Online gamers helped researchers map neuron connections involved in detecting direction of moving objects.
A vast project to map neural connections in the mouse retina may have answered the long-standing question of how the eyes detect motion. With the help of volunteers who played an online brain-mapping game, researchers showed that pairs of neurons positioned along a given direction together cause a third neuron to fire in response to images moving in the same direction.
It is sometimes said that we see with the brain rather than the eyes, but this is not entirely true. People can only make sense of visual information once it has been interpreted by the brain, but some of this information is processed partly by neurons in the retina. In particular, 50 years ago researchers discovered that the mammalian retina is sensitive to the direction and speed of moving images1. This showed that motion perception begins in the retina, but researchers struggled to explain how.
When light enters the eye, it is captured by photoreceptor cells, which convert the information into electrical impulses and transmit them to deeper layers of the retina. Individual photoreceptors are not sensitive to the direction in which an object may be moving, so neuroscientist Jinseop Kim, of the Massachusetts Institute of Technology (MIT) in Cambridge, and his colleagues wanted to test whether the answer to the puzzle could lie in the way various types of cells in the retina are connected.
Photoreceptors relay their signals via ‘bipolar neurons’, named this way because they have two stems that jut out of the cell's body in opposite directions. The signal then transits through ‘starburst amacrine cells’ — which have filaments, or dendrites, that extend in all directions similarly to light rays out of a star — before reaching the cells that form the optic nerve, which relays them into the brain.
To understand how bipolar and starburst cells are wired together, Kim and his colleagues analysed high-resolution electron microscope images of a mouse retina with the help of nearly 2,200 members of EyeWire, an online ‘citizen-science’ game set up to help with brain-mapping efforts (see 'Computer science: The learning machines'). Players traced the pathways through the layers of cells to create a high-resolution wiring diagram of part of the retina.