked. They found that even though the young mice were deaf, the auditory cortex lit up with similar activity patterns as in hearing mice, proving the signals weren't coming from the ear.
Instead, the experiments revealed a "vocalization control area" in the frontal cortex that becomes active when the animals move or produce sound. This region uses internal motor commands to send signals directly to the auditory cortex, allowing the brain to "practice" processing sound before it ever actually hears anything from the outside world.
Image caption: The developing brain prepares itself by sending internal commands directly to the auditory cortex. This bypasses the ears, allowing the brain to "practice" processing sound before the outside world is audible.
Image credit: Courtesy of the Kanold Lab
"The act of vocalizing drives activity in the auditory system," says Kanold. "What we found is that the 'squeaking'—or rather, the motor program producing the squeak—is transmitted to the auditory cortex internally through direct brain circuits before activity from the ears gets there."
The findings suggest that, in mammals, including humans, the auditory system may begin organizing before birth. When a mother speaks, a fetus often makes orofacial movements, such as mouthing, which could trigger similar internal neural activity. This may help explain how infants begin distinguishing speech from other sounds, and why newborns are already primed to recognize their mother's voice, says Kanold.
"Before a sensory area, such as the auditory cortex, establishes its adult identity, it is first awakened by motor signals during development, setting the stage for its future sensory role," says study co-author and postdoctoral fellow Didhiti Mukherjee.
The team's next steps focus on understanding how essential this pathway is for normal auditory development and what happens when it is disrupted.
The discovery, Kanold notes, reveals something fundamental about human development. "The foundation for who we are is being built long before we ever hear our first sound," he says.
"Self-vocalizations activate the developing auditory cortex via an intracortical pathway" was published July 3 in Science Advances. In addition to Kanold and Mukherjee, co-authors include graduate student Chih-Ting Chen. The Kanold Lab is part of the Department of Biomedical Engineering and the Kavli Neuroscience Discovery Institute at Johns Hopkins University. This work was supported by NIH grants R01DC009607 and R21DC020560.
