Given that the function relating current to brightness varies across electrodes, it is necessary to develop some mapping that will easily permit brightness to be equated across an entire array. As the numbers of electrodes in an array increase, it will be unfeasible to measure full brightness-rating or brightness-matching functions for each individual electrode.
Here, we find that that brightness as a function of current amplitude can be described using a power function with a single scaling factor as a free parameter. Equivalently, it is possible to describe relative brightness across electrodes using a linear scaling with a single free parameter based on the slope of the brightness-matching function using a single electrode as a reference.
Of course, these results may not hold for other stimulation waveforms (though see Ref.
19), other prosthetic devices (such as those using much smaller electrodes or subretinal implantation), or in arrays that are less proximate to the retina (among other factors). However, these results, if they hold more generally, suggest that it should be possible to normalize brightness across an entire array of electrodes by measuring a single parameter for each electrode. A single brightness rating or matching judgment, if made at relatively high amplitude, would be sufficient to describe relative sensitivity across electrodes, and thereby provide a simple method of creating the appearance of equally bright phosphenes across an entire array across a wide range of brightness levels.
Supported by Second Sight Medical Products, Inc., Fletcher Jones Foundation, National Institutes of Health Grants NEI EY012893 and EY014645, and Research to Prevent Blindness.
Disclosure:
S.H. Greenwald, Second Sight Medical Products, Inc. (I, E, P);
A. Horsager, Second Sight Medical Products, Inc. (F, P, R);
M.S. Humayun, Second Sight Medical Products, Inc. (F, I, C, P, R);
R.J. Greenberg, Second Sight Medical Products, Inc. (E, P);
M.J. McMahon, Second Sight Medical Products, Inc. (E, P);
I. Fine, Second Sight Medical Products, Inc. (F, E, C, P)
The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked “
advertisement” in accordance with 18 U.S.C. §1734 solely to indicate this fact.
The authors thank Anne-Marie Ripley and Grant Palmer for support on regulatory issues and for comments on the manuscript.