The study was conducted in cultured mouse neurons, but Cohen and his colleagues plan to use Arch to measure neuronal activity in live animals, starting with simple organisms, such as the zebrafish and the worm C. elegans. One advantage of these animals is that they're transparent, making it easy to see the fluorescent signal through a microscope.

Arch could also prove useful for imaging electrical signals in the mammalian brain, especially for experiments in mice, which could be genetically engineered to express the protein in specific neurons or at specific times in development, for example.

The challenge of transferring the approach to animals is making sure the fluorescent signal stays strong and consistent. "In the living brain, light gets absorbed—for example, by blood—so you lose light," says Ed Boyden, the researcher at MIT who led the study that used Arch to silence neurons.

The fluorescence given off by Arch also isn't as bright as some of the other available dyes, but its low toxicity makes this less of a concern, because researchers could compensate by using higher concentrations. "The fact that they got it to work well in mouse neurons bodes well," says Peterka.