How our eyes track a fluid bird's flight

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Washington, Apr 25 : While tracking a bird's flight across the sky, the eye gives a smooth and uninterrupted visual experience, and now in a breakthrough study, scientists at University of California, San Francisco have determined the underlying complex coordination of neurons that generates this tracking motion.

This finding will also lead the researchers to determine how neurons orchestrate all of the body's movements.

Researchers have denied the long-held belief that that individual neurons fire independently across the entire duration of a motor function. In fact, they coordinate their activity with other neurons, each firing at a particular moment in time.

"Scientists have known that neurons that connect to muscles initiate movement in a coordinated fashion. But they have not known how the neurons we are studying - which coordinate these front-line neurons -- commit the brain to move the eyes," said co-lead author David Schoppik, PhD.

"The new findings suggest a totally different way of looking at how movement is controlled across time," said Stephen Lisberger, PhD, at the University of California, San Francisco.

From what the researches have said, the findings could inform efforts to develop neural prosthetics to treat paralysis and motor dysfunctions, such as those resulting from stroke.

"The brain's messages don't reach the muscles in these conditions, so it's critical that the drive to these prosthetics reflect what the brain is trying to do to move muscles. Understanding how multiple neurons work together could influence the type of software created to drive these devices," said Schoppik.

Since it was difficult to study more than one neuron in action at a time during the course of a behaviour, the researchers conducted the new study on macaque monkeys that had been trained to track a moving object with their eyes.

They based their study two key pieces of information, firstly that when a neuron responds to a stimulus there is always a slight variation in its performance, a phenomenon that neuroscientists traditionally refer to as "noise," and, second, that each attempt of the eye to pursue a moving target is also unique.

They proposed that some aspects of neural variation might reflect behavioural variation, which was used as a probe. They explored how individual neurons would behave relative to their neighbours.

The investigators compared the deviations from the average spiking activity of single neurons and simultaneous deviations from the mean eye velocity. They also measured the degree to which variation shared across two pairs of concurrently active neurons.

The results showed that individual neurons encode different aspects of behaviour, controlling eye velocity fluctuations at particular moments during the course of eye movement, while the population of neurons collectively tiles the entire duration of the movement.

Their analysis also revealed the strength of correlations in the eye movement predictions derived from pairs of simultaneously recorded neurons, and suggested that either a small number of neurons are sufficient to drive the behaviour at any given time or that many neurons operate collectively at each moment.

Lisberger said that the finding underscores the importance of recording for more than one neuron at a time. "There is a lot that we can learn from how multiple neurons interact," he added.

The study is reported in the April 24 issue of Neuron.

ANI

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