Some expectant parents play classical music for their unborn babies, hoping to boost their children's cognitive capacity. While some research supports a link between prenatal sound exposure and improved brain function, scientists had not identified any structures responsible for this link in the developing brain.
A new study is the first to identify a mechanism that could explain an early link between sound input and cognitive function, often called the "Mozart effect." Working with an animal model, the researchers found that a type of cell in the brain's primary processing area during early development, long thought to have no role in transmitting sensory information, may conduct such signals after all.
The results, which could have implications for the early diagnosis of autism and other cognitive deficits, were published in the Proceedings of the National Academy of Sciences.
"Our work is the first to suggest that very early in brain development, sound becomes an important sense," said one of the principal authors. "It appears that the neurons that respond to sound play a role in the early functional organization of the cortex. This is new, and it is really exciting."
Working with young ferrets researchers observed sound-induced nerve impulses in subplate neurons, which help guide the formation of neural circuits in the same way that a scaffolding helps a construction crew erect a new building. This is the first time such impulses have been seen in these neurons. During development, subplate neurons are among the first neurons to form in the cerebral cortex--the outer part of the mammalian brain that controls perception, memory and, in humans, higher functions such as language and abstract reasoning.
The role of subplate neurons is thought to be temporary. Once the brain's permanent neural circuits form, most subplate neurons disappear. Researchers assumed that subplate neurons had no role in transmitting sensory information, given their transient nature.
Scientists had thought that mammalian brains transmit their first sensory signals in response to sound after the thalamus, a large relay center, fully connects to the cerebral cortex. Studies from some mammals demonstrate that the connection of the thalamus and the cortex also coincides with the opening of the ear canals, which allows sounds to activate the inner ear. This timing provided support for the traditional model of when sound processing begins in the brain.
However, researchers had struggled to reconcile this conventional model with observations of sound-induced brain activity much earlier in the developmental process. Until, the researchers directly measured the response of subplate neurons to sound, the phenomenon had largely been overlooked.
"Previous research documented brain activity in response to sound during early developmental phases, but it was hard to determine where in the brain these signals were coming from," senior author said. "Our study is the first to measure these signals in an important cell type in the brain, providing important new insights into early sensory development in mammals."
By identifying a source of early sensory nerve signals, the current study could lead to new ways to diagnose autism and other cognitive deficits that emerge early in development. Early diagnosis is an important first step toward early intervention and treatment.
The next step is to begin studying in more detail how subplate neurons affect brain development. There are many avenues of research: for instance, the researchers say that the neurons might play a role in some cognitive conditions such as autism.