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Fibers Measure Frogs' Eyes

Photonics Spectra
Nov 1998
Kevin Robinson

Researchers at two Tulsa universities have designed a fiber optic probe to measure distances inside a frog's eyeball, an early step in artificial vision research. Peter LoPresti, an electrical engineering professor at the University of Tulsa, and Warren Finn, a physiologist at Oklahoma State University's College of Osteopathic Medicine, designed the probe to help them accurately select regions of a frog's retina to stimulate.
As a first step toward artificial vision, researchers at the University of Tulsa and Oklahoma State University use multimode fiber to create a probe for measuring distances inside a frog's eye. Courtesy of Warren E. Finn and Peter G. LoPresti.

The team is pursuing an artificial vision system that uses electrodes to stimulate the nerve cells in the retina directly. The goal is to implant a receiver in the eye that gets signals from a transmitter mounted on special glasses. Getting a signal into the eye is not the difficult part, but turning that signal into something that the brain can use is.

Before making an artificial vision system, the researchers must understand how the brain perceives and deals with retinal stimuli.

Their plan is to stimulate a frog's retina and then watch the brain to see what happens. However, to place the stimulus more accurately, the scientists need to know how close the electrode is to the retina. For this, they use fiber optic interferometry.

The optical probe is attached to the electrode probe that provides the stimulus. It consists of two multimode optical fibers from 3M Specialty Optical Fibers of West Haven, Conn.

A 670-nm diode laser from Panasonic illuminates one fiber. The other fiber sends the reflected signal to a photodetector optimized for that wavelength and amplified by circuitry the researchers designed.

LoPresti said the group chose multimode fiber because it can carry more power and the light can be coupled into it more easily. He added that it distributes the output light better as well.

Recording the brain

A description of the experiments to build the optical measurement probe appeared in the June issue of Applied Optics. LoPresti said that the research has continued beyond what is described there.
"We have gotten to the point of reliably recording from the brain," he said. "Right now we are stimulating a small area and are looking at the difference between the signal on the left side of the brain and the right side, because there is crossover in the visual system [and sensitivity to pulse amplitude and duration]."

LoPresti said that the researchers began by using visual stimuli from an amber light-emitting diode to eliminate the possibility of electrical interference. This allowed them to refine the method for recording the brain's response. He said they have begun to use electrical stimuli as well.

Eventually the electrical stimuli will create a brain response similar to an optical stimulus. Using this, the group hopes to restore some type of sight to people with vision problems such as macular degeneration, in which the light-sensitive cells are damaged but the nerves remain intact.

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