SPIE Newsroom 10.1117/2.1200609.0423 Selecting the most effective visual information for retinal prosthesis Golshah Naghdy Image-processing algorithms are being designed to intelligently reduce high-resolution images to 25 × 25 pixels suitable for retinal implants. Each time we open our eyes, millions of photoreceptors con- vert light into neural signals and pass them to our inner nu- clear layers, and ultimately our optic nerves, via ganglion cells. 1 In visually-impaired patients with age-related macular degener- ation (AMD) and retinitis pigmentosa (RP), however, the pho- toreceptor cells cease to function although the inner layers of the retina are intact. Over the last decade, in their quest to partially restore visual perception in patients suffering from AMD or RP, researchers have been exploring the possibility of simulating the function of photoreceptors by implanting electrodes inside the retina to generate neural signals. Broadly, there are two hardware configurations proposed for a visual prosthesis. The system can be totally intraocular or can combine intraocular and extraocular hardware. 23 The latter ap- proach incorporates an extraocular camera and processing unit in addition to a retinal stimulation device 4 (see Figure 1). This topol- ogy increases the image-processing capability while reducing the amount of hardware contained within the eye. This is important because the most challenging aspect of a retinal prosthesis is the limit on the density ofthe electrode array that arises from practical considerations of power, heat generation, and space. To put this in context, an image generated by a charge-coupled device (CCD) camera could have a resolution anywhere from 336 × 244 to 3060 × 2060 pixels while the electrode array might be limited to only 25 × 25 electrodes. The image processing de- mand is therefore to reduce the resolution from at least 336 × 244 to25 × 25 while maintaining the effective visual information. 5 The activation levels of each electrode are also limited. For example, a CCD camera denotes the light intensity in eight bits represent- ing 256 distinct levels. The activation levels of each electrode, on the other hand, could be as low as four. The retinal prosthesis, Figure 1. A retinal prosthesis captures an image with a camera and transmits images to an electrode array implanted in the back of the eye. VCSEL: Vertical-cavity surface-emitting laser. PD: Photodiode. therefore, requires a reduction in the spectral resolution of the im- age from 256 grey levels to four. Such a reduction is achieved by progressively selecting pixels in the image between an upper and lower brightness threshold and setting them to a single grey level. There are many options for selecting the most useful visual information with least resolution. 6 The first step in this pro- cess is to find the region of interest (ROI) in an image, ignore the rest of the image, and then reduce the resolution of that region. Many resolution-reduction algorithms treat the entire image uniformly and reduce the resolution through averaging, sub-sampling, or through transform operations such as wavelet transform (see Figures 2-4). To effectively exploit the precious few electrodes available to represent an image, we propose a non-uniform resolution reduc- Continued on next page