Oct 19 2007
Physicists from the University of California-Santa Cruz have built a tool that studies how the eye processes the busy world around it, transforming the chaos of incoming light into elegantly moving images.
The tiny device, which detects patterns of signals sent from the eye to the brain, could someday be used to help design retinal prosthetic devices - or even artificial vision.
"We're able to see patterns of electrical activity in a large population of neurons," said Alan Litke of the Santa Cruz Institute for Particle Physics at UCSC. "There could be applications of this technology to many different areas of neuroscience."
As described in today's issue of the weekly Journal of Neuroscience, the tool helps explain why seeing is believing. Eyes are exquisite interpreters of light, able to extract life-saving information from a mere pattern of rays - such as whether an oncoming truck is about to flatten you, or has slowed down to allow you to cross safely.
Upon absorbing a single particle of light, or photon, retina cells fire off an electrical signal to the brain via the optic nerve. As few as six of these photon signals are required for the brain to perceive a flash.
But how does the retina convert light into messages to the brain?
Until now, scientists sought answers by studying the electrical behavior of a single neuron. On the other extreme, they also inspected patterns of blood flow through millions of cells in the brain.
But understanding how the brain works requires studying how systems of neurons work together. So this new effort by UCSC scientists offers an improved approach.
Their tool, called the microelectrode array, records the electrical activity of more than 250 cells simultaneously. This gives it the ability to detect patterns of activity - how a simple flicker of light can trigger an ornate symphony of electrical signals.
Although no one yet knows how the brain interprets these signals, the sensitivity to changing patterns suggests an ability to sense motion, Litke said.
When these light-collecting cells die, they take the gift of sight with them.
Could sight be re-created without these cells - by simply inducing electric impulses to the brain? If so, artificial vision could be created.
This ground-breaking work was done at three sites.
At UCSC, Litke has been conducting research at the intersection of physics and biology for over a decade.
This new project is based on expertise developed for experiments in high-energy physics. Litke recognized that the concept behind the search for fundamental particles such as the top quark and the Higgs boson could also be used to study neural systems.
"Watching my young children grow," Litke said, "I was fascinated by this thing called the brain, and how it might work."
The microarray, built at UCSC and Stanford University, is a piece of glass that holds 512 electrodes jammed into an area the size of the head of a straight pin.
At La Jolla's Salk Institute, a retina was placed on the device - and then shown a movie. Although no Academy Award winner, just a flickering checkerboard, its images were picked up by retinal cells called rods and cones. The microelectrode array monitored its electrical signals.
The La Jolla scientists identified one, never-before-seen type of cell - an upsilon cell - as the critical middleman in visual processing. It collates signals, sending them to the optic nerve and to the brain.
The microarray "read out the signals from the nerve cells. We recorded and processed those signals," Litke said.
The third part of the team was based at the University of Science and Technology in Krakow, Poland, where software for interpretation was built.
After picking up the signals on the electrode array, UCSC postgraduate researcher Dumitru Petrusca matched them with the movie, allowing him to map out the light-sensitive regions of each cell. The team found that the collection of upsilon cells forms a mosaic across the retina, with nearly continuous coverage and very little overlap.
Future research will expand the search for how cells react to light - and what they tell the brain.
"We want to see what it reacts to - to find out if it's seeing color, responding to motion or whatever it might be doing," said Litke.
"This has been a fantastic journey through high-energy physics, neurobiology, technology and human health," Litke said.