A Theoretical Three Dimensional Electromagnetic Eye: Advances Towards the Optimisation of Electroretinographic Signal Recovery

Ocular electrophysiology is the study of electrical signals produced by the retina. In the past many different techniques have been used to provide indirect, non-invasive, objective measurements of retinal function. These measurements are routinely used to predict, and confirm diagnoses of, a wide range of retinal disorders. Electrophysiology has many shortcomings and new developments are currently addressing these. In recent years, advances have been made in the simultaneous stimulation of multiple focal areas of the retina. This has led to the production of three-dimensional topographical maps of visual function from signals of only nanovolt initial amplitude. A numerical three-dimensional computer program designed to calculate the electromagnetic potential distribution within the human eye has been developed from existing two-dimensional numerical models. This has been validated against two- dimensional analytical findings and various experiements to assess its robustness have been conducted. Modifications to the three-dimensional numerical model have enabled a more accurate retinal source photoreceptor distribution to be incorporated. Focal and ring stimulation of the three-dimensional retina has been performed and characteristic corneal distributions have been found. A possible application for investigation using this model has been found. It is known that diseases affecting the photoreceptors of the retina may often be detected using electrophysiology. Predictions of comeal potential changes following simulated retinal scotomas by the three-dimensional model may therefore prove of use. By 'mapping' the potential distribution over the corneal surface optimal recording sites for a number of retinal stimulation parameters may be found. A new type of electrode has been designed and built. Measurements made using this electrode have enabled further validation of theoretical potential distribution predictions. Further adaptation of the three-dimensional model is required including the provision of a mechanism by which the model may take account of the position of a reference electrode. Additional modification of the correction for retinal photoreceptor density is also needed due to the known variation of photoreceptor density changes in all dimensions. This three-dimensional model enables predictions to be made regarding the potential distribution at the electrode measurement site. This work shows in detail the variation in potential over the corneal / scleral surface. This insight should lead to further investigations regarding more accurate measurement techniques to facilitate optimal signal recovery. In future work it may be necessary for correction factors to be applied to various parts of the field if mono electrode recording is taking place when wide field peripheral stimulation is used