Abstract
Multi-electrode recordings of local field potentials (LFPs) provide the opportunity to investigate the spatiotemporal organization of neural activity on the scale of several millimeters. In particular, the phases of oscillatory LFPs allow studying the coordination of neural oscillations in time and space and to tie it to cognitive processing. Given the computational roles of LFP phases, it is important to know how they relate to the phases of the underlying current source densities (CSDs) that generate them. Although CSDs and LFPs are distinct physical quantities, they are often (implicitly) identified when interpreting experimental observations. That this identification is problematic is clear from the fact that LFP phases change when switching to different electrode montages, while the underlying CSD phases remain unchanged. In this study we use a volume-conductor model to characterize discrepancies between LFP and CSD phase-patterns, to identify the contributing factors, and to assess the effect of different electrode montages. Although we focus on cortical LFPs recorded with two-dimensional (Utah) arrays, our findings are also relevant for other electrode configurations. We found that the main factors that determine the discrepancy between CSD and LFP phase-patterns are the frequency of the neural oscillations and the extent to which the laminar CSD profile is balanced. Furthermore, the presence of laminar phase-differences in cortical oscillations, as commonly observed in experiments, precludes identifying LFP phases with those of the CSD oscillations at a given cortical depth. This observation potentially complicates the interpretation of spike-LFP coherence and spike-triggered LFP averages. With respect to reference strategies, we found that the average-reference montage leads to larger discrepancies between LFP and CSD phases as compared with the referential montage, while the Laplacian montage reduces these discrepancies. We therefore advice to conduct analysis of two-dimensional LFP recordings using the Laplacian montage.
Highlights
Multi-electrode recordings of local field potentials (LFPs) offer the possibility to monitor cortical activity with high spatiotemporal resolution
Before systematically comparing LFP and current source densities (CSDs) phase-patterns through simulations and identifying the factors that lead to discrepancies, in Section The LFP as an Integrated Signal we provide some intuition for the effects of volume-conduction and in Section Phase-Contraction we discuss a simple example that can be analyzed mathematically
Equation (2) is the formal solution of Poisson’s equation and shows how the LFP arises from the volume density of transmembrane currents: At any particular location, the LFP is generated by integrating the CSD over the tissue volume, where the CSDs contribution is weighted by the distance to the electrode tip
Summary
Multi-electrode recordings of local field potentials (LFPs) offer the possibility to monitor cortical activity with high spatiotemporal resolution. While LFPs have traditionally been studied almost exclusively in the temporal domain, the development of multi-electrode arrays has forced us to treat cortical activity as continuous in space. This implies that all functional roles of cortical oscillations and their temporal coordination (Fries, 2005) can be recast into a broader framework (Maris et al, 2016). Propagating waves could subserve any function that relies on spatiotemporal modulation of neural excitability, such as sensory attention and, more generally, prioritizing of information streams (Wu et al, 2008; Zanos et al, 2015), phase-encoding in case of oscillatory waves (Ermentrout and Kleinfeld, 2001; Agarwal et al, 2014), and stimulus integration and segregation (Wu et al, 2008; Reimer et al, 2011; Sato et al, 2012; Zheng and Yao, 2012)
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