Toward Electrical Impedance Tomography of Functional Activity in the Retina

Abstract

Electrical Impedance Tomography (EIT) is a non-invasive electrical imaging technique that reconstructs spatial variations in tissue conductivity. Traditionally used for thoracic imaging, EIT has recently been extended to neural applications in the brain and peripheral nerves. This study investigates the feasibility of using EIT to image functional neural activity in the retina. We present computational modeling of EIT measurements for image reconstruction of small conductivity perturbations across different layers of the ex-vivo mouse retina with electrodes placed in epiretinal configuration. Experimentally, we describe the design of a multi-electrode array suitable for impedance measurements under these conditions and report preliminary recordings of light-evoked impedance changes in response to wide-field optical retinal stimulation. Our computational results suggest that small conductivity perturbations of 200{micro}m diameter and 5% conductivity variation are experimentally detectable by high-end biopotential recording equipment and could be localized by EIT imaging within {approx}13% of inter-electrode distance and {approx}22% of retinal thickness, respectively. In preliminary recordings, we observed slow impedance variations of 13.0{+/-}9.3 {micro}V (0.37{+/-}0.44%) in response to retina-wide light pulse trains, when performing impedance measurements with 10 {micro}A applied current at 2-8 kHz. These variations were characterized by a slow time course, distinct from the fast neural EIT signals associated with action potentials, and may reflect extracellular or metabolic processes linked to retinal activation. Our modelling and technical results support the feasibility of retinal EIT, while the experimental observations are preliminary and motivate further controlled studies. Retinal EIT could significantly advance retinal electrophysiology research and prosthetic development.