P151 Between near and far-fields: influence of neuronal morphology and channel density on EEG-like signals
Paula T. Kuokkanen*1, Richard Kempter1,2,3, Catherine E. Carr4, Christine Köppl5
1Institute for Theoretical Biology, Humboldt-Universität zu Berlin, 10115 Berlin, Germany 2Bernstein Center for Computational Neuroscience Berlin, 10115 Berlin, Germany 3Einstein Center for Neurosciences Berlin, 10115 Berlin, Germany 4Department of Biology, University of Maryland College Park, College Park, MD 20742 5Department of Neuroscience, School of Medicine and Health Sciences, Research Center for Neurosensory Sciences and Cluster of Excellence “Hearing4all” Carl von Ossietzky University, 26129 Oldenburg, Germany
*Email: paula.kuokkanen@hu-berlin.de Introduction
Both the near and far fields of extracellular neural recordings are well understood. The near field can be explained by models of ion channel activity in nearby compartments [1]. The far field can be approximated by current dipoles produced by the membrane currents of multicompartmental cells [2]. The dipole spanned between the dendrites and soma is typically assumed to be the basis of the electro-encephalography (EEG) signals of cortical pyramidal neurons [e.g. 3]; yet also their somatic spikes can be observed in the EEG [4]. Such potentials, measured relatively far away from the source but not strictly in the far field, are highly dependent on the morphology of the cell, its ion channel concentrations, and the electrodes’ positions [5].
Methods We simulate single multi-compartment cells with NEURON and LFPy packages to study their 'mid-field' potentials. We vary the neurons’ simplified morphologies systematically, and use combinations of channel densities to compare the mid-field potentials with the dipole moments of the cells. We especially study the spatial limitation of the far-field approximation depending on the cell properties. We verify our results with the use of experimental data [6], EEG-like single-cell recordings from the auditory nerve and the auditory brainstem Nucleus Magnocellularis in the barn owl.
Results We observe that, as expected, the dendritic-somatic dipole can determine the far and mid-fields in pyramidal cell-like morphologies. Unexpectedly, we observe that a dipole moment caused by branching axons can have a similar amplitude to the dendritic dipole in mid and far fields. Furthermore, we show that under certain conditions a somatic spike — not necessarily related to any current dipole — can contribute to fields even at a distance of 10 mm from the soma. These results match with our experimental results from the owl. Discussion Common assumptions about the distances from a neuronal source where far-field conditions predominate may not hold. Depending on the neuron type, both the morphology and differential densities of active ion channels across cell compartments can play a large role in creating their fields at varying distances. The axonal arborizations, because activated simultaneously by a single spike, can create a dipole [7] with a surprisingly large contribution to the far fields as compared to the dendritic-somatic dipoles. Furthermore, large somata with high densities of active currents can contribute to the extracellular field at distances of even 1 cm, violating the usual far-field assumption.
Acknowledgements We thank Ghadi ElHasbani for helpful discussions, and Hannah Schultheiss for preliminary modeling. This research was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) grant nr. 502188599. References 1. https://doi.org/10.1152/jn.00979.2005 2. https://doi.org/10.1016/j.neuroimage.2020.117467 3. https://doi.org/10.7554/eLife.51214 4. https://doi.org/10.1016/j.neuroimage.2014.12.057 5. http://doi.org/10.1097/00004691-199709000-00009 6. https://doi.org/10.1101/2024.05.29.596509 7. https://doi.org/10.7554/eLife.26106
