Jérémie Barral Shedding light on the auditory circuit: from the cochlea to the brain

Hair cells, the guardians of sound 

After passing through the outer and middle ears, sound waves are transmitted to the inner ear, where the mammalian hearing organ, the cochlea, is located. This structure has very specific physical and mechanical characteristics that enable it to break down the various components of sound. The sound signal is transformed in the cochlea by mechanosensory hair cells. Depending on where they are located in the cochlea, these cells pick up different sound frequencies, i.e. a given number of oscillations per second that characterizes a low-pitched or high-pitched sound. In this way, the hair cells transmit very precise information to the brain about the characteristics of the sounds they perceive. Hair cells defects are responsible for some forms of hearing loss. Cochlear implants are currently used to electrically stimulate the neurons downstream of the hair cells, enabling in some cases to partially recover hearing.

Hair cells in action 

The perception of sound, from the hair cells to the brain, has been the focus of Jérémie Barral's research since the start of his scientific career. With his team, he is looking into fundamental questions that are essential for understanding the mechanisms involved in hearing and developing therapeutic strategies. 

• How many hair cells need to be activated to perceive a sound? 
• How are the characteristics of a sound, such as intensity and frequency, transmitted between the hair cells and the brain? 
• What is the circuit that an auditory stimulus follows in the brain? 

These questions cannot be studied using your favorite piece of music, because it is still difficult to separate the different components of a sound in the cochlea. In their laboratory, Jérémie Barral and his team have developed a technique that will make it possible to stimulate hair cells specifically, thereby overcoming the complexity of sounds and being able to dissect the neural circuit of hearing.

Lighting up the auditory circuit 

Optogenetics is a technique developed in 2006 that uses light to activate a cell. As part of the project supported by Impulscience and led by Jérémie Barral, optogenetics will be used to selectively activate hair cells and observe the consequences in the brain. This specific stimulation will allow Jeremie Barral’s team to determine the number and location of cells to be stimulated to mimic a specific frequency or sound intensity. It will also help to determine how to stimulate these cells to generate a response similar to that generated by a complex sound. Moreover, they will follow the circuit of neurons responsible for propagating auditory information in the brain. 

Overall, this project will study normal hearing and reproduce auditory input as faithfully as possible. While the generation of prostheses is not a short-term objective of this project, it will certainly help to envisage the use of optogenetics to cure certain types of deafness.