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Los Alamos studies nerve activity to improve artificial retina

DOE/Los Alamos National Laboratory : 26 May, 2007  (Technical Article)
Los Alamos National Laboratory is supporting the Department of Energy's artificial retina project by developing better ways to visualize and interpret the patterns of neural activity that result when the retina is stimulated. Employing new and existing techniques, a team from Los Alamos' Biological and Quantum Physics Group has produced movies of the dynamic responses that characterize the function of the ganglion cells that make up the optic nerve.
Los Alamos' efforts to detect, record and model retinal nerve signals have two main goals:

* understanding how the retina processes information so next-generation artificial retinas can better reproduce natural visual function; and
* learning how best to apply electric currents or magnetic fields to stimulate nerve cells in the retina.

One-third of the human brain plays a role in vision, with more than 30 separate specialized areas dedicated to processing visual information. The question that Los Alamos scientists and their colleagues in the artificial retina project ultimately seek to answer is whether they can develop electronic technology to produce neural activity that is close to what the brain would normally receive from a natural retina. If so, experience with existing systems (such as the cochlear prosthesis used to restore hearing) suggests that the brain will learn to interpret the signals from the artificial retina as authentic visual information.

Los Alamos researchers employ a combination of dynamic functional imaging techniques and detailed computational simulations of visual networks to seek a better understanding of how the visual system processes information. This in turn should help optimize the processing and encoding of visual information to drive a retinal prosthetic implant.

Because little is known about the natural (or electrically stimulated) activity of retinal nerve cells, tools are needed that can probe the function of networks of neurons. Using high-performance video cameras and near-infrared illumination, the Los Alamos team can record tiny changes in the optical properties of neural tissue that are associated with function. Other optical imaging methods measure changes in blood flow and oxygenation that are slow, indirect responses to the metabolic demands of nerve firing. Los Alamos researchers can image light scattering and birefringence changes that are direct physical consequences of the electrical activity of nerve cells.

In birefringence measurements, the tissue sample is illuminated with polarized light, and the light transmitted through the sample is recorded through a rotated polarizing filter. The crossed polarizer eliminates light that isn't carrying the optical signal, greatly increasing the signal-to-noise ratio and thus the sensitivity of these measurements. However transmitted light measurements are impractical for most applications of functional imaging. Researchers cannot put a light source behind the retina, so they must rely on measurements of reflected light. Los Alamos researchers have recently demonstrated the feasibility of using reflected polarized light to record dynamic responses from isolated nerves, paving the way for new imaging techniques that eventually could be employed by medical practitioners.

Furthermore, the Los Alamos team has developed a theory of what goes on inside the nerve cells to produce these polarized light signals, minute swelling due to the electrical excitation, and supported this idea with experimental data. They developed a computer code that predicts the electrical responses of individual cells and an overall model of the retina that can directly predict the dynamics of firing in retinal neurons as a function of the location of the neuron and pattern of light applied to the retina.

Of course, the ultimate purpose of these models is to improve the retinal prosthesis. The first and largest technical challenge of the artificial retina project is to improve the interface between the retina and the electronics. To that end, Los Alamos is exploring alternative technologies for the implanted stimulating arrays. Instead of flexible electrode arrays designed to conform to retinal surface, arrays of three-dimensional electrodes manufactured to match the curvature of the retina may produce a more practical, tightly integrated interface between the prosthesis and the cells. The Los Alamos team is exploring the use of novel microfabrication techniques to grow arrays of microscopic, 3D electrodes from a variety of materials. These methods also allow the construction of more complex structures that may enable new technical strategies.

For example, Los Alamos is exploring novel approaches that might do away with electrodes altogether. Electrodes depend on an electrochemical interface, which can corrode over time. However, in principle, retinal neurons might be activated by focused magnetic stimulation transmitted through arrays of sealed magnetic microcoils, thereby avoiding such potential damage. To achieve this, the team must better understand the biophysical mechanisms that allow magnetic fields to stimulate nerve cells. This effort will depend on lab capabilities in electromagnetic theory and modeling, advanced microfabrication techniques and functional neuroimaging.

Members of the Los Alamos artificial retina team include team leader John George and members Angela Yamauchi, Beth Perry, Xin-cheng Yao, Benjamin Barrows, and Garrett Kenyon, all of Biological and Quantum Physics (P-21); Bryan Travis of Atmospheric, Climate and Environmental Dynamics (EES-2) and James Maxwell of polymers and coatings (MST-7).

The artificial retina technology was featured in Chicago at the Department of Energy's 'What's Next Expo,' an event designed to showcase the newest, most innovative, cutting-edge scientific and technological advances to interest young people in pursuing careers in math and science.'

'The Department of Energy has led the way to many scientific breakthroughs, especially when several scientific disciplines combined to make a whole greater than the sum of the parts,' Energy Secretary Spencer Abraham said. 'This project is one such example where biology, physics, and engineering have joined forces to deliver a capability that will enable blind people to see. This agreement between the DOE laboratories and the private sector will facilitate transfer of many aspects of DOE technology to a clinical device that has the potential of restoring sight to millions of blind individuals.'

Los Alamos develops and applies science and technology to ensure the safety and reliability of the U.S. nuclear deterrent; reduce the threat of weapons of mass destruction, proliferation and terrorism; and solve national problems in defense, energy, environment and infrastructure.
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