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Gita Khalili Moghaddam, Socrates Dokos, Gregg J. Suaning, Nigel H. Lovell, Robert Wilke; Electric Crosstalk Impairs Spatial Resolution when using Multielectrode Arrays for Retinal Implants. Invest. Ophthalmol. Vis. Sci. 2011;52(14):459.
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© ARVO (1962-2015); The Authors (2016-present)
To assess the spatial resolution multi-electrode arrays (MEA) can provide before electric crosstalk (CT) degrades image contrast in retinal implants.
Mathematical simulation (finite element method using COMSOL) was performed using a simplified model of the retina to assess current spread when multiple electrodes of a MEA are activated synchronously. The amount of CT was assessed as a function of the following parameters, electrode spacing (55-2500µm), electrode size (50 and 100µm diameters), return electrode configuration (monopolar, tripolar, hexagonal) and distance from assumed target structures (20 to 200µm). Image contrast was impaired by CT, leading to a loss of spatial information. This effect was evaluated as a function of the above parameters. Current thresholds for the different electrode configurations were also evaluated.
We found that electrode spacing was the main parameter affecting contrast in all three return configurations. Decrease of spacing in order to enhance resolution of a MEA leads to a decrease in contrast in accordance with a sigmoidal function. Contrast was estimated to be close to zero when exceeding an electrode spacing that would allow a resolution representing a visual acuity of 20/400 (monopolar configuration, 100µm distance to target structures). This effect can be compensated partly by close proximity of the MEA to target structures, or by the use of a hexagonal return configuration. The hexagonal configuration appears to be advantageous in cases where a close proximity to neurons cannot be achieved, such as, for example, in suprachoroidal approaches. However, this configuration exhibits higher current thresholds in comparison to monopolar and tripolar configuration. Thresholds for the hexagonal configuration were found to be ~2 times higher at an anticipated distance of 100µm and a spacing capable of resolving 20/1000.
Visual acuity that can be provided by a MEA is not simply limited by its ability to sample an image as defined by its electrode spacing. When employing closer electrode separations and smaller electrodes to increase sampling frequency, the effects of electric CT become more significant, ultimately prohibiting fine rendering of images. Methods of current focusing using alternative return configurations, such as hexagonal returns, have the potential to compensate for this effect. However this is at the expense of significantly increased thresholds, particularly at larger distances between electrodes and target cells.
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