Wiring of retina reveals how eyes sense motion
*Article by Mo Costandi, Nature, May 4th, 2014
Photo credit: Barlow, H. B. et al. J. Physiol. 173, 377–407 (1964)
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.
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.
The reconstructed map, described today in Nature, showed that while one type of bipolar cell connects to the amacrine cells’ filaments close to the cell body, another does do so farther away along the length of the filaments. And crucially, the bipolar cells that connect closer to the starburst amacrine cell bodies are known to relay their messages with a time delay, whereas the others transmit their immediately.
Because of the lag in the first type of connection, signals that hit two nearby locations on the retina at two slightly different times—as would happen when an object moves across the visual field—could reach the same amacrine-cell filament at the same time. This could explain how the retina detects motion, the authors suggest: The amacrine cell might fire only when it receives this combined information, signalling that something is moving in the direction of the filament. Stimuli not moving in the direction of the filament will produce impulses that reach the amacrine cell at different times, so that it will not fire.
“This is a very nice paper that poses a very clear and testable prediction about direction-selective computation in the retina,” says Botond Roska, a neuroscientist at the Friedrich Miescher Institute for Biomedical Research in Basel, Switzerland. “It’s an exciting idea, and I bet it’ll be followed by research from many labs trying test this hypothesis.”
Read the original study: Jinseop S. Kim, Matthew J. Greene, Aleksandar Zlateski, Kisuk Lee, Mark Richardson, Srinivas C. Turaga, Michael Purcaro, Matthew Balkam, Amy Robinson, Bardia F. Behabadi, Michael Campos, Winfried Denk, H. Sebastian Seung & the EyeWirers (2014). Space–time wiring specificity supports direction selectivity in the retina. Nature, doi: 10.1038/nature13240