Abstract
Planar intra-cortical electrode (Utah) arrays provide a unique window into the spatial organization of cortical activity. Reconstruction of the current source density (CSD) underlying such recordings, however, requires “inverting” Poisson’s equation. For inter-laminar recordings, this is commonly done by the CSD method, which consists in taking the second-order spatial derivative of the recorded local field potentials (LFPs). Although the CSD method has been tremendously successful in mapping the current generators underlying inter-laminar LFPs, its application to planar recordings is more challenging. While for inter-laminar recordings the CSD method seems reasonably robust against violations of its assumptions, is it unclear as to what extent this holds for planar recordings. One of the objectives of this study is to characterize the conditions under which the CSD method can be successfully applied to Utah array data. Using forward modeling, we find that for spatially coherent CSDs, the CSD method yields inaccurate reconstructions due to volume-conducted contamination from currents in deeper cortical layers. An alternative approach is to “invert” a constructed forward model. The advantage of this approach is that any a priori knowledge about the geometrical and electrical properties of the tissue can be taken into account. Although several inverse methods have been proposed for LFP data, the applicability of existing electroencephalographic (EEG) and magnetoencephalographic (MEG) inverse methods to LFP data is largely unexplored. Another objective of our study therefore, is to assess the applicability of the most commonly used EEG/MEG inverse methods to Utah array data. Our main conclusion is that these inverse methods provide more accurate CSD reconstructions than the CSD method. We illustrate the inverse methods using event-related potentials recorded from primary visual cortex of a macaque monkey during a motion discrimination task.
Highlights
Multi-electrode recordings of extra-cellular potentials (LFPs) provide a window into the mesoscopic organization of neuronal activity and are a valuable tool in cognitive and perceptual neuroscience [1,2,3,4,5,6,7]
Since neural correlates of cognitive and perceptual processes are to be expressed in terms of transmembrane currents, it is of importance to understand the relationship between the dynamics of current source density (CSD) and that of the ensuing local field potentials (LFPs) in any particular experimental set-up
Concerning the methods, we focus on the most commonly used distributed inverse methods: the minimum norm estimate (MNE), the weighted minimum norm estimate (WMNE), dynamic statistical parametric mapping, standardized low resolution electromagnetic tomography, and low resolution electrical tomography (LORETA) [31]
Summary
Multi-electrode recordings of extra-cellular potentials (LFPs) provide a window into the mesoscopic organization of neuronal activity and are a valuable tool in cognitive and perceptual neuroscience [1,2,3,4,5,6,7]. The physiological content of LFPs is at present not completely understood [8,9,10,11], their biophysical origin has been clarified [12]: Extra-cellular potentials reflect volume-conducted transmembrane currents that can be described by a (volume) current source density (CSD). In case of planar recordings of cortical LFPs—which are obtained by inserting a two-dimensional electrode array into cortical tissue at a certain depth—volume-conduction leads to increased propagation speeds and spatial coherence and that these discrepancies between LFP and CSD strongly depend on the inter-laminar organization of the CSD [13]. It is of considerable interest to find out if the transmembrane currents can be reconstructed from the observed extra-cellular potentials
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