Lighting Up Neurons in the Auditory Cortex

A new imaging technique from researchers at Johns Hopkins University, U.S.A., provides a way to view and map neuron activity in the auditory cortex—the area of the brain that processes sound—in mice that are alive and awake (Neuron, DOI:10.1016/j.neuron.2014.07.009). The technique allowed the team to map brain activity at multiple scales, zooming in to view individual neurons and zooming out to see the location of groups of neurons in the larger organizational structure of the brain. Other imaging techniques observe activity only at a fixed scale, in response to only one auditory tone at a time. The researchers hope their work can eventually lead to better treatments for people with hearing loss.

Lead author John Issa and colleagues studied mice genetically engineered to express neurons that glow green in response to a rise in Ca²⁺ when activated, to observe brain activity in response to different tones. Using two-photon Ca²⁺microscopy with unanaesthetized mice, the team was able to determine the spatial arrangement of where different frequencies are processed in the mouse’s auditory cortex. In a different experiment, the researchers identified regions and individual neurons in the auditory cortex that react to chirps from other mice—sounds the mice use to communicate with each other. David Yue, head of the calcium signals lab at Johns Hopkins, says that “understanding how sound representation is organized in the brain is ultimately very important for better treating hearing deficits.”

Yue also notes that two-photon Ca²⁺microscopy could be modified to examine and map other areas of the brain, like the visual cortex or areas responsible for limb movement. The technique also facilitates experimental complexity by allowing researchers to gather data from awake and active animals and to create multiscale maps from a single trial.