To date, chip-based retinal implants have only permitted a rudimentary restoration of vision. However, modifying the electrical signals emitted by the implants could change that. This is the conclusion of the initial published findings of a project sponsored by the Austrian Science Fund FWF, which showed that two specific retinal cell types respond differently to certain electrical signals - an effect that could improve the perception of light-dark contrasts.

"""Making the blind really see - that will take some time"", says Frank Rattay of the Institute of Analysis and Scientific Computing at the Vienna University of Technology - TU Wien. ""But in the case of certain diseases of the eyes, it is already possible to restore vision, albeit still highly impaired, by means of retinal implants."" Pulse Emitter To achieve this, microchips implanted in the eye convert light signals into electrical pulses, which then stimulate the cells of the retina. One major problem with this approach is that the various types of cells that respond differently to light stimuli in a healthy eye are all stimulated to the same degree. This greatly reduces the perception of contrast. ""But it might be possible"", Rattay says, ""to stimulate one cell type more than the other by means of special electrical pulses, thus enhancing the perception of contrast."" Within the framework of an FWF project, he and his team have discovered some promising approaches. Together with colleagues Shelley Fried of Harvard Medical School and Eberhard Zrenner of University Hospital Tübingen, he is now corroborating the simulated results with experimental findings. Simulated & Stimulated With the help of a sophisticated computer simulation of two retinal cell types, Rattay and his team have discovered something very exciting. They found that by selecting specific electrical pulses, different biophysical processes can actually be activated in the two cell types. For example, monophasic stimulation, where the electrical polarity of the signal from the retinal implant does not change, leads to stronger depolarisation in one cell type than in the other. ""Depolarisation means that the negative charge that prevails in cells switches briefly to a positive charge. This is the mechanism by which signals are propagated along nerves"", Rattay explains. This charge reversal was significantly weaker in the other cell type. In their simulation, the team also found as much as a fourfold difference in the response of calcium concentrations in the two cell types to a monophasic signal."

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