Abstract
Imaging of neuronal depolarization in the brain is a major goal in neuroscience, but no technique currently exists that could image neural activity over milliseconds throughout the whole brain. Electrical impedance tomography (EIT) is an emerging medical imaging technique which can produce tomographic images of impedance changes with non-invasive surface electrodes. We report EIT imaging of impedance changes in rat somatosensory cerebral cortex with a resolution of 2ms and <200μm during evoked potentials using epicortical arrays with 30 electrodes. Images were validated with local field potential recordings and current source-sink density analysis. Our results demonstrate that EIT can image neural activity in a volume 7×5×2mm in somatosensory cerebral cortex with reduced invasiveness, greater resolution and imaging volume than other methods. Modeling indicates similar resolutions are feasible throughout the entire brain so this technique, uniquely, has the potential to image functional connectivity of cortical and subcortical structures.
Highlights
There is currently great interest in imaging depolarization and spiking in neuronal cell bodies or their processes in the brain, in order to validate and refine computational models of neuronal processing (Kopell et al, 2014; Sporns, 2014)
Both optical methods have an imaging volume of b1 mm3; microelectrode arrays sample over a few cubic millimeters (Kajikawa and Schroeder, 2011), but are penetrating and cause local tissue disruption. fMRI enables imaging throughout the brain, but images hemodynamic changes over seconds (Heeger and Ress, 2002), rather than true neural activity over milliseconds
Electrical impedance tomography (EIT) arrays comprised 30 platinized, stainless-steel electrodes embedded in silicone, with contacts 0.6 mm in diameter and centers offset in a triangular pattern 1.2 mm apart
Summary
There is currently great interest in imaging depolarization and spiking in neuronal cell bodies or their processes in the brain, in order to validate and refine computational models of neuronal processing (Kopell et al, 2014; Sporns, 2014). A population level resolution of micrometers may be achieved with multiple microelectrodes as in the Utah array (Maynard et al, 1997) or optical methods such as twophoton imaging of calcium indicators or voltage sensitive optogenetic models (Ahrens et al, 2013; Hillman, 2007). Electrical impedance tomography (EIT) is a technique which has the unique potential to image fast neural activity over milliseconds throughout the brain, using non-penetrating surface electrodes. In EIT, images are reconstructed from multiple measurements of transfer impedance made with surface electrodes. For the first time, the implementation of EIT to image cortical neural activity, using a 30-electrode epicortical planar array. Functional connectivity was extracted from impedance images using dynamic analysis This revealed that the depth of largest lateral spread was at layer II/III, and occurred predominantly along barrel rows. Simulations indicate that similar resolutions are feasible throughout the entire brain and so we anticipate EIT will enable mapping of functional connectivity of cortical and subcortical structures
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