Figure 4

Threshold current versus electrode diameter/area. Computed trend for variation of threshold currents with changing electrode area (corresponding electrode diameter is shown on the top axis). Dotted line represents the charge density limit calculated for platinum electrodes using a stimulation pulse width of 0.975 ms. When electrodes are in contact with the retina, the threshold current is a power function of the square root of the electrode area (or a power law with the electrode circumference). When electrodes are not in contact with the retina, the threshold is almost independent of the electrode size until the electrode diameter is roughly equal to the electrode-retina distance, and then follows the power law. This behaviour is explained by dominance of edge effects at small electrode-retina distances. The current injection limit trend line is also plotted on the graph. It is observed that for an electrode with a radius smaller than the electrode-retina distance will typically require a stimulation current larger than the injection limit.