Design, Fabrication and Testing of a Configurable Full-Field Stimulus Source for Electroretinography
thesisposted on 2019-08-01, 00:00 authored by Giulia Crocioni
The retina is the fundamental component of vision as it contains the photoreceptors, specialized cells which convert light stimuli into electrical signals, and which represent the first stage of the visual process. The electroretinogram (ERG) is a functional test widely used in clinical practice to help diagnose diseases of the retina and optic nerve. In fact, it measures the electrical response to a light stimulus of retinal cells and different ERG tests can be used to probe different retinal areas and cells. The tests most used are the full-field ERG, which consists of flashes of variable intensity and frequency, the multifocal electroretinogram (mfERG), which consists of illuminating in a pseudo-random way hexagonal elements on a monitor, and finally the pattern electroretinogram (pERG), which consists in high-contrast reversals of a grating pattern. While the first is performed by probing the entire retina, mfERG and pERG testing is usually limited to its central portion. The stimulators most commonly used in visual electrophysiology are Ganzfeld stimulators, spherical devices used to perform full-field ERG, and flat monitors, which are able to display patterns to perform standard mfERG and pERG. Devices capable of performing all the tests of interest are obtained by integrating a Ganzfeld stimulator with an internal or external monitor. The stimulators can use different technologies, but light emitting diodes (LEDs) are becoming very popular since they are almost ideal light sources. The conventional pERG testing is limited to the central part of the human visual field and this limits the sensitivity of the test. Since pERG probes retinal ganglion cells (RGCs), and since the first cells to be damaged by glaucoma are peripheral RGCs, it may be extremely helpful in early detection of this optic neuropathy. Thus, the Neural Engingeering Vision Laboratory at UIC developed a three dimensional stimulus source able to perform peripheral pattern electroretinogram (ppERG). The flat monitors used in standard tests are not really suited for the detection of peripheral early disfunctions, since they do not probe the entire human visual field. Thus, the developed stimulator consisted in an hemispherical dome tilted with white LEDs in its peripheral part and its validation gave satisfactory results, obtaining waveforms similar to those obtained with conventional pERG. The purpose of this thesis is to build a stimulus source for electroretinography that is capable of presenting an arbitrary pattern of pixels to the entire visual field, thus bringing together the functionalities of Ganzfeld stimulators, standard monitors and ppERG stimulus source. The pattern of pixels shall be configurable to present stimuli for flash ERG, pERG, and mfERG protocols to any arbitrary sector of the visual field, giving also the possibility to use different colours.