Design, Fabrication and Testing of a Novel Stimulus Source for Visual Electrophysiology

Retinal neurodegenerative diseases represent the most common cause of irreversible low vision and blindness worldwide. The most frequent retinal diseases include age-related macular degeneration, glaucoma, diabetic retinopathy, or retinitis pigmentosa, but there are also several neurodegenerative diseases, like Alzheimer’s and Parkinson’s, which result in ocular degeneration as the disease progresses. The Electroretinogram (ERG) is a diagnostic tool that records, at the corneal level, the electrical electrical response of the eye’s light-sensitive cells, called rods and cones. Among the different ERG protocols that currently exist, the most commonly performed are the full-field ERG (ffERG), the pattern ERG (pERG), and the multifocal ERG (mfERG). As a result of the many light stimuli and patterns delivered to the eye, an objective diagnosis of the functionality of individual cells or retinal areas is possible. For example, the peripheral retinal ganglion cells (RGCs) are the first cells to be destroyed by the presence of glaucoma. Pattern ERG can be used to assess the functionality of RGCs, allowing for a more accurate diagnosis and, as a result, more effective monitoring and therapy. For the execution of a full-field ERG or pattern ERG, numerous stimulators are employed in the field of visual electrophysiology, the most frequent of which being Ganzfeld stimulators and flat-screen monitors. The Ganzfeld stimulator has the limitation of not being able to perform patterns, whilst flat-screen monitors on the market are unable to stimulate and gather responses from the more peripheral regions of the retina. This project aims to design and implement a novel stimulation source to test the central and peripheral retina, that will allow for the early detection of retinal diseases. In other words, we aim at designing and building a highly configurable device capable of completing numerous clinic protocols and collecting data from the whole visual field, thus combining the features of current stimulators into a single device. The experimental research for this thesis project has been carried out at the University of Illinois at Chicago’s Neural Engineering Vision Laboratory (NEVL). Four concentric hemispheres of different materials characterize the existing device: acrylic, polyethylene foam sheets, Optix 95-LD, and Delrin. The structure’s skeleton is built of aluminum. The device’s rows and column walls divide it into 268 sectors, each with a high-power RGBW light-emitting diode (LED) placed on a printed circuit board (PCB) with two layers of diffusive material and appropriate coatings to provide good brightness uniformity. Each of the 268 sections will be called a pixel, or check. The mechanical assembly has been performed for the first three rows of the device, which are rows L, K, and J. A new working approach has also been implemented, both in terms of hardware and software. The concept of a bit matrix has been established to maximize the device’s flexibility and potential. Each clinically recognized protocol, such as pERG, fERG, mfERG, and so on, can be completely specified by a matrix description, defined by the test’s stimulation pattern, frequency, color, and length. The hardware and software were constructed and tested on a prototype consisting of 12 RGBW LEDs, three per row of the device. Due to the small subset of LEDs for which the device has been tested, the bit matrix approach has been set aside for when the full dome will be completed. The assessment of luminance strength, luminance uniformity, contrast, and timing, included the same prototype with 12 RGBW LEDs mentioned above. The uniformity parameter is strongly dependent on the single check’s setup and the placement of diffusive material layers throughout each section. A novel combination of diffusive layers and materials, improving the device’s luminance uniformity, has been studied. Chapter one provides an introduction to the anatomy and physiology of the retina, a description of the available stimulus sources, and an overview of the most common retinal neurodegenerative diseases. Chapter two lays out a summary of the specific aims of the thesis project. The next chapter introduces and describes in detail the device developed at the Neural Engineering Vision Lab, for both the optical, mechanical, and optoelectronic assessment. Then chapter four discusses the characterization of the LEDs’ light uniformity, intra-check, and inter-check, highlighting the results and comparing them to both ISCEV standards and the previously designed device. Following up, chapter five describes the experimental settings and tests performed as proof of concept. Finally, the last chapter focuses on drawing conclusions on the thesis work, as well as including limitations of the current device and the future developments.