Studying the Unborn Brain: From Silence to Sound

Sound has a way of influencing how we feel. We live in a world full of sounds, with a diversity of sources ranging from the crumpling of paper, a basketball hitting the pavement to the howling wind. The perception of sound not only varies from one person to the next, but also develops with age: children tend to hear higher frequencies of noise such as high-pitched ringing compared to older adults. But what about a fetus in the womb?  A fetus gains the ability to hear at around 20 weeks, but how do they actually perceive sound?

There has been increasing evidence that fetuses can hear sound in the womb which has led many expecting mothers to either sing to their child or even play music for them. However, a fetus hears sound very differently. Imagine you’ve taken a dive into a deep large pool and are sitting at the bottom. The music and sounds you once heard loud and clear are now muffled. Similarly, a fetus hears muffled sounds. 

How does one assess a fetuses’ ability to respond to sound? Previous studies relied on external stimuli such as playing a sound directly onto the mother’s abdomen to assess fetal reaction. However, this technique has been deemed unsafe as intense and sustained sound placed directly over the abdomen can have serious implications for the developing fetal auditory system. The fetal environment is incredibly fragile and for the safety of the baby, a non-invasive technique is preferred. 

Dr. Sandrine de Ribaupierre’s lab in the Biophysics department at Western University, investigated whether a fetus responds to an internal stimulus, their mother’s voice. Internal stimuli are changes to the conditions inside the body. The mother’s voice produces internal vibrations through her respiratory system that the fetus can respond to. To understand the methods used to assess the fetal response to sound, it is important to first understand how a fetus develops hearing. 

The fetal brain develops rapidly, undergoing structural and functional changes throughout pregnancy. The fetus begins to develop its auditory system around 15 weeks of pregnancy and the auditory system becomes functional around 20 weeks. Auditory development is rapid, and by      32 weeks the fetus is able to differentiate voices and phenomes, as well as learn its mother’s voice and simple music that can be recognized after birth. 

Research has shown that a newborn’s auditory cortex, the area in the brain that processes sound, is more reactive to maternal sounds than environmental stimuli. Similarly, they can differentiate new sounds from familiar ones that they were exposed to as a fetus.

This pilot study showed that not only external but internal acoustic stimuli, like the mother’s voice, can activate the fetal primary auditory cortex (also called Heschl’s gyrus). These findings were discovered using functional magnetic resonance imaging (fMRI), is a non-invasive method that can detect brain activity by measuring oxygen levels in the brain.  

Primary researcher Estee Goldberg studied the fetal response to sound in nine pregnant women at 33-38 gestational weeks using fMRI. She assessed the fetus’s movement in the womb and which areas of the brain used the most oxygen when the mother sang. When exposed to the mother’ voice all nine fetal subjects demonstrated activation of the Heschl’s gyrus.   

Estee Goldberg established that an internal acoustic stimulus such as a mother’s singing can be used safely to analyze a developing auditory system. Previous research would use an external stimulus however, this method is no longer considered safe. Fetal research is difficult to conduct as fetal motion is unpredictable and hard to control. This study addresses fetal motion by using a tool that accounts for movement in the womb that occurs during the fMRI and correcting it when analyzing the results of the study.  

Goldberg’s study provides a solid foundation for further research to be safely conducted using internal acoustic stimuli. In the future, researchers can use this technique to investigate auditory development across gestation, whereas clinicians can use it to assess fetal auditory function.