Making sound connections

The complex neuronal circuits within the hearing centers of the brain are starting to be mapped out.
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VIP neurons (in red), a newly discovered neuron type, among neighboring neurons (in blue) in the inferior colliculus. Image credit: Goyer et al. (CC BY 4.0)

Our brains help us make sense of our surroundings. Our sense organs, for example the ears, receive signals from the environment, which are passed on to specialized parts of the brain, called nuclei. There, neurons process the information and send impulses to other areas in the brain, which eventually help us decide how to respond.

A nucleus called the inferior colliculus is one of the major hearing centres in the mammalian brain. In humans, it is vital for recognizing speech and pinpointing the location of sounds. It contains several different types of neurons, and links with many other areas of the brain.

While the inferior colliculus is crucial for our sense of hearing, little is known about the specific properties of the neurons within it. In particular, these cells are difficult to divide into well-defined groups. Even less is understood about the precise nature of the connections between these neurons, which likely underpin the computational power of the inferior colliculus. Goyer et al. therefore set out to identify a specific class of neurons in this region and map the circuits they formed.

Experiments were conducted on genetically modified mice whose neurons were only ‘glowing’ if they had a gene called VIP switched on. Detailed examination of the shape of the ‘VIP cells’, as well as their chemical and electrical properties, confirmed that they were indeed a distinct class of neurons.

Another set of experiments relied on a method that uses light to control the activity of brain cells. This showed that VIP cells received signals both from other neurons in the inferior colliculus, and from another hearing center in the brain. In turn, VIP cells sent signals over long distances to many other parts of the brain that handle sound signals. This suggests that VIP neurons have a wide-ranging influence on our brains’ ability to process sound.

The work by Goyer et al. has, for the first time, reliably identified specific circuits in a brain region essential for our sense of hearing. By knowing more about how the brain’s hearing centers are connected to each other, it may become possible to understand their roles in hearing loss. In this effort, the inferior colliculus may become a target for treatments for patients with hearing difficulties.