Shining laser light of different wavelengths into mouse brains reveals chatter among separate sets of neurons, showing how brain circuits produce complex behaviors.

Researchers can record neuronal activity in the brain of a living mouse using a technique called fiber photometry. To do this, they genetically engineer mice to express fluorescent molecules in a specific set of neurons. They then implant an optical fiber that transmits light from a laser into the brain. The light excites the molecules, which glow when the cells fire.

In the new method, two lasers emit different wavelengths of light: blue and yellow-green. The light travels down the fiber to excite green fluorescence in one set of neurons and red in another. A custom algorithm separates the light signals.

The researchers used the method to monitor activity in two populations of neurons in the striatum — a region that regulates movement — in eight mice. They found that one set of cells, called the direct-pathway neurons, are most active when the animals make a large turn or move across their cage. The second set, indirect-pathway neurons, are most active when mice pause.

The first set of neurons initiates a movement and calibrates how vigorous it will be, the researchers say, and the second determines whether the movement will stop or continue.

These two pathways have never been observed simultaneously in the same animal, the researchers reported 16 May in Neuron. Scientists could use the system to study how neuron ensembles communicate and produce behaviors, and to analyze alterations in that activity in conditions such as autism.

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