Article
Mapping the electrical resistivity of retinal layers
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Authors
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Anna Kochnev Goldstein
- Department of Electrical Engineering, Stanford University, Stanford, CA, USA
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S.V. Shah
- School of Medicine, Stanford University, Stanford, CA, USA
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Z.C. Chen
- Hansen Experimental Physics Laboratory, Stanford University, Stanford, CA, USA
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P. Vasireddy
- Department of Electrical Engineering, Stanford University, Stanford, CA, USA
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A.J. Phillips
- Department of Electrical Engineering, Stanford University, Stanford, CA, USA
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M. Bhuckory
- Hansen Experimental Physics Laboratory, Stanford University, Stanford, CA, USA; Department of Ophthalmology, Stanford University, Stanford, CA, USA
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D. Palanker
- Hansen Experimental Physics Laboratory, Stanford University, Stanford, CA, USA; Department of Ophthalmology, Stanford University, Stanford, CA, USA
| Published: | May 9, 2025 |
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Outline
Text
The design of retinal prostheses requires proper computational modeling of the resulting electrical stimulation of various retinal neurons. Such modeling, in turn, greatly depends on accurate characterization of the resistivity of the retinal layers. However, previous studies report a wide range of values, and many treat the retina as a uniform medium, ignoring its layered structure. We applied electrical impedance tomography to address these issues and obtain a comprehensive profile of retinal resistivity.
Electrical impedance tomography was performed ex-vivo on six Long-Evans rat retinas using a flat multielectrode array with a 30 µm electrode spacing. An electric current of 4 to 60 nA was injected from 27 electrodes, and the resulting voltage was recorded from all other electrodes. The obtained voltage measurements were calibrated using two saline solutions of known resistivity and processed for noise reduction. A 2D resistivity profile of the retinal layers as a function of depth was then reconstructed by feeding the processed data into the Res2DInv inversion software. OCT imaging was used to identify the thicknesses of the different layers.
The inversion results provide a complete 2D resistivity profile of the retina and showcase the complex nature of the retinal layers. There appears to be a clear difference between the inner and outer retinal layers. While the resistivity of the photoreceptor layer ranges from 220 to 520 Ohm*cm, the INL and the IPL layers can be as resistive as 2,500 Ohm*cm.
Electrical impedance tomography allows measuring the complete retinal resistivity profile without using penetrating probes, which distort the measured tissue. These measurements reveal the retina’s complex electrical structure, which should be accounted for when modeling electrical stimulation. Electrode size and pitch limit the axial resolution of the resistivity map, and the measurement noise limits its accuracy.
* A. Kochnev Goldstein and S.V. Shah contributed equally to the publication.
