Protti, D. | bioRxiv Science

bioRxiv2026

Toward Electrical Impedance Tomography of Functional Activity in the Retina

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.

neuroscience↗

bioRxiv2019

The temporal structure of the inner retina at a single glance

The retina decomposes visual stimuli into parallel channels that encode different features of the visual environment. Central to this computation is the synaptic processing in a dense and thick layer of neuropil, the so-called inner plexiform layer (IPL). Here, different types of bipolar cells stratifying at distinct depths relay the excitatory feedforward drive from photoreceptors to amacrine and ganglion cells. Current experimental techniques for studying processing in the IPL do not allow imaging the entire IPL simultaneously in the intact tissue. Here, we extend a two-photon microscope with an electrically tunable lens allowing us to obtain optical vertical slices of the IPL, which provide a complete picture of the response diversity of bipolar cells at a \"single glance\". The nature of these axial recordings additionally allowed us to isolate and investigate batch effects, i.e. inter-experimental variations resulting in systematic differences in response speed. As a proof of principle, we developed a simple model that disentangles biological from experimental causes of variability, and allowed us to recover the characteristic gradient of response speeds across the IPL with higher precision than before. Our new framework will make it possible to study the computations performed in the central synaptic layer of the retina more efficiently.

neuroscience↗