Abstract
This work presents and evaluates a 12-electrode intracranial electroencephalography system developed at the National Institute of Mental Health (Klecany, Czech Republic) in terms of an electrical source imaging (ESI) technique in rats. The electrode system was originally designed for translational research purposes. This study demonstrates that it is also possible to use this well-established system for ESI, and estimates its precision, accuracy, and limitations. Furthermore, this paper sets a methodological basis for future implants. Source localization quality is evaluated using three approaches based on surrogate data, physical phantom measurements, and in vivo experiments. The forward model for source localization is obtained from the FieldTrip-SimBio pipeline using the finite-element method. Rat brain tissue extracted from a magnetic resonance imaging template is approximated by a single-compartment homogeneous tetrahedral head model. Four inverse solvers were tested: standardized low-resolution brain electromagnetic tomography, exact low-resolution brain electromagnetic tomography (eLORETA), linear constrained minimum variance (LCMV), and dynamic imaging of coherent sources. Based on surrogate data, this paper evaluates the accuracy and precision of all solvers within the brain volume using error distance and reliability maps. The mean error distance over the whole brain was found to be the lowest in the eLORETA solution through signal to noise ratios (SNRs) (0.2 mm for 25 dB SNR). The LCMV outperformed eLORETA under higher SNR conditions, and exhibiting higher spatial precision. Both of these inverse solvers provided accurate results in a phantom experiment (1.6 mm mean error distance across shallow and 2.6 mm across subcortical testing dipoles). Utilizing the developed technique in freely moving rats, an auditory steady-state response experiment provided results in line with previously reported findings. The obtained results support the idea of utilizing a 12-electrode system for ESI and using it as a solid basis for the development of future ESI dedicated implants.
Highlights
In neuroscience, the rat model is widely used for studying brain disorders and investigating brain functions (Ellenbroek and Youn, 2016)
In the case of the deep testing dipoles, both algorithms again showed a similar accuracy of 2.6 mm on average. These results indicate that the 12-electrode system facilitates electrical source imaging (ESI) and it is a solid base for the development of future ESI dedicated implants
It has been shown that the proposed 12-electrode cortical EEG system allows rat brain activity to be reliably reconstructed from the potential recordings obtained using the electrode system
Summary
The rat model is widely used for studying brain disorders and investigating brain functions (Ellenbroek and Youn, 2016). While neuroimaging methods monitor brain activity as accurately as possible, one of the limitations of using these in rodents is the relatively small spatial resolution due to the small size of the brain. EEG in contrast to fMRI or PET has a relatively low spatial resolution, but a much higher temporal resolution enabling monitoring events within milliseconds. Due to the small volume of the rat brain (approximately 2 cm3), the volume conduction effect and low skull conductivity (Welniak-Kaminska et al, 2019), spatially accurate electrical source imaging (ESI) for testing small deep sources in the rat brain has not been extensively explored yet and remains a challenging and difficult task
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