Research Is an Eye-Opening Look at Retina Neurons

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A recent discovery by UC Berkeley researchers will allow scientists to take a closer look at sight itself.

Berkeley molecular and cell biology professor Frank Werblin, working with graduate students Shelley Fried and Thomas Münch, discovered the cellular mechanism that enables the eye's retina to detect directional motion.

Almost 40 years ago at UC Berkeley, researcher Horace Barlow discovered certain retina cells, called directionally selective cells, fired signals when light passed over them in one direction but not when light passed over them in the opposite direction.

After Barlow's experiments, many researchers worked to discover the cellular mechanism behind directionally selective cell signalling, Fried said.

Fried and Münch said that, while other researchers looked at the directionally selective cells' output, measured by their signalling activity, their research took the alternate path and examined the cells' input.

"People have been studying only outputs," Fried said. "By looking at input, we found that inputs of (directionally selective) cells are different."

The scientists found a balance of excitatory and inhibitory inputs from connecting cells determines whether the directionally selective cells fired signals.

The eye's retina is composed of several layers of different types of cells.

When light shines on the retina, different light-detecting cells fire off signals that filter through the layers of cells.

The researchers found that the starburst cell, a laterally reaching cell shaped like a starburst, was the crucial motion-detecting cell.

The research team discovered that starburst cells are asymmetrically connected to directionally sensitive cells, Münch said.

The team found that the starburst cell "delivers direct inhibition to directionally sensitive cells" if it is pointing in the "null direction," or the direction of light movement that does not induce signal production.

Münch affirmed the discovery that if the light stimulus moved in the null direction, the starburst cell delivered inhibition.

"It's an inhibitory connection," said Münch. "If the stimulus moves, it's inhibited and prevents the (directionally selective) cell from spiking."

Professor Werblin said that this finding has generated an "enthusiastic" response from other scientists in the field.

In an article published in "Nature," University of Pennsylvania neuroscientist Peter Sterling said that the Berkeley team's findings "represent a major intellectual and technical triumph."

However, Sterling also said that there was still more research to perform into the retina's signal-processing mechanism.

"Although they've solved the broad outline, there are still several details that are interesting and need to be investigated," Sterling said.

The UC Berkeley researchers said their findings on this relatively simple neural circuit may help scientists understand the brain's circuitry as a whole, leading to future scientific breakthroughs as a result.

Fried said the information could also be used in the future development of a prosthetic retina.

"We were able to (solve the problem) because we have techniques to look up through the (directionally selective) cells to the cells talking to it," Werblin said. "It's partially techonology and the intelligent use of technology."

The researchers said the next step in their research woud be finding out how the inherently selective starburst cell functions.

The biologists' research and findings on this topic were published in the Nov. 28 issue of the British journal Nature.


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