Even before a baby is born, the brain is already practicing how to hear. That's the discovery from researchers at Johns Hopkins University, who found that a baby's brain builds hearing circuits before birth using a hidden shortcut that bypasses the ears entirely.
The study, published in the journal Science Advances, looked at newborn mice, which are temporarily deaf at birth because their ear canals are sealed shut. This made them perfect subjects to observe how the hearing system organizes itself before sound can even enter the picture.
What the researchers found surprised them. While scientists already knew that sound normally travels from the ear to the auditory cortex—the brain's hearing center—the team discovered a second, previously unknown path. The frontal cortex, the region involved in making sounds, sends signals directly to the auditory cortex. This allows the brain to practice processing sound before the outside world becomes audible.
"Our results provide the first direct functional evidence of this biological shortcut that doesn't go through hearing," said Patrick Kanold, a professor of biomedical engineering and neuroscience at Johns Hopkins who led the study.
The scientists recorded brain activity in the young mice when they made squeaking sounds. Even though the mice were deaf, their auditory cortex lit up with the same activity patterns seen in hearing mice. This proved the signals weren't coming from the ears at all. Instead, a "vocalization control area" in the frontal cortex becomes active when animals move or produce sound, sending internal motor commands straight to the auditory cortex.
"The act of vocalizing drives activity in the auditory system," Kanold explained. "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 this same process likely happens in humans. When a pregnant woman speaks, a fetus often makes small mouth movements, which could trigger similar internal brain activity. This may help explain why newborns already recognize their mother's voice and why babies begin distinguishing speech from other sounds so early.
The discovery also has potential medical importance. The researchers say this neural signaling may not develop normally in people with autism spectrum disorders, schizophrenia, or central hearing impairments—and understanding this pathway could someday help explain those conditions.
Study co-author Didhiti Mukherjee, a postdoctoral fellow, described what the results reveal about human development: "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."
Kanold put it simply: "The foundation for who we are is being built long before we ever hear our first sound."