Habib Amgad G, Cameron Morven A, Suaning Gregg J, Lovell Nigel H, Morley John W
Graduate School of Biomedical Engineering, University of New South Wales, Sydney, Australia.
Annu Int Conf IEEE Eng Med Biol Soc. 2012;2012:2776-9. doi: 10.1109/EMBC.2012.6346540.
Retinal visual prostheses provide hope of restoring sight to patients suffering from retinal degeneration such as retinitis pigmentosa and age-related macular degeneration. Retinal prostheses are used to electrically stimulate residual neurons that are spared in these diseases, namely the retinal ganglion cells (RGCs), eliciting percepts of light termed 'phosphenes'. The elicitation of multiple phosphenes via an electrode array allows patterns to be produced, resulting in a rudimentary form of vision. For such patterns to be produced effectively, the prosthesis must generate well-defined phosphenes. To this end, the hexpolar configuration has been proposed as an alternative to the traditional monopolar or bipolar configurations. It utilizes six electrodes surrounding the stimulating electrode to serve as a combined return, or 'hex guard', purportedly localizing the activation to cells located within them. In this study, the efficacy of the hexpolar configuration in localizing activity was investigated by using patch-clamp electrophysiology to measure the activation thresholds of RGCs to electrical stimulation in isolated rabbit retina. Cells located outside the hex guard were found to have significantly higher relative hexpolar thresholds (>2 fold) as compared to cells located within the hex guard. This confirms the efficacy of the hexpolar configuration in localizing activity to within the hex guard. Furthermore, the effect of using cathodic-first versus anodic-first stimulation on hexpolar threshold and localization was investigated. No significant difference was observed between the two groups, in terms of lowering thresholds or improving localization.
视网膜视觉假体为患有视网膜变性（如色素性视网膜炎和年龄相关性黄斑变性）的患者恢复视力带来了希望。视网膜假体用于电刺激在这些疾病中得以保留的残余神经元，即视网膜神经节细胞（RGCs），从而引发被称为“光幻视”的光感。通过电极阵列激发多个光幻视可产生图案，进而形成一种基本的视觉形式。为了有效产生此类图案，假体必须产生清晰明确的光幻视。为此，已提出采用六极配置作为传统单极或双极配置的替代方案。它利用围绕刺激电极的六个电极作为组合回路，即“六极防护”，据称可将激活定位到位于其中的细胞。在本研究中，通过使用膜片钳电生理学来测量离体兔视网膜中RGCs对电刺激的激活阈值，研究了六极配置在定位活性方面的功效。结果发现，与位于六极防护内的细胞相比，位于六极防护外的细胞具有显著更高的相对六极阈值（>2倍）。这证实了六极配置在将活性定位到六极防护内的功效。此外，还研究了采用阴极优先与阳极优先刺激对六极阈值和定位的影响。在降低阈值或改善定位方面，两组之间未观察到显著差异。
