They found an algae protein that lets scientists switch chosen brain cells on and off with light.

The quick version

1. A green alga has a protein that opens a tiny gate when blue light hits it.
2. Put that protein's gene into nerve cells, and a flash of light makes those cells fire.
3. Scientists can now test which brain cells cause a memory, feeling or behavior, in living animals.

Picture it

Light-sensitive doorbells on chosen houses. Picture a city where every house looks the same from a plane. You install a doorbell that rings only when a blue flashlight shines on it, and you fit it only on houses of one kind, say bakeries. Now you can fly over, shine the light, and see what the city does when only the bakeries wake up. The analogy breaks in two places: the 'doorbell' is a gate that lets charged particles flow into the cell, and scientists fit it by delivering a gene, so the cell builds the gate itself.

For younger students

There is a tiny green creature in ponds that can sense light and swim toward it. Scientists borrowed the part it uses to sense light and put it into brain cells of mice. Now when they shine a blue light, those brain cells switch on, like pressing a button.

A 10-minute lesson

Minutes 0 to 2, opening question. Ask the class, "Your brain has about 90 billion nerve cells mixed together. If two things happen at the same time, how do you prove one caused the other?" Take two or three answers. Steer toward the idea that you need a way to switch one thing on by itself.

Minutes 2 to 7, hands-on demo. You need a flashlight, a piece of blue cellophane or a blue plastic bag, a rubber band, and a few sticky notes. Cover the flashlight with the blue layer. Tell students they are brain cells, then secretly give blue sticky notes to a few of them. Those students hold the "gene" for a light gate. Sweep the blue light across the room. Only students with a sticky note stand up when the light hits them.

Then connect it to the science. A pond alga called Chlamydomonas has a protein, channelrhodopsin, that opens like a gate when blue light hits it. Hegemann and Nagel found it. Deisseroth put its gene into nerve cells. In 2007, lighting one group of cells in sleeping mice made them more likely to wake up, which showed those cells can drive waking.

Minutes 7 to 10, check for understanding. Ask, "I shine the blue light on a student with no sticky note. What happens, and why?" The answer is that nothing happens, because light only works on cells that received the gene. Close by noting it is mainly an animal research tool, with human trials only for one inherited form of blindness.

Words to know

Ion channel

A tiny gate in a cell's outer skin that lets charged particles in or out.

Channelrhodopsin

An algae protein that is an ion channel and opens when blue light hits it.

Optogenetics

Using genes plus light to switch chosen cells on or off.

Chlamydomonas

A single-celled green alga, about 0.015 mm across, that swims toward light.

Nerve signal

A brief electrical pulse a nerve cell fires to pass a message along.