An Evaluation of the Conductivity Profile in the Somatosensory Barrel Cortex of Wistar Rats

Abstract

Microelectrode arrays used to record local field potentials from the brain are being built with increasingly more spatial resolution, ranging from the initially developed laminar arrays to those with planar and three-dimensional (3D) formats. In parallel with such development in recording techniques, current source density (CSD) analyses have recently been expanded up to the continuous-3D form. Unfortunately, the effect of the conductivity profile on the CSD analysis performed with contemporary microelectrode arrays has not yet been evaluated and most of the studies assumed it was homogeneous and isotropic. In this study, we measured the conductivity profile in the somatosensory barrel cortex of Wistar rats. To that end, we combined multisite electrophysiological data recorded with a homemade assembly of silicon-based probes and a nonlinear least-squares algorithm that implicitly assumed that the cerebral cortex of rodents could be locally approximated as a layered anisotropic spherical volume conductor. The eccentricity of the six cortical layers in the somatosensory barrel cortex was evaluated from postmortem histological images. We provided evidence for the local spherical character of the entire barrels field, with concentric cortical layers. We found significant laminar dependencies in the conductivity values with radial/tangential anisotropies. These results were in agreement with the layer-dependent orientations of myelinated axons, but hardly related to densities of cells. Finally, we demonstrated through simulations that ignoring the real conductivity profile in the somatosensory barrel cortex of rats caused considerable errors in the CSD reconstruction, with pronounced effects on the continuous-3D form and charge-unbalanced CSD. We concluded that the conductivity profile must be included in future developments of CSD analysis, especially for rodents.

Pitfalls in the dipolar model for the neocortical EEG sources

“…It has been shown that, as a consequence of two major dielectric relaxation mechanisms (i.e., the counterion and interfacial polarizations), the brain tissues are highly dispersive for the frequency range of the electrophysiological recordings (Gabriel et al 1996(Gabriel et al , 2009). In addition, the conductivity and permittivity depend on the location (i.e., inhomogeneity) and direction (i.e., anisotropy) inside the brain (Gabriel et al 2009;Goto et al 2010). Therefore, as charge moves, its effect on externally measured fields can depend on location.…”

“…The parameters used in the vCSD method were 1) the intergrid distance ⌬, which was 50 m, and 2) the radial/tangential conductivities and radii for all cortical layers, which were previously estimated by Goto et al (2010) for the somatosensory cortex of adult Wistar rats. We applied the average reference operator (Pascual-Marqui 1999) to the Green's function matrices, event-related LFPs, and SRPs to remove undesirable signals from the reference electrode (Bertrand et al 1985).…”

“…A mesoscopic cortical region (i.e., a barrel) comprises 2 tissues, which could represent supragranular (tissue A) and infragranular (tissue B) layers. In the somatosensory cortex of rats, these layers have been found to have different conductivity values at 500 Hz (Goto et al 2010). In addition, these tissues have different spectral characteristics for the electric conductivity (left), with the particularity that, for example, tissue A is less conductive than tissue B for the frequency range of the LFP.…”

Psyche, Signals and Systems

“…It turns out that this question has remained unresolved, with a number of studies reporting an anisotropic and homogeneous σ (Nicholson and Freeman 1975;Logothetis et al 2007) (Fig. 3d, e) to strongly anisotropic and inhomogeneous (Goto et al 2010;Hoeltzell and Dykes 1979;Ranck 1973) and, finally, even of capacitive nature (Bédard and Destexhe 2009;Bédard et al 2004;Gabriel et al 1996).…”

“…Yet, others found the extracellular medium to be strongly anisotropic and inhomogeneous (Hoeltzell and Dykes 1979;Ranck 1973) or even of capacitive nature (Gabriel et al 1996;Bédard et al 2004;Bédard and Destexhe 2009). In a more recent study, Goto et al (2010) used extracellular recordings to measure the conductivity profile along the entire somatosensory barrel cortex in rodents using depth multi-electrode recordings and reported that radial and tangential conductivity values varied consistently across the six neocortical laminas. Thus, they showed that the electric properties of the extracellular medium in the living animal were anisotropic and inherently inhomogeneous, agreeing with the in vitro findings of Anastassiou et al (2015).…”