Objective. Neuromodulation, particularly electrical stimulation, necessitates high spatial resolution to achieve artificial vision with high acuity. In epiretinal implants, this is hindered by the undesired activation of distal axons. Here, we investigate focal and axonal activation of retinal ganglion cells (RGCs) in epiretinal configuration for different sinusoidal stimulation frequencies. Approach. RGC responses to epiretinal sinusoidal stimulation at frequencies between 40 and 100 Hz were tested in ex-vivo photoreceptor degenerated (rd10) isolated retinae. Experiments were conducted using a high-density CMOS-based microelectrode array, which allows to localize RGC cell bodies and axons at high spatial resolution. Main results. We report current and charge density thresholds for focal and distal axon activation at stimulation frequencies of 40, 60, 80, and 100 Hz for an electrode size with an effective area of 0.01 mm². Activation of distal axons is avoided up to a stimulation amplitude of 0.23 µA (corresponding to 17.3 µC cm⁻²) at 40 Hz and up to a stimulation amplitude of 0.28 µA (14.8 µC cm⁻²) at 60 Hz. The threshold ratio between focal and axonal activation increases from 1.1 for 100 Hz up to 1.6 for 60 Hz, while at 40 Hz stimulation frequency, almost no axonal responses were detected in the tested intensity range. With the use of synaptic blockers, we demonstrate the underlying direct activation mechanism of the ganglion cells. Finally, using high-resolution electrical imaging and label-free electrophysiological axon tracking, we demonstrate the extent of activation in axon bundles. Significance. Our results can be exploited to define a spatially selective stimulation strategy avoiding axonal activation in future retinal implants, thereby solving one of the major limitations of artificial vision. The results may be extended to other fields of neuroprosthetics to achieve selective focal electrical stimulation.

中文翻译：

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通过正弦视网膜前刺激避免轴突刺激

客观的。神经调节，特别是电刺激，需要高空间分辨率才能实现高敏锐度的人工视觉。在视网膜前植入物中，这会受到远端轴突不期望的激活的阻碍。在这里，我们研究了不同正弦刺激频率下视网膜前结构中视网膜神经节细胞（RGC）的局灶和轴突激活。方法。在 40 至 100 Hz 之间的频率下测试了 RGC 对视网膜前正弦刺激的反应离体光感受器退化（rd10）孤立的视网膜。实验使用基于高密度 CMOS 的微电极阵列进行，该阵列允许以高空间分辨率定位 RGC 细胞体和轴突。主要结果。^{我们报告了有效面积为 0.01 mm 2}的电极尺寸在 40、60、80 和 100 Hz 刺激频率下局部和远端轴突激活的电流和电荷密度阈值。刺激幅度高达 0.23 时可避免远端轴突的激活µA（对应17.3µC cm ^-2 ) 在 40 Hz 和高达 0.28 的刺激幅度µ甲（14.8µC cm ^-2 )，60 Hz。局灶性和轴突激活之间的阈值比从 100 Hz 的 1.1 增加到 60 Hz 的 1.6，而在 40 Hz 刺激频率下，在测试强度范围内几乎没有检测到轴突反应。通过使用突触阻滞剂，我们证明了神经节细胞的潜在直接激活机制。最后，使用高分辨率电成像和无标记电生理轴突跟踪，我们证明了轴突束的激活程度。意义。我们的结果可用于定义空间选择性刺激策略，避免未来视网膜植入物中的轴突激活，从而解决人工视觉的主要局限性之一。该结果可能会扩展到神经修复学的其他领域，以实现选择性局部电刺激。